The effect of temperature on the stability of compounds used as UV-MALDI-MS matrix: 2,5-dihydroxybenzoic acid, 2,4,6-trihydroxyacetophenone, α-cyano-4-hydroxycinnamic acid, 3,5-dimethoxy-4-hydroxycinnamic acid, nor-harmane and harmane

The thermal stability of several commonly used crystalline matrix-assisted ultraviolet laser desorption/ionization mass spectrometry (UV-MALDI-MS) matrices, 2,5-dihydroxybenzoic acid (gentisic acid; GA), 2,4,6-trihydroxyacetophenone (THA), alpha-cyano-4-hydroxycinnamic acid (CHIC), 3,5-dimethoxy-4-hydroxycinnamic acid (sinapinic acid; SA), 9H-pirido[3,4-b]indole (nor-harmane; nor-Ho), 1-methyl-9H-pirido[3,4-b]indole (harmane; Ho), perchlorate of nor-harmanonium ([nor-Ho + H](+)) and perchlorate of harmanonium ([Ho + H](+)) was studied by heating them at their melting point and characterizing the remaining material by using different MS techniques [electron ionization mass spectrometry (EI-MS), ultraviolet laserdesorption/ionization-time-of-flight-mass spectrometry (UV-LDI-TOF-MS) and electrospray ionization-time-of-flight-mass spectrometry (ESI-TOF-MS)] as well as by thin layer chromatography analysis (TLC), electronic spectroscopy (UV-absorption, fluorescence emission and excitation spectroscopy) and H-1 nuclear magnetic resonance spectroscopy (H-1-NMR). In general, all compounds, except for CHIC and SA, remained unchanged after fusion. CHIC showed loss Of CO2, yielding the trans-/cis-4-hydroxyphenylacrilonitrile mixture. This mixture was unambiguously characterized by MS and H-1-NMR spectroscopy, and its sublimation capability was demonstrated. These results explain the well-known cluster formation, fading (vanishing) and further recovering of CHIC when used as a matrix in UV-MALDI-MS. Commercial SA (SA 98%; trans-SA/cis-SA 5: 1) showed mainly cis- to-trans thermal isomerization and, with very poor yield, loss Of CO2, yielding (3',5'-dimethoxy-4'-hydroxyphenyl)-1-ethene as the decarboxilated product. These thermal conversions would not drastically affect its behavior as a UV-MALDI matrix as happens in the case of CHIC. Complementary studies of the photochemical stability of these matrices in solid state were also conducted. Copyright (C) 2008 John Wiley & Sons, Ltd.