Direct detection of amino acids using extractive electrospray ionization tandem mass spectrometry

A novel method was developed for the direct detection of amino acids in biological fluids by extractive electrospray ionization (EESI) tandem mass spectrometry (MS) with minimal sample pretreatment. Based on our previous research, a novel EESI ion source was constructed to improve analytical performance and operation safety. The EESI-MS conditions were optimized using representative amino acid standards. The methanol-water reagent solvent (1:1, v/v) was electrosprayed at 5 μL min^-1 at a high voltage (+4 kV, positive ion detection mode). The temperature of the heated capillary was optimized to be 150 °C. Collision induced dissociation (CID) experiments were performed by applying 17%-25% of the collision energy to the precursor ions isolated with a window width of 1.5 mass/charge (m/z) units. The limit of detection (LOD) for these amino acids was in the range of 0.14-26.2 μg L ^-1, and the linear dynamic range was larger than two orders of magnitude. The average time for a single amino acid analysis using a typical human urine sample was less than 0.5 min. The recovery rates for different concentrations of amino acids ranged from 82.6% to 105.6%, and the relative standard deviations (RSD) ranged from 2.2% to 11.4%. The results showed that EESI-MS is a powerful tool for the rapid, sensitive, and quantitative detection of amino acids in complex biological samples.