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 emulsi fied 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-1 00 aqueous solution (naphtha to aqueous ratio 2:1) with constant stirring f or 20 min with a magnetic stirrer. Oxygen is used to remove the carbon buil d-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 wh en a 1 + 1 mixture of naphtha and xylene is directly introduced into the pl asma 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 fo r As. Using a semiquantitative approach the detection limits of Hg and Pb w ere found to be 0.12 and 0.09 mu g L-1, respectively. The method was applie d 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 w as used as an internal standard.