Studying the Influence of Temperature on the Emission of Ions from a Nanoscale, Electrified Meniscus

Motivated by the goal of sequencing single protein molecules, we have built a custom mass spectrometer featuring a nanocapillary ion source that emits single amino acid ions directly into high vacuum from the charged meniscus. In this talk we describe a study of the ion emission process. The mass spectrum obtained from aqueous solutions of arginine in positive ion mode shows a broad distribution of peaks corresponding to arginine clustered with different numbers of water molecules, as well as various hydronium-water clusters. An apparatus housed inside the mass spectrometer then applied heat to the ion source capillary to raise the temperature at the meniscus. Radiative cooling along the heated capillary limited the maximum temperature at the tip to about 55°C. The heated ion source changed the mass spectra in significant and interesting ways: The number of ion-solvent mass peaks decreased, as the highest mass ion clusters disappeared as well as the lightest ones corresponding to hydronium-water clusters. The emission rate of the remaining arginine-water clusters was relatively unchanged. Interestingly, this culminated in a lower overall emission current at higher temperature. A similar study of positive ion emission from aqueous sodium iodide solutions showed a decrease in the abundance of high mass ion clusters. We discuss how the ion emission mechanism may change with temperature in view of these findings.