Conductometric Gradient Ion Exclusion Chromatography for Volatile Fatty Acids

We describe a fatty acid vapor extractor (RAVE) as a postcolumn device for sensitive detection following ion exclusion chromatographic (ICE) separation of weak acids. The device consists of a single length of a permselective membrane tube surrounded by a jacket that consists of two isolated sections. The separation column effluent flows through the lumen. A suitable strong acid is put in the upstream, short section of the jacket and permeates in, rendering the lumenal flow strongly acidic (pH <= 2) that suppresses eluite weak acid dissociation. A lipophilic polysiloxane membrane is selectively permeable to volatile fatty acids (VFAs). A small fraction of the VFAs transfer to a cocurrent receptor stream of water (or a weak base, e.g., dilute hydroxylamine), flowing through the second, longer section of the jacket. Even though the transferred amount of VFAs may be very small (0.5-5%), significantly better detection limits than conventional suppressed conductometric ICE (SCICE) is possible because of the low and stable background (noise <1 nS/cm). It permits gradient elution, not possible in SCICE. The polysiloxane based FAVE device is highly selective for VFAs, it shows no response to dicarboxylic acids, hydroxycarboxylic acids, or aromatic acids. As such, trace detection of VFAs in the FAVE extractant is possible while other components can still be monitored conventionally in the FAVE lumenal effluent. Various parameters, related both to device design and operation were studied. The FAVE provides isolation from the eluent matrix and can be used for other detectors where the eluent matrix is incompatible with the detector.