甲基化辅助实时直接分析电离的机理研究

实时直接分析电离源(DART)已经广泛应用于固体、液体和气体样品的快速检测. 在使用DART对低挥发性化合物进行分析时, 样品衍生化是十分重要的. 四甲基氢氧化铵(TMAH)是强的瞬时甲基化试剂, 常用于GC-MS的分析中. 以人参皂苷及人参寡糖为例, 研究了它们在TMAH的辅助下在DART中发生甲基化及电离的过程, 并从凝聚相和气相的角度对电离过程中的甲基化机理进行了研究. 人参皂苷主要发生不完全和全甲基化, 人参寡糖的甲基化则随着糖链的增长以及羟基的增多由全甲基化主导转变为过甲基化主导. 结果表明, 凝聚相和气相的共同作用是质谱检测到甲基化及过甲基化样品分子的根本原因.

Study of Mechanism of Ionization Assisted by Methylation in Direct Analysis in Real Time Ion Source

Direct analysis in real time ion source(DART) has been applied to the rapid analysis of solid,liquid and gaseous samples.The compounds with either high volatility or high proton affinity are favorable for DART analysis.However,a lot of natural products such as ginsenosides and ginseng oligosaccharides are difficult to be directly ionized by DART due to their low volatility and proton affinity.Derivatization is very necessary in the analysis of the compounds with low volatility and proton affinity.Tetramethylammonium hydroxide(TMAH) is a frequently used methylation reagent which can be applied to improve the volatility of low volatile compound in GC-MS.In the current work,TMAH was introduced to assist the ionization of ginsenoside and ginseng oligosaccharide in DART.Small amount of ginsenoside standard or ginseng oligosaccharide powder was collected on the end of melting point tubes respectively.Then 5 μL of liquid from a solution containing 25% solution of TMAH dissolved in MeOH was applied to elute the surface of each melting point tube.Finally,all the end of melting point tubes containing sample powder and TMAH were put on a stainless holder for DART analysis.The solvents were flash evaporated in the DART source at 400 ℃ and the signals of methylated analytes were detected.The mechanism of methylation and ionization in DART was elucidated from the aspects of condensed and gas phase.On the one hand,the methylation enhanced volatility of analytes,on the other hand,a N(CH3)3H+ ion was generated to assist the ionization of methylated analytes.Incomplete and complete methylation is the major reaction of ginsenoside,but hypermethylation occurred in ginseng oligosaccharide with the increase of number of hydroxyls.The results showed that the synergy of methylation reaction both in condensed phase and gas phase is the fundamental reason of the generation of methylated ions.The methylation in condensed phase is governed by necleophilic reaction,and radical reaction is the major reaction in gas phase.