# Vacuum Ultraviolet Single-Photon Positionization of Amino Acids
## 3 Results  
### (a) VUV photon number  
The VUV photon number of each laser pulse was determined using the homemade VUV detector. The VUV photon number increased with an increase in laser energy from 43 to 60 mJ/pulse. However,  
Figure 2: Schematic of VUV light source and part of the TOF-MS system. A UV laser (355 nm) was focused into a Xe cell by using UV lens (f = 25 cm). After VUV radiation was generated, the coaxial and expanding UV and VUV laser beams were separated and focused using an off-axis LiF-VUV lens. The UV laser beam was blocked by graphite-coated stainless-steel baffles, whereas the VUV laser beam passed through stainless steel baffles and refocused on the ionization region. Ions from UV-MALDI and VUV photoionization were analyzed and detected using TOF-MS, in which the flight axis is perpendicular to the paper. the photon number did not considerably increase when the laser energy was increased from 70 to 100 mJ/pulse. The VUV photon number of each laser pulse was calculated from the measurement using [PERSON]'s law:  
\[ \begin{array}{ll}\text{photon number}&=\frac{16\text{V}}{50\text{\Omega}} \times 7\times 10^{-9}\text{s}\times\frac{1}{1.6\times 10^{-19}\text{C}} \times\frac{1}{0.01}\times\frac{1}{0.886}\\ &=1.58\times 10^{12}\,\text{photons/pulse}\end{array} \]  
where 16 V was the measured pulse height from the VUV detector with an oscilloscope impedance set at 50 \(\Omega\), and 7 ns was the measured pulse width (full width at a half maximum). The transmission efficiency of the metal mesh and photoelectron quantum efficiency of Pd at 118 nm are 0.886 and approximately 0.01, respectively [62]. **(b) Mass spectra obtained using UV-MALDI and VUV-SP-positionization**  
Figure 3a,b shows the UV-MALDI mass spectra of 5% glycine in a THAP matrix. At a low laser fluence (70 J/m\({}^{2}\)), only a small amount of protonated and sodiated THAP was observed. When the UV laser fluence was increased to 110 J/m\({}^{2}\), signals of the protonated and sodiated THAP matrix significantly increased; however, no glycine ions or protonated glycine were found. Only sodiated glycine with a very small intensity was observed, as shown in Figure 3b. Glycine ions and protonated glycine were not observed even at higher laser fluence. Figure 3c shows the UV-MALDI mass spectra of 5% tryptophan in THAP matrix. A protonated tryptophan signal was clearly observed. When both UV and VUV lasers were employed, the ions generated from VUV-SP-positionization were recorded in the same spectrum along with the ions directly generated through UV-MALDI. Because of different ion formation times and positions, the flight times of the ions produced  
Figure 3: UV-MALDI full scan mass spectrum. Red and green lines represent the sample of the pure 2,4,6-trihydroxyacetophenone (THAP) matrix and sample containing 5% analyte in THAP matrix, respectively. The analyte used in (**a**) and (**b**) is glycine and that used in (**c**) is tryptophan. through these two ionization methods were substantially different. The ions produced through VUV-SP-positionization were formed after those produced by UV-MALDI because of the delay between the UV and VUV laser pulses. Moreover, the ions produced through VUV SP were generated at a position 1-2 mm above the sample surface, where the electric potential was less than that of the ions produced through UV-MALDI. Therefore, the flight time of the ions generated through VUV-SP-positionization was longer than that of the ions generated through UV-MALDI. The ions produced using these two ionization methods are easily distinguished. Figure 4 displays the mass spectra of UV-MALDI and VUV-SP-positionization for a pure THAP matrix sample and when 5% analyte was placed in the THAP sample. For the pure THAP matrix sample at low laser fluence (70 J/m\({}^{2}\)), only a few THAP ions generated through UV-MALDI were observed (red line in Figure 4a). When both UV and VUV lasers were employed, numerous ions were observed (purple line in Figure 4a).