## 3 Results and Discussion  
Figure 1a shows the laser desorption ionization (LDI) mass spectrum of Sal (sample only) in the negative ion region. The spectrum was obtained without use of any matrix. As Sal has an absorption coefficient at 337 nm, it was ionized to yield a deprotonated species, [Sal \(-\) H]\({}^{-}\), at \(m/z\) = 137 even in the absence of a matrix. The LDI mass spectrum of Sal in the positive ion region is shown in Figure 1b. Although the S/N ratio was very low, peaks of radical cation species [Sal]\({}^{+}\) and protonated Sal ([Sal \(+\) H]\({}^{+}\)) were observed at \(m/z\) = 138 and 139. [Sal \(+\) H]\({}^{+}\) and [Sal \(-\) H]\({}^{-}\) were generated as a result of the dissociation of two Sal molecules in Sal crystals after the photoexcitation. As Sal is an acidic molecule, it could be easily deduced that proton adduction would be difficult and the peak intensity of [Sal \(+\) H]\({}^{+}\) would be low. For the generation of the radical cation species, [Sal]\({}^{+}\), the following reaction would be expected. \[2\mathrm{Sal}+2\mathrm{H}^{+}\to 2\mathrm{Sal}\cdot+2\mathrm{H}\cdot\to 2 \mathrm{Sal}\cdot+\mathrm{H}_{2}\uparrow \tag{1}\]  
Protons generated during the production of [Sal\(-\)H]\({}^{-}\) species would attack Sal to remove an electron. Hydrogen radicals would terminate each other in the gas phase and a hydrogen molecule would be released. In addition, a reaction pathway using the hemolysis reaction would be also expected. \[2[\mathrm{Sal+H}]^{+}+h\
u(heat)\to 2\mathrm{Sal}\cdot+2\mathrm{H}\cdot\to 2 \mathrm{Sal}\cdot+\mathrm{H}_{2}\uparrow \tag{2}\]  
A proton-adducted species of Sal would be first generated by the adduction of a proton to one of the oxygen atoms in Sal, which would dissociate into two radical cation species using photon or heat energy. Figure 1c shows the MALDI mass spectrum of Sal measured using CdTe nanoparticles (0.5 mg/mL) as the matrix. The peak intensity of [Sal\(-\)H]\({}^{-}\) increased; it was almost 2.9-fold higher than that in Figure 1a. In the positive ion region shown in Figure 1d, on the other hand, the peak intensities of [Sal]\({}^{+}\) and [Sal + H]\({}^{+}\) were almost equal to those in Figure 1b. This result clearly indicated that CdTe promoted the deprotonation of Sal. After the photoexcitation of CdTe, the following reaction would be expected. \[2\mathrm{RCOOH}+2e^{-}\to 2\mathrm{RCOO}^{-}+\mathrm{H}_{2}\uparrow \tag{3}\]  
Here, R represents the other groups of Sal. Electrons ejected from CdTe nanoparticles would react with the carboxyl groups of Sal and [Sal\(-\)H]\({}^{-}\) would be generated. For other compounds containing carboxyl groups, MALDI MS was carried out using CdTe nanoparticles (0.5 mg/mL) as the matrix, and the obtained spectra in the negative ion mode are shown in Figure 2. The peak intensity of Glu ([Glu\(-\)H]\({}^{-}\); \(m/z\) = 193.2) shown in Figure 2a was increased compared with that obtained without CdTe (Figure 2b). In addition, the peak intensity of  
Figure 1: Laser desorption ionization mass spectrum of salicylic acid (Sal) in the (**a**) negative and (**b**) positive ion regions. Matrix-assisted laser desorption ionization mass spectrum of Sal with CdTe nanoparticles as the matrix (0.5 mg/mL) in the (**c**) negative and (**d**) positive ion regions. Ibu ([Ibu\(-\)H]\({}^{-}\); \(m/z\) = 205.3) was slightly increased by using CdTe, as shown in Figure 2c,d. However, that of Tyr ([Tyr\(-\)H]\({}^{-}\); \(m/z\) = 180.2) almost disappeared when the CdTe matrix was used (Figure 2e,f). The pKa values were known as 2.9 for Sal, 3.2 for Glu, 4.5 for Ibu, and 2.2 for Tyr. As Tyr has the strongest acidity, the structure of the conjugate base must be the most stable among those four compounds. However, Tyr is an amino acid; it has an amino group as well as a carboxyl group. When a carboxylate anion was produced in a Tyr molecule according to Equation (3), it was expected that the amino group would catch a proton or an alkali metal cation to keep the molecular charge neutral.