Liquid Chromatographic Determination of Free Glutamic Acid in Soup, Meat Product, and Chinese Food: Interlaboratory Study

An interlaboratory study of the liquid chromatographic (LC) determination of free glutamic acid in soup, meat product, and Chinese food was performed. Homogenized food samples were extracted with hot water, filtered, and diluted. Aliquot portions were treated with N,N-dimethyl-2-mer-capto-ethyl-ammonium chloride (DMMAC) and o phtaldialdehyde (OPA) to convert glutamic acid to a stable fluorescent complex. After LC separation on a reversed-phase C18 column with acetonitrile–phosphate buffer (pH 7.O)–water (80 +180 + 740, v/v) as mobile phase, glutamic acid peaks were measured fluorometrically (excitation, 340 nm, and emission, 389 and/or 440 nm). Homocysteic acid was used as internal standard. Twelve samples (6 blind duplicates) containing about 0.3–1.3% (w/w) of glutamic acid were analyzed singly by 12 laboratories. Results from one participant were excluded. Repeatability relative standard deviations (RSDr) varied from 1.3 to 4.5%, and reproducibility relative standard deviations (RSDR) ranged from 4.1 to 7.1 %. Average recovery of glutamic acid determined at 6 levels was 101.5% (range, 98–106%).