DETERMINATION OF SMALL CARBOXYLIC-ACIDS BY ION-EXCLUSION CHROMATOGRAPHY WITH ELECTROSPRAY MASS-SPECTROMETRY

Ion exclusion chromatography (IEC) is used with electrospray mass spec trometry (ES-MS) to analyze a mixture of formic, glyoxylic, oxalic, 2- hydroxyisobutyric, and maleic acids. Compromise experimental parameter s were evaluated. Ion extraction voltages can be found that produce re asonable signals from negative parent ions ([M-H](-)) of these compoun ds as well as succinic, malic, 1-hexanoic, malonic, and tricarballylic acids. The effects of sheath gas, solvent composition, concentrated m atrix ions, and pH of solution on sensitivity and linearity were exami ned. A basic isopropanol solvent with nitrogen sheath gas yield the be st results. Detection limits are 40-200 ppb for direct infusion of sam ple into the ES-MS and 2-8 ppm for IEC-ES-MS. The presence of concentr ated matrix ions can suppress the analyte signal significantly. These matrix effects are minimized by chromatographic separation and interna l standards. In most cases, the sample is injected directly onto the c olumn with little or no preparation. A small additional flow of organi c solvent with pH adjustment is added before the sample reaches the ES -MS.