Non-aqueous spray solvents and solubility effects in desorption electrospray ionization

The use of non-aqueous solvents in desorption electrospray ionization mass spectrometry (DESI-MS) is explored by analyzing a set of 43 compounds using binary mixtures of chloroform, tetrahydrofuran, and acetonitrile as the spray solvent. Comparisons of data obtained from chloroform/tetrahydrofuran (1:1) and chloroform/acetonitrile (1:1) spray solvents with the standard aqueous-based spray solvent (methanol/water, 1:1) shows that the non-aqueous systems have practical value for DESI, especially in the analysis of hydrophobic compounds. Non-aqueous spray solvents were used to ionize thermometer molecules (benzyl pyridinium salts) and showed lower internal energies (softer DESI ionization compared with methanol/water, 1:1), a result that has parallels in known solvent effects in electrospray ionization and is explained by solvent effects on surface tension. Consideration of octanol/water partition coefficients (K(ow)) of the 43 analytes in the light of their DESI results reveals the importance of the solubility of analyte in the spray solvent in producing high quality mass spectra. This finding provides additional support for the droplet pick-up description of the DESI mechanism, which is based on analyte dissolution in the spray solvent, followed by splashing of subsequently arriving droplets in the liquid film to form microdroplets of dissolved analyte. DESI solvent optimization can be improved by the use of K(ow) of the analyte as an indication of the polarity of the most appropriate solvent system.