Letter
to
the
Editor
Artifact formation of methylmercury in sediments Monomethylmercury (MeHg) is of particular concern due to its extreme toxicity and its ability to bioaccumulate infishtissues. Recently, research laboratories have started monitoring MeHg contents in sediments for the purpose of pollution monitoring and geochemical studies. The techniques developed for determining MeHg in biological and sediment materials typically involve a succession of analytical steps, which may all be prone to systematic errors. Ten years ago, these methods were poorly validated due to a lack of evaluation programs and certified reference materials (CRMs). The situation has improved thanks to more sensitive and specific analytical techniques; interlaboratory studies; and the availability of CRMs from organizations such as the European Commission's Community Bureau of Refer- Figure 1 . MeHg in CRM 5 8 0 . The results correspond to six replicate determinations as performed by different laboratories using ence (BCR)-Programme, the National Research Council of Canada the International various methods. MEANS indicates the mean of laboratory means with 95% confidence interval (adapted from Ref. 4). Atomic Energy Agency (IAEA) the NaAbbreviations: CVAAS, cold vapor atomic absorption spectrometry; CVAFS; cold vapor atomic tional Institute of Standards and Technolfluorescence spectrometry; ECD, electron capture detection; MIP, microwave-induced plasma ogy and Japan's National Institute of Enviatomic emission spectrometry; QFAAS: quartz furnace atomic absorption spectrometry; SFE, ronmental Studies supercritical fluid extraction. Recently, sediment reference materials were certified by the IAEA (polluted marine of 202MeHg was observed after water vapor mation of MeHg was mainly observed in sediment, IAEA-356) and by BCR (estuarine distillation. Thisfindingsuggested an in samples rich in organic matter and high in sediment, CRM 580). The collaborative trials situ formation of MeHg from inorganic Hg. inorganic mercury, whereas such artifacts that led to the development of these new This was also proven by conventional anawere not observed in samples with low Hg CRMs enabled scrutiny of most of the techlytical techniques (2). It was also demoncontents. Also, no artifacts were observed niques currently used for determining strated that isolation techniques have difduring the analysis of biological materials. MeHg and the subsequent detection of the ferent capacities for transforming spiked At the 1996 conference, the discussion obvious sources of systematic errors. Hg2+ to MeHg (herein called artifact) of was enlarged to critical comments on the which distillation and alkaline dissolution value of the IAEA (2) and BCR (3) sediConcern over the possible formation of showed the highest potential. ment reference materials. The controversy MeHg artifacts during certain analytical led to a workshop, held in Wiesbaden, Gerprocedures was first expressed at the The amount of MeHg formed was very many, in May 1998, which was attended by "Mercury as a Global Pollutant" conference small, varying from about 0.005 to 0.1% of prominent experts in MeHg analysis; its in 1996 (1). Stable isotope dilution inducthe spiked Hg2+. Nevertheless, this could results will be published soon in a special tively coupled plasma (ICP) MS was used represent an important proportion of issue of Chemospherer to study the accuracy of the mercury speci- MeHg in samples that normally contain ation analysis. It was found that on spiking low MeHg concentrations (a factor of up to Many laboratories now doubt that the sediments with relatively high quantities of 1.5 was reported) and could lead to overCRMs they use for quality control are ininorganic mercury (Hg2+ in acidic aqueous estimation of or false positive MeHg meadeed properly certified and reflect the true solution) enriched with 202Hg, an increase surements. Interestingly, the spurious forvalues for MeHg. The workshop discusAnalytical Chemistry News & Features, March 1, 1999 155 A
sions examined this question in detail and concluded that the findings on artifact formation of MeHg are not sufficient to claim that MeHg results are overestimated. It is well known that free ionic Hgr is a precursor for Hg transformation mechanisms, including methylation. Normally, the quantity of free ionic Hg2* in the environment is low; probably most of the endogenous inorganic Hg in sediment is bound as humates, to sulfur, etc. Spiking a sediment sample with high quantities of acidic Hg2* seriously alters the sample composition, resulting in the transformation of the mercury species of interest. Therefore, such an analytical approach contradicts basic principles of speciation analysis, which require that the mercury species originally present in an endogenous form in the sample be preserved. To date experiments have not been conducted with enriched isotopes of Hg2* incorporated into the test material in a natural process; this would represent a much accurate experimental design to test the accuracy of analytical procedures In addition, there is excellent comparability of data obtained by different analytical methods for a number of samples, including CRM 580 (4). In most certification campaigns for MeHg, only laboratories with validated analytical techniques and outstanding performance are involved. The results thus obtained are discussed with all the participants, and discrepancies are carefully examined and discussed before the results are excluded (on technical grounds) from the final certification. Before a certificate is issued, stability testing is also performed. Figure 1 shows an example of such a certification—where the results obtained by different laboratories for CRM 580 are presented (4). Distillation-based techniques, indicated in purple, were used by four independent laboratories and were in good agreement with techniques not involving distillation. Similar results were obtained for CRM IAEA-356 (3). This, of course, is strong evidence of comparability of data and represents an excellent reference point for determinations of MeHg in sediments. However comparability of data on its own is not an absolute assurance of true values and further studies using modern techniques and appropriate experimental design are to be strongly encouraged. From these comments, it is clear that aspects of the certification of reference materials are often misunderstood and confused with research activities on analytical techniques. The suspicions about the certi156 A
fied MeHg contents in sediment reference materials originated from a small group of laboratories, which based their conclusions on limited scientific evidence. If these suspicions are confirmed, there might be serious economic consequences. At this stage, the controversial thesis is not supported by strong scientific evidence, and research on analytical methodologies should be pursued on a larger scale through new collaborative exercises. CRMs offer the means to achieve data comparability for laboratories using current techniques. Scientists should be careful not to doubt CRMs just on the basis that some research has suspected risks of systematic errors in one particular technique. Experts participating in the 1998 workshop agreed that, at present, existing CRMs fulfill the purposes of verifying the accuracy of current methods and achieving data comparability. The doubts expressed about analytical measurements should however be considered seriously and further research should be pursued to confirm or refute the concerns Philippe Quevauviller and Milena Horvat European Commission Directorate Brussels, Belgium References (1) Hintelmann, H.; Falter, R; Ilgen, G; Evans, R. D. Fresenius'J. Anal. Chem. 1997,358, 363. (2) Bloom, N. S.; Colman, J. A.; Barber, L. Fresenius'J. Anal. Chem. 1997,358,371. (3) Horvat, M.; Mandiae, V.; Liang, L.; Bloom, N. S.; Padberg, S;; Le,, Y.-L.; Hintelmann, H; Benoit, J. Appl. Organometal. Chem. 1994,8, 533. (4) Quevauviller, Ph.; Fortunati, G. U.; Filippelli, M.; Bortoli, A.; Muntau, H. Appl. Organometal. .hem. .198,12, ,51.
CAN'T ATTEND ACS MEETINGS?
But you want to know about the latest research in analytical chemistry?
IT CAN BE DONE ! Become a member of the Division of Analytical Chemistry
For US$15 per year* benefits include: *US$12 ACS members, US$6 Students
• Two issues of the Division newsletter with over 400 abstracts describing latest research to be presented at the national ACS meetings. • Information on professional issues, recognition of excellence and opportunities to recognize your peers. a Members directory listing 8,000 professionals in the field (updated every 3 years).
How to join Send check payable to DAC
Letters to the Editor We welcome your comments regarding published articles or other topics of interest to analytical chemists. Letters may be submitted by e-mail (
[email protected]), fax (202-872-4574), or regular mail (Analytical Chemistry, ACS, 1155 16th St., N.W., Washington, DC 20036). Please include your full address signature and daytime phone number. Letters should be brief and may be edited for clarity or space.
Analytical Chemistry News & Features, March 1, 1999
(include ACS member number or student id. for discount) Add US$2 for enrollment in the Subdivision of Chromatography and Separations Chemistry.
Include your name, address and mail to: Robert Thompson. Chemistry Dept. Oberlin College 130 W.Lorain St. Oberlin, OH 44074 Phone: 440-775-8305
[email protected] For more informatton visit our web site: nexus.chemistry.duq.edu/analytical/analytical.html
