Abstract Quantitative trace analysis for organic compounds in complex matrices such as oils often requires time-consuming sample pretreatment. Two examples are shown of the use of a highly selective tandem mass spectrometer as a liquid chromatographic detector for the quantification of dibenzothiophene in a crude petroleum oil and in an alternate fuel oil. No sample preparation except an appropriate dilution was required. A preconcentrating liquid chromatography—mass spectrometry interface was used, allowing detection limits of ca. 20 ng to be attained.