# Vacuum Ultraviolet Single-Photon Positionization of Amino Acids
## 2 Experiment  
In this study, homemade VUV light source and MALDI/VUV-SP-positionization time-of-flight mass spectrometer (TOF-MS) instruments were used. Figure 1 illustrates part of the experimental setup. Similar homemade instruments have been reported in previous studies [58; 59; 60; 61]. Desorbed ions and neutrals were generated by irradiating a UV-pulsed laser beam from the third harmonic of a Nd:YAG laser (355 nm, 5-ns pulse duration, Minlite II, Continuum, San Jose, CA, USA) on a solid sample surface. The laser beam was focused onto an elliptical spot with an area of 350 \(\upmu\)m \(\times\) 200 \(\upmu\)m on the sample surface. Ions generated through MALDI were analyzed using TOF-MS. Desorbed neutrals from the same UV laser pulse were ionized using a VUV (18-nm) laser pulse, and the ions produced through VUV photoionization were analyzed using the same TOF-MS system. The VUV beam was parallel to the sample surface and overlapped with the plume of desorbed neutrals at a distance of 1-2 mm above the sample surface, where photoionization occurred. The delay between the UV and VUV laser pulses was controlled by a digital delay generator (DG 535, Stanford Research Systems, Inc., Sunnyvale, CA, USA.). The delay was adjusted to 3 \(\upmu\)s to achieve optimal temporal overlap between the MALDI plume and VUV photons. The spectra presented herein were obtained by averaging five spectra (20 laser shots for each spectrum). The mass calibration and resolution of UV-MALDI TOF-MS were performed using CsI clusters. The mass calibration and resolution of a VUV-SP-positionization mass spectrometer were performed using 2,5-DHB, THAP, CHCA, SA, and FA matrices. The mass resolution of UV-MALDI and VUV SP was 523 at \(m/z\) 393 (m / FWHM\({}_{\rm m}\), where FWHM stands for the full width at half maximum) and 200 at \(m/z\) 168, respectively. The mass resolution was optimized for UV-MALDI; therefore, the resolution of VUV SP was not as good as that of UV-MALDI. Figure 2 presents a schematic of the VUV laser generation. The 118-nm VUV laser beam was generated through frequency tripling of the UV (355-nm) laser beam in xenon gas. The UV laser beam from the third harmonic of the Nd:YAG laser (355 nm, 5-ns pulse duration, 680 mW, 10 Hz, LS-213 TU, LOTIS Tii, Minsk, Belarus) was focused into a Xe cell to generate VUV radiation at the focal point in the Xe cell. Coaxial and expanding UV and VUV laser beams were directed off-axis into a LiF-VUV lens (biconvex, f = 103 mm, diameter = 38.1 mm, Crystran Ltd. Poole, Dorset, UK). The VUV and  
Figure 1: Schematic of experimental setup. The relative dimensions of each component are not drawn in scale. A UV-MALDI laser (355 nm) struck on the sample surface to generate MALDI ions and desorbed neutrals. Ions were accelerated by an electronic field, focused using an Einzel lens, and analyzed using TOF-MS. Desorbed neutrals were postionized by a VUV pulse (118 nm). Ions generated through VUV-SP-positionization were accelerated and focused by the same ion optics and analyzed using the same TOF-MS system. UV laser beams were separated by the lens because of their different refractive indices. The UV laser beam was blocked by a graphite-coated stainless-steel baffle, whereas the VUV laser beam passed through the stainless-steel baffle and was refocused on the ionization region. The VUV beam size was estimated to be less than 1 mm in diameter at the ionization region. The VUV photon number of each laser pulse was detected using a homemade Pd detector. There are many materials suitable for detecting VUV; however, to measure the absolute number of VUV photon, the quantum efficiency at the given VUV wavelength has to be known. It just happens that we have a piece of Pd, and Pd is one of the materials suitable for detecting VUV, and the quantum efficiency at 118 nm is known [62]. A metal mesh (transmission efficiency, 88.6%; BM-0117-01; Industrial Netting, Minneapolis, MN, USA), which was floated to 700 V, was placed at a distance of 5 mm in front of the Pd plate.