SGMS Meeting 2001

Atmospheric Pressure Photoionization (APPI): Selective or complementary ionization technique for LC-MS ?

Vittorio Raverdino
Agilent Technologies Schweiz AG, 39, rue de Veyrot - 1217 Meyrin, Switzerland

Although the photoionization technique is applied in some gas-chromatographic detectors (PID) to specifically detect compounds in different matrices, the development of a photoionization source for a mass spectrometer coupled with a liquid chromatograph is relatively new.

GC photoionization detectors are generally used to achieve more selectivity for aromatic compounds and organic compounds containing heteroatoms. They are considered as non-destructive detectors where an ultraviolet light lamp is a means of ionizing the analytes from a GC column. Only a very small fraction of the analyte molecules are actually ionized in this way.

For an APPI source , the ionization of compounds eluting from HPLC needs to be performed in the gas phase, and sufficient energy must be brought from the light source to the molecules (M) to produce radical-ions or promote proton exchange with the surrounding ionized mobile phase (SH) :

Therefore, in a first step is a heated nebulizer brings the mobile phase plus the substance to the vapor phase. A krypton lamp emits photons at an energy level sufficient to selectively ionize compounds eluting from HPLC in most cases, whereas the common solvents used for reverse phase separation are not efficiently ionized.

The addition of a dopant to the mobile phase is sometimes useful to promote the proton exchange in order to induce the formation of ions for the mass spectrometer.

Although all possible fields of application of APPI have not yet been completely explored, most of non-polar and medium polarity analytes in the molecular weight range roughly between 70 and 1000 can be analyzed successfully, except those that exhibit too high primary ionization potentials.

Between the most important parameters that can influence ionization efficiency of an APPI source can be mentioned: thermal degradation during the vaporization step, molecular structure, mobile phase composition, temperature, others.

The comparison of APPI with « classical » API-ES and APCI sources for LC-MS applications leads to several considerations:

(1) possibility of APPI to ionize substances that normally cannot be ionized by the classical sources
(2) obtain better detection limits for specific compounds
(3) obtain complementary information from different sources

APPI can be applied in different application areas as for example: « small » molecules for combinatorial libraries, steroids, aromatics, drug discovery, toxicology, molecules of interest in the food and beverages areas and others, vitamins and others.