Division of Pharmaceutical Chemistry, Faculty of Pharmacy, University of Helsinki, P.O. Box 56, 00014 Helsinki, Finland.
Anal Chem. 2012 Feb 7;84(3):1408-15. doi: 10.1021/ac2024574. Epub 2012 Jan 23.
A direct current induced vacuum ultraviolet (dc-VUV) krypton discharge lamp and an alternating current, radio frequency (rf) induced VUV lamp that are essentially similar to lamps in commercial atmospheric pressure photoionization (APPI) ion sources were compared. The emission distributions along the diameter of the lamp exit window were measured, and they showed that the beam of the rf lamp is much wider than that of the dc lamp. Thus, the rf lamp has larger efficient ionization area, and it also emits more photons than the dc lamp. The ionization efficiencies of the lamps were compared using identical spray geometries with both lamps in microchip APPI mass spectrometry (μAPPI-MS) and desorption atmospheric pressure photoionization-mass spectrometry (DAPPI-MS). A comprehensive view on the ionization was gained by studying six different μAPPI solvent compositions, five DAPPI spray solvents, and completely solvent-free DAPPI. The observed reactant ions for each solvent composition were very similar with both lamps except for toluene, which showed a higher amount of solvent originating oxidation products with the rf lamp than with the dc lamp in μAPPI. Moreover, the same analyte ions were detected with both lamps, and thus, the ionization mechanisms with both lamps are similar. The rf lamp showed a higher ionization efficiency than the dc lamp in all experiments. The difference between the lamp ionization efficiencies was greatest when high ionization energy (IE) solvent compositions (IEs above 10 eV), i.e., hexane, methanol, and methanol/water, (1:1 v:v) were used. The higher ionization efficiency of the rf lamp is likely due to the larger area of high intensity light emission, and the resulting larger efficient ionization area and higher amount of photons emitted. These result in higher solvent reactant ion production, which in turn enables more efficient analyte ion production.
一种直流感应真空紫外（dc-VUV）氪放电灯和一种交流、射频（rf）感应 VUV 灯，它们与商业大气压光电离（APPI）离子源中的灯基本相似，对这两种灯进行了比较。测量了灯出射窗直径上的发射分布，结果表明，rf 灯的光束比 dc 灯宽得多。因此，rf 灯具有更大的有效电离面积，并且比 dc 灯发射更多的光子。在微芯片大气压光电离质谱（μAPPI-MS）和解吸大气压光电离质谱（DAPPI-MS）中，使用相同的喷雾几何形状，用两种灯比较了灯的电离效率。通过研究六种不同的 μAPPI 溶剂组成、五种 DAPPI 喷雾溶剂和完全无溶剂的 DAPPI，获得了对电离的全面了解。用两种灯观察到的每种溶剂组成的反应物离子都非常相似，但甲苯除外，在用 rf 灯进行 μAPPI 时，甲苯比 dc 灯产生更多的源自溶剂的氧化产物。此外，用两种灯都检测到了相同的分析物离子，因此，两种灯的电离机制相似。在所有实验中，rf 灯的电离效率都高于 dc 灯。当使用高电离能（IE）溶剂组成（IE 高于 10 eV），即己烷、甲醇和甲醇/水（1:1v:v）时，灯的电离效率差异最大。rf 灯的较高电离效率可能是由于高强度发光的面积较大，导致有效电离面积较大，发射的光子数量较多。这导致更多的溶剂反应物离子生成，从而使更多的分析物离子生成更有效。
