Probe electrospray ionization of mixture solutions using metal needles with different tip conditions.

Time‐dependent mass spectra of mixture solutions of cytochrome c and cholesterol were measured by probe electrospray ionization (PESI) mass spectrometry and the PESI characteristics for several metal needles were investigated with different tip conditions. For a mixture solution of cytochrome c and cholesterol in 0.1% formic acid methanol/water (1/1) at a motion frequency of 1 Hz, an acupuncture needle with a sharp tip and 0.5‐mm‐diameter titanium and stainless‐steel wire needles gave strong cytochrome c and extremely weak cholesterol ion signals. When the frequency was lowered, and the high‐voltage duration increased, the titanium and stainless‐steel needles provided both cytochrome c and cholesterol ion signals with high intensities well separated over time. [ABSTRACT FROM AUTHOR]