Negative electrospray ionization mode in mass spectrometry: A new perspective via modeling

• We model the signal response in negative ESI-MS.
• The effect of physiochemical properties on the signal response is examined.
• The effect of the structural features is also explored.
• The signal response is studied in flow and injection analysis.

Electrospray ionization technique is used for production of gas phase ions without fragmentation and is considered as one of the most sensitive analytical methods for structural characterization of molecules. Nonetheless, the determination of some parameters (physicochemical properties or structural features) that may enhance the signal response especially in the negative ion mode has not yet been clarified. The present work is an attempt to correlate the signal response behavior of 110 compounds used as probes, with their characteristics (molecular descriptors, X variables). In order to quantify this phenomenon, Partial Least Squares which is a software capable of performing linear multivariate analysis was applied. The models derived explore the positive or negative effect of 49 X variables on the signal response of each analyte, expressed as Y variable. The process of gas phase ions formation was verified by both flow injection and column analysis. The models derived are proven reliable for the study of such mechanisms, with small number of components and good linearity (R2 > 83%, Q2 > 70%). The present study showed that parameters as pKa, ionization percentage of the analyte, PSA, HBA, COOH, water solubility and surface tension of a solid are affecting ion formation. At the same time, slight differentiations of the influence of certain parameters were observed on column injection analysis due to the chromatographic delay of some analytes.