In the past the most common reagents used for CI have been gases such as methane, isobutane and ammonia, which are introduced into ionization sources at relatively high pressures (~1 torr). In an ion trap mass spectrometer no external source is required, the reagent being introduced directly into the trap; hence CI can be carried out at low pressure (~10-5 torr). This enables the use of liquid CI reagents such as methanol and acetonitrile, the reagent gas coming from the headspace of the liquid. As well as being very low cost reagents the use of these liquids eliminates the need for large cylinders of high purity gases in the laboratory. Both acetonitrile and methanol are relatively soft reagents and give similar spectra to those generated by isobutane. Acetonitrile is proving to have important application in the analysis of long chain hydrocarbons due to the formation of high abundances of [M+CH2CN]+ species and its ability to differentiate branched and unbranched hydrocarbon chains.
This paper will discuss the use of acetonitrile for the analysis of hydrocarbons, pesticides and herbicides and terpenes using CI/MS and CI/MS/MS.