Investigations of Analyte-Specific Response Saturation
and Dynamic Range Limitations in Atmospheric Pressure Ionization Mass
Spectrometry
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Abstract
With this study, we investigated
why some small molecules demonstrate
narrow dynamic ranges in electrospray ionization-mass spectrometry
(ESI-MS) and sought to establish conditions under which the dynamic
range could be extended. Working curves were compared for eight flavonoids
and two alkaloids using ESI, atmospheric pressure chemical ionization
(APCI), and heated electrospray ionization (HESI) sources. Relative
to reserpine, the flavonoids exhibited narrower linear dynamic ranges
with ESI-MS, primarily due to saturation in response at relatively
low concentrations. Saturation was overcome by switching from ESI
to APCI, and our experiments utilizing a combination HESI/APCI source
suggest that this is due in part to the ability of APCI to protonate
neutral quercetin molecules in the gas phase. Thermodynamic equilibrium
calculations indicate that quercetin should be fully protonated in
solution, and thus, it appears that some factor inherent in the ESI
process favors the formation of neutral quercetin at high concentration.
The flavonoid saturation concentration was increased with HESI as
compared to ESI, suggesting that inefficient transfer of ions to the
gas phase can also contribute to saturation in ESI-MS response. In
support of this conclusion, increasing auxiliary gas pressure or switching
to a more volatile spray solvent also increased the ESI dynamic range.
Among the sources investigated herein, the HESI source achieved the
best analytical performance (widest linear dynamic range, lowest LOD),
but the APCI source was less subject to saturation in response at
high concentration