Understanding the Response of the Molecule to Electrospray Ionization(ESI) by Selected Reaction Monitoring (SRM) Tandem Mass Spectrometry:Defining Molecular ESI Index (MESII)

In an attempt to understand the response of the molecule to electrospray ionization (ESI), a new parameter called ‘molecular electrospray ionization index (MESII)’ is defined. Selected reaction monitoring (SRM) data acquired on an ESI tandem quadrupole mass spectrometer is utilized for calculating MESII, which we denote as eSRM: eSRM= - log (SRM intensity/Population of ions or molecules in-solution). Population of ions or molecules in-solution is estimated using molecular mass of the respective compound and Avogadro’s constant. Simvastatin acid (SVA), lovastatin (LV) and simvastatin (SV) are chosen as model compounds. SRM experiments in positive ion mode were performed on singly protonated ions ([M+H]+) of SVA, LV and SV. In negative ion mode, only SVA was investigated by SRM, using singly deprotonated ion ([M-H]-) as precursor ion. Thus estimated MESII values in positive ion mode are: e+ SRM (SVA)=7.4288, e+ SRM (LV)=7.4541 and e+ SRM (SV)=8.6833 and in negative ion mode, e- SRM (SVA)=7.2253. This newly defined index not only gives an idea about ionization potential (i.e., degree of ionization), but can also be an indicator of limit of detection (LOD) of an analyte. When utilizing SRM data recorded from different type of instruments for a compound, the variations that may arise in MESII values could help in understanding the influence of different instrument configurations/methods on the degree of ionization of that compound. Matrix effects on the extent of analyte’s ionization too can be understood from the differences in the MESII values. Further, it may be possible to utilize MESII for quantitation as well.