Use of nonvolatile buffers in liquid chromatography/mass spectrometry: advantages of capillary-scale particle beam...

The use of nonvolatile buffers like phosphate and citrate is usually incompatible with mass spectrometric detection or may heavily interfere with ion generation. In the case of conventional HPLC flow rates, the continuous buffer deposition produces fouling of the mass spectrometer ion source and may worsen the performance of any LC/MS interface as well. Our research group demonstrated that reducing the mobile phase flow rate in a particle beam interface improves the nebulization and enhances the overall performance. We have also assumed that such a modification may better handle potentially instrument harmful solvents or nonvolatile HPLC buffers. Since the presence of salts does not interfere with the electron ionization process, the particle beam interface can be considered, in principle, particularly suitable for those chromatographic separations that benefit from nonvolatile buffers. The microscale flow rate interface, developed in our laboratory, generates an aerosol from 1 microL/min of mobile phase flow rate. Under these conditions, the absolute amount of a nonvolatile buffer actually introduced into the system is approximately 1/1000 of that carried by a conventional HPLC column, slowing its deposition. This assumption was verified with a real-world application requiring the use of a phosphate buffer at a maximum concentration of 10 mM. It involved the determination of dexamethasone, an anti-inflammatory drug, in human blood after its administration to a patient. Several crucial mass spectral and chromatographic parameters were monitored during a 35-day period of intense use. Neither appreciable signal modification nor evident buffer deposition was observed during the test period, with only a normal and limited run-by-run and day-by-day variation of the instrument response.