The mechanism of charge separation and droplet breakup during electrospray has been investigated using both biological and nonbiological microparticles as probes to preserve and record the charge state of droplets in the micron size range. Charged droplets and/or microparticles were observed using two-stage image charge detection after entering a differentially pumped, atmospheric pressure inlet. Microparticle probes allowed observation of not only droplets that did not fully evaporate but also product droplets small enough to fully evaporate, as well as the ability to distinguish between these two types of droplets. In both positive- and negative-mode electrospray ionization of aqueous suspensions, we observed roughly 20% of particles carrying charges opposite to the biased voltage on the capillary. Both positive and negative modes yielded distributions of charge states with maxima in the expected polarity but with tails extending into the opposite polarity of charge. Both polarities produced similar fractions of particles with opposite charge, with no preference for either polarity. This result is consistent with electric-field-induced charge separation and droplet breakup within the high-field region between the capillary and the counter electrode. These results suggest that electric-field-induced charge separation may be the dominant mechanism, at least in the micron size range and under the present experimental conditions, in which primary charged droplets of either polarity split into smaller progeny with a range of charges spanning both polarities. Implications for electrospray ionization mass spectrometry are discussed.

中文翻译：

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使用微粒记录液滴电荷状态的电喷雾电离中电荷分离的机理研究。

已经使用生物和非生物微粒作为探针在电喷雾过程中电荷分离和液滴破裂的机理进行了研究，以保留和记录微米尺寸范围内液滴的电荷状态。进入差压泵送的大气压入口后，使用两阶段图像电荷检测观察带电的液滴和/或微粒。微粒探针不仅可以观察到没有完全蒸发的液滴，还可以观察到足够小的液滴以完全蒸发的产物，并可以区分这两种类型的液滴。在水悬浮液的正模式和负模式电喷雾电离中，我们观察到大约20％的粒子带有与毛细管上的偏压相反的电荷。正模式和负模式都产生电荷状态分布，其预期极性最大，但尾部延伸到相反的电荷极性。两种极性均产生相似部分的带相反电荷的颗粒，而没有任何一种极性可言。该结果与毛细管和对电极之间的高电场区域内的电场感应电荷分离和液滴破裂是一致的。这些结果表明，电场诱导的电荷分离可能是主要的机制，至少在微米尺寸范围内，并且在当前的实验条件下，其中任一极性的初级带电液滴分裂为较小的后代，且电荷范围跨越了两者极性。讨论了对电喷雾电离质谱的影响。