1766
Anal. Chem. 1980, 52, 1766-1766
Exchange of Comments on Determination of Iron, Manganese, and Zinc in Seawater by Graphite Furnace Atomic Absorption Spectrometry Sir: In a recent paper by Sturgeon et al. ( I ) , the authors review the literature concerning direct analysis for trace metals in seawater by flameless atomic absorption spectrophotometry. They then report development of techniques for analysis of Fe, Mn, and Zn in seawater which they claim to be significantly better than those previously reported in the literature. While we acknowledge the value of the authors’ contribution and are grateful that they have made reference to some of our own early work in this field, we believe that your readers should be aware that we have developed and published ( 2 ) several years ago the details of analytical techniques for determination of Fe, Mn, Cu, Cd and Zn in seawater which are very closely similar to those reported in the Sturgeon et al. ( 1 ) paper. The reader is misled by Sturgeon et al.’s ( I ) use of a quotation from one of our earlier papers (3)that “direct injection analysis of seawater for most trace elements is not possible as the attainable sensitivities are only marginally good enough and there is a severe matrix effect.” This statement referred to the early model Perkin-Elmer heated graphite atomizer (HGA-70),a t that time the only model available. This early model featured an open ended heated graphite tube and a gas flow pattern which permitted the exhaust gases to remain in the analysis light beam as they were carried away from the heated zone of the graphite atomizer. The condensing particles in the cooling gas cloud gave rise to copious quantities of visible smoke which was primarily responsible for the “severe matrix interferences.” More recent models of the Perkin-Elmer heated graphite atomizer have been modified specifically to address the problem of the cooling gas cloud remaining in the spectrophotometer light beam. The newer atomizers (introduced in approximately 1974 and used by Sturgeon et al. (I)) are specifically designed to remove the hot gases from the light beam before they cool, and atom recombination and condensation occurs. Using this newer design of heated graphite atomizer shortly after its introduction, we have developed methods for Mn, Fe, Cu, and Zn analysis of seawater by direct injection into the atomizer of small volumes of untreated seawater. This work was extensively described in a publication in 1975 ( 2 ) . Our methodologies for Fe and Mn and our observations of the controllable matrix effects which do remain when analyzing seawater in the newer atomizer, were essentially identical to those repeated and reported by Sturgeon et al. ( I ) . The approximate detection limits (concentration giving signal twice that of blank or amplitude of noise during blank injection, whichever is larger) that we were able to attain were also essentially identical to those claimed by Sturgeon et al. (I) for these two elements. For the analysis of Zn we were able to develop a procedure whereby, unlike Sturgeon et al. ( I ) , we had no need to add “,NO3 to the seawater sample or to dilute it for analysis. Our simpler analytical technique for Zn is about one order of magnitude more sensitive than that reported by Sturgeon et al. ( 1 ) for their method. We were also able to develop a sensitive and interference-free methodology for direct analysis of untreated seawater for Cd. Cd, a volatile element, is
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atomized in a similar manner to Zn a t temperatures close to the vaporization temperature of the potassium and sodium salts in seawater. It is these salts that can cause total attenuation of the spectrophotometer light beam during volatilization because of the extremely high density of molecular and particulate broad band absorbing species. It should be noted that Cd and Zn analysis employing our methodologies requires, as described by us (2),great care with the setting of instrumental parameters, particularly the background corrector/analysis light beam balancing and alignment. Not only have our methods for direct analysis of seawater for Fe, Mn, Zn and other metals been published for several years, but these methods have now been successfully used for the analysis of several thousands or more of seawater samples both in our laboratory at the Atlantic Oceanographic and Meteorological Laboratories, where the methods were developed, and in other laboratories. Results of these analyses have also appeared in the open literature as early as 1976 ( 4 ) . Verification of the accuracy of the direct analysis procedures is difficult owing to the lack of proven reference methods for analysis of these elements in seawater. However, the methods have provided data from several studies including that of the New York Bight ( 4 ) which show environmental distributions and variations of the metals in the oceans clearly consistent with the known physics, chemistry of, and inputs to these oceans. This correspondence of metal distribution data with known (and previously unknown but confirmable) geochemical processes is the ultimate test of the veracity and usefulness of any data set, a test all too often failed by previously used methodologies. We believe that any of your readers who wish to follow the methodologies that Sturgeon et al. ( I ) have reported should be acquainted with the earlier literature. Not only can we provide further confirmation that the previously extremely difficult and time-consuming analyses of these metals in seawater may now be replaced by reliable, fast, and accurate direct atomic absorption analysis as we have described but also we have demonstrated that direct analysis of Zn may be carried out without the dilution and addition of ammonium nitrate as suggested by Sturgeon et al. ( I ) . In addition the reader should be aware that similar methodologies exist for several other elements such as Cd and Cu ( 2 ) .
LITERATURE CITED (1) R. E. Sturgeon, S.S.Berman, A. Desaulniers, and D. S.Russell, Anal. Chem., 51, 2364 (1979). (2) D. A. Segar and A. Y. Cantillo, Adv. Chem. Ser., No. 147, 56 (1975). (3) D. A. Segar, Int. J. Environ. Anal. Chem., 3 , 107 (1973). (4) D. A. Segar and A . Y. Cantillo, Am. SOC.Limnol. Oceanogr., Spec. Symp. 2 , 171 (1976).
Douglas A. Segar* SEAMOcean P.O. Box 2234 Wheaton, Maryland 20902 Adriana Y. Cantillo NOAA/NOS/EDL Rockville, Maryland 20852 RECEIVED for review January 29,1980. Accepted May 23,1980.
8 1980 American
Chemical Society
