Evaluation of a Direct Current Arc Charge Injection Device Spectrograph for Direct Analysis of Soils

Abstract

The feasibility of using a dc arc charge injection device (CID) spectrograph for the semiquantitative determination of metals in soils has been demonstrated. Ar–O₂ gas in a proportion of 70+30 was delivered through a Stallwood jet at 5 ml min^-1 while 20 mg of sample were burned using 15 A of current and a constant 4 mm electrode gap. SPEX multi-element graphite powders were used to map wavelengths on the CID sensor and construct working curves. The CRM SO-1 was diluted with graphite powder and analysed. The analysis was accomplished using the full-frame capability of the CID, which permitted the evaluation of several analytical lines for the target elements. Several elements, including Ag, Be, Cd, Hg, Sb and Se, were present in the diluted sample at less than 5 µg g^-1, and results corroborated these low concentrations. Quantitative results were obtained for P and Pb, semiquantitative results were achieved for Ba, Mn and Sr, and results for B, Co, Cr, Cu, Ni, V and Zn were qualitative. Detection limits were determined by burning three samples of pure graphite powder. Detection limits for Ag, B, Ba, Be, Cd, Co, Hg, Mn and Ni were ≤2 µg g^-1; for Cr, P, Pb, Sb, Se, Sr, V and Zn ≤5 µg g^-1, and for As, Cu and Tl ≤10 µg g^-1. The relative precision of the method was examined by burning three separate 33 µg g^-1 multi-element SPEX standards. Most elements showed RSDs of less than 20% with the exception of As, Cd, Co, Ni, P and Zn. Ni and Zn showed the greatest imprecision at about 70% and 55% RSD, respectively. The use of Li₂CO₃ to buffer the arc temperature improved the precision for most elements, especially Ni and Zn.

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