High-resolution gas chromatography (HRGC) and gas chromatography/mass spectrometry (GC/MS) are the techniques of choice to determine the retention indices of more than 200 organic acids as their trimethylsilyl (TMS) or oxime-trimethylsilyl derivatives. Several types of apolar and semipolar fused-silica capillary columns (OV-1, SE-52, and OV-1701), used to analyze and separate organic acids isolated from urine samples, are evaluated.

No abstract

The determination of ampicillin in plasma and serum by reversed-phase high-performance liquid chromatography with UV detection suffers from poor selectivity and sensitivity. Currently, the most common approach to overcoming these problems consists of improving the compound's detectability via pre-or postcolumn derivatization. In the method that we describe, however, enhanced selectivity is afforded by sample purification by a tandem solid-phase extraction method (ion-exchange and reversed-phase). This approach permits detection at wavelengths of as low as 210 nm, which results in enhanced sensitivity (detection limit, 0.01 ,ug/ml). A second factor that affects selectivity is the addition to the chromatographic eluent of a crown ether to optimize the separation between ampicillin and polar endogenous plasma constituents. This combination of improved sample pretreatment and a more selective chromatographic system in conjunction with internal standardization forms the basis of a new assay for the quantitation of ampicillin in plasma. The overall recovery of ampicillin was 76.4% + 4.9% (n = 24), and the within-run and between-run coefficients of variation ranged from 1.6 to 7.2%. The method was applied to pharmacokinetic studies in cows and dogs after intramuscular or oral administration of the drug.The determination of ampicillin and other penicillins in plasma by high-performance liquid chromatography (HPLC) has traditionally struggled with problems of selectivity and sensitivity, mainly as a result of poor detectability (12). The molar absorption coefficient of ampicillin at its 'max (e = 3,036 M-1 cm-' at 257 cm) (7) is too low to afford sufficient sensitivity. Pre-and postcolumn reactions have been reported to convert the compound either to a derivative that displays more useful light-absorbing properties (nmax, 300 to 330 nm) (2, 3,12,17) or to a fluorescent breakdown product (10, 13). Alternatively, UV detection is carried out at 220 to 230 nm, which is more sensitive but less selective than detection at 257 nm (1,4,6,8,16). However, the loss of selectivity by these methods is not compensated for by a superior sample pretreatment. The latter merely consists of ultrafiltration (6) or a simple deproteinization of plasma without further cleanup of the extract (1,4,8,16). By the latter methods, sensitivity is further restricted by the dilution of the sample. Neither double-phase nor solid-phase methods have been reported for the extraction and concentration of ampicillin from plasma.The method described in this report approaches the basic problems of sensitivity and selectivity in ampicillin quantitation assays from the standpoint of sample pretreatment rather than detection. To this end, a tandem solid-phase extraction procedure was developed. The procedure not only ensures sufficient sample purification to permit sensitive detection at wavelengths of as low as 210 nm but also permits a twofold concentration of the sample. This new approach forms the basis of a reversed-phase HPLC method * Corresponding author.

A multi-stage screening and identification scheme for the diagnosis of inborn errors in amino acid, fatty acid, carbohydrate and intermediary metabolism is described. The method is based on a computerized analysis of data obtained with dual-column gas chromatography/FID (GC/FID) and gas chromatography/mass spectrometry (GC/MS) profiling of urinary organic acids. It involves: (1) isolation of the compounds by solvent or solid-phase extraction; (2) conversion into trimethylsilyl (TMS) or TMS-oxime derivatives; (3) GC/FID analysis on SE-52 and OV-1701 capillary columns; (4) tentative identification by comparing the methylene unit (MU) values on both columns with a user-built library of reference compounds; (5) quantitative evaluation of the excretion profile; and (6) analysis by GC/MS of samples with an abnormal profile using an automated peak identification programme.

This is a new method for isolating and concentrating urinary organic acids before gas chromatography--mass spectrometry. Sulfate and phosphate anions are removed by precipitation with Ba(OH)2, and the urine pH is adjusted to 8-8.5. The sample is then applied onto small, disposable, strong-anion-exchange columns. Neutral and basic compounds are washed out with water, which is then removed by centrifugation and by rinsing the columns with methanol and diethyl ether. The organic acids are eluted with a 4/1 (by vol) mixture of an organic solvent--either n-butanol or ethyl acetate--and formic acid containing HSO4- (0.1 mol/L) as a highly selective counter-ion, and finally with methanol alone. Sulfate ions are retained, and the eluate is evaporated before trimethylsilyl derivatization. Analytical recoveries (about 100%) of organic acids compare favorably with those obtained with solvent extraction. This convenient procedure is selective and reproducible and is a suitable alternative to the more cumbersome diethylaminoethyl-Sephadex extraction method.