Analysis of Uncertainity in Atomic Absorption

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ANALYSIS OF UNCERTAINTY

MEASUREMENT IN ATOMIC

ABSORPTIONSPECTROPHOTOMETER

NEHA S.MAHAJAN1 *

1*M.Tech Student

Department of Mechanical Engineering,G. H. Raisoni College of Engineering

Nagpur, Maharashtra, [email protected]

NITIN K. MANDAVGADE2

2Department of Mechanical Engineering,G. H. Raisoni College of Engineering


S.B. JAJU3

3Department of Mechanical Engineering,G. H. Raisoni College of Engineering


Abstract : A spectrophotometer is a photometer that can measure intensity as a function of the light source wavelength.

The important features of spectrophotometers are spectral bandwidth and linear range of absorption or reflectance measurement. Atomic absorption spectroscopy (AAS) is a very common technique for detecting

chemical composition of elements in metal and its alloy. It is very reliable and simple to use.Quality of result (accuracy) depends on the uncertainty of measurement value of the test. If uncertainty of

measurement is more there may be doubt of about the final result. The final result of Atomic AbsorptionSpectrophotometer gets affected by the number of parameters; we should take in to account will calculating thefinal result.

This paper deal with the methodology of evaluating the uncertainty of measurement of chemical compositionusing AAS. The study is useful for quality of measurement equipment and testing process.

Keywords - Uncertainty, Chemical Testing, Atomic Absorption Spectrophotometer.

1. Introduction

The objective of a measurement is to assign a magnitude to the measurand, the quantity intended to bemeasured. The assigned magnitude is considered to be the best estimate of the values of the measurand. For agiven result there is not one value, but an infinite number of values dispersed about the result within an interval.Hence measurement of a parameter is meaningless unless the width of interval within which the reported valueof the measured is expected to disperse is specified.

Material used for army purpose is to be tested in proper way because the chemical composition of material ismost affecting factor in performance of final product. While testing chemical composition the final result getaffect by various factors so the composition result may not be so correct.

The uncertainty to be truly meaningful, it must address the entire measuring process, which may haveuncertainties associated with factors such as equipment calibration, operator skill, sample variation, and

Neha S.Mahajan et al. / International Journal of Engineering Science and Technology (IJEST)

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environmental factors [4]. Uncertainty sources are classified according to the evaluation method used. Type Auncertainties are evaluated statistically [2-5].

2. Atomic Absorption Spectrophotometer

The instrument employs a double beam technique where the emitted radiation from the light source of hollow

cathode lamps of elements of interest is split into two beams, a reference beam and a sample beam. Sample beam travels through the flame while reference beam travels around the flame. The atomic absorptionSpectrophotometer is as shown in Fig.1

The liquid sample is reduced to a vapour mist of atomic population by a nebulizer with support gas. Mixed with proper fuel, it is sprayed over the flame. The reference beam going around the flame is not affected becausethe energy of sample beam going through the flame is absorbed by the sample element present in the liquid sample and amount of absorption being proportional to the element concentration. Both these beam arecombined together before entering the monochromatic region which selects the appropriate resonance line and direct it to the wide range photo multiplier tube. The electronics there after separates reference and sample

signal in time reference single is used to compensate for drift in lamp intensities and sample signal is processed for photometric computation of the result [3].

2.1 Principle of working

When a liquid sample containing a chemical constituent element of interest is reduced to a spray mist of atomicvapour, mixed with proper fuel and burn over a burner head and if light radiation of the same element from aemitting source is passed through the flame, as atoms of the element in the sample vapour are present in ground

state of unexcited condition they absorb amount of corresponding radiation passing through the flame. Amountof absorption by the analyst depend on its concentration in the sample and are directly proportional to eachother. Hence measurement of absorption value forms the base for calculation of its concentration in sample [1].

2.2 Method of Analysis

Every time when a element is estimated, the system is calibrated by aspirating a set of different strengthstandard solution of known concentration of the element and graphical representation of absorbance verses

concentration is made to get a liner graph for the standard solution .when a unknown sample is aspirated in thesame condition, with reference to its measured absorbance and the standard graph concentration of the elementin test sample is displayed in part per million (ppm).

Fig. 1.Atomic absorption spectrophotometer

3. Evaluation of Uncertainty

Uncertainty of measurement (UOM) does not imply doubt about the validity of a measurement; on the contrary,

knowledge of the uncertainty implies confidence in the validity of a measurement result. It defines a range that

could be reasonably being attributed to the measurement result at a given confidence.The UOM in atomic absorption spectrophotometer may be affected by following factors


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Qualitative factors-

Knowledge of operator.

Experience of operator.

Training of operator.

Education of operator.

Confidence of operator

If the operator is unskilled, untrained, less experience & not having knowledge of equipment may performthe test with less confidence which may affect result of test.

Test on atomic absorption spectrophotometer for chemical composition are performed by me with the helpof Mr.Ravishankar having experience more than 20 year in this field. Hence effect of qualitative factor in our case negligible.

Quantitative factor-

Repeatability

Mass for weight of sample

Volume taken

CRM sample

Calibration CRM Volume made

3.1 Repeatability

It is included in Type-A uncertainty. It is an analytical method including the calculation of standard deviationand type-A standard uncertainty. Standard deviation indicates the spread of values. Spread of values tells ussomething about the uncertainty of measurement. Repeatability factor is concerned with the instrument.

3.2 Mass for weight of sample

As the input and output estimates are not co-related, this component is evaluated as relative standard uncertainty(RSU) sample wt for AAS analysis may range from 0.1 to 10 gram. The expanded uncertainties (K=2) for both

from the certificate are close together as 0.00028 and 0.00034.Considering the fact that smaller sample wt ismore frequently used in AAS analysis the RSU of smaller wt. is general may be consider.

3.3 CRM sample

As the uncertainty state in the certificate of CRM without confidence level and K factor their standard uncertainty for rectangular distribution are given by stated U/√3 (for 1000 ppm).The highest calibration standard is considered for this aspect as a maximum possible contribution toward this component. Therefore if 1000 ppmsolution has U ± 0.2 % ppm.

All Nomenclature for the parameters is shown on Table 1.

Table1. Nomenclature for the parameters

Sr. no Parameter Nomenclature

1 UC Combined uncertainty

2 UA Type A standard uncertainty

3 UB Type B standard uncertainty

4 σ Standard deviation

5 n No. of readings

6 Effective Degree of freedom

7 K Coverage factor


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After performing the observations on AAS for measured values of Mn ten times, readings are as shown inTable-2.

Table2. Observations

Mean=X=0.6951

Standard Deviation =

2

1

i X X

n

(1)

σ= 0.0028

Standard uncertainty =n

(2)

=

Repeatability (UA) = 0.009

Degree of freedom = n-1 = 10-1= 9

Type B Contribution

Sources of uncertainty and its calculations are as shown in Table 3.

Sr. no. Readings i X % 2

i X X

1 0.6903

2 0.6953

3 0.6953

4 0.6977

5 0.6958

6 0.6977

7 0.6938

8 0.6977

9 0.6903

10 0.6977


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Table 3. Sources of uncertainty

(3)

= 0.0091

Effective Degree of freedom= eff =

n

i i

i

c

U

U

1

4

4

(4)

= 9.406

From Student’s t-distribution, for the confidence level of 95.45% and for eff =9.406, the coverage factor

K = 2.32

Expanded Uncertainty (U) =K× UC (5)U = 2.32 ×0.0091

U = 0.0211

Final result =0 .6951± 0.0211

Final uncertainty budget is as shown in Table 4.

Source Distribution DOF Calculation Value

U2 = Mass for Wt of sample

Expanded uncertaintyfor 0.1g is 0.00028

Normal ∞ Standard uncertainty=0.00028/2=0.00014

U2 = 0.00014/0.1=0.0014

Expanded Uncertainty is divided by coverage factor K=2 atconfidence level of 95.45%

0.0014

U3 = standard uncertainty due to

volume taken.Vol. =20ml

Max.calibrationerror=0.03

Triangular √6 ∞ Standard uncertainty=0.03/

=0.012U3 = 0.012/20

= 0.00006

0.00006

U4= Uncertainty duesample CRM

Reported uncertainty=0.006

Rectangular √3

∞ U4 = 0.006/

= 0.0003

0.0003

U5 = Uncertainty due tocalibration CRMReported

uncertainty=.2% for 1000 ppm

Rectangular √3∞

0.2/100=0.0020.002/(1000×√3)U5 = 0.000001

0.000001

U6 = Uncertainty due tovolume made

Max. calibration error for 100ml=0.1 ml

100ml made 2 time

Triangular √6 ∞ Standard uncertainty=0.1/

=0.04U6 = 2×0.04/100

= 0.0008

0.0008


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Table4. Uncertainty budget

Source of Uncertainty

Estimated value

Distributiondivision

Sensitivitycoefficient

Std.uncertainty

DOF

U1 0.009 Normal 1 0.009 9

U2 0.00028 Normal 1 0.0014 ∞

U3 0.03 Triangular 1 0.00006 ∞

U4 0.006 Rectangular 1 0.0003 ∞

U5 0.2% Rectangular 1 0.000001 ∞

U6 0.1 ml Triangular 1 0.0008 ∞

Fig. 2 .Effect of various factors of uncertainty of AAS

Conclusion

Even if the result of the measurement is not perfect, it is possible to obtain reliable information, since the result

of the measurement is associated with its respective uncertainty. The success in estimating uncertainty of

measurement depends on correct analysis of the whole measuring process. Estimation of measurement

uncertainty is very important for reliability of measurement data. In case of Atomic Absorption

Spectrophotometer, uncertainty gets affected by different factors such as qualitative and quantitative. This

paper shows the effects of individual factors and its importance on final result. Evaluation of uncertainty gives

idea about various factors affecting the test results. Accordingly proper actions can be taken to reduce the effect

of factor which is affecting more. This helps to improve the quality of testing and calibration lab. In case of

AAS repeatability is affecting more, so while performing the test standard procedure should be followed which

help to minimize the uncertainty of measurement


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References:

[1] Beaty Richard D. and Kerber Jack D.(1993) Concepts, Instrumentation and Techniques in Atomic Absorption Spectrophotometrey.

Second Edition[2] BIPM/IECIIFCC/ISO/OIML/IUPAC, ISBN 92 67 101889, 1993-95- Guide to the expression of uncertainty in measurement.

[3] Erxleben A (2009).–ppt on Atomic Absorption Spectroscopy.

[4] EA-4/02 (1990) Expression of the uncertainty of measurement in calibration: European Cooperation for Accreditation.

[5] GUM (1993) Guide to the expression of uncertainty in measurement, International Organization for Standardization, Geneva, ISBN9267-10188-9, First Edition.

[6] Mandavgade N.K.(2011) Mathematical modeling of effects of various factors on uncertainty of measurement in material testing,

Proceedings of 2011 International Conference on Mechanical Engineering and Technology ICMET 2011, November 2011, London,UK, DOI-http://dx.doi.org/10.1115/1.859896.paper43


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Analysis of Uncertainity in Atomic Absorption

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