Amino Acid Clusters Formed by Sonic Spray Ionization

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Abstract
A novel ion source based on the principle of sonic spray ionization has been built and used to optimize mass spectrometric conditions for generating amino acid clusters. The ion source employs a simple pneumatic spray operated at extremely high nebulizing gas flow rates. Several factors that affect the performance of the cluster source are identified, and information from these observations provides insights into the mechanisms of gas phase ion formation. Serine is used as a model system in optimizing instrumental and sample parameters to maximize cluster ion formation. The sonic spray results for this oligomer compare favorably with electrospray data, showing an order of magnitude better signal intensity and excellent signal-to-noise ratios. The performance of the system for the protonated serine octamer includes a limit of detection of 10 nM and a linear dynamic range of 4 orders of magnitude. Ion formation was observed to go into saturation above 1 mM. This result and data on pH, electrolyte concentration, and solvent composition are interpreted as supporting a charge residue model of sonic spray ionization. Other amino acids can be substituted for serine in the octamer, with a strong chiral preference in favor of homochiral cluster formation in the cases of threonine and cysteine. These amino acids show a preference for substitution of more than two serine molecules. Phenylalanine, asparagine, tryptophan, and tyrosine also substitute into the serine octamer; however, the process yields only two incorporations and only small chiral effects.