Relating chromatographic retention and electrophoretic mobility to the ion distribution within electrosprayed droplets |
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Authors: | C Fredrik Bökman Dan Bylund Karin E Markides Per J R Sjöberg |
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Institution: | Clinical Chemistry Laboratory, Falun Central Hospital, Falun, Sweden. |
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Abstract: | Ions that are observed in a mass spectrum obtained with electrospray mass spectrometry can be assumed to originate preferentially
from ions that have a high distribution to the surface of the charged droplets. In this study, a relation between chromatographic
retention and electrophoretic mobility to the ion distribution (derived from measured signal intensities in mass spectra and
electrospray current) within electrosprayed droplets for a series of tetraalkylammonium ions, ranging from tetramethyl to
tetrapentyl, is presented. Chromatographic retention in a reversed-phase system was taken as a measure of the analyte’s surface
activity, which was found to have a large influence on the ion distribution within electrosprayed droplets. In addition, different
transport mechanisms such as electrophoretic migration and diffusion can influence the surface partitioning coefficient. The
viscosity of the solvent system is affected by the methanol content and will influence both diffusion and ion mobility. However,
as diffusion and ion mobility are proportional to each other, we have, in this study, chosen to focus on the ion mobility
parameter. It was found that the influence of ion mobility relative to surface activity on the droplet surface partitioning
of analyte ions decreases with increasing methanol content. This effect is most probably coupled to the decrease in droplet
size caused by the decreased surface tension at increasing methanol content. The same observation was made upon increasing
the ionic strength of the solvent system, which is also known to give rise to a decreased initial droplet size. The observed
effect of ionic strength on the droplet surface partitioning of analyte ions could also be explained by the fact that at higher
ionic strength, a larger number of ions are initially closer to the droplet surface and, thus, the contribution of ionic transport
from the bulk liquid to the liquid/air surface interface (jet and droplet surface), attributable to migration or diffusion
will decrease. |
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