Proton transfer chemical ionization mass spectrometry of fatty acid methyl esters separated by gas chromatography: Quantitative aspects

The propensity of esters to accept protons in the gas phase during chemical ionization (CI) mass spectrometry (MS) has been well known for some time. This proton transfer CI almost exclusively produces the [M+H]⁺ quasimolecular ion for most esters. While it has been suggested that this could serve as the basis for particularly sensitive quantitation of fatty acid methyl esters (FAME) separated by gas chromatography (GC), the application of this approach to quantitate FAME has not been fully explored. Here, we briefly address some considerations pertinent to the application of GC with proton transfer CI-MS to quantitative analysis of FAME. Limits of detection and relative response factors (RF) for a number of FAME separated by GC and detected by CI-MS are presented and discussed. While the detection limits are comparable or superior to other detection methods, it was noted that the RF for unsaturated FAME were dramatically reduced with increasing degree of unsaturation.