Investigation of the unusual molecular isotope patterns of trimethylsilyl esters under GC-ESI/MS

• Main Authors: Yu-Meng Ou, 歐育孟
• Other Authors: Guor-Rong Her
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/64271569094300472984

碩士 === 國立臺灣大學 === 化學研究所 === 102 === Gas Chromatography (GC) combined with Mass Spectrometry (MS) is widely used for the analysis of metabolomics. With the use of the GC-ESI/MS interface that was developed by our laboratory to analyze the TMS (Trimethylsilyl) derivatives of organic acids, we have observed unusual high abundance of [M+3]+ ions. The abundance of [M+3]+ ions were as high as a third of the protonated molecule [M+H]+ ions. Based on the speculation of reduction of the C=O double bond (from R-CO2-Si(CH3)3 to R-H(COH)-O-Si(CH3)3), we proposed a possible structure of the unusual [M+3]+ ions. When these [M+3]+ ions were studied by Tandem Mass Spectrometry (MS/MS), a [M+3-H2O]+ product ion was observed and this ion was not detected from the fragmentations of the protonated molecules [M+H]+. With the use of deuterium solvent as the ESI solvent followed by MS/MS, the deuterated [M+5]+, and [M+4]+ ions, and the protonated [M+3]+ ions could respectively produce [M+5-D2O]+, [M+4-DHO]+, and [M+3-H2O]+ fragments, suggesting the presence of a hydroxyl group in the [M+3]+ ions. With the use of the IT-TOF mass spectrometer to identify the [M+3]+ ions, we found that the mass difference between the detected mass and the proposed structure is about 200 ppm. The most possible elemental composition R-C3H7O3Si, predicted by the computer database, was about 20 ppm from the detected mass. It was speculated that not only the unusual ions but also the 30Si isotopes of the derivatives were included in the [M+3]+ peak due to the limit of the resolution of the IT-TOF mass spectrometer. After calculation, R-C3H7O3Si was believed the reasonable elemental composition of the unusual [M+3]+ ions. The elemental composition of [M+3]+ ions were identified using a Q-Orbitrap mass spectrometer having a resolution about 100,000. Based on the elemental composition, a possible structure was proposed. The [M+3]+ ion was produced due to the exchange of a methyl group in the TMS group with a hydroxyl group. A possible mechanism for the formation of the [M+3]+ ions was also proposed.