267 Chapter 11 COUPLING MASS SPECTROMETRY WITH LIQUID CHROMATOGRAPHY (LC-MS) FOR HYDROCARBON RESEARCH Chang Samuel Hsu ExxonMobil Research and Engineering Co., Process Research Laboratories, Baton Rouge, LA 70821 1. INTRODUCTION Petroleum and other fossil fuels, such as coals and shale oils, are complex hydrocarbon mixtures that contain thousands of components. 1 Numerous analytical techniques have been developed to characterize bulk properties and composition of petroleum and its products. 2 However, much more detailed molecular-level compositional analysis of petroleum and its fractions is often needed for the understanding of the chemistry involved in refining processes and the correlation and prediction of the properties and performance of products. 3 It is often desirable to separate petroleum crude oils into fractions to facilitate the analysis in manageable complexity. The most common approach has been to distill petroleum into fractions (distillates) corresponding to the product streams in refineries, such naphtha, middle distillates, gas oils and residua (resids). The other approach is to fractionate petroleum and its distillates according to the polarity of various compound types in the mixture. In this aspect, supercritical fluid chromatography (SFC) has been developed for more volatile fractions, such as naphtha and middle distillates (650F-), and liquid chromatography (LC) for less volatile fractions including gas oils and resids (650F+). Petroleum and its fractions are generally fractionated into saturated, aromatic and polar fractions using open columns or normal-phase high-performance liquid chromatography (HPLC) although reversed-phase HPLC has also been used to separate more polar fractions, such as polycyclic aromatic hydrocarbons. The LC fractions are then analyzed by various analytical techniques for characterization, including mass spectrometry (MS). However, samples