Matrix problems in the determination of lithium by flameless (HGA) atomic-absorption spectrometry, and their solution.
Flameless atomic-absorption analyses for Li in geological materials (limestones, carbonate-bearing cherts, calcium containing subsurface brines and sweet waters) result in low values, which may amount to less than 10% of the real lithium concentration in the sample. The observed signal decrease is caused by the reaction between gaseous Li and Cl and the consequent formation of LiCl (b.p. = 1325-1360 degrees ). The Cl vapour is produced by the dissociation of CaCl(2) molecules in the graphite-tube cavity. The interfering capacity of CaCl(2) is marked because of its survival in the graphite tube at temperatures above the boiling points of both LiCl and Li metal (= 1317 degrees ). Hydrochloric acid represses the Li signal in a similar manner but to a lesser extent because most of it is removed during the pre-atomizing drying and ashing stages. The addition of Ca(2+) to the analytical solution lowers the absorption signal only if chloride is present. This interference of Cl has been completely overcome by addition of sulphuric acid in moderate excess relative to the quantity of chloride present. The release from interference is in strict accord with the stoichiometry of the reaction: H(2)SO(4) + CaCl(2) --> 2HCl + CaSO(4). The same remedy overcomes Cl interference introduced by the presence of hydrochloric acid in the analytical solution. The addition of sulphuric acid itself has no effect on the intensity of the atomic-absorption signal of lithium. An analogous reaction with phosphoric acid takes place, but the Li signal is weaker and less reproducible. Solutions in which the Na/Li ratio exceeds 3 x 10(4) cannot be analysed for Li by the method described since a molecular sodium peak is superimposed on the major Li peak.
Katz
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Department of Geology, The Hebrew University, Jerusalem, Israel.
Taitel
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