Rapid analysis of underivatized fatty acids by electrospray-ionization-ion mobility spectrometry

Abstract

The current analytical method for the determination of fatty acids is by GC-FAMEs (gas chromatography-Fatty Acid Methyl Esters). GC-FAMEs consist of a time-consuming esterification process to increase the volatility and decrease the polarity of fatty acids before gas chromatographic separation. The work reported here evaluates ion mobility spectrometry as alternative method for the analysis of FAMEs. Electrospray ionization—ion mobility spectrometry (ESI-IMS) enabled the rapid qualitative analysis of both the non-volatile and volatile fatty acids (FAs) and FAMEs. For the volatile FAME compounds, a ⁶³Ni ionization source was also evaluated. While individual analytes worked well, a mixture of the FAME analytes experienced ion suppression. Introduction of a FAME analyte mixture by electrospray-ionization improved both detection and separation. The introduction of the FAs by direct ESI with ion mobility separation, however, provided the best results producing a rapid method for the characterization of FAs that did not require derivatization. Reduced mobility values (K₀) were reported for the first time for six FA analytes: heneicosanoic acid (1.03 ± 0.02 cm²V⁻¹ s⁻¹), cis-11-eicosenoic acid (1.07 ± 0.02 cm²V⁻¹ s⁻¹) oleic acid (1.13 ± 0.0 cm²V⁻¹ s⁻¹),cis-vaccenic acid (1.13 ± 0.02 cm²V⁻¹ s⁻¹), γ-linolenic acid (1.16 ± 0.02 cm²V⁻¹ s⁻¹), and lauric acid (1.31 ± 0.02 cm²V⁻¹ s⁻¹), as well as their corresponding FAME analytes: methyl heneicosanoate (0.90 ± 0.02 cm²V⁻¹ s⁻¹), methyl-cis-11-eicosenate (0.96 ± 0.02 cm²V⁻¹ s⁻¹), cis-11-vaccenic acid methyl ester (1.03 ± 0.02 cm²V⁻¹ s⁻¹), methyl oleate (1.05 ± 0.02 cm²V⁻¹ s⁻¹), methyl linolenate (1.12 ± 0.02 cm²V⁻¹ s⁻¹), methyl laurate (1.21 ± 0.02 cm²V⁻¹ s⁻¹).