More inclusive or selective ionization for mass spectrometry using obstructive sonic spray ionization and voltage polarity switching.

RATIONALE

A new variation of sonicspray ionization (SSI) significantly enhances its sensitivity with an advantage of producing selective or more inclusive ionization by changing the polarity of the potential applied to an obstruction in the path of the SSI spray. This technique also provides high sensitivity with water solutions, which is difficult for electrospray ionization (ESI) without added organic solvent.

METHODS

An Orbitrap Exactive mass spectrometer with an IonMax source heated electrospray ionization (ESI) probe operating at its maximum sheath gas setting was used for SSI. Both positive and negative polarities varying from 0.1 to 4 kV were applied to a metal obstruction positioned in the path of the spray using an atmospheric solid analysis probe. All mass spectra for this study were acquired in the positive ion mode.

RESULTS

SSI using a variety of obstructions improved the observed analyte ion abundance of small molecules, peptides, and proteins by approximately two orders of magnitude. The addition of a DC or AC voltage to a metal obstruction further enhanced the abundance of analyte ions by an additional two orders of magnitude. The relative abundances of the detected positive ions were considerably altered by switching the voltage polarity on the obstruction.

CONCLUSIONS

SSI with an obstruction improves the sensitivity of various samples compared with SSI. An applied voltage onto the obstruction further enhances the analyte ion abundances to approximately equal that of ESI but has the added advantage that switching the voltage polarity from positive to negative on the obstruction emphasizes different compounds present in the sample.