Internal energy of thermometer ions formed by femtosecond laser desorption: Implications for mass spectrometric imaging

Abstract

Mass spectrometry (MS) imaging of biological samples would greatly benefit from improved lateral resolution and depth profiling which may be possible with femtosecond (fs), near-IR laser desorption microprobes to form ions directly in laser desorption ionization (fs-LDI) or via vacuum ultraviolet postionization (fs-LDPI). The use of fs-LDI-MS or fs-LDPI-MS for imaging, however, requires a determination of the magnitude of internal energy imparted by the ultrashort desorption pulse because the amount of internal energy determines the extent of ion fragmentation, and extensive fragmentation degrades MS imaging by complicating the mass spectra. This paper estimates internal energies imparted to 4-chlorobenzylpyridinium (CBP) thermometer ions desorbed from both simulated bacterial biofilms and bovine eye lens tissue samples by ∼75 fs, 800 nm laser pulses. Both direct ions and photoions formed by 10.5 eV single photon ionization of desorbed neutrals are analyzed by time-of-flight MS. Survival yields (SYs) of CBP varied from 0.2 to 0.8 and depended upon desorption laser fluence, overlap between desorption laser pulses, and whether direct ions or photoions were detected. SYs for photoions additionally depended on time delay between desorption and photoionization laser pulses, with the highest SYs seen at longer delay times. CBP internal energies were estimated using previously published computational results and compared with those from several other common MS imaging ion sources. The results are discussed in terms of their implications for MS imaging by fs-LDI and fs-LDPI.