Investigations on the coupling of high-performance liquid chromatography to direct analysis in real time mass spectrometry.

A novel hyphenated technique namely the combination of HPLC with direct analysis in real time (DART) mass spectrometry (MS) is presented. The coupling of HPLC to DART-MS was achieved by a simple interface design with a capillary end piece transferring the HPLC effluent to the ionization region of the DART. Flow rates (0.3-1.6 mL min(-1)) and inner diameters of the capillary (50-150 microm) were harmonized to provide a stable liquid jet. Ionization-related parameters optimized included positioning of the capillary end piece with respect to He outlet of the ionization source and the MS inlet, He heater temperature and He flow rate. Among DART-MS voltages, the grid electrode voltage proved to have the most pronounced effect on signal intensities. A major benefit of this setup is the possibility to employ (commonly not MS-compatible) HPLC eluents such as phosphate buffers up to a phosphate concentration of 120 mM even at typical HPLC flow rates such as 1 mL min(-1) and above without negative side effects like contamination of the ion source or ion suppression. Experiments evaluating the correlation of signal intensity to mass-flow and concentration revealed that DART-MS can be seen as a mass flow sensitive detector. The usability of this hyphenated technique has been tested on the example of four parabenes (measured in the negative ion mode) as well as a set of pyrazine derivatives (measured in the positive ion mode). For the parabenes limits of detection (LOD) in the range of 20-55 microg L(-1) and linear ranges from at least 200-10000 microg L(-1) with correlation coefficients better than 0.997 were obtained.