Quantitative bioanalysis utilizing high-performance liquid chromatography/electrospray mass spectrometry via selected-ion monitoring of the sodium ion adduct [M+Na]+.

A high-performance liquid chromatography (HPLC)/electrospray mass spectrometric method for quantitative determination of a compound in dog plasma was developed and validated via the selected-ion monitoring of the electrospray-generated [M+Na]+ adduct of the compound. The plasma samples were acidified with HCl and then extracted with methyl tert-butyl ether. The reconstituted extracts were injected into an HPLC/positive-ion electrospray ionization mass spectrometry system. The HPLC mobile phase consisted of acetonitrile, water, formic acid (3 mM) and sodium acetate (0.3 mM). This composition of mobile phase provided the optimum electrospray condition for the formation of the [M+Na](+)-ion. This work demonstrates that the addition of sodium acetate into the HPLC mobile phase and the subsequent selected-ion monitoring of the sodium ion adduct of the analyte is a viable approach in quantitative bioanalysis. The facile formation of the sodium ion adduct of the analyte, which does not contain functional groups that are known to be strong proton acceptors, appears to be a function of the particular electrospray instrument used.