title: (18O)quinolinic acid: its esterification without back exchange for use as internal standard in the quantification of brain and CSF quinolinic acid.
abstract: In the process of developing a high-sensitivity negative chemical ionization gas chromatographic/mass spectrometric assay for brain and cerebrospinal fluid (CSF) levels of quinolinic acid (QUIN, 2,3-pyridine dicarboxylic acid), (18O4)QUIN was prepared. Its properties as an internal standard were compared with those of the structural isomer 2,4-pyridine decarboxylic acid (2,4-PDC) previously used by others. All oxygen atoms in QUIN were labeled by heating in 3 N HCl/(18O)water for 48 h at 80 degrees C. Back-exchange of (18O4)QUIN was prevented during derivatization to an electron-capturing dihexafluoropropanol ester by using trifluoroacetylimidazole as catalyst instead of perfluroacyl anhydrides. When mixtures of QUIN and (18O4)QUIN and/or 2,4-PDC were followed through a procedure to isolate and quantify brain and CSF QUIN, the variability in the ratio of QUIN:2,4-PDC was greater than for QUIN: (18O)QUIN. We conclude that (18O)QUIN is the preferred internal standard in gas chromatographic/mass spectrometric quantification of brain and CSF QUIN, and that (18O)-labeled carboxylic acids may be esterified effectively without back-exchange using acylimidazole reagents.
authors.affiliation: Laboratory of Neurophysiology, NIMH, Bethesda, Maryland 20892.
authors.forename: M P
authors.initials: MP
authors.lastname: Heyes
authors.forename: S P
authors.initials: SP
authors.lastname: Markey
mesh_terms: D000818:Animals; D001923:Brain Chemistry; D004952:Esters / Q000032:analysis / Q000138:chemical synthesis; D008401:Gas Chromatography-Mass Spectrometry; D007202:Indicators and Reagents; D011725:Pyridines / Q000032:analysis*; D017378:Quinolinic Acid; D011805:Quinolinic Acids / Q000032:analysis* / Q000134:cerebrospinal fluid; D051381:Rats; D012015:Reference Standards
chemical_list: D004952:Esters; D007202:Indicators and Reagents; D011725:Pyridines; D011805:Quinolinic Acids; D017378:Quinolinic Acid