Prepared by the Health and Safety Laboratory for the Health and Safety Executive 2010

Health and Safety Executive

Testing of high flow rate respirable samplers to assess the technical feasibility of measuring 0.05 mg.m-3 respirable crystalline silica

RR825Research Report

Peter Stacey and Andrew Thorpe Health and Safety LaboratoryHarpur HillBuxton DerbyshireSK17 9JN

Testing of high flow rate samplers to assess the technical feasibility of measuring 0.05 mg.m-3 respirable crystalline silica.

This report describes testing of five personal respirable dust samplers operating with flow rates of 4 l/min or greater, available in 2008. Three were commercially available, one a prototype and one adapted at HSL to operate at a higher flow rate. Testing compared these samplers with a reference sampler, operating at 2.2 l/min, to ascertain if an increase in the mass of dust sampled could improve the reliability of measurements of respirable crystalline silica (RCS). None of the samplers satisfied all of the success criteria for the project, which included, the ability to maintain the specified flow rate over 4-hours, ease of use in the workplace, and an improvement in the measurement precision without additional complications caused by the increased mass of sampled dust. Infrared analysis is not recommended for samples with dust mixtures, because it was difficult to obtain a reliable result when the loading exceeds 1 mg. The samplers with the best performance were the PGP10 and the modified GK2.69 samplers. The other samplers tested either under-sampled or there was lost sample during transfer onto the analysis filter. When field tests were conducted, air sampling pumps operating with the modified GK2.69 samplers failed to maintain a consistent flow rate, and the PGP 10 samplers were heavy and caused discomfort for the workers The report recommends the use of the PGP 10 and GK2.69 samplers after further work to resolve the minor issues and changes in the sampling and measurement strategies to accommodate new procedures for use of higher flow rate samplers.

This report and the work it describes were funded by the Health and Safety Executive (HSE). Its contents, including any opinions and/or conclusions expressed, are those of the authors alone and do not necessarily reflect HSE policy.

Testing of high flow rate respirable samplers to assess the technical feasibility of measuring 0.05 mg.m-3 respirable crystalline silica

HSE Books

Health and Safety Executive

© Crown copyright 2010

First published 2010

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CONTENTS

1 INTRODUCTION......................................................................................... 1 1.1 Background ............................................................................................. 1 1.2 Principles of the analysis techniques....................................................... 1

2 STAGE 1: SELECTION OF EQUIPMENT INCLUDED IN THE STUDY..... 4 2.1 Selection of Samplers.............................................................................. 4 2.2 Selection of filters .................................................................................... 4 2.3 Selection of sampling trains..................................................................... 4

3 CALIBRATIONS ....................................................................................... 10 3.1 Calibrations for x-ray diffraction ............................................................. 10 3.2 Calibrations for infrared analysis ........................................................... 10 3.3 Discussion ............................................................................................. 10

4 STAGE 2B: ABSORPTION AND DEPTH EXPERIMENTS...................... 12 4.1 Evaluation of absorption and depth effects in x-ray diffraction analysis. 12 4.2 Evaluation of the effect of absorption on FTIR analysis......................... 20

5 STAGE 3: ASSESSMENT OF THE BIAS OF SAMPLERS...................... 23 5.1 Sampling tests with Arizona road dust................................................... 23 5.2 Gravimetric Analysis .............................................................................. 24 5.3 RCS Analysis by X-ray Diffraction ......................................................... 25 5.4 RCS Analysis by Direct on Filter Infrared Analysis ................................ 26 5.5 Recovery for the PGP 10 cyclone using cellulose nitrate filters............. 27

6 STAGE 4: ASSESSMENT OF BIAS OF ANALYTICAL TECHNIQUES .. 29 6.1 Workplace tasks examined.................................................................... 29 6.2 Particle SIZE distribution of the generated aerosol................................ 30 6.3 Gravimetric analysis .............................................................................. 30 6.4 X-ray Diffraction analysis ....................................................................... 33

7 FIELD TRIALS.......................................................................................... 36 7.1 Approach ............................................................................................... 36 7.2 Analytical Results .................................................................................. 36 7.3 Practical experience .............................................................................. 39 7.4 Comments recorded from the Workers.................................................. 39

8 OVERALL PERFORMANCE OF SAMPLERS ......................................... 44 8.1 Modified GK 2.69 cyclone...................................................................... 44 8.2 IOM sampler with foam separator.......................................................... 448.3 IPP Impactor.......................................................................................... 44 8.4 PGP 10 Cyclone .................................................................................... 45 8.5 CIP 10 SAMPLER ................................................................................. 45

9 PRECISION OF XRD MEASUREMENTS AT 0.05 MG.M-3 ...................... 46

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10 PROJECT SUCCESS CRITERIA.......................................................... 49

11 REFERENCES ...................................................................................... 52

12 APPENDIX 1: PRESSURE DROP WITH FLOW RATE ACROSS AN MIXED CELLULOSE ESTER FILTER............................................................. 54

13 APPENDIX 2: INSTRUMENTAL PARAMETERS................................. 56 13.1 X-ray Diffraction..................................................................................... 56 13.2 FTIR....................................................................................................... 56

14 APPENDIX 3: CALIBRATIONS FOR X-RAY DIFFRACTION .............. 57 14.1 SIMPEDS Calibrations........................................................................... 5714.2 GK 2.69 cyclone Calibrations ................................................................ 57 14.3 IOM sampler wih foam separator........................................................... 58

15 APPENDIX 4: CALIBRATION FOR THE INDIRECT ANALYSIS PROCEDURE ..................................................................................................59

16 APPENDIX 5: CALIBRATIONS FOR INFRARED – DIRECT ON-FILTER ANALYSIS ....................................................................................................... 60 16.1 SIMPEDS .............................................................................................. 60 16.2 GK 2.69 cyclone .................................................................................... 60 16.3 IOM sampler with foam separator.......................................................... 61

17 APPENDIX 6: CALIBRATIONS FOR INFRARED – INDIRECT ANALYSIS ....................................................................................................... 62

18 APPENDIX 7: XRD SCANS OF ABSORPTION TEST MATERIALS ... 64

19 APPENDIX 8: INFRA RED ABSORNACES ......................................... 67

20 APPENDIX 9: PARTICLE SIZE DISTRIBUTIONS FROM SIMULATED WORK TASKS................................................................................................. 69

21 APPENDIX 10: GRAVIMETRIC ANALYSIS WITH SIMULATED WORK ACTIVITIES...................................................................................................... 71

22 APPENDIX 11. DUST LOSSES FROM PVC FILTERS ........................ 72

23 APPENDIX 12: STAGE 4 XRD COMPARISON WITH SIMPEDS ........ 75

24 APPENDIX 13 STAGE 4: COMPARISON WITH PGP 10 CYCLONES 77

25 SITE 1: FOUNDRY VISIT...................................................................... 79

26 SITE 2: POT/BRICK MANUFACTURE................................................. 94

27 SITE 3: CERAMICS MANUFACTURE................................................ 105

28 SITE 4 CONSTRUCTION.................................................................... 115

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29 SITE 5: QUARRY VISIT ...................................................................... 123

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EXECUTIVE SUMMARY

Objectives

This project investigated the technical feasibility of using high volume personal respirable dust samplers (> 4 l/min) to improve the reliability of measurements of respirable crystalline silica (RCS) using x-ray diffraction (XRD) and Fourier Transform Infra Red (FTIR) analysis in order to support further reductions in the United Kingdom’s (UK) Workplace Exposure Limit (WEL).

The objective was to evaluate the samplers against the following criteria:

• Can the nominal flow rate of the sampler be maintained over typical sampling periods (within ± 5%)?

• Does the sampler have a performance comparable with the SIMPEDS respirable dust sampler recommended in the Health and Safety Executive (HSE) method MDHS 14 for dust sampling (HSE 2000)?

• Is the sampler be comfortable to wear without interfering with the activity of the worker and is it applicable for use in UK workplaces?

• Does the increased mass of dust collected with the sampler affect RCS measurement by XRD and FTIR?

• Does the increased mass of dust collected with the sampler increase interferences?

• Would the precision of measurement at the proposed WEL of 0.05 mg.m-3 be equal to or better than the precision of ± 12% (2σ) at the current WEL of 0.1 mg.m-3?

Five high flow rate (> 4 l/min) personal samplers were evaluated in this project: a GK 2.69 cyclone, modified to use 25 mm rather than 37 mm diameter filters, operating at 4 l/min; the PGP 10 cyclone, operating at 10 l/min; a prototype IPP impactor, operating at 8 l/min; an IOM sampler with a foam separator selected to operate at 4 l/min; and the CIP 10 sampler, operating at 10 l/min.

Main Findings

Sampling

a) Pump performance

• For a flow rate of 4 l/min through a filter with a 0.8 μm pore size, the sampling pump needs to cope with a backpressure greater than 25 inches of water under load.

• The sampling pump used with the modified GK 2.69 cyclone failed to maintain the nominal flow rate within ± 5% in the field tests when conducting 4 hour sampling.

• The pumps used with the PGP 10 and IPP samplers in the field trails maintained their nominal flow rates during the sampling periods.

b) Respirable dust collection

• The GK 2.69 cyclone, operating at 4 l/min, and the PGP 10 cyclone, operating at 10 l/min, gave comparable results to the SIMPEDS for respirable dust.

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• The prototype IPP impactor tended to slightly under sample respirable dust compared with the SIMPEDS (~15%), possibly associated with the potential for loss of dust from handling the filter.

• The IOM sampler with a foam separator for respirable dust selected to operate at 4 l/min tended to under sample compared with the SIMPEDS (~22%) at high loadings (> 1.26 mg) and results were more variable than for other samplers. It is suspected that the collection of dust eventually changes alters the performance of the foam separator.

• In these tests, the CIP 10 sampler tended to under sampled respirable dust compared with the SIMPEDS (28 – 34%).

c) Practical use

• The PGP 10 cyclones and their pumps were heavy and interfered with the worker’s activities.

• The IPP impactor is small and compact but required the same heavy pumps that were

used for the PGP 10 cyclones. • The GK 2.69 cyclones and their pumps were lighter compared with the PGP 10 cyclone

and pump, and did not interfere with the workers activities. Measurement a) X-ray diffraction

• The GK 2.69 cyclone and the PGP 10 cyclone gave RCS results that closely matched those obtained with the SIMPEDS.

• The IOM sampler with foam separator gave RCS results that closely matched the SIMPEDS, when the sample loading was < 1.26 mg. However, the XRD results reported less respirable quartz.

• The IPP impactor obtained RCS results that were comparable to those obtained with the SIMPEDS when measuring Arizona Road Dust (ARD), which contains about 70% crystalline silica. However, when sampling in simulated workplace conditions it tended to report less crystalline quartz than the SIMPEDS, PGP 10 cyclone and GK 2.69 cyclone. This could have been due to losses during recovery of dust for analysis.

• Using a silver filter can reduce the calibration uncertainty of by about half.

• XRD response to crystalline silica can be reduced for samples with heavy dust loadings (> 2 mg) because of absorption by sample matrix. This study found that accurate corrections for the intensity are possible using the reflections from a silver filter as an internal standard with an indirect analysis approach, in which the dust on the sample filter is recovered and deposited onto a silver filter for analysis using filtration apparatus with 15 mm diameter funnel.

• The study also found that the response from the measurement of 2% respirable quartz, in a material with a mass absorption coefficient of 156 cm2/g, across a 15 mm diameter deposit on a filter was a linear up to 2 mg of dust. The absorption and depth effects will

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not be as significant when measuring RCS in an air sample filter with the deposit spread over an active diameter of 20-22 mm. This finding indicates that the current guidance in HSE method MDHS 101 (HSE 2005) is unnecessarily cautious for XRD measurements and should be increased to 3 mg.

• The response of 2% quartz in a material with mass absorption coefficient similar to quartz (34.8 g/cm2) was linear up to about 3 mg when analysing a 15 mm diameter deposit with the instrumental parameters used at HSL.

• Little benefit is gained by using high volume samplers when the percentage of quartz in a matrix with an absorption coefficient of 71 cm2/g is less than 1% since no improvement in intensity is made despite the greater mass of dust. This is the almost same percentage of quartz in an air sample containing 0.05 mg.m-3 of RCS in 4 mg.m-3 of respirable dust.

b) Infrared analysis

• Infrared analysis should not be used as an analysis technique for samples with high loadings (> 2 mg) because the absorbance, when using the direct on-filter analysis procedure, is dependent on the matrix.

• FTIR gave comparable results to XRD when analysing ARD.

Can a lower WEL for RCS be measured reliably with the available apparatus?

None of the samplers fully met all of the success criteria, but not all the difficulties are insurmountable.

The two samplers that had most consistent performance in these tests, were the PGP 10 cyclone and the modified GK 2.69 cyclone.

The PGP 10 cyclone provides reasonably accurate results when the process of recovery from the sample filter is done carefully. However, the sampler and pump are heavy and its sample train can interfere with the workers activity.

The modified GK 2.69 cyclone gives a very similar measurement performance in laboratory tests, but the pumps used with it failed to maintain a consistent flow (± 5%) in the field tests.

The IPP impactor tended to under sample respirable dust compared with the SIMPEDS. This sampler could meet the respirable dust sampling convention prescribed in EN 481 and ISO 7708, if used carefully to avoid sample losses when handling the sampling filters, but greater confidence in its performance would be needed before it could be put in widespread use. The IPP is compact but requires use of the same heavy pump as the PGP 10 cyclone. Options

Four possible options for are outlined below taking into account the following considerations:

• The performance of the samplers in the tests.

• The potential additional costs and consequences of HSE changing its measurement philosophy.

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• Potential future developments.

Option 1: Retaining the existing samplers and using a silver filter for sampling could achieve a further reduction in the WEL

Sampling using silver filters could halve measurement uncertainty. In turn, this could enable a reduction in the WEL towards 0.05 mg.m-3 if short sampling times (< 4 hours) were no longer permitted for making measurements to assess compliance with the WEL. This option would allow HSE to retain its current measurement philosophy of using the direct on-filter analysis technique specified in MDHS 101 (HSE 2005). The use of silvers filters and a direct on-filter analysis procedure is already used in Italy where an insurance limit of 0.05 mg.m-3 exists.

However, many of the pumps used at present might not operate successfully with the backpressures experienced when samplers are used with silver filters and some industries might have to purchase new pumps. Furthermore, the exact improvement in measurement precision with silver filters has not yet been fully quantified and the improvement could be sampler dependent. The use of FTIR analysis is not possible with silver filters and many small laboratories cannot afford to replace their FTIR spectrometers with XRD instruments.

Option 2: Use the modified GK 2.69 cyclone

The flow rate used with the modified GK 2.69 cyclone is sufficient to collect enough dust to achieve a significant improvement in measurement precision and the use of silver filters would provide a further enhancement by allowing accurate corrections for heavy sample loadings. The use of the modified GK 2.69 cyclone will allow HSE to retain its current measurement philosophy of using direct on-filter analysis. The use of a silver filter with a larger pore size (1.2 µm) could reduce backpressure and cooperation with industry may improve pump performance. A newly available pump, known as the SG 10–2, should be able to work with the backpressures experienced by the modified GK 2.69 cyclone, but at the time of the publication of this report this pump would cost about £1k per unit.

The success of using this option is dependent on:

a) A manufacturer adopting the design and making the sampler commercial available

b) The availability of a pump to operate with the backpressures experienced.

Both factors will require cooperation with industry.

Option 3: Use the unmodified GK 2.69 cyclone

This sampler is commercially available and the use of a 37 mm filter reduces the backpressure.

However, an indirect analysis procedure is needed which could introduce inaccuracies due to sample losses and HSE would have to change its measurement philosophy, which would incur additional costs. Option 4: Use the PGP 10 cyclone

The flow rate is more than sufficient to collect enough dust to improve the measurement precision and the sampler and pump are commercially available. It is however, very expensive and the purchase of a single unit with pump (> £2k) would represent a significant cost to a small company. The sampler is also considered heavy, interferes with the worker’s activity and received many adverse comments from workers. Many of the concerns raised could be

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overcome with the use of a harness and backpack to support the sampling equipment on the worker.

Recommendations Recommendations for policy

There is a history of occupational hygienists taking half shift (~4 hour) samples in the UK when assessing a worker’s exposure to RCS. HSE should work with the British Occupational Hygiene Society (BOHS) to encourage occupational hygienists to take full shift samples, whenever possible, even if the task involving work with silica containing materials is for less than 4 hours. The differences in strategies for task specific sampling exercises and the taking measurements to demonstrate compliance with the law should be emphasised in HSE’s guidance. Measurements taken for less than a full shift should not be considered as sufficient to check compliance with a WEL. Sampling for the majority of the working day will reduce the uncertainty of the reported result and a longer sampling period is more representative of an individual’s exposure as it makes better consideration of failures of controls from other processes or contamination. Additional time is needed to assess if the manufacturers of pumps can improve the performance of pumps operating under load, to enhance the options for sampling in future and reduce the incidence of pump failure experienced at present. HSL also needs to work with the manufacturers of the GK 2.69 cyclone, when and if a suitable pump is available, to assess if the modified version can be made commercially produced. A short ANOVA evaluation of the improved precision when using silver filters with the existing equipment is underway and the results will soon be available. The use of these samplers with silver filters, combined with longer sampling times and better pump performance, could be sufficient to reduce the WEL for RCS. This would be the most cost effective approach for HSE and industry. If the pumps cannot be improved to cope with the backpressure of the modified GK 2.69 cyclone and a manufacture cannot be found then HSE should encourage the use of the unmodified GK 2.69 cyclone and adopt a different measurement strategy.

However, change to a measurement strategy reliant on the use of samplers with filters greater than 25 mm in diameter would inevitably increase costs for industry/HSE because:

a. An indirect analysis approach is required and the analytical process is more time consuming.

b. The analytical procedure could require the purchase of additional specialised equipment to ash or to weight 37 mm diameter filters in most laboratories.

c. Additional funding would be needed for HSE to communicate its expectations to the occupational hygiene community and industry.

The PGP 10 cyclone is commercially available but the introduction of this sampler into widespread use is unlikely, since the equipment procurement costs are significant and its use is more limited than the GK 2.69 cyclone. The PGP 10 cyclone will probably find a role with specialist activities such as task-specific sampling with short sampling times (< 2 hours), static and verification of enforcement sampling by HSE, where the levels of dust are low. Further work is needed to assess if the difficulties of using this device could be overcome through the use of a harness a new pump.

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Recommendations for sampling and analysis

Consideration should be given to creating a stronger link between the sampling environment, working patterns, selection of the sampler and the type of analysis procedure required to measure the samples, so that the sampler and measurement approach can be linked to specific sampling applications. HSE should consider the development of a guidance document for sampling and analysis, similar to HSG 248 "Asbestos: The analysts' guide for sampling, analysis and clearance procedures'.

The CIP 10 sampler should not be used in the UK since it can potentially under sample respirable dust compared with the SIMPEDS when carrying out some common workplace tasks.

The Workplace Analysis Scheme for Proficiency (WASP) proficiency-testing programme should trial the use of silver filters to confirm any improvement in the reliability of measurements between laboratories.

If direct on-filter XRD analysis is used, sampling should be carried out using 25 mm diameter, 0.8 µm or 1.2 µm pore size, silver filters, since these offer advantages over the use of the GLA 5000 PVC filters.

FTIR analysis should not be used for the analysis of RCS except in specific specialist applications (e.g. for foundry samples and for coal samples, where the carbon is anthracite) and then only when the loading of the sample is < 1 mg.

The HSE method for the analysis of RCS should be rewritten to exclude FTIR analysis. Originally, the XRD and FTIR analysis methods were separate before they were combined as MDHS 101 (HSE 2005). The FTIR and the XRD analysis procedure should be separate MDHS methods as they were previously, so that the different analytical procedures for different sampling equipment with XRD analysis and the limitations of the FTIR analysis procedure are more clearly defined. FTIR analysis can still have a specialist role.

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1

1 INTRODUCTION

‘The Commission considered …………… a lower level (WEL for respirable crystalline silica) was not practicable at present, if it could not be measured, but advised that HSE should be aiming to achieve a further reduction’. (Health and Safety Commission, Minutes 2006)

1.1 BACKGROUND This document details the results from a project to investigate the technical feasibility of using high volume samplers to improve the measurement precision to enable the Health and Safety Executive (HSE) to reduce the Workplace Exposure Limits (WEL) for respirable crystalline silica (RCS) from its present level of 0.1 mg·m-3 to 0.05 mg·m-3. HSE document EH 74/4 (HSE 2004) describes the risk of developing silicosis after 15 years of exposure to RCS as about 0.5% (1 person in 200) at an air concentration of 0.04 mg.m-3 and 2.5% at 0.1 mg.m-3. Other studies indicate a similar risk to worker health at lower concentrations (NIOSH 1998). This has increased the pressure on legislative authorities to reduce exposure limits to a level where the studies suggest that the risk of damage to the health of an individual is minimal. The European Scientific Committee for Occupational Exposure Limits (SCOEL) has recommended that, to eliminate silicosis, European occupational exposure standards should be set below 0.05 mg.m-3. However, evidence presented to the Health and Safety Commission (HSC) and HSE’s Advisory Committee on Toxic Substances (ACTS) demonstrated the poor reliability of measurements of concentrations of RCS in air at 0.05 mg.m3 made when collecting samples at the flow rates of the samplers then available (HSE 2006). HSC therefore accepted HSE’s recommendation that the WEL for RSC should be lowered from its earlier value of 0.3 mg.m-3 to 0.1 mg.m-3, rather than 0.05 mg.m-3. If a WEL is set at a level where the analytical method approaches its limit of quantification, exposure measurements in this region would be unreliable because of high analytical variability. To enforce the law the regulatory authorities have to be confident that measurements between laboratories are reliably consistent. This is especially important when considering implementing lower exposure limits than were previously enforced. Most samplers in the United Kingdom (UK) operate at flow rates between 1.7-2.2 l/min. This project investigated the use of higher flow rate samplers to collect more dust for measurement. Samplers to be tested were selected on the basis that they should be able to double the amount of dust collected, so potentially halving measurement precision. Therefore, samplers with flow rates less than 4 l/min were excluded from this project. The sensitivity of the instrumental techniques used to determine the mass of RCS in an air sample is dependent on the physical properties of the dust collected, such as the depth of the deposition, the absorption of radiation by the sample matrix, the distribution of the dust across the filter and the particle size. Some of these factors can reduce, rather than increase, the sensitivity of the analysis. This study investigated the performance of measurements for quartz (the most common polymorph of RCS) when using the high volume personal samplers available in 2008.

1.2 PRINCIPLES OF THE ANALYSIS TECHNIQUES

The two most frequently used instrumental techniques for the analysis of respirable crystalline silica are x-ray diffraction (XRD) and Fourier Transform Infra Red (FTIR) spectrometry.

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1.2.1 X-ray diffraction

Powder XRD measures the x-ray radiation reflected from a material. When a crystalline material is placed in focal point of an x-ray beam and the orientation of a plane of atoms within the structure of crystals in the material fulfils the Bragg equation nλ=2dSinθ, where λ is the wavelength of the radiation, d is the lattice spacing and θ is the diffraction angle (the angle between the plane of atoms and the detected signal), constructive interference of the radiation occurs and a peak is observed above the background signal. Amorphous or semi-crystalline materials influence the background in a variety of ways but do not produce scans with defined peaks. The penetration of the x-ray beam into the sample is dependent on the intensity of the radiation, the density of the material and how the material absorbs the radiation. For example, copper radiation generally penetrates 100 µm into the surface of a powder sample. Devices called slits and masks are used to control the area of the sample illuminated and the amount and type of radiation that is accepted by the detector. Figure 1 shows the arrangement of the optics in XRD spectrometer.

Figure 1: Optics in an XRD spectrometer (reproduced with the permission of Panalytical)

1.2.2 Infra red analysis

Infra red analysis measures the amount of infrared energy that is absorbed by the sample. When chemical bonds within a molecule are subjected to infrared radiation many will vibrate or stretch and absorb the infrared energy. Each bond vibration or stretch has distinct absorption within the infrared spectrum and a chemical is identified from the spectrum pattern. Respirable quartz has two distinct absorbencies at 800 cm-1 and 780 cm-1. FTIR instruments use an interferometer to simultaneously scan the infrared range. This enables the FTIR to have a fast analysis time compared with XRD analysis and the instrument is cheaper to manufacture and

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maintain. However, a sample needs to be thin enough to allow the transmission of the infrared radiation through the sample, so heavily loaded air sample filters cannot be analysed reliably. Also the infrared beam measures a small proportion of the total area of the filter (7-8 mm) and repeatability of the measurement is dependent on the homogeneity of the dust distributed on the air sample filter or in a disc of potassium bromide.

1.2.3 Analytical approaches

Generally, the analytical procedure employed to analyse RCS in airborne dust is dependent on the type of sampler used to collect the aerosol. In countries where samplers using filters with a diameter greater than 25 mm, or where foam is used as the dust collection medium, methods were developed to recover the dust from the filter or foam and to concentrate the dust onto a smaller filter, so that all the collected dust deposit is analysed by the instrument. These procedures are referred to as indirect analysis procedures since they involve a process to recover the dust from the air sample filter. In many countries where samplers with a 25 mm diameter filter are used, the air filter is analysed for RCS without any pre-treatment. This is referred to as direct on-filter analysis and is the approach adopted by HSE in its published method MDHS 101 (HSE 2005). If smaller diameter filters are used in samplers that operate at higher flow rates this can lead to higher backpressures that can strain the pump and cause failures. Therefore, designers of higher flow rate devices generally opt to reduce the backpressure by using a filter with a larger surface area.

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2 STAGE 1: SELECTION OF EQUIPMENT INCLUDED IN THE STUDY

2.1 SELECTION OF SAMPLERS

The samplers selected for study are shown in Table 1.

When this project started, there were few commercially available samplers designed for personal sampling of respirable dust that operated at flow rates of 4 l/min or greater. Two of the five samplers included in the study were adapted at HSL. The GK 2.69 cyclone was modified to operate with a 25 mm diameter filter for use with a direct on-filter analysis approach; and the IOM sampler was used with a foam separator, selected at HSL to operate at 4 l/min rather than 2 l/min. The other samplers tested were a prototype IPP impactor, operating at 8 l/min, which was loaned to HSL by the manufacturer, and the PGP 10 cyclone and the CIP 10 sampler, both operating at 10 l/min,. The reference sampler used in the study was the Safety in Mines Personal Dust Sampler (SIMPEDS), manufactured by Casella Ltd, a version of the Higgins-Dewell cyclone, operating at 2 l/min. The SIMPEDS was used as the reference sampler because it is the sampler recommended for respirable dust in MDHS 14/3 (HSE 2000).

Manufacturers, other institutes and HSL have tested the samplers evaluated in this study to assess their conformance with the ISO/CEN convention for respirable dust (ISO 1995, CEN 1993) and all have a reasonable match to the respirable convention. The purpose of this study, therefore, was not to assess this, but to examine differences in measurements of RCS in the dust collected by the samplers. In most experiments, two samplers of each type were used to sample each test material. However, only one prototype IPP impactor was available.

2.2 SELECTION OF FILTERS

Both 25 mm diameter 0.8 µm pore size, silver filters and 25 mm diameter, 5 µm pore size, Gelman GLA 5000 PVC filters were included in the study for those samplers able to be used with the direct on-filter analysis approach, such as the SIMPEDS, the adapted GK 2.69 cyclone and the IOM sampler with the modified foam separator. The filters or foams recommended by the manufacturers were used for the other samplers. These included 37 mm diameter, 5 µm pore size, Gelman GLA 5000 PVC filters for the IPP impactor and 37 mm diameter, 8 µm pore size, cellulose nitrate filters for the PGP 10 cyclone.

2.3 SELECTION OF SAMPLING TRAINS

2.3.1 Assessment of backpressure on filters at 4 litres/minute

In order to assess the ability of sampling pumps to operate with the backpressure caused by a 25 mm diameter filter operating at 4 l/min, pressure differentials were measured for 0.8 µm, 1.2 µm and 3 µm pore size mixed cellulose ester (MCE) filters mounted in an in-line filter holder with a calibrated micro-manometer connected to each side (Figure 2).

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Table 1: Samplers tested in the project Apparatus Make Nominal Flow

Rate (l/min)

Comment Picture

Adapted GK 2.69 cyclone

HSL/BGI 4 GK 2.69 cyclone adapted at HSL

for use with 25 mm diameter

filters

IOM sampler with foam separator

HSL/SKC 4 IOM sampler with foam

selected by HSL to separate

respirable dust at 4 l/min

IPP impactor (prototype)

SKC 8 Impactor onto PVC filter

PGP 10 cyclone GSA

Messgerätebau GmbH

10 Specified for use with celluose nitrate filters

CIP 10 sampler Arelco 10 Spinning foam in

head Integral pump

unit with battery

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Figure 2 Apparatus for the testing of backpressure across filters

Charts showing the relationship between pressure drop and flow rate through each pore size of MCE filter are shown in Appendix 1.

Table 2 lists the pressure drop, in inches of water, derived from the charts in Appendix 1, for the different pore sizes of MCE filter, when subject to a flow rate of 4 l/min.

Table 2: Pressure drop at 4 l/min Filter pore size (µm)

Pressure drop (inches of water)

0.8 23 1.2 16.4 3.0 9.6

The results indicate that if filters with a pore size of 0.8 µm are used then pumps that cannot cope with a backpressure of 23 inches of water are not likely to achieve or maintain a flow rate of 4 l/min. This eliminated from the project the majority of pumps currently used at HSL. Three sets of pumps from three different manufacturers were run with the 3 µm pore size MCE filter at 4 l/min and of these only one set of pumps ran for more than six hours. This highlights the need for high quality pumps.

2.3.2 Pressure drop across silver filters

Silver filters offer advantages of lower background, lower calibration line uncertainty and better measurement repeatability when compared with the membrane filters based on cellulose or other polymers that are also used for XRD analysis. Their disadvantage is that they are relatively expensive compared with the other membrane filters, cannot be used with infrared analysis and slowly absorb sulphur dioxide and chlorine from the environment, so slightly increasing their weight from day to day. Silver filters of a number of different pore sizes are available but the particle collection efficiency for respirable dust of silver filters with larger pore sizes might not be adequate (Lee K and Ramamurthi M 1993) Also it is thought that for silver filters of larger pore size some dust could be masked from the x-ray beam if the dust is drawn into the pores of the material. It was therefore decided to test the backpressure associated with

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0.8 µm pore size silver filters with a view to using these in this work. Figure 3 shows the results of the tests concerned.

y = 0.0031xR2 = 0.9782

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0 1000 2000 3000 4000 5000

Flow rate (cm3 min-1)

Pre

ssur

e dr

op (I

nche

s w

ater

)

Figure 3 Pressure drop across a 0.8 µm pore size silver filter

Surprisingly, the back pressure on 0.8 µm pore size silver filters operating at a flow rate of 4 l/min was only 12.4 inches of water, which is not much greater than 3 µm pore size silver filter and better than 0.8 µm pore size MCE filters. They were therefore fit for purpose.

2.3.3 Performance of sampling train under load

Pumps were purchased that operated at the required backpressure and a series of tests were performed to assess now these pumps would also operate under load at the nominal flow rates of the samplers. Buck Libra pumps were used for the devices with a flow rate of 4 l/min and Leyland legacy pumps from SKC were used with the IPP impactor and PGP 10 cyclone. The samplers , fitted with pre-weighed filters and operated at their nominal flow rates, were exposed to an aerosol of fine Arizona Road Dust (ARD) in a small dust tunnel (Figure 4) at an approximate concentration in air of 40 mg.m-3, estimated using a real time dust monitor.

8

.

Figure 4 Samplers tested in dust tunnel

The flow rate was checked periodically with a Gilibrator bubble flow meter and the mass collected on each filter weighed to assess the loading at which the sampling train would not be able to meet the requirement, prescribed in EN 1232 (BSI 1997), that the flow rate should be maintained within 5% of the quoted value.

The results are shown in Figure 5

-2.50

0.00

2.50

5.00

7.50

10.00

12.50

15.00

17.50

0.00 5.00 10.00 15.00 20.00 25.00

Dust load (mg)

% d

rop

in fl

ow ra

te (l

min

-1)

GK2.69 modified - 0.8µm silver filter IOM with foam - 0.8µm silver filter

PPI08 impactor - 5µm PVC filter PGP 10 - 8 µm cellulose nitrate filter

SIMPEDS - 0.8 µm silver filter

.

Figure 5 Drop in flow rate with dust loading

All the samplers, except the IOM sampler with foam separator, maintained their required flow rates up to a loading of about 5 mg or greater. The reduced performance of the IOM sampler was believed to be due to the collection of dust on both the foam separator and the filter increasing the backpressure to a greater extent than just the filter. One modified GK 2.69 cyclone failed the criteria with just 1.5 mg of dust on the filter but it was decided to

Location of the samlpers

9

continue its use because in another run more than 5 mg of dust was collected on the filter before the flow rate drop exceeded 5% and the recorded back-pressure at maximum loading was well within the pump’s specification of 55 inches of water. Figure 6 shows the backpressures obtained with each sampler during the loading of the filters. The performance of the new pumps purchased for the modified GK 2.69 cyclone and IOM sampler with foam separator, applying the criteria developed in 2.3.1 that they should cope with a backpressure of 23 inches of water, was better than the existing equipment at HSL.% Table 3 list the samplers, the filter medium and the maximum loading before failure. Table 3: Maximum loading of sampling trains Sampler Medium Pore size

(µm) Maximum loading

(mg) GK 2.69 cyclone Silver filter 0.8 ~5 IOM with foam separator Silver filter 0.8 ~ 2 PGP 10 PVC filter 5 ~ 8.3 PGP 10 Cellulose nitrate 8 > 10 PPI impactor PVC filter 5 > 15

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

0.00 5.00 10.00 15.00 20.00

Dust load (mg)

Pre

ssur

e dr

op (i

nche

s w

ater

)

GK modified - 0.8µm silver filter

IOM with 75ppi foam - 0.8µm silver filter

PPI08 impactor - 5µm filter

PGP10 - 5 µm pore size PVC filter

SIMPED - 0.8 µm pore size silver filter

Figure 6 Change in backpressure with loading

10

3 CALIBRATIONS

3.1 CALIBRATIONS FOR X-RAY DIFFRACTION

3.1.1 Quartz x-ray diffraction pattern

Quartz has four reflections in its x-ray diffraction pattern that are commonly used by laboratories for quantification. When using copper Kα radiation, these are: 1) the 100 reflection (20.9 degrees 2θ) with a relative intensity of about 25%; 2) the 101 reflection (26.6 degrees 2θ) with a relative intensity of 100%; 3) 112 reflection (50.1 degrees 2θ) with a relative intensity of about 9-14%; and 4) the 211 reflection (60.1 degrees 2θ) with a relative intensity of about 7-9%. The XRD instrument was calibrated for the 100, 101 and 112 reflections. A programme, established in x-pert industry software (Appendix 2), was applied to all the calibration samples so that all the samples were analysed using the same instrumental parameters.

3.1.2 Direct on-filter analysis

Calibration samples were prepared and analysed for the SIMPEDS, the modified GK 2.69 cyclone and the IOM sampler using the procedure described in MDHS 101 (HSE 2005). Separate calibrations were generated for the 25 mm diameter, 0.8 µm pore size, silver filters and the 25 mm diameter, 5 µm pore size, GLA 5000 PVC filters. XRD calibrations for both filter types are shown in Appendix 3.

3.1.3 Indirect filter analysis

Separate calibrations were established for the IPP impactor and the PGP 10 cyclone following the procedure outlined in NIOSH 7500 (NIOSH 2003). A suspension of HSE quartz standard A9950 in isopropanol was prepared and aliquots of the suspension were filtered onto 25 mm diameter, 0.45 µm pore size, silver filters or 25 mm diameter, 5 µm pore size, GLA 5000 PVC filters to produce calibration samples with loadings in the range 1 mg – 6 mg. The calibrations established by the analysis of these samples are shown, corrected for depth effects using the reflection from a silver filter as an internal standard, and uncorrected, in Appendix 4.

3.2 CALIBRATIONS FOR INFRARED ANALYSIS

The calibration samples prepared on GLA 5000 PVC filters for indirect filter analysis by XRD (3.1.3) were also analysed on a Perkin Elmer FTIR spectrometer using the principal quartz infrared absorptions of 800 cm-1 and 780 cm-1 to establish separate calibrations for each sampler type. The instrumental parameters used in the analysis are given in Appendix 3 and the calibrations for the direct on-filter analysis methods and for the indirect analysis methods are shown in Appendix 5 and Appendix 6.

3.3 DISCUSSION

It has been suggested that the absorption of copper x-ray radiation by silver might reduce the intensity of the signal detected by the XRD instrument when using silver filters. However, it can be seen that the trend lines in Appendix 3 are very similar. The silver filter calibrations have slightly better regression coefficients than the GLA 5000 PVC filter calibrations, except for the samples from the IOM sampler. The IOM sampler has a metal sample cassette and difficulties extracting silver filters without damage could have led to sample loss . It is also possible that the IOM cassette might not seal as effectively when using a silver filter as when compared with other filter types and this could cause migration of dust to the edges of the filters and subsequent loss when the top part of sampling cassette is removed (in a circular motion) to gain access to

11

the filter beneath. All results from the FTIR analysis gave straight trend lines except for the calibrations for the indirect analysis procedure which were curved and more variable as the depth of dust on the calibration samples reaches the point at which transmittance of the infrared radiation is so small that the relationship of absorbance with mass on the filter is no longer linear (the Beer-Lambert law).

12

4 STAGE 2B: ABSORPTION AND DEPTH EXPERIMENTS

4.1 EVALUATION OF ABSORPTION AND DEPTH EFFECTS IN X-RAY DIFFRACTION ANALYSIS

4.1.1 Factors that influence the measurement of crystalline silica using X-ray diffraction

Two major factors that influence the measurement of crystalline silica in a significant amount of another dust on a filter by XRD are the depth of the sample and the absorption of the matrix. The current HSE method for the analysis of respirable crystalline silica (HSE 2005) assumes the depth of the sample is so thin that the x-ray radiation penetrates the whole sample and reflected radiation is not absorbed. This assumption is satisfactory if samplers operating at 2 l/min are used and if the sample does not include a high absorbing element, such as the absorption of copper radiation by iron. However, when an increased mass of dust is collected this assumption is broken. A limited study was developed to examine these effects on the measurement of a small amount of crystalline silica in a matrix because crystalline silica is a trace component of many mineral products.

4.1.2 Materials

Three different matrix materials were selected for study that:

a. were minerals encountered in industry

b. offered a range of absorption coefficients (an absorption coefficient is a measure of the ability of the mineral to absorb x-ray radiation, with high numbers ~200 cm2/g indicating a very strong absorbance)

c. did not have peaks that interfere with the 101 reflection of quartz.

Table 4 provides details of the minerals selected as matrix materials and of the quartz standard used in the study.

Table 4: Minerals involved in the absorption study Mineral Formula Mass Absorption Coefficient Origin Olivine (MgFe)2SiO4 154.8 cm2/g Single Crystal

Richard Taylor Minerals Calcite CaCO3 71.5 cm2/g Alfa Aesar 99.99% Talc Mg3Si4O10(OH)2 31.4 cm2/g BDH Chemicals Ltd fine

Talc purified by acids Quartz SiO2 34.8 cm2/g HSL Standard A9950

XRD scans of each of the selected matrix materials (see Appendix 7)did not show the presence of crystalline silica.

The olivine and talc samples were ground for more than 8 hours in an attempt to obtain powder within the respirable size range. This was not completely effective as a proportion of the ground material had particle sizes larger than 20 µm, although the larger particles sizes could have been due to unresolved agglomerations. The calcite was not ground because, when examined through a microscope and compared against a calibrated micrometer, it appeared to be within the

13

respirable size range. However, results from particle size measurements performed using a laser light scattering instrument (an Horiba 390L manufactured by Particle Analysis Ltd) showed a distribution of particles that was slight larger than the respirable range. The median particle sizes for each mineral are shown in Table 5.

Table 5: Median particle size

Mineral Median Particle Size (µm) Calcite 23.2 Olivine 8 Talc 13.9 Quartz 2

4.1.3 Procedure

Two suspensions of each matrix material in isopropanol were prepared by following the procedure outlined in NIOSH 7500 (NIOSH 2003). A known mass of mineral dust was mixed with an amount of HSE quartz standard A9950 and the mixture was then dispersed in a known volume of isopropanol (determined from the weight and the density of isopropanol). The proportion of HSE standard A9950 used in each of the two mineral dust suspensions was equivalent to quartz at the current WEL (0.1 mg.m-3) and at half the WEL (0.05 mg.m-3) in matrix material at the WEL for respirable dust (4 mg.m-3). This resulted in suspensions containing approximately 1% and 2% crystalline silica. Fortuitously, these percentages of quartz are levels that are sometimes encountered in commercially available products of these minerals .

Aliquots of each suspension were then taken to produce loadings within the range from 1 mg – 6 mg on each filter. Each aliquot of suspension was filtered onto a 25 mm diameter, 0.45 µm pore size, silver filter for analysis by XRD, or onto a 25 mm diameter, 5 µm pore size, GLA 5000 PVC filter for analysis by FTIR. However, the deposit did not extend across the whole filter because the filter funnel diameter was only approximately 15 mm. The expected mass of crystalline silica on each filter was calculated from the total mass of dust deposited and the percentage of quartz mixed with the matrix material. An example of the pipetting accuracy is given in Figure 7. The slope of the regression line for the preparation of a series of test samples of 2% RCS in talc is very close to the ideal value of 1.00, indicating the suspension was mixed uniformly and that the pipetting of aliquots produced repeatable test samples.

14

Recorded mass = 1.0129xR2 = 0.9982

0

1

2

3

4

5

6

0 1 2 3 4 5 6

Predicted Mass (mg)

Rec

orde

d M

ass

(mg)

Results

Linear (1:1Relationship)

Figure 7 Accuracy of loading test samples

4.1.4 XRD results for 1% quartz in talc

Figure 8 compares the mass of quartz recorded by the XRD instrument for the measurement of approximately 1% crystalline quartz in talc, with the expected value of quartz in the test sample derived from the weighed mass of dust collected on the filter and the percentage of quartz standard in the mineral suspension. There are two data sets shown on the chart in Figure 8. One data set shows the results uncorrected and the other the results corrected for depth and absorption effects. The measurement for quartz was corrected following the procedure outlined in NIOSH 7500 (NIOSH 2003) using the silver line at 38.12 degrees 2θ as an internal standard. The recorded value is the average of the results from the 100, 101 and 112 reflections. Occasionally, the 100 reflection was excluded because of inconsistency with the results from the other two reflections and a suspected interference.

15

Corrected = 0.9981xR2 = 0.9824

0

10

20

30

40

50

60

0 10 20 30 40 50 60Expected Value (µg)

Rec

orde

d V

alue

(µg)

1 % Quartz in Talc Corrected

1% Quartz in Talc Uncorrected

Linear (1:1 Relationship)

Figure 8 Measurement of 1% quartz in talc

Figure 9 compares the results for a mixture of dust containing 2.45% crystalline quartz in talc. Only the 101 and 100 reflections were used to obtain the recorded value because the reflection at 211 was subjected to interference.

16

Corrected = 0.9392xR2 = 0.9772

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Expected Mass of Quartz (µg)

Rec

orde

d M

ass

of Q

uartz

(µg)

2.5 % Quartz in TalcUncorrected

2.5 % Quartz in TalcCorrected

Linear (1:1Relationship)

Linear (2.5 % Quartzin Talc Corrected)

Figure 9 Measurement of 2.5% quartz in talc

Although, many of the corrected results do lie on the ideal 1:1 relationship line it can be seen that a number of results slightly under record the mass of RCS in the sample. It is thought that some of the variability is due to the inconsistency of the deposition of dust on the filter surface after filtration. Some of the deposits are slightly off centre. However, a correlation of 0.98 for the analytical range 20–120 µg should be considered reasonably good because this range is at the lower end of the validated calibration range for this procedure of 20–2000 µg (NIOSH 2003).

4.1.5 XRD results for calcite

4.1.5.1 XRD results for 1% quartz in calcite

The XRD results for 1% quartz in calcite, which has a mass absorption coefficient of 71.5 g/cm2, twice that of talc, have been separated into different charts for the 100 and 101 with the 211 reflections. This has been done to demonstrate the unexpected difficulties encountered when measuring quartz in this matrix. Figure 10 shows results for the 100 reflection of quartz (secondary peak at 20.9 degrees with a relative intensity of 25% compared with the 101 reflection) and Figure 11 shows the combined results for the 101 and 211 reflection (primary quartz peak at 26.67 and tertiary peak at 50.1 degrees) for a mixture of 0.93% crystalline silica in calcite. The results for the 101 and 211 were more consistent, although produced slightly low values compared with the expected results. The results for the 100 reflection produced very high results.

17

R2 = 0.9695

0

20

40

60

80

100

120

0 50 100 150

Expected Mass (µg)

Mas

s of

Qua

rtz M

easu

red

(µg)

1% Quartz in Calcite100 reflectioncorrected1% quartz in calcite100 reflectionuncorrectedLinear (1:1Relationship)

Linear (1% Quartz inCalcite 100 reflectioncorrected)Linear (1% quartz incalcite 100 reflectionuncorrected)

Figure 10 XRD response for the 100 reflection (1% quartz in calcite)

y = 0.8874xR2 = 0.9748

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Expected Mass (µg)

Mas

s of

Qua

rtz M

easu

red

(µg)

1 % quartz in calcite 101and 112 Corrected

1% quartz in calcite 101and 112 Uncorrected

Linear (1:1 Relationship)

Linear (1 % quartz incalcite 101 and 112Corrected)Linear (1% quartz incalcite 101 and 112

Figure 11 XRD response for the 101 and 112 reflections (1% quartz in calcite)

18

4.1.5.2 XRD results for 2% quartz in calcite

The results for the analysis of 2% quartz in calcite are shown in Figure 12

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Expecetd Mass of Quartz (mg)

Rec

orde

d M

ass

of Q

uartz

(mg)

2.5 % quartz incalcite corrected2.5 % quartz incalcite uncorrected1:1 Relationship

Linear (1:1Relationship)

Figure 12 Measurement of 2% quartz in calcite

In Figure 12, the two most consistent results used to record the measured value were those from the 100 and 211 reflections. The most sensitive 101 reflection generally recorded a lower value compared with the other two results.

4.1.6 XRD results for 2% quartz in olivine

Figure 13 shows the results obtained when analysing 2% quartz in olivine, which has an absorption coefficient of 154 g/cm-1, four times that of quartz. The results were obtained from the 101 reflection since this reflection was the only peak relatively free from interference.

19

0

20

40

60

80

100

120

0 20 40 60 80 100 120

Expected Mass

Rec

orde

d M

ass

101

Ref

lect

ion

2 % quartz inolivine corrected

2 % quartz inolivine -uncorrectedLinear (1:1relationship)

Figure 13 measurement of 2% quartz in olivine

4.1.7 Discussion

It proved very difficult to grind all the crystals in the powders below the respirable size, and this could have affected absorption because of differences in the packing density of the particles on the filters. Larger particles could have ‘shadowed’ the smaller particles of respirable quartz powder or build deposits with a greater depth, since they pack less efficiently. This could have led to an excessively pessimistic impression of the effect of absorption and sample depth than is in fact the case.

The mixture of quartz in Talc represents the simplest case in this study because the absorption coefficient of Talc is similar to that of quartz (see Table 4). Effectively, the results shown in Figure 8 and Figure 9 show the effect of depth on the measurement of quartz rather than depth and absorption. The results for the line, corrected for depth and absorption, have a correlation coefficient of 0.982, which is very reasonable considering proximity the measurement to the limit of quantification of the analytical procedure. The results tend to deviate from the ‘ideal’ 1:1 relationship between the expected and the recorded loading when approximately 3.5 mg of dust are loaded on the filter. This is slightly above the calibration range specified for XRD analysis of crystalline silica in air in NIOSH 7500 (NIOSH 2003). The good correlation for Talc indicates that accurate results for the measurement of RCS are obtained when analysing a material with a mass absorption coefficient similar to quartz and when the matrix causes few interferences.

The corrected relationships for trace amounts of quartz in calcite are more variable. This is partly attributable to the effect of calcite peaks on the background of the adjacent silver reflections used for absorption correction and also to the proximity of the large calcite peak to the 100 reflection. An additional factor for the mixture containing 1% quartz in calcite was the low intensity of the quartz peaks measured in every sample, which added to the variability of

20

these measurements. Few results recorded greater than 20 µg for the measurements of the test samples containing 1% quartz in calcite, even for the most intense 101 reflection. The uncorrected trend lines for 1% quartz in calcite (see Figure 10 and Figure 11) do not show a significant relationship with the expected mass, indicating that, without correction, the limit of detection cannot be described in terms absolute mass but rather as a percentage of quartz in a matrix. This is because, although each test sample contained more quartz, the intensity of the quartz peaks did not change significantly.

The uncorrected lines for the 101 reflections in the charts for calcite and olivine with 2% quartz are similar (see Figure 12 and Figure 13). The uncorrected line tends to move away from the ideal relationship when there is about 40 µg of quartz in 2 mg of dust, which matches the upper limit of the calibration range given in the NIOSH 7500. This contrasts with the results in Figure 9, in which uncorrected results from samples with 60 µg of quartz in 3 mg of talc gave the expected result.

The test samples used in this work were prepared using an aliquot of suspension deposited onto a filter using a filter funnel with a diameter of about 15 mm. In practice, therefore, the dust loading at which absorption correction is required would be different from that determined because the diameter of the deposit is approximately 20 - 22 mm for air samples. A 20 - 22 mm diameter deposit has almost twice the area of a 15 mm diameter deposit, so absorption effects would be less for the same mass of particles on the filter. Theoretically, therefore, if the dust were uniformly spread over a filter with an active area of 20 - 22 mm diameter, then approximately 4 mg would need to be collected before absorption would affect the measurement result. However, the dust is not usually uniformly spread over the filter due to the sampling characteristics of the apparatus.

The results obtained in this work confirm the upper limits of the calibration range for a heavily loaded filter using the indirect analysis approach specified in NIOSH 7500 (NIOSH 2003).

4.2 EVALUATION OF THE EFFECT OF ABSORPTION ON FTIR ANALYSIS

4.2.1 Materials

The same materials were used in the evaluation of the effect of absorption on FTIR analysis as in the experiments carried out to evaluate absorption and depth effects in XRD analysis (see 4.1.2).

4.2.2 Procedure

Test samples containing dust mixtures of 2% quartz in each of the minerals and 1% quartz in calcite were prepared on GLA 5000 PVC filters for FTIR analysis. Each filter was scanned before loading using the same instrumental parameters as the for calibration samples. The test samples were prepared with loadings of about 1–6 mg of dust. Figure 14 plots the peak height of the 780 cm-1 absorbance obtained for each mineral.

21

R2 Olivine = 0.9287

R2 Calcite = 0.9016

R2 Talc = 0.9806

0

0.02

0.04

0.06

0.08

0.1

0.12

0 20 40 60 80 100 120 140

Absorbance (cm-1)

Exp

ecte

d M

ass

of Q

uartz

(µg)

2.5% quartz in talc2.5 % quartz in olivine1 % quartz in calcite2.5% quartz in calcite

Figure 14 infrared responses at 780 cm-1

0

20

40

60

80

100

120

140

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07

Absorbance (cm-1)

Exp

ecte

d M

ass

of Q

uartz

(µg)

2.5 % quartz in talc2.5 % quartz in olivine1 % quartz in calcite2.5 % quartz in calcite

Figure 15 FTIR responses at 800 cm-1

4.2.3 Discussion

Appendix 8, compares a scan of a calibration sample containing 60 µg of A9950 quartz on a filter with scans obtained from test samples of a similar mass of quartz on a filter in a matrix of

22

talc, calcite or olivine. Figure 14 and Figure 15 show that the sensitivity of the peak height absorbance is dependent on the matrix of the material and that, although straight trend lines were obtained, for a successful analysis the FTIR calibration has to be matrix specific because of the different response of quartz in each mineral. The straight-line relationships shown in Figure 15 were obtained for the 800 cm-1 absorbance for quartz, despite an uncertainty about where to place the background points for the calculation of the peak height (Appendix 8). The ratio of the 780 cm-1 and 800 cm-1 absorbances for talc were not the same as for the calibration sample and this suggests possible talc interference at 800 cm-1. It is possible that the slope of the background had an effect on results and that use of software to deconstruct the absorbance of silica from the matrix could have produced a more reliable measurement. Although all the scans were corrected for the filter absorbance, this probably had little effect because of the magnitude of the absorbance of the matrix. These data indicate that FTIR analysis would not be a suitable analytical technique for very heavily loaded filters unless the calibrations were matched to the sample matrix.

23

5 STAGE 3: ASSESSMENT OF THE BIAS OF SAMPLERS

5.1 SAMPLING TESTS WITH ARIZONA ROAD DUST Aerosols to challenge the performance of the samplers were generated in the calm air dust chamber originally developed at HSL by Kenny et al (1999) for work on inhalable samplers (Figure 16). This is now the standard apparatus used to test aerosol samplers at HSL and is similar to the apparatus used by manufacturers of dust monitoring equipment. The calm air test chamber comprised two steel sections (1m x 1m x 1m) stacked on top of the other. The sections were earthed to reduce any effects caused by charge on the aerosol. Typically, the volume flow rate was set to 220 l.min-1, which equated to an air velocity through the chamber of 0.4 cm.s-1. The samplers were therefore tested within calm air conditions (assumed to be air movements of < 10 cm.s-1). Although the temperature and relative humidity inside the chamber were not regulated, they were fairly constant between 21-23°C and 30-35%, respectively, throughout the tests. The dust was introduced into the chamber using the rotating brush generator model RBG 1000 manufactured by PALAS GmbH.

Figure 16 Calm air dust chamber

The main disadvantage of this method is that the resultant aerosol can have an extremely high charge produced by tribo-electrification effects; however, an ionizing fan on the chamber helped to reduce the charge on the aerosol. The main advantage is that it can produce a very constant and reproducible feed of dust. The aerosol is generated and mixed with the air in the top of the chamber. Honeycomb sections of foil between each section help create a lamina flow of air and aerosol towards the samplers in the bottom chamber. The samplers are rotated at the bottom to reduce the effects of any variability of the concentration of dust in the aerosol. The homogeneity

24

of the aerosol in the chamber, when tested gravimetrically, using six GK 2.69 cyclones at three different concentration levels, was calculated as ± 6%. Arizona road dust (ARD) supplied by Powder Technology Incorporated was selected for sampling tests because the powder conforms to the requirements of ISO 12103 Part 1 (1997), is commonly used as a test material for respirable samplers and contains about 70% crystalline silica.

The samplers were challenged to six different concentrations of ARD by maintaining all the parameters except the length of the sampling time of each sampling exercise.

5.2 GRAVIMETRIC ANALYSIS

The results for the gravimetric determination of the mass of aerosol collected by each sampler are shown in Figure 17, which compares the concentration of dust collected by each sampler with the concentration of dust collected by the reference sampler ((SIMPEDS).

Figure 17 Gravimetric comparison of respirable samples

The results shown on Figure 17 indicate that, when challenged to the ARD used in this exercise, the PGP 10 cyclone had the best relationship with the SIMPEDS and the CIP 10 sampler under estimated the gravimetric dust by about 35%. It is known that the CIP 10 sampler can under sample very small respirable dust sizes ( P Görner 2001). The trend lines for the modified GK 2.69 cyclone and the IPP impactor were possibly influenced by the underestimation at the highest air concentration. The reason for the underestimation with the GK 2.69 cyclone is attributed to the losses from the highest loaded filter material.

Comparison of respirable samplers

PGP10 = 1.0073xR2 = 0.997

GK2.69 = 0.9219xR2 = 0.9972

PPI = 0.853xR2 = 0.9785

IOM Foam = 0.9622xR2 = 0.9622

CIP10 = 0.6689xR2 = 0.9984

0.00

2.00

4.00

6.00

8.00

10.00

12.00

14.00

16.00

0.00 5.00 10.00 15.00Reference SIMPEDS Concentration (mg m-3)

Sam

pler

con

cent

ratio

n (m

g m

-3)

GK2.69 IOM foam

PGP10 PPI8

CIP(10)

25

5.3 RCS ANALYSIS BY X-RAY DIFFRACTION

Figure 18 shows the results for RCS obtained using XRD and the calibrations developed in Appendix 3 and Appendix 4.

CIP1 = 0.564xR2 = 0.9487

CIP2 = 0.5439xR2 = 0.9414

IOM = 1.0042xR2 = 0.9599

PPI = 0.9329xR2 = 0.9656

GK2.69 = 1.0657xR2 = 0.9957

0

2

4

6

8

10

12

14

0 2 4 6 8 10 12

Reference SIMPEDS Sampler Air Concentration mg.m-3

Sam

pler

Air

Con

cent

ratio

n m

g.m

-3

GK IOM PPI CIP1 CIP2

Figure 18 Measurement of quartz by x-ray diffraction

The filters from the IPP impactor were ashed in a furnace brought to a temperature of 400°C for 4 hours. Those samples from the PGP 10 cyclone and CIP 10 sampler were ashed following the French standard method NF X 43-295 (AFNOR 1995) One SIMPEDS sample sustained slight damage on its surface as it was removed from the filter cassette in the sampler and was excluded from the evaluation.

The modified GK 2.69 cyclone and the IOM sampler with foam separator obtained the closest relationships with the reference sampler (SIMPEDS). The IOM sampler with foam separator obtained a slightly poorer correlation coefficient, probably due to one very low result The IPP impactor obtained a better slope than with the gravimetric tests, suggesting a better recovery when analysed by XRD. Its results closely matched the IOM sampler and SIMPEDS. The CIP 10 sampler under recorded the mass of quartz in the respirable sample by almost 45% on average compared with the SIMPEDS, when exposed to the fine ARD. This suggests that the CIP 10 sampler under sampled respirable quartz or that 10-15% of dust was lost during the analytical procedure. The results for the PGP 10 cyclone are not shown because the recovery of dust using the procedure at HSL was very poor and they were too variable.

26

5.4 RCS ANALYSIS BY DIRECT ON FILTER INFRARED ANALYSIS

The three GLA 5000 PVC filters were analysed using the direct on-filter FTIR analysis approach described in MDHS 101 (HSE 2005). Figure 19 shows the relationship obtained for the modified GK 2.69 cyclone and the IOM sampler with foam separator against the values obtained for the reference sampler (SIMPEDS). Only the absorbance at 780 cm-1 was used because the peak from the 800 cm-1 absorbance was very small and did not compare well with the profile of the calibration test samples.

0

0.5

1

1.5

2

2.5

3

3.5

4

0 1 2 3 4

Reference Sampler (SIMPEDS) Air Concentration (mg.m-3)

Sam

pler

Air

Con

cent

ratio

n (m

g.m

-3)

GK2.69IOM HeadsLinear (1:1 Relationship)

Figure 19 Comparison of FTIR analysis results

The RCS concentration obtained using the 780 cm-1 absorbance is compared with the RCS concentration obtained from the average XRD result obtained from the 100, 101 and 112 reflections in Figure 20. The slope of the relationship line is not significantly different from 1 (95% confidence level is from 0.77 to 1.16) indicating no significant bias is observed between the XRD and the FTIR results.

27

Figure 20 Comparison of analysis techniques

5.5 RECOVERY FOR THE PGP 10 CYCLONE USING CELLULOSE NITRATE FILTERS

The results for the PGP 10 cyclones are not shown in Figure 18 because they were extremely variable due to the very poor recovery of some samples. The recommended filter for the PGP 10 cyclone causes recovery problems because it is fabricated from cellulose nitrate, which can ‘pop’ causing a loss of sample when placed in a furnace or in a low temperature asher. As no published procedure for the recovery of dust from cellulose nitrate filters for RCS analysis exists at the time of writing this report, the procedure described in the French method NF 243 (AFNOR 1995) for collapsing plastic foams from CIP 10 samplers was used. This procedure involves ignition of the air sample filter in isopropanol before placing the residue in a furnace or low temperature asher to complete the combustion process. The purpose of the ignition process is to melt and burn the plastic foam at a relatively low temperature. However, if burning of the filter is not completely effective any remaining filter material will ignite rapidly causing a loss of sample. Markus Mattenklott of the Institut für Arbeitsschutz der Deutschen Gesetzlichen Unfallversicherung (IFA), who has worked with the PGP 10 cyclone, was approached for assistance with this problem. The method employed at IFA uses 1,3 butanediol to burn the filter as the furnace is heated to the temperature required, removing the ash. This process is smoky and the furnace needs suitable ventilation. Initially, low recoveries were still obtained because some of the material was ‘sticking’ to the crucible. However, improved recoveries were obtained when the crucible with sample was placed in a beaker with isopropanol and put in an ultrasonic bath in isopropanol for about 5 minutes to disperse agglomerates rather than the usual practice of washing the contents of the crucible into the beaker before using ultrasound and filtering. After this treatment it was filtered in the usual manner, which is to wash the contents of the crucible into the beaker several times and then disperse the agglomerates. Figure 21 shows the gravimetrically determined recoveries following this adapted method. The slope

Comparison All Data= 1.0093xR2 = 0.9512

0

0.5

1

1.5

2

2.5

3

3.5

4

0 0.5 1 1.5 2 2.5 3 3.5 4

Air Concentration Obtained by XRD (mg.m-3)

Air

Con

cent

ratio

n O

btai

ned

by F

TIR

(mg.

m-3

)

GK2.69 ResultsIOM Heads ResultsSIMPEDS ResultsLinear (1:1 Relationship)

28

coefficient suggests a slight under recovery of about 2%, which suggests that the improved procedure is capable of giving satisfactory results for the PGP 10 cyclone in future tests.

Recovered = 0.9811xR2 = 0.9983

0

2

4

6

8

10

0 2 4 6 8 10

Mass Loaded (mg)

Mas

s R

ecov

ered

(mg)

Linear (1:1Relationship)

Figure 21 Recovery for cellulose nitrate filters

29

6 STAGE 4: ASSESSMENT OF BIAS OF ANALYTICAL TECHNIQUES

6.1 WORKPLACE TASKS EXAMINED

The calm air chamber was again used to contain and sample dust generated from two simulated work tasks in experiments to assess the bias of the analytical techniques used in RCS measurement. The purpose of the exercise was to generate dust with two different particle size distributions in order to assess whether this influenced the performance of the analytical techniques. Two workplace tasks were simulated on two materials in the top of the calm air chamber and the samplers involved in this study were used to collect the aerosolised dust in the bottom chamber. The first tool used was a power chisel, but this caused health concerns (noise and vibration issues) and it was replaced with a hammer hand drill. The other tool used in the experiments was an angle grinder. Two tests were performed with the angle grinder, two tests with the hammer drill on the kerbstone, two tests were performed with the angle grinder and one test with the hammer drill on the sandstone. Figure 22 and Figure 23 show the types of tools and materials employed in these experiments.

Figure 22 Angle grinder and kerbstone in the top of the calm air chamber

Figure 23 Hammer Drill and kerbstone in the top of the calm air chamber

30

Except for the IPP impactor, two samplers of each type were placed in the sampling chamber. For those samplers with 25 mm diameter filters that could be used for the direct on-filter analysis procedure, one sampler contained a 5 µm pore size Gelman GLA 5000 PVC filter and the other contained a 0.8 µm pore size silver filter.

6.2 PARTICLE SIZE DISTRIBUTION OF THE GENERATED AEROSOL

The particle size distribution of the aerosol generated from each task and test material was measured by collecting a sample of aerosol onto an open-faced filter with a conductive cowl. The dust collected on the filter was then analysed for its particle size distribution using an AeroSizer and AeroDisperser (Amherst Process Instruments Inc, [API], MA, USA), which is an aerodynamic particle sizer incorporating a time-of flight aerosol beam spectrometer.

The results are shown in Table 6 and figures showing the distributions of the particles are shown in Appendix 9.

Table 6: Summary of mean particle sizes for dusts produced by different work activities and different materials Activity/Material Run No Aerodynamic particle size (µm) (Aerosizer) Number Volume Mean STDEV Mean STDEV Drilling Concrete 175 4.06 2.54 18.47 1.54 Cutting Concrete 187 3.94 2.22 14.69 1.61 Drilling Sandstone 179 4.17 2.37 20.77 1.68 Cutting Sandstone 183 4.51 2.78 29.29 1.79 Arizona Road Dust 193 4.51 2.56 26.74 1.58 Activity/Material Run No Geometric particle size (µm) (Aerosizer) Number Volume Mean STDEV Mean STDEV Drilling Concrete 175 2.64 2.57 12.06 1.53 Cutting Concrete 187 2.54 2.23 9.56 1.61 Drilling Sandstone 179 2.75 2.48 13.79 1.67 Cutting Sandstone 183 2.93 2.78 19.03 1.88 Arizona Road Dust 193 2.73 2.59 16.43 1.57

Unexpectedly, the simulated work tasks produced very similar bimodal distributions in terms of number of particles for each size fraction. The mean values for particle size in Table 6 are all probably located near the mid point of the bimodal distribution (in the dip). The study was not able to ascertain the particular size fraction of the respirable quartz particles in the concrete dusts, so it has to be assumed that they are evenly distributed over the whole range and that the two tools have similar mechanical actions that cause a bimodal distribution of particles.

6.3 GRAVIMETRIC ANALYSIS

6.3.1 Comparison of results to the SIMPEDS

The figures in Appendix 10 compare the gravimetric results obtained using each of the tested samplers with those obtained with the SIMPEDS reference sampler.

The mass of dust loaded onto each filter was greater than in the tests carried out in Stage 3 of the project. Gravimetrically, the PGP 10 cyclone, the IPP impactor and the GK 2.69 cyclone

31

were the samplers giving results closest to the average SIMPEDS value , whilst the IOM sampler with a foam separator and CIP 10 sampler gave results with a consistent negative bias.

6.3.2 Comparison of results obtained with silver filters and GLA 5000 PVC filters

Silver filters and GLA 5000 filters used in the same type of sampler were exposed to the same concentration of dust in a series of sampling exercises. Figure 24 compares the gravimetric results obtained on silver filters with those obtained on GLA 5000 PVC filters.

0.5

1

1.5

2

2.5

3

3.5

0.5 1 1.5 2 2.5 3 3.5

Mass on PVC Filters (mg)

Mas

s on

Silv

er F

ilter

s (m

g)

SIMPEDS

GK2.69

IOM

Linear (1:1Relationship)

Figure 24 Comparison of silver with PVC filters Caption The results obtained with the SIMPEDS exhibited an almost 1:1 relationship comparing use of the two filter types. Excluding one pair of results for which one of the filters was recorded as damaged, the slope for the SIMPEDS was 1.03 with a correlation coefficient of 0.956, suggesting a slight positive bias towards the silver filters. On the whole, the silver filters appeared to collect slightly more dust. . The results obtained for the GK 2.69 cyclone and the IOM sampler with foam separator were more variable

6.3.3 Comparison of results from weighing of filters and weighing of filters in plastic filter cassettes

Several of the samplers tested have an internal cassette that is intended to be sent to the laboratory without unloading the filter. Wall deposits form part of the sample and the filter and filter cassette are intended to be weighed together rather than the filter being removed from the cassette and weighed separately. However, due to the difficulty of obtaining reproducible results when weighing filter cassettes, filters are often weighed separately to improve accuracy. Figure 25 compares the results obtained for the GK 2.69 cyclone and SIMPEDS when weighing the filter and filter cassette together and when weighing the filter removed from the cassette. The results are highly scattered around the 1:1 relationship and this is a reflection of the difficulty of weighing filter and filter cassette together, something that is further illustrated by the fact that some negative values were obtained. It should also be noted that it would be

32

anticipated that the loss of sample reported in 6.3.4 would lead to higher results from weighing of the filter and filter cassette together. However, such a pattern cannot be discerned in Figure 25, suggesting that a larger proportion of the variability of the weighing is from the plastic cassettes rather than the visually observed losses.

-2

-1

0

1

2

3

4

0 0.5 1 1.5 2 2.5 3 3.5

Mass from filter weighing (mg)

Mas

s fro

m fi

lter a

nd fi

lter c

asse

tte w

eigh

ing

(mg)

SIMPEDS PVC FiltersSIMPEDS Silver FiltersGK2.69 PVC FiltersGK2.69 Silver FiltersLinear (1:1 Relationship)

Figure 265 Weighing of filters in plastic cassettes

6.3.4 Loss of sample in filter cassettes before XRD analysis There was visual evidence of sample loss onto the internal surfaces of the cassettes containing the GLA 5000 PVC filters. Photographs illustrating these losses and comparing them to the situation for cassettes containing silver filters are shown in Appendix 11. Some of the cassettes containing GLA 5000 PVC filters showed signs of dust having fallen into the space behind the filter and some showed dust on the o-ring that secures the filter in position in the cassette. There was no indication of similar losses occurring for the cassettes containing silver filters, but, occasionally, the edge of a silver filter was distorted, if it was put in the cassette slightly askew. This could theoretically cause a problem if the surface of the filter was uneven during analysis but it was possible to press the metal back into shape without affecting the filter deposit.

There are two potential reasons for the loss of sample from the surface of the GLA 5000 PVC filters:

1. Loss of sample due to the filters flexing as they are removed from the cassettes for weighing.

2. Filters becoming charged during sampling, leading to increased deposition of aerosol on to the internal surfaces of the cassette.

The problem of filter charge with GLA 5000 PVC filters is known and it has been recommended to soak the filters in a 1% solution of surfactant and dry before use (Blackford et al 1985). The action of the machine tools on the test materials could have charged the particles generated in this work and there was no mechanism employed to eliminate the static charge in the dust as it was generated. However, the tasks simulated in this exercise are typical workplace

33

processes and the experiments are valid because charge is not eliminated from the aerosol when sampling in the field.

6.4 X-RAY DIFFRACTION ANALYSIS

6.4.1 Comparison of RCS results from direct on-filter analysis of silver filters and GLA 5000 PVC filters by XRD

Figure 26 shows the relationship, for the SIMPEDS, GK 2.69 cyclone and IOM sampler, between the average RCS mass recorded on the silver filters and the average RCS mass recorded on the GLA 5000 PVC filters.

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Mass Recorded on PVC Filters (mg)

Mas

s R

ecor

ded

on S

ilver

Filt

ers

(mg)

GK2.69

SIMPEDS

IOM Sampler

Linear (1:1 Relationship)

Figure 27 Comparison of XRD results on silver filters and GLA 5000 PVC filters

Figure 26 shows that the majority of results on silver filters were higher than on GLA 5000 PVC filters. It was therefore decided not to include the results from the GLA 5000 PVC filters when comparing the performance of different samplers for the XRD determination of respirable quartz because the potential losses of dust from the GLA 5000 PVC filters could compromise any conclusions. Unfortunately, the variability of results from the GLA 5000 PVC filters also meant that it was not possible to assess differences in the performance of the FTIR and XRD techniques because silver filters cannot be used for FTIR analysis.

6.4.2 Comparison of results to the SIMPEDS

The figures in Appendix 12 compare XRD results for RCS obtained using each of the tested samplers with those obtained using the SIMPEDS reference sampler. All results are corrected for recovery and absorption, using the formula stipulated in the NIOSH 7500 (NIOSH 2003).

The XRD results compared to the SIMPEDS are not as good as the gravimetric comparison (6.3.1), which is probably due to dust losses occurring between the two analyses being

34

performed. The best regressions are for the PGP 10 cyclone (1.02) and the modified GK 2.69 cyclone (0.94).

Another approach to comparing the samplers was to examine the correlations between pairs of results. Pearson correlation values and their probabilities of no relationship existing, shown in Table 7, were obtained for each pair of samplers, excluding one pair of results for which the silver filter from the SIMPEDS was damaged. The smaller the probability the better the relationship between each set of paired results. A value of p = 0.01 represents a probability level of about 99% that a relationship exists.

Table 7: Pearson correlation coefficients of paired x-ray diffraction results

Sampler SIMPEDS PGP 10 GK 2.69 IOM IPP CIP 10

SIMPEDS 0.89

(p=0.035)

0.91

(p=0.022)

0.45

(p=0.15)

0.78

(p=0.024)

0.76

(p=0.037)

PGP 10 0.98

(p=

35

0.0

0.5

1.0

1.5

2.0

0.0 0.5 1.0 1.5 2.0Uncorrected Result GK 2.69 sampler

(mg)

Cor

rect

ed R

esul

t GK

2.69

sam

pler

(mg)

Linear (1:1Relationship)

Figure 28 Comparison of results with and without absorption correction

It can be seen that an absorption correction for the samples included in this study is not necessary and the result is unlikely to