Hadamard Transform Proton Transfer Reaction Mass Spectrometry for Real-Time Atmospheric VOC Measurements

点击翻译按钮获取中文摘要

Trace gases in air are important indicators of the state of our environment. The ideal instrument for measuring these gases would offer high accuracy and precision, good temporal resolution, and high sensitivity. It should also be able to distinguish between the various components of a complex mixture. Proton transfer reaction mass spectrometry (PTR-MS) is a relatively new trace gas measurement technique which goes some way towards achieving the ideal qualities described above. The technique is based on chemical ionization in which a proton, usually from H3O+, is captured by a neutral molecule in the analyte gas to generate a protonated ion. Charge transfer takes place in a drift tube and the product ion(s) are subsequently detected by mass spectrometry. Only those molecules with proton affinities exceeding that of H2O can accept a proton from H3O+, a criterion that excludes the major components of air such as N2, O2 and CO2, but includes most volatile organic compounds (VOCs). This discrimination enables trace VOCs to be detected without complications from intense background signals and confers high sensitivity. A wide variety of applications of PTR-MS have already been demonstrated, including environmental gas monitoring, and the growing importance of this technique is reflected in the large and increasing number of research groups with access to PTR-MS instrumentation. Quadrupole mass spectrometry (QMS) is employed in most existing PTR-MS instruments but in the past five years several have been built that use time-of-flight mass spectrometry (TOF-MS), and indeed a commercial version has recently been marketed. TOF-MS offers several improvements over QMS, including a much higher mass resolution, a wider mass range, and the ability to collect data in all mass channels simultaneously, i.e. it is a multichannel device. However, the downside of TOF-MS is its limited duty-cycle, which arises because ions are injected into the flight tube in 'packets' which must reach the detector before the next 'packet' is injected. This gives typical duty-cycles of < 5%, which means that >95% of the potential signal is wasted. The aim of the work proposed here is to take PTR-MS to the next level, where all the advantages of TOF-MS as the mass detector are retained while solving the low duty-cycle problem. This will be achieved using a new form of TOF-MS that has been developed in the past decade, Hadamard transform TOF-MS. We will demonstrate, through laboratory-based studies and through field deployment, that a PTR-MS instrument utilising Hadamard transform TOF-MS offers important advantages over existing PTR-MS instruments for the detection of trace VOCs. The student employed on this project will adapt and refine an existing prototype instrument, will formally establish its capabilities, and will use its strengths to tackle a fundamental problem in trace gas measurement, namely to determine the extent and importance of rapid variations of VOC concentrations in the urban environment.