Study of the thermal stability of compounds commonly used as UV-MALDI- TOF-MS matrices

O. I. TARZI; R. ERRA-BALSELLS; H. NONAMI,; ERRA BALSELLS, ROSA

Hiroshima, Japón.

Congreso; 55th Annual Conference on Mass Spectrometry Hiroshima,; 2007

Hiroshima, Japón.

Study of the thermal stability of compounds commonly used                       as UV-MALDI-TOF-MS matrices (1CIHIDECAR-CONICET, Departamento de Quimica Organica, Facultad de Ciencias Exactas, Universidad de Buenos Aires, Argentina · 2College of Agriculture, Ehime University, Japan) Olga I. Tarzi 1, Rosa Erra-Balsells 1, Hiroshi Nonami 2Short Abstract: The non-emissive deactivation of gentisic acid (GA), 2,4,6-trihydroxyacetophenone (THA), nor-harmane (nHo), harmane (Ho) and a-cyano-4-hydroxycinnamic acid (CHC) was analyzed by laser induced optoacoustic spectroscopy, LIOAS. An efficient heat release was observed in all cases except for CHC, where a very small and unstable acoustic signal was obtained.  Due to the differences observed and the lack of studies in literature regarding the thermal stability of UV-MALDI matrices, we studied their chemical stability when heated to their melting point.  Keywords: UV-MALDI-TOF-MS, matrices, LIOAS, ESI-TOF-MS.             After studying the photochemical stability of gentisic acid (GA), 2,4,6-trihydroxyacetophenone (THA), nor-harmane (nHo), harmane (Ho) and a-cyano-4-hydroxycinnamic acid (CHC), we decided to investigate their radiationless deactivation from the electronic excited state by using laser induced optoacoustic spectroscopy, LIOAS. GA and THA behaved as a calorimetric reference, releasing the energy absorbed as prompt heat in a time shorter than 200 ns (a= 0.96 and 1, respectively; calorimetric reference: 2-hydroxybenzophenone (2-HBP), a=1), generate efficiently a triplet state without forming singlet oxygen (Figure 1). For nHo and Ho, the heat release process is less efficient as there also exist radiative deactivation. Comparing their behavior when bubbling N2 to the one of the reference, there is a species that stores energy, while in O2 atmosphere, an additional mechanism for thermal deactivation is operating together with the formation of singlet oxygen to some extent. This species was characterized during the experiments monitoring its emission at 1269 nm. On the other hand, CHC showed almost no heat release compared to the reference (Figure 2). Moreover, an hypsochromic shift in the absorption spectra was observed after the experiment. Figure 1. Photoacoustic signal of GA and 2-HBP in MeCN (298 K; lexc = 355 nm.) Figure 2. Photoacoustic signal of CHC in MeCN (298 K; lexc = 355 nm.)               We also performed thermal stability studies, heating the matrices in the solid state to their melting point. The experiments were monitored by thin layer chromatography (tlc), electronic spectroscopy (UV-vis absorption and fluorescence), NMR-1H, EI-MS, UV-LDI and ESI-TOF-MS, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC).             GA, THA, nHo and Ho were stable throughout the experiments, while CHC showed an hypsochromic shift in the absorption spectra (Figure 3) and chemical changes in the NMR-1H  (Figures 4.a and b). These changes were also observed in the UV-LDI and ESI-TOF-MS of the melted CHC sample. The results obtained could explain the lack of shot-to-shot and sample-to-sample reproducibility, formation of clusters and vanishing of the analyte signals after several laser shots when CHC is used in UV-LDI-MS and as a matrix in UV-MALDI-MS. Figure 3. UV-vis absorption spectra of commercial CHC and melted sample. Figure 4.a  NMR-1H (500 MHz) of commercial CHC (solvent: CD3OD). Figure 4.a  NMR-1H (200 MHz) of melted CHC (solvent: CD3OD).