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

Abstract

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.

Figure 1. SAW device consisting of 10 pairs of inter-digitated electrode on a sectioned piezoelectric lithium niobate wafer

The width of each electrode was ~100μm, fabricated with a pitch of 200μm to provide a surface acoustic with a wavelength of 400μm. The aperture the IDT was 10mm. The IDT was excited at it’s resonance frequency of ~ 9.56MHz in a pulse mode where surface acoustic waves were radiated for a 20ms period of a 50ms duty cycle, the power used was 2W, nebulization was achieved under these conditions. .

Figure 2. Percentage of the droplet volume nebulized versus pulse duration

The percentage of droplet nebulized as a function of pulse plume at ~800 mW power for three solutions is shown, namely (△) water, (□) 50:50 methanol:water and (○) 10μM GluFib in 50:50 methanol:water with 0.1% formic acid. The droplet size was 1 μL. Results were collected in triplicate and mean values (with standard errors) are shown.

Figure 3. Angiotension relative ion current versus time

Single ion current trace as a function of time for angiotensin “ionized” by surface acoustic wave nebulization (bottom) and electrospray ionization (top).

Figure 4. Angiotensin precursor ion mass spectra

Angiotensin was ionized by surface acoustic wave nebulization (top) and electrospray ionization (bottom) and Figures created by averaging one minute of data from Figure 3.

Figure 5

Angiotensin tandem mass spectra. Tandem mass spectra were generated by collision induced dissociation of the [M+2H]²⁺ ion of angiotensin after ionization by SAW nebulization (top) and electrospray ionization (bottom). Major fragment ions are labeled according to Roepstorf and Fohlman nomenclature. Both spectra were generated by averaging one minute of data.