Charge competition and the linear dynamic range of detection in electrospray ionization mass spectrometry
An experimental investigation and theoretical analysis are reported on charge competition in
electrospray ionization (ESI) and its effects on the linear dynamic range of ESI mass
spectrometric (MS) measurements. The experiments confirmed the expected increase of MS
sensitivities as the ESI flow rate decreases. However, different compounds show somewhat
different mass spectral peak intensities even at the lowest flow rates, at the same concentration
and electrospray operating conditions. MS response for each compound solution shows good
linearity at lower concentrations and levels off at high concentration, consistent with analyte
“saturation” in the ESI process. The extent of charge competition leading to saturation in the
ESI process is consistent with the relative magnitude of excess charge in the electrospray
compared to the total number of analyte molecules in the solution. This ESI capacity model
allows one to predict the sample concentration limits for charge competition and the on-set of
ionization suppression effects, as well as the linear dynamic range for ESI-MS. The implications
for quantitative MS analysis and possibilities for effectively extending the dynamic range
of ESI measurements are discussed. (J Am Soc Mass Spectrom 2004, 15, 1416–1423) © 2004
American Society for Mass Spectrometry
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