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Practical aspects of the uncertainty and traceability of spectrochemical measurement results by electrothermal atomic absorption spectrometry
Spectrochimica Acta Part B 62 (2007) 337 – 343 www.elsevier.com/locate/sab
Practical aspects of the uncertainty and traceability of spectrochemical measurement results by electrothermal atomic absorption spectrometry S. Duta a,b,⁎,1 , P. Robouch a,1 , L. Barbu c , P. Taylor a,1 a
Institute for Reference Materials and Measurements, Joint Research Centre, European Commission, Retieseweg 111, B-2440 Geel, Belgium b National Institute of Metrology, 042122 Vitan Barzesti 11, sector 4 Bucharest, Romania c Coca-Cola Entreprise, Analytical Department, Bucharest, Romania Received 1 September 2006; accepted 12 February 2007 Available online 22 February 2007
Abstract The determination of trace elements concentration in water by electrothermal atomic absorption spectrometry (ETAAS) is a common and well established technique in many chemical testing laboratories. However, the evaluation of measurement uncertainty results is not systematically implemented. The paper presents an easy step-by-step example leading to the evaluation of the combined standard uncertainty of copper determination in water using ETAAS. The major contributors to the overall measurement uncertainty are identified due to amount of copper in water sample that mainly depends on the absorbance measurements, due to certified reference material and due to auto-sampler volume measurements. The practical aspects how the traceability of copper concentration in water can be established and demonstrated are also pointed out. © 2007 Elsevier B.V. All rights reserved. Keywords: Measurement uncertainty; Traceability of results; Trace elements in water; ETAAS
1. Introduction The maximum allowable levels of different trace chemical elements in water are set-up by European legislation [1]. It requires reliable and accurate results to be reported. According to ISO/IEC [2] the laboratories should fulfil some technical requirements, such as the evaluation of the measurement uncertainty and the assurance of the traceability of measurement results. The measurement uncertainty estimation associated with low levels of concentration determined by electrothermal atomic absorption spectrometry (ETAAS) depends on the correct identification and quantification of the main sources of ⁎ Corresponding author. Institute for Reference Materials and Measurements, Joint Research Centre, European Commission, Retieseweg 111, B-2440 Geel, Belgium. Tel.: +32 14571963; fax: +32 14571806. E-mail addresses: [email protected] (S. Duta), [email protected] (P. Robouch), [email protected] (P. Taylor). 1 Tel.: +32 14571963; fax: +32 14571806. 0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.02.004
uncertainty. Some of them concern the reference materials used in the analytical process, the calibration of analytical procedure, instrumental performances, etc. The Guide to the Expression of Uncertainty in Measurement [3] and Quantifying Uncertainty in Analytical Measurement [4] are the reference documents used for the evaluation and expression of uncertainty in routine chemical measurements. The uncertainty of ETAAS results should be evaluated by the laboratory itself. The traceability, is a property of the measurement results, allowing the comparability of trace elements concentration in water to well stated references. According to the European legislation [5], 10 μg/L is the maximum concentration of copper allowed in water intended for human consumption. Considering the above directive requirement, the comparability of results provided by different laboratories relies in the traceability to common and internationally accepted references. To asses the compliance of copper concentration in water sample with the statutory limits set-up by different specifications, a careful investigation should be considered for the uncertainty evaluation as well as for the establishing traceability of the reported results.
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concentration in water sample is further corrected by recovery factor and dilution factor.
2. Experimental In this work copper determination in water is investigated, stressing the uncertainty evaluation and establishing traceability issues. 2.1. Analytical procedure Copper determination in water by ETAAS is a routine analytical experiment for many chemical testing laboratories. The standardized analytical procedure is described in [6]. Other analytical procedures are described in the manual methods of atomic absorption spectrometer manufacturers. In this application, the experimental protocols for copper determination in water consist of: (i) (ii) (iii) (iv) (v) (vi) (vii)
stabilisation of water sample; preparation of the standard solutions used for calibration; calibration of the analytical procedure; injection of a sub-sample water into the ETAAS; determination of copper content from calibration data; determination of copper concentration in water sample; report of results.
An aliquot of water sample (100 mL) is stabilised with 0.5 mL nitric acid. The copper sample concentration is determined from the calibration data of atomic absorption spectrometer. In practice, the calibration technique of copper determination in water samples is performed either by standard calibration technique or by standard addition technique. In this application the standard calibration technique is used; three calibration solutions (1; 5 and 10 μg/L) were used to calibrate the analytical procedure. Five replicate measurements are performed for each calibration solutions and the blank solution. The absorbance values of the calibration solutions are corrected by subtracting the absorbance value of blank. The calibration curve is plotted based on the absorbance peak height against the concentration of calibration solutions. The concentration of copper concentration is calculated using the two calibration solutions by bracketing the unknown sample concentration. The copper
2.1.1. Copper standard solutions The copper standard solutions have an important role both for uncertainty evaluation and establishing traceability of reported result. Hence, the preparation of the working and calibration solutions is presented in more details. A copper standard stock solution with nominal concentration Cstock = (1000 ± 3) mg/L is employed (Merck, Darmstad, Germany). The working solution, cwork, is prepared by diluting Cstock twice: 10 mL stock solution to 1000 mL volumetric flask and 1 mL from this solution to 1000 mL volumetric flask. The concentration of working solution obtained is cwork = 10 μg/L. The first calibration solution is prepared by dilution of 5 mL of working solution to 50 mL volumetric flask resulting in a copper concentration ccal1 of 1 μg/L. The second calibration solution is prepared by dilution of 25 mL of working solution to 50 mL volumetric flask resulting in a copper concentration ccal1 of 5 μg/L. When 20 μL of first and, respectively, second calibration solution are injected into the graphite furnace, the corresponding copper content are wcal1 = 2 ⁎ 10− 5 μg and wcal2 = 10 ⁎ 10− 5 μg. The model equations that describe the preparation of the working solution and the calibration solutions are presented in Table 1. 2.2. Instrumentation An atomic absorption spectrometer type Ati Unicam SOLAAR 939 with GF 90 Plus furnace and FS95 auto-sampler is used. The sample is collected and dispensed via an inert PTFE capillary with rapidly interchangeable tips. All facilities are programmed from the atomic absorption spectrometer software and sampling facilities. The temperature programme used for the graphite furnace is described in Table 2. The deuterium background wavelength correction and the following optimized parameters of the instrument were applied: wavelength 324.8 nm, spectral bandpass 0.5 nm, sample volume 20 μL.
Table 1 Preparation of working and calibration solutions Step
Model equations
Explanation of the input quantities
Preparation of working solution, cwork
cwork ¼ ½Cstock ⁎ðV10 =V1000 Þ⁎ðV1 =V1000 Þ ð1Þ
cwork—concentration of working solution, mg/L Cstock—concentration of stock solution, mg/L V1000 is the final volume of the working solution, mL V10—the volume of the stock solution taken for the first dilution step, mL V1—the volume of the intermediate solution taken for the second dilution step, mL
Preparation of calibration solutions, ccal1 and ccal2
ccal1 ¼ cwork ⁎V5 =V50
ð2Þ
Cstock—concentration of stock solution, mg/L
ccal2 ¼ cwork ⁎V25 =V50
ð3Þ
ccal1 and ccal2—concentration of first and second calibration solutions V50—the final volume of the calibration solution, mL V5—volumes of the working solution taken for the first dilution, mL V25—volumes of the working solution taken for the second dilution, mL
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3. Results and discussion This work was mainly focused on the evaluation of measurement uncertainty and establishing traceability of copper concentration in water sample. 3.1. The evaluation of measurement uncertainty According to [7] the measurement uncertainty of an analytical result is defined as one important ‘parameter associated with the measurement results that characterizes the dispersion of values that could reasonably be attributed to the measurand.’ The well accepted and standardized procedure to evaluate the measurement uncertainty is based on the appropriate model equations that describes the measurement procedure. The standard uncertainty for each input quantities are identified and quantified. By combining the individual uncertainty components using the law of uncertainty propagation, the measurement uncertainty is evaluated. The contribution of the major contributors to the overall measurement uncertainty is useful to improve the measurement procedure performances. The steps considered to evaluate the measurement uncertainty according to ISO-GUM are: - define the measurand; - describe the measurement procedure by the corresponding model equations; - identify the possible sources of uncertainty; - estimate all the input quantities; - evaluate the standard uncertainty of each input quantity; - estimate the measured value; - evaluate the combined standard uncertainty of the result; - evaluate the uncertainty sources contribution/index; - calculate the expanded uncertainty; - report the result with the associated uncertainty. The practical aspects of the uncertainty evaluation are presented taking into consideration the experimental data, information from the literature and from the manufacturer certificates. 3.1.1. The determination of copper content from the calibration data The measurand in this application is copper concentration in water, expressed in μg/L. The calibration curve obtained for copper determination by SOLAAR 939 spectrometer is expressed as: A = 0.0385 +
Table 2 The temperature programme for copper determination by SOLAAR 939 Step
Temperature, °C
Time, s
Ramp, °C/s
Drying
100 120 850 2100 2500
1.0 10 20 3.0 3
30 1 150 0 0
Pyrolysis Atomization Cleaning
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0.0306 ⁎ c where A is the instrumental absorbance and c is the concentration value. In respect of measurement uncertainty evaluation, the determination of copper content is necessary taking into consideration the autosampler effect on the volume injected into furnace. The copper content in sample (wsample) is estimated by interpolation of the observed signal Asample from the two calibration data, respectively, ccal1 and ccal2. The bracketing calibration between Acal1 and Acal2 is used. The model equation for bracketing calibration is described by the following model equation: wsample ¼ twcal1 ⁎ðAcal2 −Asample Þ þ wcal2 ⁎ðAsample −Acal1 Þb=ðAcal2 −Acal1 Þ
ð4Þ
where: wcal1 wcal2 Acal Acal2 Asample
is the copper content contained in the calibration solution ccal1 is the copper content contained in the calibration solution ccal2 is the instrumental absorbance corresponding to wcal1 is the instrumental absorbance corresponding to wcal1 is the instrumental absorbance for water sample
3.1.2. The determination of copper concentration in water sample To determine the copper concentration in water sample, the copper content in water is reported to the intaken volume (V20u). The result is also corrected by the recovery factor (Cobs / Ccrm) and by dilution effect (V100 + V0.5) / V100 as well. The following model equation is used: CCu ¼ ðwsample =V20u Þ⁎ðCobs =Ccrm Þ⁎ðV100 þ V0:5 Þ=V100
ð5Þ
where: wsample V20u Cobs Ccrm V100 V0.5
is copper content in water sample is the volume of sample injected into graphite furnace is the certified concentration of reference material is the observed concentration of reference material is the volume of water sample is the volume of nitric acid added for water stabilisation
3.2. The estimation of input quantities and their standard uncertainty The input quantities may be quantities whose values and uncertainty are directly determined in the current measurement (Type A, statistical analysis of series of observation) or brought into measurement from external sources (Type B, previous experiments, literature data, information from manufacturer etc.). In any analytical application, the possible sources of uncertainty are due to the storage conditions, the reagent purity, the measurement conditions, the interferences from other components etc. The main identified sources of uncertainties taken into
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Table 3 Input quantities with their standard uncertainty Quantity
V20u V0.5 V1 V5 V10 V25 V50 V100 V1000 ccrm Cstock cobs Acal1 Acal2 Acal3 Asample
Units
μL mL mL mL mL mL mL mL mL μg/L mg/L μg/L
Value
20.00 0.5000 1.0000 5.000 10.000 25.000 50.000 100.000 1000.00 1.81 1000.0 1.728 0.0707 0.2253 0.4153 0.1227
Uncertainty due to temperature variation correction is evaluated as:
ucert (±)
urep (±)
uΔT (±)
Standard uncertainty (±)
RSu, % (±)
0.5 0.007 0.007 0.030 0.050 0.100 0.060 0.100 0.250 0.08 3 – – – – –
0.2 0.001 0.001 0.006 0.009 0.015 0.012 0.019 0.133 – – – 0.0015 0.0015 0.0023 0.0015
– 1.6–04 3.3–04 1.6–03 3.3–03 8.2–03 0.016 0.033 0.33 – – – – – – –
0.35 0.0042 0.0042 0.018 0.030 0.060 0.040 0.069 0.38 0.05 1.7 0.013 0.0015 0.0015 0.0023 0.0015
1.8 0.8 0.4 0.4 0.3 0.2 0.08 0.07 0.04 2.6 0.17 0.76 2.2 0.7 0.6 1.2
uDT ¼ b⁎V ⁎DT
ð6Þ
where: β V ΔT
thermal expansion coefficient volume of volumetric glassware temperature variation
The thermal expansion coefficient for aqueous solution taken into account is β = 1.9 ⁎ 10− 4 K− 1. It is also assumed that the temperature variation is ΔT = 3 °C. The combined standard uncertainty of the volume is evaluated by the square root of the quadratic summation of the standard uncertainties arising from these three sources, respectively, (ucert), (urep) and (uΔT), based on the law of propagation of uncertainty and the corresponding distribution of each uncertainty component.
consideration for the copper determination in water are summarised in Table 3: - the volume measurements and its influence quantities (i.e. temperature); - the absorbance measurements of calibration solutions and water sample; - the concentration of stock standard solution, reference material used to evaluate recovery of the analyte in water samples. In Table 3 the uncertainty of the corresponding input quantity from the calibration certificates of the glassware manufactures, or from the certified chemical composition of the reference materials and certified concentration of the stock solution is marked as ucert. The uncertainty component due to the repeatability of measurements is indicated as urep and uΔT is the uncertainty component due to temperature influence. The input quantities presented in Table 3 are: The volume as input quantity The volumetric glassware used during this experiment (Table 3) are pipettes, micropipettes, volumetric flasks, calibrated against the recognised traceable measurement standards. The corresponding values and related uncertainty components (ucert and urep) are derived from the manufacturer documents [8].
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ucert 2 u 2 pffiffiffi þðurep Þ þ pDTffiffiffi uV ¼ 3 3
ð7Þ
Following this algorithm, the standard uncertainty as well as the relative standard uncertainty (RSu) of volume measurements are summarised in Table 3. The concentration input quantity The concentration of the stock standard solution Cstock, the certified concentration of the matrix reference materials ccrm as well as the observed concentration of the reference material cobs are considered as concentration input quantities (Table 3). The concentration of stock solution is considered as an input quantities with a rectangular distribution, as a consequence its standard uncertainty is evaluated as 3 / √3 = 1.7 mg/L (Table 3). The certified reference material SLRS-4 provided by National Research Council (NRC, Ottawa, Canada) was used. The certified copper concentration is (1.81 ± 0.08) μg/L. In accordance with the information from the certification campaign, for copper certification four independent measurements were performed by ETAAS, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). The standard uncertainty associated to the certified value is
Table 4 Concentration and standard uncertainty of working solution Parameter
Unit
Value
Standard uncertainty (±)
RSu, % (±)
Cstock
V1
V10
V1000
Cstock V1 V10 V1000 cwork Index
μg/L L L L μg/L
1.00 + 06 0.001 0.01 1.00 10.00
1.73 + 03 4.18–06 3.04–05 3.83–04 0.06
0.2 0.4 0.3 0.0 0.60
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
9.9%
57.6%
30.6%
1.9%
S. Duta et al. / Spectrochimica Acta Part B 62 (2007) 337–343
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Table 5 Copper content and standard uncertainty of calibration solutions wcal1 Parameter
Units
Value
Standard uncertainty (±)
RSu % (±)
cwork
V20u
V5
V50
cwork V20u V5 V50 wcal1 Index
μg/L L L L μg
10.00 2.00–05 5.00–03 0.05 2.00–05
0.055 3.51–07 1.84–05 4.02–05 3.76–07
0.6 1.8 0.4 0.1 1.9
10.10 2.00–05 5.00–03 0.050
10.00 2.04–05 5.00–03 0.05
10.00 2.00–05 5.02–03 0.05
10.00 2.00–05 5.00–03 0.05
8.6%
87.4%
3.8%
0.2%
considered as a type B uncertainty with a rectangular distribution, as a consequence its standard uncertainty is approximated to (0.08 / √3 ≅ 0.05 μg/L). The observed value of the reference material and its associated standard uncertainty are obtained as average value of four independent measurements and the corresponding standard deviation of the average value (1.728 ± 0.013) μg/L. The absorbance input quantity The absorbance of the calibration solutions, Acal1 and, respectively, Acal2 as well the absorbance for water sample Asample, are considered as input quantities (Table 3). Their standard uncertainties are evaluated based on the five replicate measurements. 3.3. The evaluation of the combined standard uncertainty The spreadsheet approach developed by Kragten [9] is applied to compute the combined uncertainty. This approach, as the general formula of the law of uncertainty propagation, takes into account the dependences within the equations. 3.3.1. The combined standard uncertainty of the standard solutions concentration The combined standard uncertainty evaluation of the working solution concentration due to the preparation process, is based on the model equations (1), (2) and (3) described in Table 1. The estimated value is: cwork = (10.00 ± 0.06) μg/L. The spreadsheet calculation for the concentration of working solution is presented in Table 4. It is important to notice that the contribution of the uncertainty of volume measurements, V1 and V10, to the uncertainty of the concentration of working solution are the dominant components. They represent 57.6% and, respectively, 30.6%.
The spreadsheet calculation for the standard uncertainty of the copper content calibration solutions wcal1 and, respectively, wcal2 are presented in Table 5 and, respectively, Table 6. The estimated values are: wcal1 ¼ ð2:000F0:037Þ⁎10−5 Ag and wcal2 ¼ ð10:000F0:186Þ⁎10−5 Ag In both cases, the dominant contributor is the volume injected into furnace by the auto-sampler system (cca. 90%). The second significant contributor is the concentration of working solution (cca. 9%). 3.3.2. The combined standard uncertainty of copper content The combined standard uncertainty evaluation of the copper content in water sample due to the instrumental measurements is based on the model equations (4). The spreadsheet approach is applied to the bracketing calibration to evaluate the combined standard uncertainty for copper content in water sample. The calculation of the associated standard uncertainty of the copper content in water is presented in Table 7. The estimated value of copper content obtained is: wsample = (4.69 ± 0.119) ⁎ 10− 5 μg. The major uncertainty contributor is the sample absorbance measurement (cca. 45%). Furthermore the absorbance measurements for calibration solution Acal1 and for working solution content, wcal2, contribute to 20% and 27%, respectively to the combined uncertainty. 3.3.3. The combined standard uncertainty of copper concentration in water sample The combined standard uncertainty evaluation of the copper concentration in water sample due to the instrumental measurements is based on the model equations (5).
Table 6 Copper content and standard uncertainty of calibration solutions wcal2 Parameter
Units
Value
Standard uncertainty (±)
RSu % (±)
cwork
V20u
V25
V50
cwork V20u V25 V50 wcal2 Index
μg/L L L L μg
10.00 2.00–05 0.025 0.05 1.00–04
0.055 3.51–07 6.02–05 4.02–05 1.86–06
0.6 1.8 0.2 0.1 1.9
10.10 2.00–05 0.025 0.05
10.00 2.04–05 0.025 0.05
10.00 2.00–05 0.0251 0.05
10.00 2.00–05 0.025 0.05
8.8%
89.4%
1.7%
0.2%
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Table 7 Copper content and standard uncertainty in water sample Parameter
Units
Value
Standard uncertainty (±)
RSu, %, (±)
wcal1
wcal2
Acal1
Acal2
Asample
wcal1 wcal2 Acal1 Acal2 Asample wsample Index
μg μg
2.00–05 1.00–04 0.0707 0.225 0.123 4.69–05
3.76–07 1.86–06 1.53–03 1.53–03 0.153 1.19–06
1.9 1.9 2.2 0.7 1.2 2.5
2.04–05 1.00–04 0.0707 0.225 0.123
2.00–05 1.02–04 0.0707 0.225 0.123
2.00–05 1.00–04 0.0722 0.225 0.123
2.00–05 1.00–04 0.0707 0.227 0.123
2.00–05 1.00–04 0.0707 0.225 0.124
4%
27%
20%
5%
44%
μg
In Table 8 the spreadsheet calculation for the associated standard uncertainty of the copper concentration in water is presented. The concentration of copper in water sample with their combined standard uncertainty is (2.249 ± 0.090) μg/L. The major uncertainty contributors are: (i)—the amount of copper in sample (40%) that mainly depends on the sample absorbance measurement; (ii)—the certified value of reference material used (38%) and (iii)—the in taken volume of sample by the auto-sampler (18%). The reported result (2.249 ± 0.180) μg/L is expressed as a range that may encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. The expanded uncertainty is reported by multiplying the combined standard uncertainty by a coverage factor of k = 2 to give a level of confidence of approximately 95%. To improve the result quality more effort should be spent to decrease the effect of the major identified contributors to the overall measurement uncertainty. 4. Practical aspects on the traceability of copper concentration According to [7] traceability is ‘the property of result of a measurement or the value of a standard whereby it can be related to stated reference, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties’. In respect of comparability of copper concentration in water sample as described in this example with the results provided by other chemical testing laboratories, the traceability of the results should be proven and demonstrated. To establish traceability of the measurement results, in practice several aspects should be
considered. Some of them were already mentioned in this paper during the evaluation of measurement uncertainty process. First of all, the measurand should be clearly specified, in this case, it is the concentration of copper in water sample, expressed in μg/ L. Then suitable model equations should be fully designed to calculate the result. All the input quantities are then identified and quantified. Establishing traceability for each input quantity is an important requirement to establish traceability of the final reported result. As a consequence, the traceability of all the input quantities mentioned in the model equation (5) for copper concentration in water is well documented, such as: - the concentration value of the standard solutions used to calibrate the analytical system is traceable to SI units, traceability documented by the Certificate of standard solution provided; - the volume measurement values are traceable to the SI units, traceability demonstrated by the Certificate glassware Laborgerate manufacturer ; - the chemical composition of the reference materials used to evaluate the recovery of the analytical process is traceable to SI units and is demonstrated by the Certificate of the reference material. Considering the practical aspects on the measurement uncertainty evaluation as well on the establishing the traceability of the results presented in this paper, the measurement procedure of copper determination in water by ETAAS can be easily improved. To minimise the measurement uncertainty contribution due to the calibration data, that mostly affect the amount of copper in water sample, the laboratory should consider: (i) the
Table 8 Copper concentration and standard uncertainty in water sample Parameter
Units
Value
Standard uncertainty (±)
Rsu, % (±)
wsample
V20u
V0.5
V100
Cobs
Ccrm
wsample V20u V0.5 V100 Cobs Ccrm CCu Index
μg L L L μg/L μg/L μg/L
4.69–05 2.00–05 5.00–04 0.1 1.73 1.81 2.249
1.19–06 3.51–07 4.17–06 6.91–05 0.0131 0.0462 0.09
2.5 1.8 0.8 0.1 0.8 2.6 4.03
4.81–05 2.00–05 5.00–04 0.1 1.73 1.81
4.69–05 2.04–05 5.00–04 0.1 1.73 1.81
4.69–05 2.00–05 5.04–04 0.1 1.73 1.81
4.69–05 2.00–05 5.00–04 0.1 1.73 1.81
4.69–05 2.00–05 5.00–04 0.1 1.74 1.81
4.69–05 2.00–05 5.00–04 0.1 1.73 1.86
40%
18%
0%
0%
4%
38%
S. Duta et al. / Spectrochimica Acta Part B 62 (2007) 337–343
absorbance measurements that should be optimize, i.e. by measuring the peak area instead of height peak; (ii) the concentration of the calibration solutions should be carefully prepared, i.e. by adequate successive dilution steps. To minimise the measurement uncertainty contribution due to the recovery factor, first, the laboratory should be aware on this effect, and when possible to improve the reliability of the measurement procedure using a suitable certified reference materials with a smaller associated uncertainty. 5. Conclusions This transparent and practical way to evaluate the measurement uncertainty of copper measurements in water sample by graphite furnace atomic absorption spectrometry (ETAAS) can serve as a useful exercise for chemical testing laboratories. The laboratory identifies and is awarded on the effect of different input quantities and is able to quantify the major contributions to the overall measurement uncertainty. In order to improve the analytical procedure, the effect of these contributors can be minimised. The approach developed in this paper for uncertainty evaluation and establishing traceability of copper concentration in water can be easily generated for other heavy metal determinations by ETAAS.
343
Acknowledgement The authors are thankful to all members of Training in Metrology in Chemistry programme [10] (TrainMiC) for the valuable discussion on the uncertainty and traceability issues. References [1] European Directive 2000/60/EC of the Establishing a Framework for Community Action in the Field of Water Policy, 2000. [2] ISO/IEC 17025: General Requirements for the Competence of Calibration and Testing Laboratories, ISO, Geneva, 2005. [3] Guide to the Expression of Uncertainty in Measurement (GUM), second ed.,ISO, Geneva, 1995. [4] EURACHEM/CITAC Guide: Quantifying Uncertainty in Analytical Measurement, second ed., 2000. [5] Council Directive 98/83/EC Water Intended for Human Consumption, 1998. [6] ISO 15586: Water Quality: Determination of Trace Elements Using Atomic Absorption Spectrometry with Graphite Furnace, 2003. [7] International Vocabulary of Basic and General Terms in Metrology, ISO, Geneva, 1993. [8] Hirschman Laborgerate, Certificate of Performance, Germany, 1996. [9] J. Kragten, Calculating Standard Deviation and Confidence Intervals with a Universally Applicable Spreadsheet Technique, Analyst 119 (1994) 2161–2165. [10] www.trainmic.org.
Practical aspects of the uncertainty and traceability of spectrochemical measurement results by electrothermal atomic absorption spectrometry S. Duta a,b,⁎,1 , P. Robouch a,1 , L. Barbu c , P. Taylor a,1 a
Institute for Reference Materials and Measurements, Joint Research Centre, European Commission, Retieseweg 111, B-2440 Geel, Belgium b National Institute of Metrology, 042122 Vitan Barzesti 11, sector 4 Bucharest, Romania c Coca-Cola Entreprise, Analytical Department, Bucharest, Romania Received 1 September 2006; accepted 12 February 2007 Available online 22 February 2007
Abstract The determination of trace elements concentration in water by electrothermal atomic absorption spectrometry (ETAAS) is a common and well established technique in many chemical testing laboratories. However, the evaluation of measurement uncertainty results is not systematically implemented. The paper presents an easy step-by-step example leading to the evaluation of the combined standard uncertainty of copper determination in water using ETAAS. The major contributors to the overall measurement uncertainty are identified due to amount of copper in water sample that mainly depends on the absorbance measurements, due to certified reference material and due to auto-sampler volume measurements. The practical aspects how the traceability of copper concentration in water can be established and demonstrated are also pointed out. © 2007 Elsevier B.V. All rights reserved. Keywords: Measurement uncertainty; Traceability of results; Trace elements in water; ETAAS
1. Introduction The maximum allowable levels of different trace chemical elements in water are set-up by European legislation [1]. It requires reliable and accurate results to be reported. According to ISO/IEC [2] the laboratories should fulfil some technical requirements, such as the evaluation of the measurement uncertainty and the assurance of the traceability of measurement results. The measurement uncertainty estimation associated with low levels of concentration determined by electrothermal atomic absorption spectrometry (ETAAS) depends on the correct identification and quantification of the main sources of ⁎ Corresponding author. Institute for Reference Materials and Measurements, Joint Research Centre, European Commission, Retieseweg 111, B-2440 Geel, Belgium. Tel.: +32 14571963; fax: +32 14571806. E-mail addresses: [email protected] (S. Duta), [email protected] (P. Robouch), [email protected] (P. Taylor). 1 Tel.: +32 14571963; fax: +32 14571806. 0584-8547/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2007.02.004
uncertainty. Some of them concern the reference materials used in the analytical process, the calibration of analytical procedure, instrumental performances, etc. The Guide to the Expression of Uncertainty in Measurement [3] and Quantifying Uncertainty in Analytical Measurement [4] are the reference documents used for the evaluation and expression of uncertainty in routine chemical measurements. The uncertainty of ETAAS results should be evaluated by the laboratory itself. The traceability, is a property of the measurement results, allowing the comparability of trace elements concentration in water to well stated references. According to the European legislation [5], 10 μg/L is the maximum concentration of copper allowed in water intended for human consumption. Considering the above directive requirement, the comparability of results provided by different laboratories relies in the traceability to common and internationally accepted references. To asses the compliance of copper concentration in water sample with the statutory limits set-up by different specifications, a careful investigation should be considered for the uncertainty evaluation as well as for the establishing traceability of the reported results.
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S. Duta et al. / Spectrochimica Acta Part B 62 (2007) 337–343
concentration in water sample is further corrected by recovery factor and dilution factor.
2. Experimental In this work copper determination in water is investigated, stressing the uncertainty evaluation and establishing traceability issues. 2.1. Analytical procedure Copper determination in water by ETAAS is a routine analytical experiment for many chemical testing laboratories. The standardized analytical procedure is described in [6]. Other analytical procedures are described in the manual methods of atomic absorption spectrometer manufacturers. In this application, the experimental protocols for copper determination in water consist of: (i) (ii) (iii) (iv) (v) (vi) (vii)
stabilisation of water sample; preparation of the standard solutions used for calibration; calibration of the analytical procedure; injection of a sub-sample water into the ETAAS; determination of copper content from calibration data; determination of copper concentration in water sample; report of results.
An aliquot of water sample (100 mL) is stabilised with 0.5 mL nitric acid. The copper sample concentration is determined from the calibration data of atomic absorption spectrometer. In practice, the calibration technique of copper determination in water samples is performed either by standard calibration technique or by standard addition technique. In this application the standard calibration technique is used; three calibration solutions (1; 5 and 10 μg/L) were used to calibrate the analytical procedure. Five replicate measurements are performed for each calibration solutions and the blank solution. The absorbance values of the calibration solutions are corrected by subtracting the absorbance value of blank. The calibration curve is plotted based on the absorbance peak height against the concentration of calibration solutions. The concentration of copper concentration is calculated using the two calibration solutions by bracketing the unknown sample concentration. The copper
2.1.1. Copper standard solutions The copper standard solutions have an important role both for uncertainty evaluation and establishing traceability of reported result. Hence, the preparation of the working and calibration solutions is presented in more details. A copper standard stock solution with nominal concentration Cstock = (1000 ± 3) mg/L is employed (Merck, Darmstad, Germany). The working solution, cwork, is prepared by diluting Cstock twice: 10 mL stock solution to 1000 mL volumetric flask and 1 mL from this solution to 1000 mL volumetric flask. The concentration of working solution obtained is cwork = 10 μg/L. The first calibration solution is prepared by dilution of 5 mL of working solution to 50 mL volumetric flask resulting in a copper concentration ccal1 of 1 μg/L. The second calibration solution is prepared by dilution of 25 mL of working solution to 50 mL volumetric flask resulting in a copper concentration ccal1 of 5 μg/L. When 20 μL of first and, respectively, second calibration solution are injected into the graphite furnace, the corresponding copper content are wcal1 = 2 ⁎ 10− 5 μg and wcal2 = 10 ⁎ 10− 5 μg. The model equations that describe the preparation of the working solution and the calibration solutions are presented in Table 1. 2.2. Instrumentation An atomic absorption spectrometer type Ati Unicam SOLAAR 939 with GF 90 Plus furnace and FS95 auto-sampler is used. The sample is collected and dispensed via an inert PTFE capillary with rapidly interchangeable tips. All facilities are programmed from the atomic absorption spectrometer software and sampling facilities. The temperature programme used for the graphite furnace is described in Table 2. The deuterium background wavelength correction and the following optimized parameters of the instrument were applied: wavelength 324.8 nm, spectral bandpass 0.5 nm, sample volume 20 μL.
Table 1 Preparation of working and calibration solutions Step
Model equations
Explanation of the input quantities
Preparation of working solution, cwork
cwork ¼ ½Cstock ⁎ðV10 =V1000 Þ⁎ðV1 =V1000 Þ ð1Þ
cwork—concentration of working solution, mg/L Cstock—concentration of stock solution, mg/L V1000 is the final volume of the working solution, mL V10—the volume of the stock solution taken for the first dilution step, mL V1—the volume of the intermediate solution taken for the second dilution step, mL
Preparation of calibration solutions, ccal1 and ccal2
ccal1 ¼ cwork ⁎V5 =V50
ð2Þ
Cstock—concentration of stock solution, mg/L
ccal2 ¼ cwork ⁎V25 =V50
ð3Þ
ccal1 and ccal2—concentration of first and second calibration solutions V50—the final volume of the calibration solution, mL V5—volumes of the working solution taken for the first dilution, mL V25—volumes of the working solution taken for the second dilution, mL
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3. Results and discussion This work was mainly focused on the evaluation of measurement uncertainty and establishing traceability of copper concentration in water sample. 3.1. The evaluation of measurement uncertainty According to [7] the measurement uncertainty of an analytical result is defined as one important ‘parameter associated with the measurement results that characterizes the dispersion of values that could reasonably be attributed to the measurand.’ The well accepted and standardized procedure to evaluate the measurement uncertainty is based on the appropriate model equations that describes the measurement procedure. The standard uncertainty for each input quantities are identified and quantified. By combining the individual uncertainty components using the law of uncertainty propagation, the measurement uncertainty is evaluated. The contribution of the major contributors to the overall measurement uncertainty is useful to improve the measurement procedure performances. The steps considered to evaluate the measurement uncertainty according to ISO-GUM are: - define the measurand; - describe the measurement procedure by the corresponding model equations; - identify the possible sources of uncertainty; - estimate all the input quantities; - evaluate the standard uncertainty of each input quantity; - estimate the measured value; - evaluate the combined standard uncertainty of the result; - evaluate the uncertainty sources contribution/index; - calculate the expanded uncertainty; - report the result with the associated uncertainty. The practical aspects of the uncertainty evaluation are presented taking into consideration the experimental data, information from the literature and from the manufacturer certificates. 3.1.1. The determination of copper content from the calibration data The measurand in this application is copper concentration in water, expressed in μg/L. The calibration curve obtained for copper determination by SOLAAR 939 spectrometer is expressed as: A = 0.0385 +
Table 2 The temperature programme for copper determination by SOLAAR 939 Step
Temperature, °C
Time, s
Ramp, °C/s
Drying
100 120 850 2100 2500
1.0 10 20 3.0 3
30 1 150 0 0
Pyrolysis Atomization Cleaning
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0.0306 ⁎ c where A is the instrumental absorbance and c is the concentration value. In respect of measurement uncertainty evaluation, the determination of copper content is necessary taking into consideration the autosampler effect on the volume injected into furnace. The copper content in sample (wsample) is estimated by interpolation of the observed signal Asample from the two calibration data, respectively, ccal1 and ccal2. The bracketing calibration between Acal1 and Acal2 is used. The model equation for bracketing calibration is described by the following model equation: wsample ¼ twcal1 ⁎ðAcal2 −Asample Þ þ wcal2 ⁎ðAsample −Acal1 Þb=ðAcal2 −Acal1 Þ
ð4Þ
where: wcal1 wcal2 Acal Acal2 Asample
is the copper content contained in the calibration solution ccal1 is the copper content contained in the calibration solution ccal2 is the instrumental absorbance corresponding to wcal1 is the instrumental absorbance corresponding to wcal1 is the instrumental absorbance for water sample
3.1.2. The determination of copper concentration in water sample To determine the copper concentration in water sample, the copper content in water is reported to the intaken volume (V20u). The result is also corrected by the recovery factor (Cobs / Ccrm) and by dilution effect (V100 + V0.5) / V100 as well. The following model equation is used: CCu ¼ ðwsample =V20u Þ⁎ðCobs =Ccrm Þ⁎ðV100 þ V0:5 Þ=V100
ð5Þ
where: wsample V20u Cobs Ccrm V100 V0.5
is copper content in water sample is the volume of sample injected into graphite furnace is the certified concentration of reference material is the observed concentration of reference material is the volume of water sample is the volume of nitric acid added for water stabilisation
3.2. The estimation of input quantities and their standard uncertainty The input quantities may be quantities whose values and uncertainty are directly determined in the current measurement (Type A, statistical analysis of series of observation) or brought into measurement from external sources (Type B, previous experiments, literature data, information from manufacturer etc.). In any analytical application, the possible sources of uncertainty are due to the storage conditions, the reagent purity, the measurement conditions, the interferences from other components etc. The main identified sources of uncertainties taken into
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Table 3 Input quantities with their standard uncertainty Quantity
V20u V0.5 V1 V5 V10 V25 V50 V100 V1000 ccrm Cstock cobs Acal1 Acal2 Acal3 Asample
Units
μL mL mL mL mL mL mL mL mL μg/L mg/L μg/L
Value
20.00 0.5000 1.0000 5.000 10.000 25.000 50.000 100.000 1000.00 1.81 1000.0 1.728 0.0707 0.2253 0.4153 0.1227
Uncertainty due to temperature variation correction is evaluated as:
ucert (±)
urep (±)
uΔT (±)
Standard uncertainty (±)
RSu, % (±)
0.5 0.007 0.007 0.030 0.050 0.100 0.060 0.100 0.250 0.08 3 – – – – –
0.2 0.001 0.001 0.006 0.009 0.015 0.012 0.019 0.133 – – – 0.0015 0.0015 0.0023 0.0015
– 1.6–04 3.3–04 1.6–03 3.3–03 8.2–03 0.016 0.033 0.33 – – – – – – –
0.35 0.0042 0.0042 0.018 0.030 0.060 0.040 0.069 0.38 0.05 1.7 0.013 0.0015 0.0015 0.0023 0.0015
1.8 0.8 0.4 0.4 0.3 0.2 0.08 0.07 0.04 2.6 0.17 0.76 2.2 0.7 0.6 1.2
uDT ¼ b⁎V ⁎DT
ð6Þ
where: β V ΔT
thermal expansion coefficient volume of volumetric glassware temperature variation
The thermal expansion coefficient for aqueous solution taken into account is β = 1.9 ⁎ 10− 4 K− 1. It is also assumed that the temperature variation is ΔT = 3 °C. The combined standard uncertainty of the volume is evaluated by the square root of the quadratic summation of the standard uncertainties arising from these three sources, respectively, (ucert), (urep) and (uΔT), based on the law of propagation of uncertainty and the corresponding distribution of each uncertainty component.
consideration for the copper determination in water are summarised in Table 3: - the volume measurements and its influence quantities (i.e. temperature); - the absorbance measurements of calibration solutions and water sample; - the concentration of stock standard solution, reference material used to evaluate recovery of the analyte in water samples. In Table 3 the uncertainty of the corresponding input quantity from the calibration certificates of the glassware manufactures, or from the certified chemical composition of the reference materials and certified concentration of the stock solution is marked as ucert. The uncertainty component due to the repeatability of measurements is indicated as urep and uΔT is the uncertainty component due to temperature influence. The input quantities presented in Table 3 are: The volume as input quantity The volumetric glassware used during this experiment (Table 3) are pipettes, micropipettes, volumetric flasks, calibrated against the recognised traceable measurement standards. The corresponding values and related uncertainty components (ucert and urep) are derived from the manufacturer documents [8].
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 ucert 2 u 2 pffiffiffi þðurep Þ þ pDTffiffiffi uV ¼ 3 3
ð7Þ
Following this algorithm, the standard uncertainty as well as the relative standard uncertainty (RSu) of volume measurements are summarised in Table 3. The concentration input quantity The concentration of the stock standard solution Cstock, the certified concentration of the matrix reference materials ccrm as well as the observed concentration of the reference material cobs are considered as concentration input quantities (Table 3). The concentration of stock solution is considered as an input quantities with a rectangular distribution, as a consequence its standard uncertainty is evaluated as 3 / √3 = 1.7 mg/L (Table 3). The certified reference material SLRS-4 provided by National Research Council (NRC, Ottawa, Canada) was used. The certified copper concentration is (1.81 ± 0.08) μg/L. In accordance with the information from the certification campaign, for copper certification four independent measurements were performed by ETAAS, inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma atomic emission spectrometry (ICP-AES) and isotope dilution inductively coupled plasma mass spectrometry (ID-ICP-MS). The standard uncertainty associated to the certified value is
Table 4 Concentration and standard uncertainty of working solution Parameter
Unit
Value
Standard uncertainty (±)
RSu, % (±)
Cstock
V1
V10
V1000
Cstock V1 V10 V1000 cwork Index
μg/L L L L μg/L
1.00 + 06 0.001 0.01 1.00 10.00
1.73 + 03 4.18–06 3.04–05 3.83–04 0.06
0.2 0.4 0.3 0.0 0.60
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
1.00 + 06 0.001 0.01 1.00
9.9%
57.6%
30.6%
1.9%
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Table 5 Copper content and standard uncertainty of calibration solutions wcal1 Parameter
Units
Value
Standard uncertainty (±)
RSu % (±)
cwork
V20u
V5
V50
cwork V20u V5 V50 wcal1 Index
μg/L L L L μg
10.00 2.00–05 5.00–03 0.05 2.00–05
0.055 3.51–07 1.84–05 4.02–05 3.76–07
0.6 1.8 0.4 0.1 1.9
10.10 2.00–05 5.00–03 0.050
10.00 2.04–05 5.00–03 0.05
10.00 2.00–05 5.02–03 0.05
10.00 2.00–05 5.00–03 0.05
8.6%
87.4%
3.8%
0.2%
considered as a type B uncertainty with a rectangular distribution, as a consequence its standard uncertainty is approximated to (0.08 / √3 ≅ 0.05 μg/L). The observed value of the reference material and its associated standard uncertainty are obtained as average value of four independent measurements and the corresponding standard deviation of the average value (1.728 ± 0.013) μg/L. The absorbance input quantity The absorbance of the calibration solutions, Acal1 and, respectively, Acal2 as well the absorbance for water sample Asample, are considered as input quantities (Table 3). Their standard uncertainties are evaluated based on the five replicate measurements. 3.3. The evaluation of the combined standard uncertainty The spreadsheet approach developed by Kragten [9] is applied to compute the combined uncertainty. This approach, as the general formula of the law of uncertainty propagation, takes into account the dependences within the equations. 3.3.1. The combined standard uncertainty of the standard solutions concentration The combined standard uncertainty evaluation of the working solution concentration due to the preparation process, is based on the model equations (1), (2) and (3) described in Table 1. The estimated value is: cwork = (10.00 ± 0.06) μg/L. The spreadsheet calculation for the concentration of working solution is presented in Table 4. It is important to notice that the contribution of the uncertainty of volume measurements, V1 and V10, to the uncertainty of the concentration of working solution are the dominant components. They represent 57.6% and, respectively, 30.6%.
The spreadsheet calculation for the standard uncertainty of the copper content calibration solutions wcal1 and, respectively, wcal2 are presented in Table 5 and, respectively, Table 6. The estimated values are: wcal1 ¼ ð2:000F0:037Þ⁎10−5 Ag and wcal2 ¼ ð10:000F0:186Þ⁎10−5 Ag In both cases, the dominant contributor is the volume injected into furnace by the auto-sampler system (cca. 90%). The second significant contributor is the concentration of working solution (cca. 9%). 3.3.2. The combined standard uncertainty of copper content The combined standard uncertainty evaluation of the copper content in water sample due to the instrumental measurements is based on the model equations (4). The spreadsheet approach is applied to the bracketing calibration to evaluate the combined standard uncertainty for copper content in water sample. The calculation of the associated standard uncertainty of the copper content in water is presented in Table 7. The estimated value of copper content obtained is: wsample = (4.69 ± 0.119) ⁎ 10− 5 μg. The major uncertainty contributor is the sample absorbance measurement (cca. 45%). Furthermore the absorbance measurements for calibration solution Acal1 and for working solution content, wcal2, contribute to 20% and 27%, respectively to the combined uncertainty. 3.3.3. The combined standard uncertainty of copper concentration in water sample The combined standard uncertainty evaluation of the copper concentration in water sample due to the instrumental measurements is based on the model equations (5).
Table 6 Copper content and standard uncertainty of calibration solutions wcal2 Parameter
Units
Value
Standard uncertainty (±)
RSu % (±)
cwork
V20u
V25
V50
cwork V20u V25 V50 wcal2 Index
μg/L L L L μg
10.00 2.00–05 0.025 0.05 1.00–04
0.055 3.51–07 6.02–05 4.02–05 1.86–06
0.6 1.8 0.2 0.1 1.9
10.10 2.00–05 0.025 0.05
10.00 2.04–05 0.025 0.05
10.00 2.00–05 0.0251 0.05
10.00 2.00–05 0.025 0.05
8.8%
89.4%
1.7%
0.2%
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Table 7 Copper content and standard uncertainty in water sample Parameter
Units
Value
Standard uncertainty (±)
RSu, %, (±)
wcal1
wcal2
Acal1
Acal2
Asample
wcal1 wcal2 Acal1 Acal2 Asample wsample Index
μg μg
2.00–05 1.00–04 0.0707 0.225 0.123 4.69–05
3.76–07 1.86–06 1.53–03 1.53–03 0.153 1.19–06
1.9 1.9 2.2 0.7 1.2 2.5
2.04–05 1.00–04 0.0707 0.225 0.123
2.00–05 1.02–04 0.0707 0.225 0.123
2.00–05 1.00–04 0.0722 0.225 0.123
2.00–05 1.00–04 0.0707 0.227 0.123
2.00–05 1.00–04 0.0707 0.225 0.124
4%
27%
20%
5%
44%
μg
In Table 8 the spreadsheet calculation for the associated standard uncertainty of the copper concentration in water is presented. The concentration of copper in water sample with their combined standard uncertainty is (2.249 ± 0.090) μg/L. The major uncertainty contributors are: (i)—the amount of copper in sample (40%) that mainly depends on the sample absorbance measurement; (ii)—the certified value of reference material used (38%) and (iii)—the in taken volume of sample by the auto-sampler (18%). The reported result (2.249 ± 0.180) μg/L is expressed as a range that may encompass a large fraction of the distribution of values that could reasonably be attributed to the measurand. The expanded uncertainty is reported by multiplying the combined standard uncertainty by a coverage factor of k = 2 to give a level of confidence of approximately 95%. To improve the result quality more effort should be spent to decrease the effect of the major identified contributors to the overall measurement uncertainty. 4. Practical aspects on the traceability of copper concentration According to [7] traceability is ‘the property of result of a measurement or the value of a standard whereby it can be related to stated reference, usually national or international standards, through an unbroken chain of comparisons all having stated uncertainties’. In respect of comparability of copper concentration in water sample as described in this example with the results provided by other chemical testing laboratories, the traceability of the results should be proven and demonstrated. To establish traceability of the measurement results, in practice several aspects should be
considered. Some of them were already mentioned in this paper during the evaluation of measurement uncertainty process. First of all, the measurand should be clearly specified, in this case, it is the concentration of copper in water sample, expressed in μg/ L. Then suitable model equations should be fully designed to calculate the result. All the input quantities are then identified and quantified. Establishing traceability for each input quantity is an important requirement to establish traceability of the final reported result. As a consequence, the traceability of all the input quantities mentioned in the model equation (5) for copper concentration in water is well documented, such as: - the concentration value of the standard solutions used to calibrate the analytical system is traceable to SI units, traceability documented by the Certificate of standard solution provided; - the volume measurement values are traceable to the SI units, traceability demonstrated by the Certificate glassware Laborgerate manufacturer ; - the chemical composition of the reference materials used to evaluate the recovery of the analytical process is traceable to SI units and is demonstrated by the Certificate of the reference material. Considering the practical aspects on the measurement uncertainty evaluation as well on the establishing the traceability of the results presented in this paper, the measurement procedure of copper determination in water by ETAAS can be easily improved. To minimise the measurement uncertainty contribution due to the calibration data, that mostly affect the amount of copper in water sample, the laboratory should consider: (i) the
Table 8 Copper concentration and standard uncertainty in water sample Parameter
Units
Value
Standard uncertainty (±)
Rsu, % (±)
wsample
V20u
V0.5
V100
Cobs
Ccrm
wsample V20u V0.5 V100 Cobs Ccrm CCu Index
μg L L L μg/L μg/L μg/L
4.69–05 2.00–05 5.00–04 0.1 1.73 1.81 2.249
1.19–06 3.51–07 4.17–06 6.91–05 0.0131 0.0462 0.09
2.5 1.8 0.8 0.1 0.8 2.6 4.03
4.81–05 2.00–05 5.00–04 0.1 1.73 1.81
4.69–05 2.04–05 5.00–04 0.1 1.73 1.81
4.69–05 2.00–05 5.04–04 0.1 1.73 1.81
4.69–05 2.00–05 5.00–04 0.1 1.73 1.81
4.69–05 2.00–05 5.00–04 0.1 1.74 1.81
4.69–05 2.00–05 5.00–04 0.1 1.73 1.86
40%
18%
0%
0%
4%
38%
S. Duta et al. / Spectrochimica Acta Part B 62 (2007) 337–343
absorbance measurements that should be optimize, i.e. by measuring the peak area instead of height peak; (ii) the concentration of the calibration solutions should be carefully prepared, i.e. by adequate successive dilution steps. To minimise the measurement uncertainty contribution due to the recovery factor, first, the laboratory should be aware on this effect, and when possible to improve the reliability of the measurement procedure using a suitable certified reference materials with a smaller associated uncertainty. 5. Conclusions This transparent and practical way to evaluate the measurement uncertainty of copper measurements in water sample by graphite furnace atomic absorption spectrometry (ETAAS) can serve as a useful exercise for chemical testing laboratories. The laboratory identifies and is awarded on the effect of different input quantities and is able to quantify the major contributions to the overall measurement uncertainty. In order to improve the analytical procedure, the effect of these contributors can be minimised. The approach developed in this paper for uncertainty evaluation and establishing traceability of copper concentration in water can be easily generated for other heavy metal determinations by ETAAS.
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Acknowledgement The authors are thankful to all members of Training in Metrology in Chemistry programme [10] (TrainMiC) for the valuable discussion on the uncertainty and traceability issues. References [1] European Directive 2000/60/EC of the Establishing a Framework for Community Action in the Field of Water Policy, 2000. [2] ISO/IEC 17025: General Requirements for the Competence of Calibration and Testing Laboratories, ISO, Geneva, 2005. [3] Guide to the Expression of Uncertainty in Measurement (GUM), second ed.,ISO, Geneva, 1995. [4] EURACHEM/CITAC Guide: Quantifying Uncertainty in Analytical Measurement, second ed., 2000. [5] Council Directive 98/83/EC Water Intended for Human Consumption, 1998. [6] ISO 15586: Water Quality: Determination of Trace Elements Using Atomic Absorption Spectrometry with Graphite Furnace, 2003. [7] International Vocabulary of Basic and General Terms in Metrology, ISO, Geneva, 1993. [8] Hirschman Laborgerate, Certificate of Performance, Germany, 1996. [9] J. Kragten, Calculating Standard Deviation and Confidence Intervals with a Universally Applicable Spreadsheet Technique, Analyst 119 (1994) 2161–2165. [10] www.trainmic.org.