Matrix-assisted laser desorption and ionization in the OH and CO absorption bands of aliphatic and aromatic matrices: dependence on laser wavelength and temporal beam profile

Abstract

A tunable free-electron laser (FEL) was used to initiate infrared (IR) matrix-assisted laser desorption and ionization (MALDI) of small proteins in aliphatic and aromatic matrices. The laser wavelength was scanned from 2.65 to 4.2 μm and from 5.5 to 6.5 μm, covering the absorption bands of the OH and CO stretching vibrations found in such commonly used IR matrices as succinic, fumaric and nicotinic acids. The temporal profile of the laser pulse was also varied using a broadband electro-optic switch (Pockels cell) to study the effects of fluence and irradiance. Although there are absorption peaks at 3.3 μm for succinic acid and fumaric acid, and at 4.1 μm for nicotinic acid, the lowest threshold-fluence for IR MALDI in this region was around 2.94 μm for all matrices. Moreover, the threshold-fluence increased with increasing absorption up to a value five times that of the 2.94 μm value. This result raises questions about the relative contributions of the different sample constitutents to the absorption and the role of resonant absorption in IR MALDI. The threshold-fluences are typically one order of magnitude higher than those for ultraviolet (UV) MALDI, while extinction coefficients of the IR matrices are 100–1000 times smaller than for UV matrices. Therefore, the absorbed energies per unit volume at the MALDI threshold are 10–100 times smaller than in UV MALDI. All these facts clearly indicate that a different desorption/ionization process must be operative in IR MALDI. Variations in temporal profile of the FEL pulse also revealed that ion desorption depends on laser irradiance rather than laser fluence, a result which cannot be explained simply by energy loss due to heat conduction. Two possible models for IR desorption are suggested based on these observations.