Vocus PTR-TOF mass spectra are dominated by protonated peaks and free of significant contributions from non-PTR reaction and therefore easily interpreted and quantified.
Felipe Lopez-Hilfiker，梁朱，曼努埃尔·赫特利，卢卡·卡帕林
Tofwerk，Thun，瑞士
在线CI-MS的空气分析需求很好地理解离子化学
含有挥发性有机化合物（VOC）的空气样品通常是多元或数百种不同的痕量化合物的复杂混合物。用于分析这些样品的传统技术需要劳动密集的样品制备和色谱分离，从而限制测量产量和/或VOC测量的时间分辨率。相反，化学电离质谱法（CI-MS）可以用秒第二时间分辨率定量测量VOC。这些快速的在线能力源于易于理解的离子化学，允许电离和质谱检测VOC，而无需样品制备或色谱分离。
In CI-MS, air that contains neutral, gas-phase analyte molecules is directly sampled into the mass spectrometer. Analyte molecules are ionized by way of chemical reactions with high-concentration reactant ions (aka primary ions or reagent ions) under controlled conditions. The analyte ions produced in these reactions are subsequently measured by a mass analyzer, which determines the ions’ mass/charge values and abundances. The level of difficulty of interpreting the mass spectrum depends on the complexity of the ion chemistry – which is largely determined by the reagent ion of choice – and the purity of the ion source. For a given ion chemistry, the purer the source of reactant ions, the easier the spectrum is to interpret.
High Purity Reactant Ions Make Vocus Data Readily Interpretable
Proton transfer reaction mass spectrometry (PTR-MS) is a type of CI-MS that is widely used for online detection of VOCs. The Vocus PTR-TOF is an ultra-sensitive PTR-MS that combines TOFWERK’s proprietary Vocus reactor technology and a time-of-flight mass analyzer. High abundance H3O+反应物离子在专用离子源区中的低压等离子体中用水蒸汽产生。在进入离子分子反应区域之前将产生的反应物离子转移到复合区域中，其中H3O+ions transfer protons to neutral VOC analyte molecules, creating VOC analyte ions that can be measured by the mass analyzer.
实际上，Ptr质谱可以通过源自寄生反应物离子的光谱峰损坏（例如，没有+, O2+), generated from either back-diffusion of air into the primary plasma region or the reaction of excited radicals exiting the ion source and meeting the sample flow. The presence of spurious O2+and NO+离子复杂化光谱，因为两者都能够通过电荷转移途径电离共同VOC，以产生不同（非质子化）产物离子或驱动碎片反应。
用于PTR-MS的试剂离子源的纯度可以通过A）直接比较来确定O.2+and NO+信号到h3O+or b) the ratio of the protonated and non-protonated signals of targeted compounds. The latter approach better reflects the reaction conditions in the IMR as additional quenching time during transit through the reactor can distort the actual reactor conditions and the observed relative abundances of H3O+, O2+, and NO+。
Figure 1 Comparing protonated and non-protonated mass spectral signals for various VOCs to measure Vocus PTR-TOF ion source purity.三个VOC的数据显示主要质子化峰（MH +），小电荷转移峰（M H +）和同位素峰与分子组合物一致。基于向非质子峰的所显示的呈现的比率，在最大灵敏度下操作时的离子源纯度大于98％，从而能够简单地解释VOCU PTR-TOF数据。
图1对VOCUS PTR-TOF数据中的非质子峰的质子化对多种质量/充电的一系列VOC的非质子峰进行了反应，并具有不同的官能团。在所有情况下，VOC由质子化主要收取，只有杂散电荷转移反应的轻微贡献（〜1-3％），证明了VOCUS离子源的高纯度。在这种高纯度下，VOCUS质谱由质子化峰支配，并没有来自非PTR反应的显着贡献，因此容易解释和量化。
来源：https://www.tofwerk.com/