Group-type Analysis of Crude Oil by Using GC/FI-TOFMS Part 1: Determination of average molecular weights
Introduction
Field Desorption (FD) and Field Ionization (FI) are both techniques that ionize analytes by electron tunneling from the analyte molecules to a solid surface (emitter) in a high electric field. In the case of FD, the sample is applied directly onto the emitter and heated by applying an electric current through the emitter for desorption and ionization. In the case of FI, vaporized analyte molecules are introduced into the proximity of the emitter.
Both FI and FD are soft ionization methods that generally yield intact molecular ions and, in most cases, produce very few fragment ions. Generally, these two techniques are used to ionize analytes that are easy to fragment and do not generally produce molecular ions during electron ionization (EI), such as hydrocarbons in crude oil.
For complex mixtures such as crude oils or synthetic polymers, molar mass distributions and average molecular weights are important chemical properties. By analyzing an FI mass spectrum that mostly consists of molecular ions, even for complex hydrocarbon mixtures, and using a group-type analysis software, one can obtain the molar mass distributions and average molecular weights for the various hydrocarbon types (e.g., paraffin, naphthene, olefin, aromatics) present in the sample mixture.
In this work, we analyzed a crude oil sample by using the JMS-T100GC “AccuTOF-GC” GC/FI method and then processed the resulting data using a group-type analysis software package.