The effect of liquid chromatography eluents and additives on the positive ion responses of cocaine, benzoylecgonine, and ecgonine methyl ester using electrospray ionization

Abstract

We investigated the effect of common chromatography eluents and additives on the positive ion responses of ecgonine methyl ester (EME), benzoylecgonine (BZE), and cocaine (COC) using electrospray ionization (ESI). Primarily [M+H] + ions were observed, although decomposition of EME and COC to ecgonidine methyl ester gave a sizable peak at m/ z 182.0. The results showed that the sensitivity for the test analytes was greatest in a mobile-phase consisting of a 1:1 mixture of 60% acetonitrile/40% acetone:100 mM ammonium acetate. There was no evidence of a correlation between sensitivity of [M+H] + ions and solution pH. Adducts derived from addition of ammonium salts and ammonium hydroxide, along with cluster ions were not observed, although cationization did occur for BZE (<1.0–23%). Signal intensities for COC (p K a=8.61) obtained under acidic conditions (pH=2.55–2.80) and basic conditions (pH=9.19–10.02) did not vary, suggesting that mechanisms other than in-solution ionization maybe key in formation of ions by the electrospray process.