# Vacuum Ultraviolet Single-Photon Positionization of Amino Acids
## 3 Results
### (a) VUV photon number
In particular, the ion intensity in VUV-SP-positionization is considerably higher than that in UV-MALDI for amino acids with low proton affinities, such as glycine. Figure 4: UV-MALDI full scan mass spectrum of a pure THAP matrix sample (red) and UV-MALDI full scan along with VUV-SP-positionization (purple), and UV-MALDI full scan of a sample containing 5% analyte in THAP matrix (green) and UV-MALDI full scan with VUV-SP-positionization (blue). Cyan-colored peaks represent the background signal contributed by the THAP matrix. The analyte used in (**a**) and (**b**) is glycine and that used in (**c**) is tryptophan. The _m_/z values of the peaks in the red and green curves must be read from the X axis for UV-MALDI. If the peaks in the purple and blue curves coincide with peaks in the red and green curves, they represent ions generated through UV-MALDI. Therefore, the _m_/z values of these peaks must be read from the X axis for UV-MALDI. If the peaks in the purple and blue curves do not appear in the red and green curves, they represent ions generated through VUV-SP-positionization and the _m_/z values of these peaks must be read from the X axis for VUV-SPI. \begin{table}
\begin{tabular}{l c c c c c} \hline \hline
**Amino Acids** & \begin{tabular}{c} **Parent Ion (Relative Ion** \\ **Abundance)**\({}^{\text{a,b}}\) \\ \end{tabular} & \
\begin{tabular}{c} **Fragment Ion (Relative Ion** \\ **Abundance)**\({}^{\text{a,b}}\) \\ \end{tabular} & **Amino Acids** &
\begin{tabular}{c} **Parent Ion (Relative Ion** \\ **Abundance)** \\ \end{tabular} & **Fragment Ion (Relative Ion** \\ \hline Glycine & 75 (300) & 30 (1370) & Serine & 105 \({}^{\text{a}}\) & 43, 60 (4), 74/75 (2) \\ Alanine & 89 (7) & 44 (800) & Threonine & 119 \({}^{\text{e}}\) & 57 (445), 74/75 (4200) \\ Valine & 117 (15) & 72 (160) & Phenylalanine & 165 \({}^{\text{e}}\) & 74 (1750), 91 (500), 120 (1640) \\ Proline & 115 \({}^{\text{e}}\) & 70 (2000) & Trypothon & 204 (27) & 130 (3600) \\ Leucine & 131 (15) & 86 (530) & Glutamic acid & 147 (5) & 102 (25) \\ Isolecucine & 131 (0.4) & 75 (13), 86 (10) & Aspartic acid \({}^{\text{e}}\) & 133 \({}^{\text{e}}\) & 88 (3) \\ Tyrosine & 181 \({}^{\text{e}}\) & 107 (10) & Glutamine & 146 (0.7) & 84, 101 (30) \\ Histidine & 155 \({}^{\text{e}}\) & 82 (10) & Asparagine \({}^{\text{e}}\) & 132 \({}^{\text{e}}\) & 74, 87 (13) \\ Argin & 174 \({}^{\text{e}}\) & 60, 74 (6) & Cysteine & 121 (23) & 74/75/76 (110) \\ Lysine & 146 (5) & 30, 44 (50), 56, 72 (180), 84, 101 (75) & Methionine & 149 (18) & 75 (155), 88, 101, 104 (90), \\ \hline \end{tabular}  
* The values in front of parentheses represent _n/z_ values. The values in parentheses show the relative abundance; \({}^{\text{b}}\) Relative ion abundance = ion signal intensity from VUV ionization/protonated amino acid from MALD; \({}^{\text{c}}\) THAP was used as the matrix for all amino acids, except aspartic acid and asparagine, for which 2.5-DHB was employed; \({}^{\text{d}}\) The signal of the parent ion was not observed in the VUV-SF-postionization MS spectrum; therefore, the relative ion abundance was not calculated; \({}^{\text{e}}\) The signal of the parent ion overlaps with the background signal generated from THAP in the VUV-SF-postionization MS spectrum.  
\end{table} 
Table 1: Relative ion abundance of 20 common amino acids, obtained using VUV-SF-postionization and UV-MALDI data. The parent and dominant fragment ions are listed.