Improvement of Detection Sensitivity of Triglyceride with Methylamine Formate as Ionization Enhancer in Reversed Phase Liquid Chromatography-Electrospray Ionization Mass Spectrometry

A novel ionization enhancer, methylamine formate, was proposed for improving the detection sensitivity of triglyceride in edible oil by reversed phase liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). The commonly used isopropanol-acetonitrile-methanol-water and isopropanol-acetonitrile were selected as the mobile phase. By using a reversed phase C18 column, and taking the corn oil in isopropyl alcohol as sample solution, we compared methylamine formate with ammonium formate, as ionization enhancers, for their effect on the detection sensitivity of triglyceride by LC-ESI-MS after screening other different ionization enhancers, such as formic acid, acetic acid, ammonium formate, ammonium acetate, butyl formate, dibutylamine formate, triethylamine formate, diethylamine formate, methylamine formate, and ethylamine formate. The result indicated that, using methylamine formate in mobile phase, the mass spectral peak response and the signal to noise ratio of trilinolein component were all about 5 times higher than that using ammonium formate. The effect of the concentration of methylamine formate ionization enhancer, the flow rate of the mobile phase and the flow rate of nebulizing gas on the detection of methylamine formate was investigated. The concentration of triglyceride components in corn oil starting to form aggregates was similar in different mobile phases in the electrospray process according to measurement of the relationship between corn oil concentrations and the total ion chromatogram peak area of triglyceride. In particular, the peak area of trilinolein was linear with its concentration in the range of 7 x 10(-7) -2 x 10(-4) M with the correlation coefficient R-2 = 0.9997, but increased slower in the higher concentration range. According to the experimental data, the mechanism for improvement of detection sensitivity of methylamine formate was suggested as that the addition of methylamine mono-charged ions with hydrophobic groups had lower solvation energy, which made the enriched addition ions easily evaporation from the droplet surface, thus improving the electrospray ionization efficiency. This method provided an effective way to improve the detection sensitivity of triglyceride in edible oil by LC-ESI-MS.