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

空气中的微量气体是我们环境状况的重要指标。测量这些气体的理想仪器应具有较高的准确度和精密度、良好的时间分辨率和高灵敏度。它还应该能够区分复杂混合物的各种成分。质子转移反应质谱（PTR-MS）是一种相对较新的微量气体测量技术，它在一定程度上实现了上述理想质量。该技术基于化学电离，通常来自h30 +的质子被分析气体中的中性分子捕获，产生质子化离子。电荷转移发生在漂移管中，产物离子随后通过质谱法检测。只有质子亲和力超过H2O的分子才能接受来自h30 +的质子，这一标准排除了空气中的主要成分，如N2、O2和CO2，但包括大多数挥发性有机化合物（VOCs）。这种识别使得检测到痕量挥发性有机化合物而不会受到强烈背景信号的影响，并且具有高灵敏度。PTR-MS的各种应用已经得到证明，包括环境气体监测，并且越来越多的研究小组可以使用PTR-MS仪器，这反映了这项技术日益增长的重要性。四极杆质谱法（QMS）在大多数现有的PTR-MS仪器中使用，但在过去的五年中，已经建造了一些使用飞行时间质谱法（TOF-MS）的仪器，并且最近确实有一个商业版本已经上市。与QMS相比，TOF-MS提供了一些改进，包括更高的质量分辨率，更宽的质量范围，以及同时在所有质量通道中收集数据的能力，即它是一个多通道设备。然而，TOF-MS的缺点是其有限的占空比，这是因为离子以“包”的形式注入飞行管，这些离子必须在注入下一个“包”之前到达探测器。这给出了典型的< 5%的占空比，这意味着95%的潜在信号被浪费了。本文提出的工作目的是将PTR-MS提升到一个新的水平，在解决低占空比问题的同时保留了TOF-MS作为质量检测器的所有优点。这将使用在过去十年中开发的一种新形式的TOF-MS来实现，即Hadamard变换TOF-MS。我们将通过基于实验室的研究和现场部署证明，利用Hadamard变换TOF-MS的PTR-MS仪器在检测痕量挥发性有机化合物方面比现有的PTR-MS仪器具有重要优势。该项目雇用的学生将对现有的原型仪器进行调整和改进，将正式建立其功能，并将利用其优势解决微量气体测量中的基本问题，即确定城市环境中VOC浓度快速变化的程度和重要性。

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.