Electrospray ionization/atmospheric pressure photoionization multimode source for low-flow liquid chromatography/mass...

Area of Science:

• Analytical Chemistry
• Mass Spectrometry
• Chromatography

Background:

• Traditional analytical techniques face challenges in analyzing diverse compound polarities and stability.
• Electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) are powerful ionization methods, but often require separate sources or specific conditions.
• Optimizing ionization parameters is crucial for enhancing sensitivity and selectivity in complex sample analysis.

Purpose of the Study:

• To develop and evaluate a novel dual electrospray ionization/atmospheric pressure photoionization (ESI/APPI) source, termed ESPI.
• To investigate the influence of various source parameters on ionization efficiency and compound detection.
• To compare the performance of the ESPI source against standard Electrospray Chemical Ionization (ESCI) for a range of compounds, including unstable drugs.

Main Methods:

• A newly developed dual ESI/APPI (ESPI) source was utilized for compound analysis.
• Key source variables including ESI probe heater temperature, solvent flow rate, dopant effects, repeller voltage, source geometry, and photon energy (Kr vs. Ar lamp) were systematically studied.
• Direct photoionization ([M](*+)) and indirect photo-induced chemical ionization (PCI) mechanisms were investigated under varying conditions.
• The ESPI source was evaluated for the analysis of 12 compounds in methanol, with a focus on low-flow conditions (10 µL/min) and comparison to the standard ESCI source.

Main Results:

• Direct photoionization dominated at high temperatures (>400°C) and low flow rates (<200 µL/min), yielding molecular radical cations ([M](*+)).
• Indirect photo-induced chemical ionization (PCI) became significant at lower temperatures and higher flow rates, with an Ar lamp enhancing PCI signal compared to a Kr lamp.
• The dual ESPI source demonstrated favorable performance for most compounds compared to standard ESCI, and notably superior performance for unstable drugs like flurbiprofen under low-flow conditions.

Conclusions:

• The dual ESPI source effectively combines the advantages of ESI and APPI, offering a versatile platform for analyzing both polar and non-polar compounds.
• Optimized source parameters, including temperature and flow rate, allow for control over ionization mechanisms (direct photoionization vs. PCI).
• The ESPI source provides enhanced analytical capabilities, particularly for sensitive and unstable analytes, outperforming standard ESCI in specific applications.