# Vacuum Ultraviolet Single-Photon Positionization of Amino Acids
## 1 Introduction  
Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, introduced in the late 1980s by [PERSON] et al. [1; 2], has been widely used in the characterization of biomolecules. Various small matrix molecules can be used to desorb and ionize biomolecules, such as peptides, proteins, saccharides, and DNA, as intact ions. MALDI is one of the most crucial ionization methods for the mass spectrometry of biomolecules because of its soft ionization and high sensitivity. The quality of MALDI-MS spectra is strongly dependent on numerous factors, such as the photophysical and photochemical properties of the matrix and analytes [3; 4; 5; 6; 7; 8; 9; 10; 11; 12] and laser irradiation conditions [13; 14; 15; 16; 17; 18]. Moreover, selecting a suitable matrix for the target analyte is crucial. The selection of matrices for various analytes has been investigated. For example, 2,5-dihydroxybenzoic acid (2,5-DHB), \(\alpha\)-cyano-4-hydroxycinnamic acid (CHCA), and sinapic acid (SA) are suitable for the analysis of peptides and proteins [19; 20; 21]. 2,5-DHB is suitable for the analysis of carbohydrates and lipids [22; 23]. The matrix of 2,4,6-trihydroxyacetophenone (THAP) is used for polar lipids [24], carbohydrates [25; 26; 27; 28], and functional peptides [29; 30]. For DNA analysis, 3-hydroxypicolinic acid (3-HPA) is suitable [31; 32], and 4-chloro-\(\alpha\)-cyanocinnamic acid (CI-CCA) is appropriate for proteomic analyses [33; 34; 35]. Studies have indicated that the amount of desorbed analyte neutrals in a MALDI plume is 2 to 8 orders of magnitude higher than that of desorbed analyte ions [36; 37], depending on the analyte's properties. Similar orders of magnitudes were found in MALDI and surface-assisted laserdesorption/ionization-mass spectrometry [38]. This large amount of desorbed neutrals has received attention and been investigated. Desorbed neutrals can be ionized by postionization and detected by the same mass spectrometer. The optimal conditions for the postionization of analytes are not as difficult to achieve as those required for MALDI. If postionization efficiency is high, the number of ions produced during postionization can be comparable or even higher than the number produced during conventional UV-MALDI. Positionization provides an alternative approach to obtaining the mass spectra of analytes. The infrared (IR) laser desorption of a matrix-free sample followed by UV laser multiphoton ionization was previously used to examine the composition of different exogenous materials [39]. [PERSON] demonstrated the desorption of pure peptides by using a 266-nm laser pulse followed by 255-nm resonance-enhanced multiphoton ionization (REMPI) [40; 41]. [PERSON] et al. observed parent and fragment ions of tryptophan through postionization using 266-nm REMPI, following the desorption from a rhodamine B/glycerol matrix by using a 532-nm laser pulse [42]. [PERSON] et al. employed an IR laser for desorption and another IR laser with a different wavelength for REMPI [43]. Single-photon (SP) ionization using vacuum ultraviolet (VUV) irradiation provides an alternative soft ionization method for mass spectrometry [44; 45; 46; 47]. SP ionization induces little or no fragmentation, unlike REMPI. Resonance excitation through an intermediate state, which is essential in REMPI and strongly dependent on the target molecule, is not required in SP ionization. Therefore, VUV SP ionization is a universal ionization method. A molecular fluorine excimer laser that emits approximately pulses of 10-ns pulse width at 157 nm (7.87 eV) is an intensive, convenient VUV source. SP postionization with VUV radiation from a molecular fluorine laser was used to detect antibiotics within intact bacterial colony biofilms [46]. However, most molecules with ionization energy (IE) in the range of 8-10 eV cannot be ionized by photons from a molecular fluorine laser. Chemical derivatization with low-IE tags, such as anthracene, fluorescein, and tryptophan, is required to detect peptides using a 7.87-eV photon source [44; 48; 49].