Universal acoustic flame detection for modified supercritial fluid chromatography

A novel detector, based on the frequency of acoustic emissions from an oscillating premixed hydrogen/oxygen flame, has been characterized for use in supercritical-fluid chromatography (SFC). When an organic analyte is introduced, the steady pitch of the acoustic flame detector (AFD) increases proportionally to the carbon content of the molecule. Using standard hydrocarbon analytes, the SFC–AFD system provided a linear response over about 3 orders of magnitude with a detection limit (S/σ = 3) of 18 ng of carbon per second. The detector sensitivity was uniform for all analytes and did not change when using either pure or methanol modified supercritical-carbon dioxide (SC-CO2) as a mobile phase. While a stable baseline could be obtained for a variety of constant conditions, density gradients did cause it to shift due to the changing flow rate encountered when using a passive restrictor. While these changes were small for a pure SC-CO2 mobile phase, they were larger when using a methanol modifier. Qualitatively, the AFD response compared well to a flame ionization detector (FID). Overall, the results indicate that the AFD may be a useful, inexpensive universal detector for SFC applications that require organic modifiers and are unable to use an FID.