Surface acoustic wave nebulization of peptides as a microfluidic interface for mass spectrometry

We describe the fabrication of a surface acoustic wave (SAW) device on a LiNbO(3) piezoelectric transducer for the transfer of nonvolatile analytes to the gas phase at atmospheric pressure (a process referred to as nebulization or atomization). We subsequently show how such a device can be used in the field of mass spectrometry (MS) detection, demonstrating that SAW nebulization (SAWN) can be performed either in a discontinuous or pulsed mode, similar to that for matrix assisted laser desorption ionization (MALDI) or in a continuous mode like electrospray ionization (ESI). We present data showing the transfer of peptides to the gas phase, where ions are detected by MS. These peptide ions were subsequently fragmented by collision-induced dissociation, from which the sequence was assigned. Unlike MALDI mass spectra, which are typically contaminated with matrix ions at low m/z, the SAWN generated spectra had no such interference. In continuous mode, the SAWN plume was sampled on a microsecond time scale by a linear ion trap mass spectrometer and produced multiply charged peptide precursor ions with a charge state distribution shifted to higher m/z compared to an identical sample analyzed by ESI. The SAWN technology also provides the opportunity to re-examine a sample from a flat surface, repeatedly. The process can be performed without the need for capillaries, which can clog, reservoirs, which dilute the sample, and electrodes, which when in direct contact with sample, cause unwanted electrochemical oxidation. In both continuous and pulsed sampling modes, the quality of precursor ion scans and tandem mass spectra of peptides was consistent across the plume's lifetime.