Ultrasonication-assisted Spray Ionization Mass Spectrometry (UASI MS)
Ultrasonic has been widely used in various fields including medicine, biology, industry and household appliance. Owing to the usefulness of ultrasonic in assisting liquid nebulization, it has been used to assist sample analysis in electrospray ionization mass spectrometry (ESI MS). Lately, ultrasonic is also employed to the development of two-stage ambient mass spectrometry. In this study, a novel ambient mass spectrometry based on the use of ultrasonic, so called ultrasonication-assisted spray ionization (UASI) MS, was explored. The setup of the UASI MS is extremely simple. A low-frequency ultrasonicator, a tapered capillary, and a sample container are required components for the setup. The capillary inlet was immersed into a sample vial, which was subjected in the ultrasonicator, and the tapered capillary outlet was placed close to the front of the orifice of a mass spectrometer within ~4 mm. Upon the ultrasonicator was switched on, ultrasonic spray containing charged species was readily formed in the capillary outlet. After solvent evaporation, the gas charged species was directly monitored by the mass spectrometer. It is believed that the charges attached on the gas species originally derive from the sample solution. The detection of positively or negatively charged ions mainly depends on the selection of positive/negative modes in the mass spectrometer. In this study, it has been demonstrated that UASI MS is suitable for the analysis of analytes including small molecules such as amino acids and large molecules such as peptides and proteins. The patterns obtained from multiply charged protein ions appearing in UASI mass spectra are similar to those obtained in ESI mass spectra. UASI MS can provide much cleaner background than ESI MS does. Additionally, the parameters that may affect the UAS MS performance have been examined in this study. Furthermore, on-line monitoring of ultrasonication-assisted organic reactions by UASI MS was also demonstrated in this study. The results show that UASI MS is a suitable technique to monitor reaction intermediates and products in real time without much comprising when combining with ultrasonication-assisted organic reactions. The potential for further extending the applications of UASI MS in chemistry and biochemistry researches can be expected.
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