Speciation of sulfur in petroleum liquids by gas chromatography/inductively coupled plasma mass spectrometry

Abstract

A speciation method for sulfur in petroleum liquids by gas chromatography/inductively coupled plasma mass spectrometry (GC/ICP-MS) was developed. From an investigation of the effects of the ICP-MS operating conditions on the background intensities at m/z 32 and 34, the origin of the contaminated sulfur and the interference of the oxygen molecular ion O 2+ with the background were clarified, and countermeasures, such as an alternative GC/ICP-MS interface without using silver brazing, purification of Ar gas by a getter, and alternate cone materials, were quantitatively evaluated for decreasing this contamination and interference. The dependence of the sensitivity and the matrix effect on the ICP output power were also examined, and high sensitivity and low matrix effects were obtained at higher output power. The detection limit was around 0.6 ng S mL-1, corresponding to 0.05 pg at the ICP-MS detector, taking account of the GC split ratios, which were 2 or 3 orders of magnitude superior to those with GC/FPD and GC/AED and equivalent, or 10 times superior to those with GC/pulsed FPD and GC/SCD. The detector response to sulfur is considered to be essentially equimolar for all sulfur species, but in practice, the sensitivity decreased slightly for a species with a high boiling point. It was speculated that the reason for the low sensitivity was not due to an inefficient ionization in ICP, but due to discrimination at the GC injection port, or decomposition in the GC column. The present method was successfully applied to petroleum liquids, such as naphtha, gasoline, kerosene and light oil. These results were also compared with those by GC/AED and GC/SCD, and showed satisfactory agreement. Although the quenching effects by concomitant hydrocarbons were found to be slightly larger than those observed with SCD, the present method was superior in sensitivity to other conventional methods, and is expected to contribute to the development of low-sulfur gasoline for automobiles and fuels for fuel cells. © 2007 The Japan Society for Analytical Chemistry.