Atmospheric Pressure Photoionization. 1. General Properties for LC/MS

In this work, we describe the performance of an atmospheric pressure photoionization (APPI) source for sampling liquid flows. The results presented here primarily
focus on the mechanism of direct photoionization (PI),
as compared to the dopant mechanism of PI. Measured
detection limits for direct APPI were comparable to
atmospheric pressure chemical ionization (APCI; e.g., 1
pg for reserpine). The ion signal is linear up to 10 ng
injected quantity, with a useful dynamic range exceeding
100 ng. Evidence is presented indicating that APPI
achieves significantly better sensitivity than APCI at flow
rates below 200 μL/min, making it a useful source for
capillary liquid chromatography and capillary electrophoresis. Results are presented indicating that APPI is
less susceptible to ion suppression and salt buffer effects
than APCI and electrospray ionization (ESI). The principal
benefit of APPI, as compared to other ionization sources,
is in efficiently ionizing broad classes of nonpolar compounds. Thus, APPI is an important complement to ESI
and APCI by expanding the range and classes of compounds that can be analyzed. In this paper, we also
discuss the role of direct APPI vs PI-induced APCI using
dopants.