# Vacuum Ultraviolet Single-Photon Positionization of Amino Acids
## 4 Discussion  
The degree of success of VUV-SP-positionization is strongly dependent on the properties of the VUV photons, including their wavelength, pulse duration, and intensity. The IE of most organic molecules is generally less than 10 eV [64], whereas the IE of amino acids and several biomolecules is approximately 8-10 eV [65; 66; 67]. The ninth harmonic of a Q-switched Nd:YAG laser provides suitable photon energy of 10.49 eV, which is higher than the IE of most molecules and is not sufficiently large to induce severe fragmentation. The VUV pulse generated from a typical Q-switched nanosecond-pulse Nd:YAG laser has duration of less than 10 ns. Pulse duration appropriately matches the TOF-MS and neutral desorption duration of laser desorption.  
When a suitable VUV wavelength and pulse duration are used, the ion intensity from VUV-SP-positionization is primarily dependent on the VUV intensity. The ion signal (\(S\)) of cations  
Figure 6: **Left**: Relative UV-MALDI ion signals of protonated tryptophan and phenylalanine as a function of molar ratio. **Right**: Relative VUV-SP-positionization ion signals of tryptophan and phenylalanine as a function of molar ratio. The fragment ions at \(m\)/z 74 and 120 for phenylalanine and fragment ion at \(m\)/z 130 for tryptophan were used in analysis of VUV-SP-positionization data. The dynamic range of VUV-SP-positionization for quantitative analysis is demonstrated one order of magnitude larger than that of UV-MALDI.  
produced by VUV-SP-positionization depends on the ionization cross-section (\(\sigma_{SPI}\)), VUV intensity (\(I\)), and density of gaseous molecules (\(N\)). The ion signal can be calculated as \(S=\sigma_{SPI}IN\)[69; 70]. In general, the ionization cross-section is approximately 2-20 megabarns (1 Mb = \(10^{-18}\) cm\({}^{2}\)) [71]. Aromatic species and alkynes, dienes, and alkenes exhibit ionization cross-sections of approximately 20 Mb, 15 Mb, and 8 Mb, respectively, whereas aldehydes/ketones and alkanes have lower ionization cross-sections of approximately 3-4 Mb [69; 70; 71; 72]. A typical ion-to-neutral ratio of analytes in UV-MALDI is \(10^{-8}\)-\(10^{-3}\), depending on the properties (e.g., proton affinity) of the analyte. The overlap of the VUV laser beam with the desorbed neutral plume is estimated to be approximately one tenth. To yield higher ion intensity from VUV-SP-positionization than that from UV-MALDI, the VUV photon number in each laser pulse must be large. For analytes with an ion-to-neutral ratio of \(10^{-5}\) in UV-MALDI, the VUV photon number of each laser pulse must be larger than \(10^{12}\), assuming that the ionization cross-section is 10 Mb. The VUV photon number generated from a midsize commercial Nd:YAG pulse laser is as high as \(1.58~{}\times~{}10^{12}\). The ionization efficiency of VUV-SP-positionization using such a Nd:YAG pulse laser is comparable with that of conventional MALDI-MS. Compared with the MALDI technique, in which the ionization efficiency can vary by several orders of magnitude depending on the proton affinity analyte and selected matrix [67], VUV-SP-positionization enables more readily quantifiable analysis.  
Conceptualization, C.-K.N.; Methodology, C.-K.N. and H.C.H.; Formal Analysis, H.C.H.; Data Curation, H.C.H.; Writing-Original Draft Preparation, H.C.H.; Writing-Review & Editing, C.-K.N.; Project Administration, C.-K.N.; Funding Acquisition, C.-K.N.  
We acknowledge support from the Ministry of Science and Technology, Taiwan (NSC 106-2113-M-001-023-MY3).  
The authors declare no conflict of interest.