CHEMICAL IONIZATION WITH GASEOUS AMMONIA FOR NORMAL-PHASE LIQUID-CHROMATOGRAPHIC THERMOSPRAY MASS-SPECTROMETRIC APPLICATIONS

Addition of gaseous ammonia to a thermospray ion source, followed by filament-on ionization, provides chemical ionization reagent ions. The abundance of reagent ions is sufficient to obtain spectra which contain predominantly [M + NH4]+ ions. The advantage of this method of ionization lies in normal-phase LC MS applications, where ''traditional'' ionization methods such as volatile buffer, filament-on or discharge-on ionization yields either no ionization or too much fragmentation, making identification very difficult or impossible, After modification of a Finnigan TSQ 70 ion source. the source pressure and effluent flow were optimized. The formation of [M + NH4]+ ions is dependent on the voltage applied to the repeller electrode. Low voltages yield spectra with predominantly [M + NH4]+ ions, where as high voltages yield spectra showing mostly fragmentation and a small percentage of chemical ionization. The formation of the [M + NH4]+ ion of cyclohexanone is linear over two decades ranging from 0.2 to 20 ng of the compound injected into a normal-phase column. Higher concentrations give rise to a deviation of the linear relationship. Quantitative results are obtained, with a repeatability of 8% (R.S.D.).