Desorption electrospray/metastable-induced ionization: a flexible multimode ambient ion generation technique.

Presented here is a novel multimode ambient ion source termed desorption electrospray/metastable-induced ionization (DEMI), which integrates the benefits and circumvents some of the limitations of desorption electrospray ionization (DESI, polarity range limited) and direct analysis in real time (DART)-type metastable-induced chemical ionization (MICI, molecular weight limited). This ion source allows three unique operation modes, each with unique capabilities, including spray (DESI-like)-only, MICI-only, and DEMI (multimode), and can be thus operated in each of these modes allowing the detection of a wider range of analytes of interest. Ion source operation in the MICI-only mode is particularly well suited for the analysis of low-polarity, low-molecular weight compounds in powdered, solid, or dissolved samples. Operation of the ion source in spray-only mode shows superior performance for the analysis of high-molecular weight, high-polarity compounds over the MICI-only mode. Heating the nebulizer gas in spray-only mode allows improved analyte solubility in the spray solvent, enabling up to an order of magnitude improvement in sensitivity. Perhaps the most appealing mode of operation of the ion source is the DEMI mode which allows the simultaneous detection of compounds within a much broader range of polarities and molecular weights than each of the individual modes. For drug quality screening and counterfeit detection applications, operation in the DEMI mode results in the generation of both protonated and sodiated analytes. The observation of such complementary ionic species facilitates compound identification when investigating unknowns.