Dual detection for non-aqueous capillary electrophoresis combining contactless conductivity detection and mass spectrometry.

Coupling of two detectors is a powerful tool to enhance the overall analytical performance generating complementary information and overcoming the limitations of the single detectors. In this work, capacitively coupled contactless conductivity detection (C4D) and electrospray ionization mass spectrometry (ESI-MS) were coupled in conjunction with non-aqueous capillary electrophoresis (NACE). Non-aqueous electrolytes are highly compatible with ESI due to their volatility. Moreover, they exhibit low background conductivity, which is essential for the detection with C4D. A NACE-C4D-MS method was developed using an acetonitrile buffer containing 2 M HAc and 4 mM NH4Ac as background electrolyte. The influence of the inner diameter of the separation capillary on the C4D was studied and taken into account. A capillary with 50 µm inner diameter was found to be best suited. The complementarity of the two detectors was shown by determining a sample mixture containing choline, thiamine, nitrate, and chloride as well as bromide and acetylcholine as internal standards. The C4D was the detector of choice for the inorganic ions, which were not detectable with the MS whereas the MS had much lower limits of detections for the organic biomolecules. The method was applied on an extract of a food supplement containing the model analytes.