Letter
to
the
Methyl mercury revisited Recently, some of our European Union colleagues involved in the production and certification of environmental reference materials have declared that the potential for artifact methyl mercury formation during analysis is of little concern in the certification of these materials (1). They believe that, in the absence of more valid certified reference materials (CRMs), interlab comparability of measurements of these materials is more important than having no reference materials at all. In addition, they say that the magnitude of the reported artifact is small and "unsubstantiated". We strongly disagree with this stance, because it undermines the scientist's obligation to strive for the greatest accuracy possible. Several of us have independently established that artifact methyl mercury formation occurs, especially during the distillation and hot alkaline extraction of sediments (2,3). The values of all current sediment methyl mercury CRMs have been demonstrated to be biased high. Whether the bias is 10% or 50% is less important than the response of the analytical community to this problem. By taking the stance that intercomparability is paramount, the standards and certification community perpetuates the long-time myth of the superiority of precision over accuracy in assessing data quality. Such a position could reduce the incentive to develop and validate more accurate analytical methods. In this particular case, satisfactory "performance" on the recovery of methyl mercury in these biased CRMs may result in false confidence in the measurement of the methyl mercury content of unspiked natural sediments for which the artifact DO tential has been found to be as much as an order of magnitude greater than that of commercial CRMs (Table 1) We believe that the only responsible action is to revoke the methyl mercury
Editor
certification of all current sediment CRMs and to replace it with this notation: "ConCorrection sensus value, probably biased high. CertiThe incorrect address and phone numfication pending new interlab validation ber were listed for the review of exercise." The new certification proceNuGenesis 1.1 software in the July 1 isdure should include an explicit evaluation sue, p 482 A. The correct address is Manof artifact formation and be based on artitra Software, 1900 West Park Dr., Westt fact-free and/or artifact-corrected results. borough, MA, 01581 (508-616-9876). Nicolas S. Bloom ResearchScientists,Frontier Geosciences (Seattle, WA) Letters to the Editor Douglas Evans We welcome your comments reHolger Hintelmann Environmental and Resource Studies garding published articles or other Program, Trent University topics of interest to analytical (Ontario, Canada) chemists. Letters may be submitRolf-Dieter Wilken ted by e-mail (
[email protected]), ESWE-Institute for Water Research and fax (202-872-4574), or regular Water Technology (Wiesbaden Germany)mail (Analytical Chemistry, ACS, 1155 16th St, N.W., WashReferences ington, DC 20036). Please in1. Quevauviller, P.; Horvat, M. Anal. Chem. clude your full address, signature, 1999, 71,155 A. and daytime phone number. Let2. Bloom, N. S.; Colman, J. A.; Barber, L, Fres. J. Anal. Chem. 1997, 358, 371. ters should be brief and may be 3. Hintelmann, H.; Falter, R.; ;lgen, G.; snare euiieu for lor riaritv udriiy or space. Evans, R. D. Fres. J. Anal. Chem. 1997, edited 358, 363. T a b l e 1 . M e t h y l m e r c u r y in l a k e s e d i m e n t s . Methyl mercury in freshwater lake sediments (contaminated ca. 1880), using the artifact-prone (distillation) and low-artifact (HBr/solvent) extraction procedures (2). IAEA-356 and CRM 580 iare two of the "certified" reference materials in question. Mercury Concentration, ng/g (ppb) Sediment core depth (cm) 24-28 28-32 32-36 56-60 60-64 64-68 76-80 80-85 85-90 CRM 580 IAEA-356 'Corrected for a
Total
Methyl Hg (distillation)
11,006 3.553 13,289 3.890 9,457 3.803 3.316 4,996 2.462 7,096 7,981 2.631 334 0.446 610 0.752 295 0.430 127,619 105.0 ±8.1 (6) NA 6.84 ± 0.40 (3) small (10%) observed artifact even
Methyl Hg (extraction)
% Artifact
0.471 654 749 0.458 0.407 834 0.387 757 684 0.314 0.272 867 0.155 188 0.183 311 231 0.130 70.5 ± 7.5 (6)* 49 4.69 + 0.22 (3) 46 with the HBr extraction procedure.
Analytical Chemistry News & Features, September 1, 1999 5 7 5 A
