Selectivity and sensitivity in supercritical fluid and liquid chromatography, using capillary columns and microelectrochemical detection

Sammanfattning: The obtainable selectivity of capillary column supercritical fluid chromatography wasstudied by the direct and indirect enantiomeric separation of some medium polarcompounds, i.e., benzodiazepines. The possibility to complement this chromatographictechnique with sensitive detection was pursued by the development of a micro-electrochemical detector, which was based on the impinging-jet principle and a 25 μmplatinum electrode. The detector was evaluated in both capillary column supercritical fluidand liquid chromatography (SFC and LC).In capillary column SFC, the amperometric detector was shown to be compatible withboth oxidative and reductive detection, as well as, pressure programmed conditions usingcarbon dioxide, modified with small amounts (i.e., less than 8%) of water, acetonitrile ormethanol, as the mobile phase. The compatibility with various modifiers is important asthe use of modified mobile phases in SFC generally results in altered selectivity, shorteranalysis time and the possibility to analyse more polar compounds. Using the presentdetector, a total number of about 20 different compounds, including, e.g., ferrocenes,phenols and nitroaromatic compounds, have been detected in carbon dioxide basedmobile phases with medium to low picogram detection limits. The detector response wasfound to be linear for injections of between ~0.1 and 4 ng of the analytes for bothoxidative and reductive amperometric detection. The microelectrode assembly could beemployed for at least six days of continuous operation in the SFC system. The relativestandard deviation in the peak height for ng injections of analyte was found to be ca. 5%under both pressure programmed and isobaric conditions. The stability, detection limitand reproducibility of the response of the present detector are notably better than thosepreviously reported.The study of the detector in LC was focused on the detection problems associated witha miniaturisation of the separation system. It was found that the detector could be placedup to one column inner diameter (i.e., 200 μm) from the column outlet with preservedsensitivity and efficiency. No significant dependence of the peak current on the positionof the electrode in the y- and z-directions versus the column outlet was observed.Moreover, the detector current was found to be insensitive to variations of the mobilephase linear velocity and concentration of supporting electrolyte. These features shouldfacilitate a rapid and reproducible positioning of the electrode versus the column outletand, additionally, render detection in normal-phase LC possible.