A critical limitation of electrospray ionization (ESI) liquid chromatography/mass spectrometry (LC/MS) sources is the susceptibility to blockage of interface orifices due to the deposition of involatile components from the sample and/or mobile phase. These components, including salts, buffers, and ion-pairing agents, can be essential to the performance of the chosen analytical method. We report here the performance enhancements provided by a novel atmospheric pressure ionization (API) source in the analysis of erythromycin A (ERY) using mobile phases that contain involatile components. The enhanced robustness of the new source is derived from the use of a continuous flow of aqueous solvent at the sampling cone orifice that maintains unobstructed ion transmission. The ESI mass spectral responses measured for ERY, using an LC separation that incorporates 10 mM sodium phosphate with and without 10 mM octane sulfonate, were monitored by repeated injections over 13-15 h total analysis time. Minimal effects on ESI mass spectral responses (integrated peak area) or chromatographic performance (peak shape, retention time) were observed during these studies. In the absence of the aqueous cleaning flow, complete loss of mass spectral responses and total blocking of the sampling cone was observed in less than 30 min. Responses for ERY spiked into chicken and beef liver, and catfish muscle at or below the regulatory level of interest (100 ppb), were quantified by internal standard calibration using this procedure. These results demonstrate the ability of a novel API-MS ion source to perform analyses that require the use of involatile mobile phase additives.
Copyright 2000 John Wiley & Sons, Ltd.