Direct Detection of Amino Acids Using Extractive Electrospray Ionization Tandem Mass Spectrometry

A novel method, based on extractive electrospray ionization (EESI) tandem mass spectrometry (MS), was developed for the direct detection of amino acids in biological fluids with minimal sample pretreatment. Based on our previous research, we constructed a new EESI ion source to improve the analytical performance and operation safety. We optimized EESI-MS conditions using representative amino acid standards. The reagent solvent (methanol :water=1:1) was electrosprayed at 5 mu L/min at high voltage (+4 kV, positive ion detection mode). The temperature of the heated capillary was optimized to be 150 degrees C. Collision induced dissociation (CID) experiments were done 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) of these amino acids was in the range of 0. 14 -26. 2 mu g/L, and the linear dynamic range was larger than two orders of magnitude. The amino acid concentration in human urine samples was measured by the method, and the average time for a single sample analysis was less than 0. 5 min. The recovery rates for different concentrations ranged from 82. 6% to 105. 6%, and the relative standard deviations (RSDs) ranged from 2. 2% to 11. 4%. This study showed that EESI-MS was a powerful tool for the rapid, sensitive, and quantitative detection of amino acids in complex biological samples.