Sensitivity Improvement Of Reversed Phase Liquid Chromatographv-Electrospray Ionization Mass Spectrometry Analysis Of Triglycerides Using Methylamine Formate As Mobile Phase Additives

Analysis of the composition of triglycerides (TAGs) in edible oils is an important basis for the identification of adulteration in edible oils. Liquid chromatography - mass spectrometry (LC-MS) method is frequently used for separation and analysis of TAGs. When common electrospray ionization (ESI) used, the low concentration detection sensitivity is poor,and at the high concentration, the TAGs are difficult to ionize efficiently as the hydrophobic lipid aggregate. There is a linear response only below the aggregation concentration.Therefore, it is necessary to improve the detection sensitivity of triglyceride.In the thesis, we use methylamine formate as the mobile phase additives for improving the sensitivity of triglyceride and the ESI ionization efficiency in reversed phase liquid chromatography electrospray ionization mass spectrometry (LC-ESI-MS). The results are as follows:When using a reversed phase C18 column, two common mobile phases of isopropanol-acetonitrile-methanol-water and isopropanol-acetonitrile were selected. And when using the phenylpropyl column, methanol-isopropanol-water was selected as the mobile phase. Taking the corn oil in isopropyl alcohol as sample solution, we compared different electrolyte modifiers for their effect on the detection sensitivity of triglyceride, such as formic acid, acetic acid, ammonium formate, ammonium acetate, butyl formate, dibutylamine formate, triethylamine formate, diethylamine fonnate, methylamine formate, ethylamine fonnate. As result, the total ion chromatogram (TIC) intensity and the signal-to-noise ratio were all the highest when the methylamine formate was used, which the ethylamine formate was followed. In addition to the methylamine, other organic amine additives presented residue in instrument tubes at different degree. For solving the residual problem, the washing method for flow path is also proposed.Using reversed phase LC-ESI-MS, methylamine formate was selected and compared with the commonly used ammonium fonnate, the effects of two kinds of mobile phase additives on the detection sensitivity of triglyceride in edible oil were investigated. The experimental result shows that the triglycerides produce [M + NH4]+, [M + H]+ and [M +H-RCOOH]+ ions in ESI positive ion mode when ammonium fonnate is used in the mobile phase, in which [M + N114]+ is the base peak. When methylamine formate is used,triglycerides produce only [M + CH3NH3]+ ions in ESI positive ion mode. At the same sample concentration, methylamine formate made a component get the mass spectral peak response and signal to noise ratio of [M+ CH3NH3]+ 5 times as those of [M + NH4]+ using ammonium formate. According to the experimental phenomena, the mechanism of methanamine for improvement of triglyceride ionization was discussed. It is suggested that the addition of methylamine mono-charged ions with hydrophobic TAGs showed lower solvation energy,which made the enriched addition ions easily evaporation from the droplet surface, which improved the ESI ionization efficiency. The mass spectrometry detection sensitivity is then improved.A series of cor oil samples with different concentrations in the range of 0.001?L / mL to 10?L / mL are prepared. After analyzing the relationship between the peak area of triglyceride fraction total ion chromatogram and the concentration of the sample,the linear correlation coefficient was calculated as more than 0.99 at low concentration, while the peak area of LC-MS increased nonlinearly when the concentration is more than the specific concentration(aggregation concentration). According to the relative content of each component, the aggregation concentration of triglyceride component in corn oil was calculated to be about 0.1?0.3 ?L / mL in ESI process.Finally, the detective sensitivity of other different 26 compounds using ammonium formate compared with that using methylamine formate was investigated using ESI-MS. The results show that methylamine formate as an electrolyte modifier can also improve the detection sensitivity of saccharides.The thesis provides a new way for LC-MS to improve the detection sensitivity of triglycerides in edible oil.