Accred Qual Assur (2005) 10:98–106 DOI 10.1007/s00769-004-0891-1
M. S. Al-Masri Y. Amin
Received: 11 August 2003 Accepted: 2 September 2004 Published online: 20 October 2004 © Springer-Verlag 2004
M. S. Al-Masri (✉) · Y. Amin Department of Protection and Safety, Atomic Energy Commission of Syria, P.O. Box 6091 Damascus, Syria e-mail:
[email protected] Tel.: +963-1161-11926 Fax: +963-1161-12289
PRACTITIONER’S REPORT
Use of the Eurachem guide on method validation for determination of uranium in environmental samples
Abstract Three analytical methods for determination of uranium in environmental samples by a fluorescence technique have been validated and compared in accordance with the Eurachem Guide on method validation. The first method depends on uranium separation from iron using the mercury anode technique; in the other two methods uranium is separated from iron on an anion exchange column by use of either a solution of hydrochloric acid containing ascorbic acid and hydrazine hydrate or a dilute sulfuric acid solution. Detection limits, repeatability, reproducibility, and recovery coefficient were the main validation characteristics. The results showed that
Introduction Determination of total uranium is important in any program of phosphate mining and processing and is also important in environmental safety-monitoring studies. Uranium determination can be achieved using different techniques, viz. fluorimetry, colorimetry, ICP–MS, and nuclear techniques such as alpha spectrometry and gamma spectrometry. Fluorescence has been used for many years as a screening method for uranium determination [1, 14, 15]. This technique depends on separation of uranium from environmental sample using different methods [1, 15]. The mercury anode method is usually used to separate uranium from iron, but the method is not suitable for separation of all iron from the uranium solution; iron ions have a quenching effect on uranium fluorescence production [1, 15]. An anion exchange column with a solution of hydrochloric acid containing
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better statistical values can be achieved by using the third method. Control charts for in-house control samples and international intercomparison samples have also shown that the third method is more statistically stable with time. In addition, uncertainties of measurement were estimated and compared for the three methods. It was found that the Eurachem Guide and comparison of quality statistical validation data can be good tools for selection of the appropriate method for an application. Keywords Method validation · Determination · Uranium · Fluorimeter · Environmental samples
hydrazine hydrate and ascorbic acid as mobile phase can be used to separate uranium from iron. Although this method has been used successfully [14], it is not applicable for analyzing large volumes of sample. To overcome this problem, another method has used dilute sulfuric acid for separation of uranium from iron [14]. However, justification for use of the appropriate method according to international standards is required. Over the past four years, the Environmental Protection Division of the Atomic Energy Commission of Syria has developed and validated all radiochemical and analytical methods used for environmental monitoring in accordance with the Eurachem Guide [4]. This was done to ensure that all services and research activities carried out by environmental measurement groups are of known quality. In addition, this validation process has been used to justify selection of appropriate methods to fit the purpose. According to Eurachem Guide on method vali-
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dation, method-validation studies rely on determination of overall method performance, viz. limit of detection, repeatability, recovery coefficient, etc.; this enables chemists to demonstrate that a defined method is fit for purpose [5, 9, 10, 11]. Moreover, analytical quality-control statistics are also used to analyze the variability inherent in analytical measurements [2, 4, 6, 8, 12, 13, 15]. This paper describes the use of the Eurachem Guide on method validation for determination of uranium in environmental samples, as a tool for method development and selection in our laboratories. Three methods for uranium determination have been validated and compared.
Method 2 1. Dissolve the digested sample in 20 mL HCl (7 mol L−1). 2. Run the sample solution through an anion-exchange column (5 g AG 1×4). 3. Use 80 mL HCl (7 mol L−1) to elute the co-absorbed ions. 4. Use 30 mL HCl solution (4.5 mol L−1) containing 1.8% hydrazine hydrate and 7% ascorbic acid to elute iron from the column. 5. Wash the resin with 100 mL HCl (6 mol L−1). 6. Elute uranium with 50 mL HCl (0.5 mol L−1). 7. Evaporate the eluent to near dryness and dissolve in 5 mL HCl (0.5 mol L−1). Method 3
Experimental Description of the analytical method The three methods studied here have the same pretreatment (ashing, digesting,...) and measurement techniques but differ in the procedure used for purification of uranium from other elements such as iron. Solid sample (soil, sediment; 0.2 g) is usually ashed at 600°C and decomposed totally with a mixture of mineral acids (HF, HCl, HNO3). The solution is evaporated to near dryness and transformed to the chloride form. Uranium is then purified by one of the following procedures and the resulting solution is evaporated and dissolved with 5 mL dilute hydrochloric acid. Two fractions of the sample solution (0.2 mL) are collected; one is fluxed directly with uranium carrier (Na2CO3, K2CO3, NaF) and the other is fluxed after adding a known amount of uranium as internal standard solution (0.1 µg). The two fluxes are measured by means of a Jarrel-Ash 27-000 fluorimeter (Advanced Technical Services, Switzerland). Purification procedures The following purification procedures used by several workers [1, 14] and have been examined here.
1. Dissolve the digested sample in 20 mL dilute HCl. Reduce excess acidity of the sample solution by addition of concentrated ammonia until turbidity appears, add H2SO4 (2 mol L−1) dropwise until the turbidity disappears. 2. Transform the anion-exchange resin to the sulfate form by passage of 40 mL H2SO4 (2 mol L−1). 3. Wash the resin briefly with distilled water. 4. Run the sample solution through the resin bed under the action of gravity. 5. Use 30 mL H2SO4 (0.25 mol L−1) to elute the coabsorbed ions including iron. 6. Use 30 mL concentrated hydrochloric acid to substitute the sulfate ions by chloride ions and wash out the remaining interfering ions (mainly Th). 7. Elute uranium with 50 mL HCl (0.05 mol L−1). The uranium concentration is calculated by use of Eq. (1). (1) where R1 is the average of two successive fluorescence readings of the sample, R2 the average of two successive fluorescence readings of the sample with internal uranium standard addition, Ua the amount of uranium added (µg), V the total volume of sample solution (mL), v the volume of the sample measured by fluorimetry (mL), and ms the mass of sample analyzed (g).
Method 1 1. Dissolve the digested sample in 20 mL HCl (7 mol L−1). 2. Run the sample solution through an anion-exchange column (5 g AG 1×4). 3. Use 80 mL HCl (7 mol L−1) to elute the co-absorbed ions. 4. Elute uranium and iron with 50 mL HCl (0.5 mol L−1). 5. Separate uranium from iron by use of a mercury anode cell (1.5 A, 13 V, 30 min). 6. Evaporate the eluent to near dryness and dissolve in 5 mL HCl (0.5 mol L−1).
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Results and discussion Method validation data Limit of detection The limit of detection is the smallest amount that can be measured with reasonable statistical certainty [2, 4]. To estimate the method detection limit (MDL) of each method according to Eurachem Guide, ten duplicate
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Table 1 Comparison of statistical validation data
Method property
First method
Second method
Third method
Detection limit
MDL (µg g−1)
0.5
0.5
0.5
Repeatability
(µg g−1) Sr (µg g−1) CL (µg g−1)
1.29 0.22 0.20
1.90 0.14 0.13
1.23 0.09 0.08
Reproducibility
(µg g−1) Sr (µg g−1) CL (µg g−1)
1.49 0.28 0.25
1.88 0.12 0.11
1.32 0.08 0.07
Marginal recovery coefficient
MRC (%)
samples of real soil with known concentration (about 5–7 times the theoretical MDL) were analyzed. The standard deviation of these ten duplicates was calculated by use of the formula:
87
95
97
The result can be expressed as in ISO-5725 [9] and ISO-2602 [7] (5) where
(2) where I is the sample index, n is the sample number, Xi is the concentration of uranium in sample i, and is the mean concentration of uranium in the sample. (3) The MDL values obtained for the three methods are presented in Table 1; no difference among the three methods was observed (0.5 µg g−1). This is because the same instrument was used for measurement and most of the factors affecting the MDL are the same for the three methods. However, MDL is related to chemical recovery of uranium and initial sample volume. Therefore, MDL is expected to be relatively lower using the third method, for which the value of the recovery coefficient was 97% (Table 1). In addition, larger sample volumes can be analyzed using the third method, without interference from iron. Repeatability limits Repeatability [9, 13], precision under repeatability conditions, is used to assess ability of a measuring instrument to provide closely similar indications for repeated applications of the same measurement. The repeatability limit (rL) was estimated by analyzing ten duplicates of a real soil sample with known uranium concentration high enough to get a good signal-to-background ratio under repeatability conditions. Standard deviation of these ten duplicates (Sr) was calculated according to Eq. (2) and (rL) is calculated as follows: (4) where t is Student’s t-value at 95% confidence level and n is the number of samples.
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The results of the analysis showed that the standard deviation coefficients for repeatability decreased from 0.22 µg g−1 to 0.09 µg g−1 and hence the limit of repeatability (CL) also decreased; the values were 0.20, 0.13 and 0.09 µg g−1, respectively, for the three methods. This means that the results are more precise when using the third method, even though the results showed that all the methods studied give closely similar results when the procedures are performed under the same conditions. Reproducibility limits (R) The reproducibility limit [3] (performing the analysis a number of times to ensure that the method is capable of giving repeatable results) was estimated by analyzing ten replicates of real samples with known uranium concentration, but with at least one condition changed in the analytical system (analyst, day of analysis, instrument,...). The standard deviation of ten replicates (Sr) was calculated by using Eq. (2) and (rL) was calculated from Eq. (4). Table 1 shows that the third method produces results that are more reproducible. The standard deviation coefficient for reproducibility decreased from 0.28 µg g−1 to 0.08 µg g−1 and hence the limit of reproducibility (CL) decreased from 0.25 µg g−1 to 0.07 µg g−1. Marginal recovery coefficient (MRC) The recovery coefficient is usually used to evaluate the effect of sample matrix on the results using a defined procedure, in which a known amount of uranium is added to the sample and the analysis is performed before and after the addition so that the amount recovered can be calculated [4]. To estimate this property (MRC) five
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real samples of different matrix with known uranium concentration are spiked with a known amount of uranium and the MRC was calculated as follows: (6) where AS is the measured uranium concentration after spike; OC is the uranium concentration in the sample; and S is the amount of added uranium. The recovery coefficient was found to increase from 87% to 97%. This increase is because of the reduced effect of iron on uranium determination–iron can be removed completely by using the third method and losses due to the application of the mercury technique are also avoided. Therefore, application of the other two methods give good MRC values that indicate better separation of iron and elimination of the matrix effect. However, higher values for MRC using the first method might be obtained if the iron concentration in the sample is very low; much lower values than that reported here (87%) are also expected if the iron concentration in the sample is high.
Method
ISE No
ISE value (µg g−1)
AECS value (µg g−1)
Z-score value
1
921 949 954 955 956 957 963 965 971 979 981 983 984 985 987 921 950 951 955 964 981 988 920 921 952 955 956 958 961 962 964 965 987 988 990 992
2.32 0.93 2.13 1.90 1.44 1.54 1.57 2.02 2.29 0.98 1.31 3.13 1.41 1.83 2.50 2.80 1.80 3.04 1.82 2.97 1.32 1.15 2.24 2.79 4.60 2.33 1.58 1.24 2.98 2.79 269 2.59 1.74 2.19 3.34 2.72
2.92 0.60 2.04 1.61 1.35 2.05 0.12 1.62 2.28 1.01 0.87 1.59 0.89 1.20 1.23 2.60 1.04 2.54 1.48 2.18 0.66 0.76 1.87 2.73 3.59 1.42 1.50 0.79 2.00 1.89 247 2.09 1.06 0.80 2.89 2.17
0.70 −0.92 −0.08 −0.40 −0.15 1.04 −0.70 −0.75 0.01 0.05 0.51 −0.62 0.81 −1.43 0.96 0.16 0.73 0.56 0.46 0.85 1.08 0.53 1.48 0.05 1.19 1.36 0.42 0.58 1.04 1.34 0.54 0.57 1.08 0.51 0.37 0.89
2
3
Quality-control charts The quality-control procedures used for most of the validated methods at AECS are analysis of in-house control samples and reference materials in addition to participation in national and international intercomparison and proficiency tests. The in-house control samples are routinely used during analysis of real samples; the results from these control samples were used to establish a control chart. The procedure used at AECS establishes the central line from the mean of a number of measured values of a property and the experimentally estimated standard deviation to define the warning and action limits. The Z-score approach is used here as a property and it was calculated for all analytical data according to the formula: (7) where xi is the measured value of the sample concentration, A is the true value for the sample, and Sr is the selected standard deviation. Acceptable (A): Z≤2, acceptable with warning; 2
