CHARACTERISATION OF TRACE ELEMENTS AND METHYLMERCURY IN AN ESTUARINE SEDIMENT REFERENCE MATERIAL, IAEA-405
INTRODUCTION
A fundamental requirement for the monitoring and assessment of marine contamination is accurate analytical data for pollutant concentrations in the various environmental compartments. For this purpose, the analytical methods used by the laboratories need to be tested and validated. Proper analytical quality control requires the regular use of representative reference materials; i.e., material similar in composition and concentration to the subject sample matrix, but having known concentrations of the analytes of interest. Without a representative and reliable reference material, it is difficult if not impossible to verify the accuracy of a result. Another essential aspect of quality control is periodic external performance assessments, e.g., via regular participation in blind interlaboratory analytical comparison exercises. Interlaboratory comparisons are not only essential for checking the accuracy of a laboratory's analytical results, but also serve to stimulate better analytical performance.
METHODOLOGY
A global intercomparison exercise was conducted during 1998 and 1999 for the determination of trace elements and methylmercury in an estuarine sediment sample. The study was intended to give laboratories responsible for trace element analyses of marine biota an opportunity to check analytical performance.
The subject material was collected in 1998 from the intertidal mudflats of the Tagus estuary (Portugal). It was deep-frozen, freeze-dried, ground and sieved. The sediment fraction of particle size less than 150 µm was further homogenised by mixing in a stainless steel rotating drum for two weeks. After establishing homogeneity of the sample material, aliquots of about 35 g were packed into cleaned brown borosilicate glass bottles with Teflon lined screw caps and sealed in plastic bags. A total of 530 bottles was produced.
In May 1998, the bottled material was dispatched to about 180 laboratories. In total, 120 laboratories from 55 countries participated in this intercomparison exercise and reported results for 58 elements. Methylmercury results were provided by 14 laboratories.
DATA EVALUATION
For each of the data sets, the range of determinations, arithmetic mean and standard deviation were compiled for each element, based on the laboratory means. Laboratory means were subjected to four statistical tests: Dixon, Grubbs, Skewness and Kurtosis. Any result failing one or more test was rejected as an outlier and the remaining data was retested until no further outliers were detected. These tests were applied at a significance level of 95%. The Cochran test was applied in some cases in order to point out extreme values of within laboratory variance that should be considered as outliers.
In order to establish the reference values, the quality assurance data reported by the laboratories (i.e., results of analyses of representative-matrix CRMs) were also taken into consideration. Results submitted by laboratories that did not report any quality assurance information were pointed out and the reported means were systematically considered as "outliers". Data from laboratories reporting CRM results that far exceeded the acceptable range were scrutinised and were also rejected as "outliers".
The analytical methods influenced the laboratory means in some cases. In particular, destructive methods (i.e., digestion) without the use of HF systematically resulted in lower values for 4 refractory elements (i.e., Al, Cr, Fe, V) in comparison to results obtained using total digestion techniques with HF or to non-destructive methods (e.g., NAA). This "method" effect was considered for the establishment of the reference values. Extremely low results obtained with incomplete digestion methods were considered as "outlier" values.
The remaining laboratory means were used to calculate an overall arithmetic mean, standard deviation and 95% confidence interval of the mean for each element. A summary of the results is presented in Table 1.
Table 1. Summary of the results of the intercomparison exercise IAEA-405. Classifications are given for Recommended (A&B) and Information (C) values.
Recommended values are given in bold; Information values given in bold italics. Non-classified values are italicized, and are not considered reference values.
RESULTS AND DISCUSSION
The summary of the results including the classification of the data is indicated in Table 1. Recommended values were assigned for 17 elements (9 class "A" and 8 class "B") and information values for 15 elements (class "C"). In addition, a recommended value was established for methylmercury (class A). The largest number of results (³ 20) was reported for Al, As, Cd, Co, Cr, Cu, Fe, Hg, Li, Mg, Mn, Ni, Pb, Sb, Se, Sn, Sr, V and Zn. For 24 elements only a few results were obtained and reference values could not be assigned. Figure 1 depicts an S-plot showing all laboratory mean values reported in increasing concentration for a representative class "A" element (Cu).
Figure 1. Example of a 'A'-classified element: S-plot of all Cu concentrations
The analytical methods used by the different laboratories are summarised in Figure 2. The wide range of methods employed permits a statistically valid comparison of the principal methods used, particularly the instrumental methods following sample digestion and the "non-destructive" techniques such as neutron activation analysis (NAA) and X-ray fluorescence (XRF).
Figure 2. Percentage of the analytical methods used by the different laboratories.
Most of the laboratories employed wet ashing as the sample pre-treatment procedure, which was the preferred method for analysis by AAS and ICP. Most analysts used total decomposition with a mixture of acids including hydrofluoric acid (HF). However, a number of laboratories did not include HF in their mineralisation procedure and this caused incomplete dissolution of the sediment matrix and lower metal concentrations for Al, Cr, Fe, and V.
In order to examine the relative precision and accuracy of different instrumental techniques, the complete data set for selected elements (i.e., all reported laboratory means) was categorised according to the different analytical methods and represented as multiple box-and-whisker plots. The plot for arsenic is shown in Figure 3 as an example. To be included in the plot, a method required values from at least 4 laboratories. On these graphs, a box encloses the middle 50% of the data, the median is represented as a horizontal line crossing the box and the mean is plotted as a filled square. The whiskers represent the 10 and 90 percentiles, but are only displayed when there are at least 10 measurements in the data set. The number of measurements represented by each data set is reported above the box.
Figure 3. Comparison of analytical methods for As.
(F-AAS: flame atomic absorption spectrophotometry; GF-AAS: graphite furnace atomic absorption spectrophotometry; ZGF-AAS: graphite furnace AAS with Zeeman background correction; ICP-AES: inductively coupled plasma atomic emission spectrometry; ICP-MS: inductively coupled plasma mass spectrometry; NAA: neutron activation analysis; Xray: X-ray Fluorescence; CV-AAS: cold vapour AAS; Hydr-AAS: hydride generation AAS; CV-AFS: cold vapour atomic fluorescence spectrophotometry); SAAS: solid sample AAS.
ORGANOMERCURY COMPOUNDS
The aim of the study was to compare results for methylmercury (MeHg) as obtained by different methods. Fourteen laboratories reported results for MeHg using various isolation procedures and detection systems. In the very first step, when MeHg was released from the binding sites, three techniques were used: distillation, alkaline digestion and acid leaching. Further processing included additional separation using ion-exchange separation of organic and inorganic mercury, solvent extraction with or without a clean-up step (using equilibration into aqueous cysteine or thiosulfate solution) and derivatisation by aqueous phase ethylation and GC separation. The detection systems included cold vapour atomic absorption spectrophotometry (CV-AAS), gas chromatography (GC) combined with cold vapour atomic fluorescence spectrophotometry (CV-AFS), GC with electron capture detector (GC-ECD) and HPLC with CV-AFS.
The consensus value for methylmercury compounds was calculated taking into account the results obtained by 12 laboratories/methods. Accepted laboratory means varied between 0.00393 and 0.00701 mg kg-1. The recommended value is 0.00549 mg kg-1 with the 95% confidence interval from 0.00496 to 0.00602 mg kg-1. This intercomparison exercise has shown that because a substantial number of laboratories world-wide are performing methylmercury analyses using various specific separation methods and sensitive detection systems, certification of methylmercury compounds in different sediment and environmental samples should not be a problem in the future, even at these relatively low levels.
CONCLUSIONS
The IAEA-405 worldwide intercomparison exercise attracted many participants for the determination of trace elements in an estuarine sediment sample. As specified statistical and technical criteria for assigning reference values were fulfilled for 32 elements and methylmercury, the sample can now be used as a Reference Material for quality control of data in the determination of trace elements and methylmercury in coastal sediment samples. Moreover, this exercise underscored the importance of complete decomposition of sediment material with HF for obtaining accurate results for trace element content. Effects of a partial digestion (using no HF) were indeed detected on the IAEA-405 sample for Al, Cr, Fe and V.
ACKNOWLEDGMENTS
The IAEA, Marine Environment Laboratory operates under an agreement between the International Atomic Energy Agency and the Government of the Principality of Monaco. This work was supported by the Inter-agency Programme on Marine Pollution agreed upon between UNEP, IAEA and IOC-UNESCO.
REFERENCE
EJ Wyse, M Coquery, S Azemard, SJ de Mora. Characterisation of Trace Elements and Methylmercury in an Estuarine Sediment Reference Material, IAEA-405. J. Environ. Monit., 2004, 6 (1), 48 - 57.
