Comparison of atmospheric pressure photoionization and atmospheric pressure chemical ionization for normal-phase LC/MS chiral analysis of pharmaceuticals

In this work, we compared APPI and APCI for normal-phase LC/MS chiral analysis of five pharmaceuticals. Performance was compared both by FIA and by on-column analysis using a ChiralPak AD-H column under optimized conditions. By comparison, APPI generated more reproducible signals and was less susceptible to ion suppression than APCI. APPI generated higher peak area and lower baseline noise, and therefore much higher S/N ratios. APPI sensitivity (i.e., S/N ratio) was approximately 2-130 times higher than APCI by FIA and was approximately 2.6-530 times higher than APCI by on-column analysis depending on specific compounds. The better APPI sensitivity as compared to APCI was more dramatic by on-column analysis than by FIA. APCI sensitivity was degraded by ion suppression caused by LC column bleeding components and by elevated APCI baseline noise relative to APPI. On-column APPI LODs (at S/N = 3) were 83, 16, 17, 95, and 7 pg for enantiomer #1, and 104, 23, 19, 122, and 17 pg for enantiomer #2 for benzoin, naringenin, mianserin, mephenesin, and diperodon, respectively, on a Waters ZQ. APPI offers no concern of explosion hazard relative to APCI corona needle discharge or ESI high voltage discharge when flammable solvents (e.g., hexane) are used as mobile phases. Whether APPI dopants are required depends on the IP(s) of mobile-phase solvent(s) and solvent complexes, and photon energies of VUV lamps. Dopant was not necessary for hexane-based mobile phases due to their self-doping effects. Dopants did enhance Kr lamp APPI sensitivity when MeOH was used as the mobile phase. However, dopants became unnecessary for the MeOH mobile phase when the Ar lamp was used.