Infrared laser desorption and ionization of polypeptides from a polyacrylamide gel

We observed direct desorption and ionization of angiotensin II and bovine insulin from a frozen polyacrylamide gel without the addition of an exogenous matrix, using picosecond pulses from a tunable, mid-infrared free-electron laser tuned to strong absorption bands of the gel. At 5.7, 5.9, 6.1 and 6.3 microm we were able to desorb and ionize both analyte molecules, with the strongest analyte signal generated at 5.9 microm. However, no analyte signal was observed at 5.5 microm. Consistent with a previous report, we did not observe ions of either polypeptide at 2.9 microm, in spite of strong overall absorption. We discuss the implications of this wavelength-dependent ionization, including possible ablation mechanisms and energy partitioning between competing vibrational modes.