Simple and practical derivatization procedure for enhanced detection of carboxylic acids in liquid chromatography–electrospray ionization-tandem mass spectrometry

A simple and practical derivatization procedure for increasing the detection responses of carboxylic acids in liquid chromatography–electrospray ionization-tandem mass spectrometry (LC–ESI-MS/MS) has been developed. 2-Hydrazinopyridine (HP) and 2-picolylamine (PA) rapidly reacted with biologically and clinically important carboxylic acids [chenodeoxycholic acid, glycochenodeoxycholic acid, prostaglandin E2, 2-(β-carboxyethyl)-6-hydroxy-2,7,8-trimethylchroman (γ-CEHC), α-lipoic acid, homovanillic acid (HVA) and 5-hydroxyindole-3-acetic acid] in the presence of 2,2′-dipyridyl disulfide and triphenylphosphine. The resulting HP- and PA-derivatives were highly responsive in ESI-MS operating in the positive-ion mode and gave characteristic product ions during MS/MS, which enabled the sensitive detection using selected reaction monitoring. Among the two reagents, PA was of more practical use; the detection responses of the PA-derivatives were increased by 9–158-fold over the intact carboxylic acids and the limits of detection were in the low femtomole range (1.5–5.6 fmol on column). The PA-derivatization was successfully applied to a biological sample analysis; the derivatization followed by LC–ESI-MS/MS enabled the detection of trace amounts of bile acids, γ-CEHC and HVA in human saliva with a simple pretreatment, small sample volume and short analysis time.