Department of Chemistry and Applied Biosciences, ETH Zurich, Zurich, Switzerland.
J Am Soc Mass Spectrom. 2010 Mar;21(3):378-85. doi: 10.1016/j.jasms.2009.10.022. Epub 2009 Nov 6.
Solvent polarity plays an important role in electrospray ionization-mass spectrometry (ESI-MS), one of the most widely used analytical methods for biochemistry. To have a comprehensive understanding of how solvent polarity affects ESI-MS measurements, we systematically investigated the polarity change in the ESI plume formed from an ethanol solution using laser-induced fluorescence (LIF) spectroscopy. Two solvatochromic dyes (i.e., dyes whose fluorescence emission is sensitive to solvent polarity), Nile red and DCM (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), were used as probes. The peak emission wavelengths of these two dyes exhibited significant red shifts (8-12 nm) when the measuring spot was moved away from the spray tip and in radial direction in the plume, indicating a dramatic polarity change during shrinking of the droplets. The emission intensities were also measured with a polarity-insensitive dye as a reference. The results are consistent with the peak wavelength measurements. Two key mechanisms responsible for the change of solvent polarity in the plume were considered, water entrainment from the surrounding air and solvent evaporation. Furthermore, quantitative analysis of the solvent polarity change was performed by using the Lippert-Mataga polarity parameter Deltaf. The value of Deltaf reached 0.305-0.307 at the periphery of the ESI plume, which means that the solvent polarity in the smaller droplet is close to that of a mixture of 30% water and 70% ethanol (Deltaf = 0.307), even though the bulk solvent was ethanol containing less than 1% water as an impurity.
溶剂极性在电喷雾电离质谱（ESI-MS）中起着重要作用，ESI-MS 是生物化学中最广泛使用的分析方法之一。为了全面了解溶剂极性如何影响 ESI-MS 测量，我们使用激光诱导荧光（LIF）光谱系统地研究了乙醇溶液形成的 ESI 羽流中极性的变化。两种溶致变色染料（即其荧光发射对溶剂极性敏感的染料），尼罗红和 DCM（4-二氰基-2-甲基-6-p-二甲基氨基苯乙烯-4H-吡喃），被用作探针。当测量点从喷雾尖端移开并在羽流中径向移动时，这两种染料的峰值发射波长发生了显著的红移（8-12nm），表明在液滴收缩过程中极性发生了剧烈变化。还使用一种非极性敏感染料作为参考测量了发射强度。结果与峰波长测量一致。考虑了两种导致羽流中溶剂极性变化的关键机制，即从周围空气中夹带的水和溶剂蒸发。此外，通过使用 Lippert-Mataga 极性参数 Deltaf 对溶剂极性变化进行了定量分析。Deltaf 的值在 ESI 羽流的外围达到 0.305-0.307，这意味着较小液滴中的溶剂极性接近于水和 70%乙醇的混合物（Deltaf = 0.307），尽管主体溶剂是含有不到 1%水的乙醇，杂质含量为 1%。
