Ultrasonication-assisted spray ionization mass spectrometry for the analysis of biomolecules in solution

Abstract

In this paper, we describe a novel technique-ultrasonication-assisted spray ionization (UASI)-for the generation of singly charged and multiply charged gas-phase ions of biomolecules (e.g., amino acids, peptides, and proteins) from solution; this method employs a low-frequency ultrasonicator (ca. 40 kHz) in place of the high electric field required for electrospray ionization. When a capillary inlet is immersed into a sample solution within a vial subjected to ultrasonication, the solution is continually directed to the capillary outlet as a result of ultrasonication-assisted capillary action; an ultrasonic spray of the sample solution is emitted at the outlet of the tapered capillary, leading to the ready generation of gas-phase ions. Using an ion trap mass spectrometer, we found that singly charged amino acid and multiply charged peptides/proteins ions were generated through this single-step operation, which is both straightforward and extremely simple to perform. The setup is uncomplicated: only a low-frequency ultrasonicator and a tapered capillary are required to perform UASI. The mass spectra of the multiply charged peptides and proteins obtained from sample solutions subjected to UASI resemble those observed in ESI mass spectra. Using these techniques, intense laser irradiation is generally required to provide sufficient energy to direct analytes from the condensed phase to the gas phase. Notably, the development of desorption electrospray ionization (DESI) [6]-using a fine spray of charged droplets as the desorption/ionization source for analytes under ambient conditions-has opened up new avenues for progress in desorption/ionization mass spectrometry. Apparently, as long as analyte molecules are provided with sufficient energy, which is not limited to the use of lasers, gas-phase ions can be readily generated from condensed phase samples. Furthermore, this discovery has also provided a fillip to the development of ambient mass spectrometry [7][8][9][10][11]. For example, high-energy laser [8], heated gas jets [9], low-temperature plasma [10], and nitrogen gas [11] were successfully employed to desorb analytes at ambient condition. However, post-ionization was generally required for these techniques.Ultrasonically assisted electrospray ionization [12,13] was first demonstrated more than a decade ago, using an ultrasonic transducer to enhance the nebulization efficiency of analytes eluted from a liquid chromatography column and then subjecting them to ESI. A method, so called sonic spray ionization (SSI), for small organics and drugs [14 -17] has been developed, in which a solution from a fused-silica capillary is sprayed with a sonic gas flow coaxial to the capillary. When analyzing large molecules such as proteins, an electric field is applied to the solution in the capillary to increase the charge density of produced droplets and multiply charged ions of proteins are then generated [18,19]. Lately, ultrasonic transducer-based nebulizers (operated at megahertz frequencies or greater) [20,21] have been e...

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