The method is absolute in the determination of single organic substances of known nature, but it is not absolute in the determination of the carbon content of water containing unknown impurities. However, in this case, it may be a very sensitive comparison method. The sensitivity and the precision may be affected by residual organic impurities in blank solutions. This limitation, also affecting the determination of a simple organic substance, can be overcome in industrial water quality evaluation if the Cali-
bration curve can be obtained with water at different carbon content but coming from the same plant. In this case, the nature of pollution is always the same and no dilution is needed to prepare samples of different carbon content.
RECEIVED for review November 16,1967. Accepted February Work was supported by the Consiglio Nazionale delle Ricerche, Rome, Italy. 9,1968.
Role of Contamination in Trace Element Analysis of Sea Water David E. Robertson Battelle Memorial Institute, Pacifc Northwest Laboratory, Richland, Wash. 99352 A wide variety of solvents, reagents, and other materials normally encountered in the trace element analysis of sea water have been analyzed for trace element impurities by neutron activation analysis and multidimensional gamma-ray spectrometry. The concentrations of up to 10 trace elements, including Sc, Cr, Fe, Co, Cu, Zn, Ag, Sb, Cs, and Hf, were measured in these materials. Many of these substances contained extremely high impurity levels of various elements. These analyses provide an indication of which materials may contribute to the contamination of sea water samples which come in contact with these substances. The contamination of sea water during several typical chemical separations has been estimated. Suggestions for minimizing the sources of contamination are given. SEAWATER is a complex solution containing, either in a dissolved or particulate state, all of the elements of the earth’s crust. Yet, only 13 of these elements exist at concentrations greater than 1 ppm, while the majority are present at less than 1 ppb. The sampling and analytical problems involved in measurements at these low concentrations have limited studies of trace element behavior in the oceans. For many of the oceans’ trace elements the literature contains a wide range of reported concentrations, some of which vary by several orders of magnitude ( I ) . For the most part, these discrepancies seem to have resulted from contamination of the samples, either during collection and storage or subsequent analysis. Cooper (2) has discussed some of the difficulties inherent in sea water sampling. These include contamination of a sample by material leached from the sampling apparatus itself, which may contain metallic components, rubber washers, stoppers, and tubing. and these each contain significant amounts of various metal impurities. Contamination may also arise because of leaching of materials from the bottle in which the sample is stored. Contamination may result from the rust and corrosion products which accumulate on the hydrowires and become sloughed off during the movement of messengers down the wire. Samples are subject to contamination from smoke and ash from ship’s smokestacks, from sewage and waste discharged from the ship, or from corrosion products and paint leached from the ship itself. The soiled hands of an (1) J. P. Riley, “Chemical Oceanography,” Vol. 2, J. P. Riley
and G. Skirrow, Eds., Academic Press, London and New York, 1965. (2) L. H. N. Cooper, J. Marine Res., 17, 128-32 (1958).
operator who is taking the sample may also be a source of contamination. Most methods which have been reported for the analyses for trace elements in sea water require chemical preseparations for concentration or removal of interfering elements before an analysis can be performed. These procedures greatly increase the risk of contaminating the sea water sample. Contamination may come from reagents, vessels, filters, other apparatus used for the separations, or even from airborne particulate material in the laboratory. These sources are briefly discussed by several investigators ( I , 3 , 4 ) . In our initial trace element analyses of sea water by neutron activation analysis and multidimensional y-ray spectrometry (5, 6 ) , it was observed that serious contamination of several elements could occur unless rather extreme precautionary measures were enforced. Contaminants originated mainly from impurities present in irradiation containers, from filters, and from airborne particulate material in the laboratory. Because this analytical method was purely instrumental and required only a minimum of sample handling and no chemical separations, the magnitude of the contamination problem in the trace element analyses of sea water became quite apparent. This present study was conducted to determine the potential sources and levels of contamination one might encounter in the trace element analysis of sea water. Samples of container materials, reagents, solvents, and materials used in the direct construction of sea water samplers and in related material were analyzed for trace element impurities using neutron activation analysis in conjunction with multidimensional ?-ray spectrometry. This method provided the required sensitivity and selectivity for the direct instrumental measurement of up to 10 trace elements in these various materials. (3) A. Mizuike, “Trace Analysis-Physical Methods,” G. H. Morrison, Ed., Interscience, New York, 1965, pp 105-15. (4) J. F. Slowey, D. Hedges, and D. W. Hood, Progress Report
of Research Conducted through the Texas A&M Research Foundation for the Atomic Energy Commission, Second Progress Report, Aug. 1, 1961 to Nov. 1, 1962, TID-22660, Texas A & M University, College Station, Texas, Nov. 1962. (5) R. W. Perkins, D. E. Robertson, and H. G. Rieck, Pacific Northwest Laboratory Annual Report for 1965 in the Physical Sciences, Vol 2: Radiological Sciences, BNWL-235 2, pp 108-15, Pacific Northwest Laboratory, Richland, Wash., May 1966. (6) D. E. Robertson and R. W. Perkins, “Symposium on Trace Characterization-Chemical and Physical,” National Bureau of Standards, Gaithersburg, Md., Oct 2-7, 1966, paper No. 57. VOL. 40, NO. 7, JUNE 1968
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EXPERIMENTAL
Sample Preparation. For the neutron irradiations, the samples were encapsulated in either polyethylene vials or ampoules made from the synthetically produced quartz of high chemical purity, “Spectrosil” (American Thermal Fused Quartz Co.). The polyethylene vials were precleaned by rinsing with doubly distilled nitric acid and doubly distilled water. The quartz ampoules were cleaned by refluxing them for 24 hours in doubly distilled nitric acid, rinsing with doubly distilled water, and centrifuging to dryness. The solid materials, such as quartz, glass, Teflon (made by Du Pont), polyethylene, and rubber, were cleaned both before and after the irradiations by rinsing with dilute doubly distilled nitric acid and doubly distilled water. The solid chemical reagents were encapsulated in clean polyethylene vials, while the liquid reagents and solvents were evaporated to dryness under a heat lamp in cleaned “Spectrosil” quartz
ampoules or polyethylene vials. The analyzed liquid reagents included water, nitric acid, hydrochloric acid, chloroform, and carbon tetrachloride, all purified by distillation from a borosilicate glass still. These aqueous reagents were stored prior to their analyses in polyethylene bottles, while the chloroform and carbon tetrachloride were stored in borosilicate glass bottles. Quartz-distilled water was also evaluated and purified in a commercially available all-quartz distillation apparatus. Most of the sample preparation and handling was performed in a positive pressure laminar flow clean hood (Agnew-Higgins). Neutron Activation Analyses and Sample Counting. The samples were irradiated in a well-moderated neutron flux to an integrated thermal neutron exposure of from 2 X lo1’ to 2 x 1019 n cm-2. The neutron flux was monitored by irradiating dilute standard solutions of cobalt simultaneously with the samples. After the irradiation the solid samples were mounted directly on standard 1-inch stainless steel plan-
Table I. Comparison of Trace Element Concentrations in Sea Water with Those in Concentration (Parts-per-Billion) Sb co Cr sc cs Ag Cu Hf 0.3 0.05 0.5 0.04 0.3 0.2 10 Unknown
Sample Zn Fe Average sea water 10 10 Construction materials Source” Teflon (a) 9.3 35 0.4 Plexiglas (b)
