Zhou S, Cook KD
Department of Chemistry, University of Tennessee, Knoxville 37996-1600, USA.
J Am Soc Mass Spectrom. 2000 Nov;11(11):961-6. doi: 10.1016/S1044-0305(00)00174-4.
The term "wrong-way-round ionization" has been used in studies of electrospray ionization to describe the observation of protonated or deprotonated ions when sampling strongly basic or acidic solutions (respectively) where such ions are not expected to exist in appreciable concentrations in solution. Study of the dependence of ionization of the weak base caffeine on the electrospray capillary potential reveals three distinct contributors to wrong-way-round ionization. At near-neutral pH in solutions of low ionic strength, protonation of caffeine results from the surface enrichment of electrolytically produced protons in the surface layer of the droplets from which ions are desorbed. For solutions made strongly basic with ammonia, gas-phase proton transfer from ammonium ions can create protonated caffeine. These two mechanisms have been discussed previously elsewhere. For solutions of high ionic strength at neutral or high pH, the data suggest that discharge-induced ionization is responsible for the production of protonated caffeine. This mechanism probably accounts for some of the wrong-way-round ionization reported elsewhere.
“反向电离”一词在电喷雾电离研究中被用于描述在分别采样强碱性或酸性溶液时观察到质子化或去质子化离子的情况，而在这种溶液中预计这些离子不会以可观的浓度存在。对弱碱咖啡因的电离对电喷雾毛细管电势的依赖性研究揭示了反向电离的三个不同成因。在低离子强度溶液的近中性pH条件下，咖啡因的质子化源于从其解吸离子的液滴表面层中电解产生的质子的表面富集。对于用氨调至强碱性的溶液，气相中铵离子的质子转移可产生质子化咖啡因。这两种机制先前已在其他地方进行过讨论。对于中性或高pH的高离子强度溶液，数据表明放电诱导电离是产生质子化咖啡因的原因。这种机制可能解释了其他地方报道的一些反向电离现象。
