A new approach to the selective comparative metabolite profiling of carboxylic acids in rat urine was established using CE-MS and a method for positively pre-charged and (2)H-coded derivatization. Novel derivatizing reagents, N-alkyl-4-aminomethyl-pyridinum iodide (alkyl = butyl, butyl-d9 or hexyl), containing quaternary amine and stable-isotope atoms (deuterium), were introduced for the deriva...