Neutral loss analysis of amino acids by desorption electrospray ionization using an unmodified tandem quadrupole mass spectrometer

Abstract

A new method to analyze free amino acids using desorption electrospray ionization (DESI) has been implemented. The method is based on the neutral loss mode determination of underivatized amino acids using a tandem quadrupole mass spectrometer equipped with an unmodified atmospheric interface. Qualitative and quantitative optimization of DESI parameters, including ESI voltage, solvent flow rate, angle of collection and incidence, gas flow and temperatures, was performed for amino acids detection. The parameters for DESI analysis were evaluated using a mixture of valine, leucine, methionine, phenylalanine and tyrosine standards. A few microliters of this mixture were deposited on a slide, dried and analyzed at a flow rate of 2 microL/min. The optimal ionization response was obtained using laboratory glass slides and an equivalent solution of water/methanol doped with 2% of formic acid. The method specificity was evaluated by comparing product ion spectra and neutral loss analysis of amino acids obtained either by DESI or by electrospray ionization flow injection analysis (ESI-FIA). To evaluate the quantitative response on amino acids analyzed by DESI, calibration curves were performed on amino acid standard solutions spiked with a fixed amount of labelled amino acids. The method was also employed to analyze free amino acids from blood spots, after a rapid solvent extraction without other sample pretreatment, from positive and negative subjects. The method enables one to analyze biological samples and to discriminate healthy subjects from patients affected by inherited metabolic diseases. The intrinsic high-throughput analysis of DESI represents an opportunity, because of its potential application in clinical chemistry, for the expanded screening of some inborn errors of metabolism.