直链烷烃在质子转移反应质谱中的裂解规律研究

Study on Fragmentation of Linear Alkanes in Proton-Transfer-Reaction Mass Spectrometry

通过自制的液体进样装置,利用质子转移反应质谱(Proton-transfer-reaction mass spectrometry,PTR-MS)对10种直链烷烃(C6~C15)的高纯标准品进行了检测。实验结果表明,直链烷烃在PTR-MS中与H3O+反应产生了碎片离子,其分布特征类似于电子轰击源(Electronionization,EI)。质谱分析表明,主要裂解产物为一系列[CiH2i+1]^+、[CiH2i-1]^+和[CiH2i-3]^+(i≥3)离子。实验中未观测到质子化离子[M+H]^+,而[M-H]^+离子和其它碎片离子的出现,表明直链烷烃的质子化离子形成后即发生脱氢或者直接裂解。研究还发现,碎片离子的相对丰度受E/N(E为电场强度,N为气体分子数密度)值的影响较大。在80~170Td范围内,随着E/N值升高,质荷比较大的碎片离子的相对丰度率先降低,直至信号低于检出限而无法被检测;质荷比较小的碎片离子(如[C4H9]^+(m/z57)、[C3H7]^+(m/z43)等)的相对丰度呈现先升高后降低的趋势,而碎片离子[C3H5]^+(m/z41)的相对丰度一直呈上升趋势。实验结果表明,较大质荷比的碎片离子在高E/N值条件下会发生二次裂解,逐级生成质荷比更小的碎片离子。

The pure standards of 10 linear alkanes(C6-C15)were detected by proton-transfer-reaction mass spectrometry(PTR-MS)through a self-made liquid sampling device.The experimental results showed that the fragmentation of linear alkanes in PTR-MS underwent similar patterns in the electron impact ionization source(EI).It could be seen from the mass spectrum that the main products were a series of[CiH2i+1]^+,[CiH2i-1]^+and[CiH2i-3]^+(i≥3)ions.In the experiment,protonated molecular ions[M+H]^+were not detected,and the appearance of[M-H]^+ions and other fragment ions indicated that once the protonated product of the alkane was formed,dehydrogenation or direct cleavage occurred.Furthermore,it was found that the relative abundance of fragment ions was affected greatly by E/N(the ratio of electric field strength,E,and number density of particles in the drift tube,N).In the range of 80-170 Td,as the E/N value increased,the relative abundances of the fragment ions with high m/z would decrease first,until the signal was below the detection limit of the instrument and could not be detected.The relative abundance of the fragment ions with low m/z(such as[C4H9]^+(m/z 57),[C3H7]^+(m/z 43),etc.)first increased and then decreased,while the relative abundances of the fragment ions[C3H5]^+(m/z 41)always showed an upward trend.This indicated that the fragment ions with high m/z would undergo secondary cracking under high E/N conditions,and the fragment ions with low m/z were generated step by step.