| Mass spectrometric analyses were compared for matrix-assisted laser desorption ionization (MALDI) and matrix-free, soft laser desorption ionization (soft LDI), with ultraviolet (UV) and infrared (IR) lasers, various matrices and target materials, using standard analytes and biological tissues. Analyses were performed using a linear time-of-flight mass spectrometer with an Nd:YAG laser, and a commercial reflectron time-of-flight mass spectrometer with both a 2.94 mum Er:YAG laser and a 337 nm nitrogen laser. Soft infrared LDI experiments using silicon targets produced mass spectra for analytes up to 18 kDa, no fragmentation, and sample quantities in the low femtomole range. The detection limit was similar to that for UV-MALDI (∼30 femtomole). As with ultraviolet desorption ionization from porous silicon (DIOS), IR soft laser desorption from silicon and other targets does not produce matrix ions that can interfere with analysis of low-mass analytes. However, in contrast to UV DIOS, surface porosity is not required for ionization using an IR laser. Of the target materials tested using bradykinin as an analyte, silicon and aluminum surfaces gave the best performance in terms of signal level and low mass interference. Targets transparent to the IR wavelength, such as germanium, zinc, alkali halides, and glass, also produced mass spectra with little fragmentation. UV and IR-MALDI mass spectra were compared for tissues from the sea slug, Aplysia californica, and whole bacterial cells. IR-MALDI analyses used succinic acid as a matrix, and UV-MALDI analyses used alpha-cyano-4-hydroxy cinnamic acid (CHCA). Although UV-MALDI analyses consistently produced more peaks than IR-MALDI, the choice of matrix, solvent conditions, and amino acid composition of the sample, all influenced the relative effectiveness of UV and IR-MALDI. Salt adducts of peptide peaks present in UV-MALDI spectra were absent in IR-MALDI; hence, IR-MALDI may be a better method for biological samples with high salt content. UV-MALDI, IR-MALDI, and soft infrared LDI mass spectra for bacteria showed distinctively different protein and peptide profiles, which could be used to identify different strains and species. Overall, UV-MALDI, IR-MALDI, and soft infrared LDI methods provided complementary information that will be useful in future studies of biomolecules and tissue. |
