Quantitative Analysis of Amino Acids by CE-LIF

Aqueous solutions prepared in either LC-MS grade water or ultrapure water (Milli-Q Direct Water Purification System, MilliporeSigma, Burlington, MA, USA) were used for CE-LIF measurement. For CE-LIF detection, d/l-Ser and d/l-Asp were derivatized by reaction with naphthalene-2,3-dicarboxaldehyde (NDA) (Invitrogen, Carlsbad, CA, USA). A 4 μL mixture of an aliquot of the islet extract, 20 mM potassium cyanide (KCN) in 100 mM borate buffer, and 20 mM NDA in acetonitrile (ACN) was prepared in a 1:2:1 volume ratio. The mixture was allowed to react for 2 min in the dark at room temperature and then diluted to 10 μL of total volume by water. The samples were further desalted using a procedure similar to one previously used by our group [28 (link)] (see the Supplementary Materials). For quantitation using linear calibration curves, d/l-Ser or d/l-Asp standards of different concentrations ranging from 0.025–100 μM for d and 0.125–500 μM for l were prepared, NDA-derivatized as previously described, and diluted to 100 μL by water for CE-LIF analysis.