Determination of trace elements in naphtha by inductively coupled plasma mass spectrometry using water-in-oil emulsions

A simple method for the determination of trace metals such as V, Co, Ni, As, Hg and Pb in naphtha by ICP-MS is described. The samples are first emulsified using Triton X-100 and then introduced into the plasma. A water-in-oil emulsion is prepared by mixing 2 mL of naphtha and 1 mL of 2.5% Triton X-100 aqueous solution (naphtha to aqueous ratio 2:1) with constant stirring for 20 min with a magnetic stirrer. Oxygen is used to remove the carbon build-up. By this method the background due to carbon measured at the (ArC+)-Ar-40-C-13 m/z is only 15 000 counts compared with 780 000 counts obtained when a 1 + 1 mixture of naphtha and xylene is directly introduced into the plasma under similar conditions. The limits of detection obtained were 0.6 mu g L-1 for V, 0.05 mu g L-1 for Co, 0.7 mu g L-1 for Ni and 0.1 mu g L-1 for As. Using a semiquantitative approach the detection limits of Hg and Pb were found to be 0.12 and 0.09 mu g L-1, respectively. The method was applied to two samples and the results were in good agreement with those obtained by a digestion procedure. The precision obtained was within 2-3%. Rh-103 was used as an internal standard.