Atmospheric-pressure photoionization

Atmospheric pressure photoionization (APPI) is a soft ionization method used in mass spectrometry (MS) usually coupled to liquid chromatography (LC).

It is also less prone to ion suppression and matrix effects compared to ESI and APCI and typically has a wide linear dynamic range.

Sample ions then enter into the MS interface region, frequently a capillary through the combination of a decreasing pressure gradient and electric fields.

[3] The photoionization mechanism is simplified under vacuum conditions: photon absorption by the analyte molecule, leading to electron ejection, forming a molecular radical cation, M•+.

The unpredictable fate of ions is generally detrimental to LC/MS analysis, but like most processes, once they are better understood, these properties can be exploited to enhance performance.

The most common reaction is the abstraction of a hydrogen atom from the abundant solvent to form the stable [M+H]+ cation, which is usually the observed ion.

[6] In dopant-APPI (or photoionization-induced APCI), a quantity of photoionizable molecules (e.g., toluene or acetone) is introduced into the sample stream to create a source of charge carriers.

Compared to the more commonly used predecessor ionization sources ESI and APCI, APPI ionizes a broader range of compounds with the benefit increasing toward the non-polar end of the scale.

This latter example also highlights the benefit of APPI vs. ESI in that the HPLC conditions were for non-polar normal-phase in this case using n-hexane solvent.

APPI works well under normal-phase conditions since many of the solvents are photoionizable and serve as dopant ions, which allows specialized applications such as separation of enantiomers (right figure).

This has been suggested to be due to absorption of photons by the increasing density of solvent molecules.,[20][21] However, this leads to the benefit that APPI can extend to very low flow rates (e.g., 1 μL/min domain) allowing for effective use with capillary LC and capillary-electrophoresis.

The benefits of APPI-ASAP vs. APCI-ASAP are similar to those observed in LC/MS, namely higher sensitivity to lower polarity compounds and less background signal for samples in complex matrices.

In fact, a swab-APPI-MS system designed for explosives and narcotics detection for security applications performs very well for all types of ambient analysis using a sampling wand and swab (right figure).