An interface for sensitive analysis of monoamine neurotransmitters by ion-pair chromatography-electrospray ionization-mass spectrometry with continuous online elimination of ion-pair reagents.

A challenge in coupling ion-pair chromatography (IPC) online with electrospray ionization-mass spectrometry (ESI-MS) is that the nonvolatile ion-pair reagent (e.g., alkyl sulfate for amines or tetrabutylammonium for carboxylic acids) in the mobile phase suppresses the ESI-MS signals in the gas phase and their accumulation can clog the MS sampling interface. Consequently, IPC-ESI-MS is conducted either with a volatile ion-pair reagent, which could compromise the analyte separation efficiency, or with a downstream ion-exchange column to rid the ion-pair reagents of the mobile phase. In the latter approach, the limited capacity of ion-exchange columns requires frequent off-line column regeneration, which affects the separation throughput and prohibits long separations from being performed. A dual-valve, dual-ion exchange column interface of IPC-ESI-MS is designed for undisrupted separations and simultaneous column regeneration. Owing to the efficacy in removing the ion-pair reagent, the detection of eluents of monoamine neurotransmitters by an ion trap MS results in the limits of detection of 0.03 μM for dopamine or DA and 0.01 μM for 5-hydroxytryptamine or 5-HT. These values are lower than those obtained with ion trap MS of similar sensitivity when combined with the use of specialized chromatographic columns or sample preconcentration. Excellent reproducibility was attained with repeatedly regenerated ion-exchange columns (RSD = 4-6%) for an extended period of time (RSD < 6% for 6 days). DA and 5-HT in rat straital extracts were analyzed, and our data demonstrate that interferences inherent in the tissues and the ion-pair reagent have been successfully eliminated. This simple interface should be readily amenable to the separation and MS analysis of other types of polar compounds in complex sample media.