Prazeller Peter, Palmer Peter T, Boscaini Elena, Jobson Tom, Alexander Michael
Pacific Northwest National Laboratory, 3020 Q Avenue, K8-93, Richland, WA 99352, USA.
Rapid Commun Mass Spectrom. 2003;17(14):1593-9. doi: 10.1002/rcm.1088.
Proton transfer reaction mass spectrometry is a relatively new field that has attracted a great deal of interest in the last few years. This technique uses H(3)O(+) as a chemical ionization (CI) reagent to measure volatile organic compounds (VOCs) in the parts per billion by volume (ppbv) to parts per trillion by volume (pptv) range. Mass spectra acquired with a proton transfer reaction mass spectrometer (PTR-MS) are simple because proton transfer chemical ionization is "soft" and results in little or no fragmentation. Unfortunately, peak identification can still be difficult due to isobaric interferences. A possible solution to this problem is to couple the PTR drift tube to an ion trap mass spectrometer (ITMS). The use of an ITMS is appealing because of its ability to perform MS/MS and possibly distinguish between isomers and other isobars. Additionally, the ITMS duty cycle is much higher than that of a linear quadrupole so faster data acquisition rates are possible that will allow for detection of multiple compounds. Here we present the first results from a proton transfer reaction ion trap mass spectrometer (PTR-ITMS). The aim of this study was to investigate ion injection and storage efficiency of a simple prototype instrument in order to estimate possible detection limits of a second-generation instrument. Using this prototype a detection limit of 100 ppbv was demonstrated. Modifications are suggested that will enable further reduction in detection limits to the low-ppbv to high-pptv range. Furthermore, the applicability of MS/MS in differentiating between isobaric species was determined. MS/MS spectra of the isobaric compounds methyl vinyl ketone (MVK) and methacrolein (MACR) are presented and show fragments of different mass making differentiation possible, even when a mixture of both species is present in the same sample. However, MS/MS spectra of acetone and propanal produce fragments with the same molecular masses but with different intensity ratios. This allows quantitative distinction only if one species is predominant. Fragmentation mechanisms are proposed to explain the results.
质子转移反应质谱法是一个相对较新的领域，在过去几年中引起了广泛关注。该技术使用H(3)O(+)作为化学电离（CI）试剂，以测量体积分数在十亿分之一（ppbv）至万亿分之一（pptv）范围内的挥发性有机化合物（VOCs）。用质子转移反应质谱仪（PTR-MS）获得的质谱图很简单，因为质子转移化学电离是“软”电离，几乎不会导致碎裂或完全不产生碎裂。不幸的是，由于等压干扰，峰识别仍然可能很困难。解决这个问题的一个可能办法是将PTR漂移管与离子阱质谱仪（ITMS）联用。使用ITMS很有吸引力，因为它能够进行串联质谱分析（MS/MS），并且有可能区分异构体和其他等压物。此外，ITMS的占空比远高于线性四极杆，因此可以实现更快的数据采集速率，从而能够检测多种化合物。在此，我们展示了质子转移反应离子阱质谱仪（PTR-ITMS）的首批结果。本研究的目的是研究一台简单原型仪器的离子注入和存储效率，以便估计第二代仪器可能达到的检测限。使用该原型仪器证明了检测限为100 ppbv。文中提出了一些改进建议，这些改进将使检测限进一步降低至低ppbv至高pptv范围。此外，还确定了MS/MS在区分等压物种方面的适用性。给出了等压化合物甲基乙烯基酮（MVK）和甲基丙烯醛（MACR）的MS/MS谱图，结果表明，即使同一样品中同时存在这两种物质，不同质量的碎片也使得区分成为可能。然而，丙酮和丙醛的MS/MS谱图产生的碎片具有相同的分子量，但强度比不同。只有当一种物质占主导时，这才允许进行定量区分。文中提出了碎裂机制来解释这些结果。
