Patent ID: 7993931

Claim:
A method of measuring the amount of an unlabeled organic acid in a biological sample, comprising: a) adding to a biological sample suspected of containing the unlabeled organic acid to be measured an amount of a standard comprising one or more oxygen-18 labeled organic acids, wherein at least one of the oxygen-18 labeled organic acids belongs to an organic acid class selected from the group consisting of dihydroxy mono-acid, dicarboxyl organic acid, hydroxyl dicarboxyl acid, tricarboxyl acid, glycine conjugate, glyoxylic acid, hydroxyl mono-acid selected from the group consisting of glycolic acid, lactic acid, 3-hydroxypropionic acid, 2-hydroxybutyric acid, 3-hydroxyisobutyric acid, 3-hydroxybutyric acid, 4-hydroxybutyric acid, 2-hydroxyisovaleric acid, 3-hydroxy-2-methylbutyric acid, 3-hydroxy isovaleric acid, 3-hydroxy-2-ethylpropionic acid, 3-hydroxyvaleric acid, 4-hydroxyisovaleric acid, 5-hydroxyhexanoic acid, 2-hydroxyisocaproic acid, 2-hydroxy-3-methylvaleric acid, 5-hydroxyhexanoic acid, 3-hydroxy-2-methylvaleric acid, 2-hydroxyphenylacetic acid, 4-hydroxy phenylacetic acid, 4-hydroxycyclohexylacetic acid, phenyllacetic acid, 4-hydroxyphenylpropionic acid, 5-hydroxyindoleacetic acid, indoleacetic acid and 3-hydroxydodecanoic acid, and keto acid, and wherein at least one of said oxygen-18 labeled organic acids is structurally similar or identical to the unlabeled organic acid to be measured; b) processing the sample to increase the concentration of and/or chemically modify the unlabeled organic acid to be measured; c) measuring the amount of unlabeled organic acid and oxygen-18 organic acid in the processed sample by mass spectrometry; and d) using the amount of oxygen-18 organic acid measured in step c) to adjust the measured amount of unlabeled organic acid measured in the processed sample so as to reflect the amount of unlabeled organic acid originally present in the sample.