Development Of A Sonic Spray Ionization Source For Mass Spectrometric Analysis Of Proteins

A sonic spray source (SSI), developed in our lab, was coupled to a commercially available linear ion trap mass spectrometer for protein analysis. Mass spectra of proteins such as ribonuclease A(RNase A) and lysozyme were obtained with substantially enhanced abundant peaks of low charge states. The effect of the operation parameters, including nebulization gas pressure, desolvation temperature, sample infusion rate, sample components etc. of the SSI source on the charge state distribution were systematically investigated. The experimental data show that SSI is apt to form multiply charged ions of proteins, but with charge states lower than those obtained with a conventional ESI source. The ion signal of RNASE is optimized under the operation conditions: the nebulization gas pressure is 3.4 - 3.6 MPa, the distance between the spray tip and the MS inlet is 4 - 6 min, temperature of capillary is 250 - 300 degrees C, sample flow rate of the solution (2% - 5 % formic acid) is 50 - 100 mu L/min. It is found that the low charge states of ions is favored with high temperature of the heating capillary and high pressure of the nebulization gas pressure or high contenct of volatile constituents (e. g. methanol) of the solution.