The effects of heating rate and sample size on the direct exposure/chemical ionization mass spectra of some biological conjugates

A direct exposure probe has been used to obtain mass spectra of underivatized guanosine, deoxyguanosine, sucrose and the p‐nitrophenyl‐β‐D‐glucronide. In all cases, a protonated molecular ion is produced with good relative abundance. The effects of heating rate and sample size on the production of the [MH]⁺ ion are examined in detail from total ion and single ion currents produced during rapid, repetitive scanning of the spectra after probe insertion. From this data we conclude that protonated molecular ions are produced as a result of the enhanced volatility of neutral molecules on the probe surface, followed by chemical ionization, and not by surface ionization.