Global Soil Laboratory Network

Basic guidelines for preparing a sample for internal

quality control

Global Soil Laboratory Network

Basic guidelines for preparing a sample for internal quality control

by

Charles Gowing British Geological Survey, United Kingdom

Rob de HayrDepartment of Environment and Science, Australia

Food and Agriculture Organization of the United NationsRome, 2020

Required citation: Gowing, C. and de Hayr, R. 2020. Global Soil Laboratory Network. Basic guidelines for preparing a sample for internal quality control. Rome, FAO.

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Acknowledgements

The GLobal Soil Laboratory Network (GLOSOLAN) thanks the following for taking leadership on

writing these guidelines: Charles Gowing of the British Geological Survey, United Kingdom; Rob de

Hayr of the Department of Environment and Science, Australia.

GLOSOLAN acknowledges the following for providing input on the content and structure of these

guidelines: Winnie van Vark of the Wageningen Evaluating Programmes for Analytical Laboratories,

Netherlands; João Coutinho of the Laboratório de Solos e Plantas, University of Trás-os-Montes and Alto Douro, Portugal; Nopmanee Suvannang, GLOSOLAN Chair, Kingdom of Thailand; Rich Ferguson and Christopher Lee of the United States Department of Agriculture, Natural Resources Conservation Service, National Soil Survey Center, Kellogg Soil Survey

Laboratory, United States of America.

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Basic guidelines for preparing a sample for internal quality control

This document provides basic guidelines to laboratories on preparing a sample for internal quality

control (QC). It specifies the adequacy of resource requirements, the importance of documentation,

the requirements for preparation of samples, the need for assessment of homogeneity and stability,

and the need for defined methods of labeling and packaging.

Uses for internal quality control samples The principal reason to utilize QC samples in the laboratory is to provide a cost effective means

of monitoring the performance of routine procedures for precision each time it is performed.

While QC samples may be used to establish estimates of precision and total measurement of

method uncertainty, they cannot be used to establish metrological traceability as there is no

requirement to assign metrological values.

The use of QC samples include:

• preparation of QC charts for monitoring and demonstrating that the test is in control or

confirm the laboratory’s QC processes are effective over time;

• comparing results between batches;

• validation of method development;

• repeatability and reproducibility studies through use over time including changes in operators

and instruments;

• inter-laboratory checking;

• impacts of changes in environmental conditions in the laboratory.

Personnel requirements Staff employed at each stage should be suitably qualified and trained to carry out the task at hand.

Facility requirements At each stage, use equipment suitable for the purpose. Check the suitability of equipment if it

will impact the quality of the final product.

Control the environmental conditions for specific stages to eliminate potential contamination

and maintain integrity of the sample.

Sourcing and selecting bulk material The reference soil selected for internal QC should as much as practical be representative of the soils

most commonly analysed by the laboratory. It is considered best practice to select soil that also

reflects the range of commonly expected values for the test being performed. (i.e. selecting a

reference soil with a concentration towards the low end and another towards the higher end of

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normally expected values). For convenience and efficiency, it would be advantageous to select soils

that will be useful in this regard for a number of different analyses.

Sample identification Use of a specific sample shall be authorized by the local manager or by another person that has the

authority to ensure all resources (staff and facilities) are suitable and available. Identify the sample in

unique and unambiguous terms.

Identify and record any potential health and safety considerations.

Provide the analyst with pertinent information in the description of the sample.

Documentation At each stage in the process, record and store pertinent information in a system that allows for

retrieval.

Submit to the central scheme coordinator records of the preparation along with the prepared samples.

Physical preparation process Plan all stages in the process.

Sample collectionRecord the method of sample collection. Include information about the sampling site (location,

vegetation cover, etc.) and its proximity to urban and industrial centers. This information is used to

identify potential contamination of the soil and assess suitability for use.

Pre-preparation of material

Screen the sample prior to processing. For example, remove foreign objects from the sample before

packaging for transport to preparation facility.

The field sample should be free of organic matter that was on the surface of the soil (i.e., organic

residue not incorporated into the mineral soil) and free of contaminating material.

Record specific steps of pre-preparation.

Initial drying

Dry the sample before processing. Record the method of drying.

Methods of drying may include air drying if the ambient temperature is warm enough and

the humidity is suitable, oven drying to a maximum of 40°C, or freeze drying to a maximum of 40°Cin a well ventilated area. Other methods also may be used.

Disaggregation

After the initial drying, gently break down hard clumps using a method that prevents crushing of

individual grains.

Record the method of disaggregation, target endpoint, and apparatus used, including its material. For example, wooden pestle and mortar, ceramic pestle and mortar, hardened steel mallet, or fly press.

Screening/sieving

Remove foreign objects that are not part of the actual soil sample.

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Record foreign objects and other unnecessary components, if any, that were removed. Record the

method of particle-size standardization (e.g., sieving to 2 mm with a nylon or stainless steel sieve

mesh).

Secondary drying of screened fraction

Optional use of a secondary drying stage should be recorded, e.g. if the screened material is

discernably moist.

Milling

Milling is often required to reduce and ensure uniformity of particle size to improve homogeneity and

therefore the method used should provide a consistent for the entire sample.

Record the processes for milling. Include the type of mill used, material of the milling vessel (e.g.,

agate, stainless steel, wood), maximum final particle size, and procedures for controlling dust and

preventing contamination.

N.B. If the internal reference soil is to be used to determine the performance characteristics of the

method (method validation or use of QC charts to estimate method uncertainty) it must be

processed in such a way as to be representative of samples on which analytical tests are carried out.

i.e. if the method is normally performed on a <2 mm sample then the reference material should be

<2 mm. If an internal reference sample with a different grain size is used then the control charts

cannot be used to determine the performance of the method.

Homogenization Homogenize the entire bulk volume of the sample at one time.

Record the process and duration of homogenization (e.g., V-blender, rotating barrel, or roller blender

for two, eight, or 24 hours.)

Sub dividing Subdivide the bulk soil sample so that each portion is representative and variation among portions is

minimized.

Record the process for subdividing (e.g., riffle splitter or rotary divider).

Assessment of homogeneity and stability The analytical and statistical methods used to assess the homogeneity of a sample must be suitable

for the purpose and should be recorded. Specify the elements or parameters that have been

monitored, the statistical procedures used, and the criteria for acceptance.

Use the following procedure to assess the homogeneity of a sample:

At least ten containers of each sample are selected at random, batched and analysed in duplicate for

the parameters for which the reference material is to be used to monitor according to the principles

described by Thompson et. al. (2006). Samples for homogeneity testing shall be taken from prepared

samples in final form. All homogeneity testing shall provide sufficient precision for which the QC

material is to be used. Assess the homogeneity of the sample initially by calculating the standard

deviation. If the standard deviation is less than five percent, the sample is suitable as QC material.

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Typically, soil samples that are dried and prepared are considered stable for at least five years.

Some material may be stable longer; therefore, the usable life of QC material may be extended if its

stability is verified.

Packaging Store the prepared material in inert, leak-proof containers that are sufficient to withstand routine

handling in the laboratory.

Record the portion sizes and type of packaging (e.g., plastic sachet, plastic jar, glass jar).

Labelling Identify the samples with clear and unambiguous labels. Labelling should include:

the name and description of the material (e.g., silty soil);

date of preparation and expiry date if appropriate;

reference or batch number;

unit size and unit number (may assist in determining bottle trends if unexpected results are

obtained).

Record the method used to identify the samples (i.e., information provided on labels).

Storage Record the storage conditions at the preparation facility and the duration of storage. Preferably,

store samples at a temperature of 5 to 30°C.

Prolonged storage of soil may result in settling and separation of soil particles. It is most important

that soil aliquots are thoroughly mixed before use and bulk samples are also thoroughly mixed prior

to a new subsample being taken.

References ISO. 2014. ISO Guide 80:2014(E) Guidance for the in-house preparation of quality control

material (QCMs)

Thompson, M., Ellison, S.L.R. & Wood, R. 2006. The International Harmonized Protocol for

the Proficiency Testing of Analytical Chemistry Laboratories. Pure Applied Chemistry 78: 145.

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GLOSOLANGLOBAL SOIL LABORATORY NETWORK

The Global Soil Partnership (GSP) is a globally recognized mechanism established in 2012. Our mission is to position soils in the Global Agenda through collective action. Our key objectives are to promote Sustainable Soil Management (SSM) and improve soil govern-ance to guarantee healthy and productive soils, and support the provision of essential ecosystem services towards food security and improved nutrition, climate change adapta-tion and mitigation, and sustainable develop-ment.

GLOSOLAN is a Global Soil Laboratory Network which aims to harmonize soil analysis methods and data so that soil informa-tion is comparable and interpretable across laboratories, countries and regions. Established in 2017, it facilitates networking and capacity development through coopera-tion and information sharing between soil laboratories with di �erent levels of experience. Joining GLOSOLAN is a unique opportunity to invest in quality soil laboratory data for a sustainable and food secure world.

Thanks to the financial support of

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