Analysis of ergosteroids: VIII: Enhancement of signal response of neutral steroidal compounds in liquid chromatographic–electrospray ionization mass spectrometric analysis by mobile phase additives

The signal response of moderately polar to nonpolar neutral steroidal compounds in positive ion mode was significantly improved in electrospray ionization mode by addition of volatile organic acids (trifluoroacetic acid, acetic and formic) at concentrations much lower than those normally employed for high-performance liquid chromatographic separations of ionic compounds. Each of the three acids enhanced the sensitivity, the order being: formic acid (∼50–200 ppm, v/v)>acetic acid (100–500 ppm)>trifluoroacetic acid (5–20 ppm). Higher concentrations caused decrease in the sensitivity. The extent of increase in the sensitivity was compound specific and also depended on the nature of organic modifier present in the mobile phase. Acetic acid was the acid of choice for the ‘wrong-way-round’ ionization of sulfate conjugates. The postcolumn addition of silver nitrate produced highly stable (M+Ag)+ adducts with concomitant increase in signal response and reduction in baseline noise.