Determination of Fatty Acids in Foods Using Gas Chromatography with Positive-ion Chemical Ionization Tandem Mass Spectrometry

For standard sample analysis, GC-EI-MS showed lower limit of quantification (LOQ) to saturated fatty acids. For example, the LOQ of methyl behenate (C22:0) was 0.007 ug/L for GC-EI-MS and 0.023 ug/L for GC-PICI-MS. The reason is that GC-EI-MS produces an intense molecular ion which could be used for selective ion monitoring. Adversely, GC-PICI-MS showed lower LOQs to unsaturated fatty acids. For example, the LOQ of methyl linolenate (C18:3n-3) was 0.213 ug/L for GC-EI-MS and 0.024 ug/L for GC-PICI-MS. GC-EI-MS produced a molecular ion and a variety of fragment ions from unsaturated fatty acids, showing low abundance for selective ion monitoring. In contrast, GC-PICI-MS produced mainly a quasi-molecular ion which could be used for selective ion monitoring. In most methyl fatty acids, both GC-EI-MS/MS and GC-PICI-MS/MS did not showed lower LOQs compared with GC-EI-MS or GC-PICI-MS. MS/MS significantly improves selectivity but the number of detected ions is decreased. Since the standard sample analysis is not required separation from co-eluting contaminants, MS/MS was not optimal.
For real sample analysis, MS/MS, especially GC-PICI-MS/MS, successfully removed interferences caused by co-eluting contaminants, and provided unambiguous selective detection and accurate quantitation of the fatty acids in food matrices.
The results demonstrated that GC-PICI-MS/MS is highly selective and sensitive for the analysis of fatty acids in food compared to GC-PICI-MS, GC-EI-MS and GC-EI-MS/MS.