Comparison of electrospray ionization and atmospheric pressure photoionization for coupling of micellar electrokinetic chromatography with ion trap mass spectrometry

The performance of dopant-assisted atmospheric pressure photoionization (DA-APPI) and electrospray ionization (ESI) for the coupling of micellar electrokinetic chromatography (MEKC) with ion trap mass spectrometry (ITMS) was compared using a set of test drugs comprising basic amines, steroids, esters, phenones and a quaternary ammonium compound. The influence of the surfactant sodium dodecyl sulfate (SDS) on analyte signals was studied by infusion of sample through the CE capillary into the respective ion sources. It was found that background electrolytes (BGEs) containing 20–50 mM SDS in 10 mM sodium phosphate (pH 7.5) caused major ionization suppression for both polar and apolar compounds in ESI-MS, whereas APPI-MS signal intensities remained largely unaffected. ESI gave rise to the formation of SDS clusters, which occasionally may cause space-charge effects in the ion trap. Furthermore, extensive sodium-adduct formation was observed for medium polar compounds with ESI-MS, whereas these compounds were detected as their protonated molecules with APPI-MS. Using the BGE containing 20 mM SDS, MEKC-ESI-MS still provides slightly lower limits of detection (LODs) (2.6–3.1 μM) than MEKC-APPI-MS (4.3–6.4 μM) for basic amines. For less polar compounds, highest S/Ns were obtained with APPI-MS detection (LODs, 4.5–71 μM). For BGEs containing 50 mM SDS, the limits of detection for MEKC-APPI-MS were more favorable (factor 1.5–12) than MEKC-ESI-MS for nearly all tested drugs. Spray shield contamination by SDS was lower in DA-APPI-MS than in ESI-MS. It is concluded that DA-APPI shows the most favorable characteristics for MEKC–MS, especially when compounds of low polarity have to be analyzed.