Ambient ionization and direct identification of volatile organic compounds with microwave‐induced plasma mass spectrometry

Abstract

An innovative method of volatile organic compounds analysis by using microwave-induced plasma ionization (MIPI) source in combination with an ambient ion trap mass spectrometer is presented here. Using MIPI for direct sample vapor, analysis was achieved without any sample preparation or subsequent heating. The relative abundance of the target compounds can be obtained almost instantly within a few seconds. The ionization processes of different volatile compounds was optimized, and the limits of detection were identified in the range of 0.15-4.5 pptv or 0.73-8.80 pg ml(-1). The relative standard deviation (RSD) is in the range of 4-14%, while correlation coefficients of the working curves (R(2)) are better than 0.98. The new method possesses advantages of ease operation, time-saving, high sensitivity and inexpensive setup. In addition, the ionization processes of short n-alkane chains were investigated with the MIPI technique, and a unique [M + 13](+) was detected, which has not been reported in detail by any other related ionization techniques. An ionization mechanism was proposed on the basis of the experimental results obtained in this work and available information in literatures, in which the n-alkanes in the plasma environment possibly generate protonated cyclopentadiene [M - 5](+) or alkyl-substituted analogues as well as hydrous ions [M + 13](+) and [M + 13 + 18](+), as shown in Scheme 1 in the main text.

Investigation and Applications of In-Source Oxidation in Liquid Sampling-Atmospheric Pressure Afterglow Microplasma Ionization (LS-APAG) Source

“…This is inconsistent with the report that the ion [M + 13] + produced by microwave-induced plasma ionization (MIPI) [27] is the water adduct of 1,3-cyclopentadiene or its derivatives. Unfortunately, specific molecules to produce the ions at m/z 117 have not been identified.…”

“…These detected background ions will promote efficient protontransfer ionization, the formation of [M + NH 4 ] + ions, and even the in-source oxidation of some species, in agreement with the results obtained by APAG. In APAG negative mode, the predominant species observed included NO 3 -(m/z 62), NO 2 -(m/z 46), and [HNO 3 + NO 3 ] -(m/z 125), resulting from the ionization of the moist air [27]. The NO 3 -ions can attach to some analytes to form nitrate adducts evidenced by the APAG spectra of two explosives, RDX and PETN, dominated by [RDX + NO 3 ] -(m/z 284) and [PETN + NO 3 ] -(m/z 378), respectively.…”

“…+ as MS/MS spectrum gave a major fragment at m/z 99 attributing to a neutral loss of H 2 O (18 Da) [27]. However, the formation of another intense fragment at m/z 100 attributable to CID of the same precursor ion (m/z 117) appears to be inconsistent with this explanation (see details in Supplementary Figure S21 One reasonable explanation is that the ions at m/z 117 are generated by further oxidation of n-hexane, which can be accelerated by elevated temperature.…”

“…In 2010, oxidation peaks generated during desorption electrospray ionization (DESI) discharge [28] were employed to provide MW information of alkanes, although requiring an extra derivatization reagent and resulting in a relatively complex mass spectrum. Recently, microwaveinduced plasma ionization mass spectrometry (MIPI-MS) [27] was applied to small volatile alkanes to generate water adducts of dehydrogenation oxidation products (1,3-cyclopentadiene derivatives).…”

Mass spectrometric monitoring of oxidation of aliphatic C6–C8 hydrocarbons and ethanol in low pressure oxygen and air plasmas

“…They suggested a role of atmospheric water vapor, as previously assumed as hydrated analyte ions, e.g., [M À 3H] + ⋯H 2 O. [4,5] The hydrates for carbenium ions were also proposed by Li et al [7] They analyzed volatile organic compounds by microwave-induced plasma mass spectrometry. For small n-alkanes (n-pentane, n-hexane, n-heptane and n-octane), [M + 13] + was detected as the major ion as reported by Cody.…”