Probe Electrospray Ionization Mass Spectrometry with Discontinuous Atmospheric Pressure Interface

Abstract

Probe electrospray ionization (PESI) using a 0.2 mm outside diameter titanium wire was performed and the generated ions were introduced into the mass spectrometer via a discontinuous atmospheric pressure interface using a pinch valve. Time-lapse PESI mass spectra were acquired by gradually increasing delay time for the pinch valve opening with respect to the start of each electrospray event when a high voltage was applied. The opening time of the pinch valve was 20 ms. Time-resolved PESI mass spectra showed marked differences for 10 mM NaCl, 10⁻⁵ M gramicidin S and insulin in H₂O/CH₃OH/CH₃COOH (65/35/1) with and without the addition of 10 mM CH₃COONH₄. This was ascribed to the pH change of the liquid attached to the needle caused by electrochemical reactions taking place at the interface between the metal probe and the solution. NaCl cluster ions appeared only after the depletion of analytes. For the mixed solution of 10⁻⁵ M cytochrome c, insulin, and gramicidin S in H₂O/CH₃OH/CH₃COOH (65/35/1), a sequential appearance of analyte ions in the order of cytochrome c → insulin → gramicidin S was observed. The present technique was applied to three narcotic samples, methamphetamine, morphine, and codeine. Limits of detection for these compounds were 10 ppb in H₂O/CH₃OH (1/1) for the single sampling with a pinch valve opening time of 200 ms.