Universal Detection for SFC and LC

     A Universal Detector in chromatography responds to any solute with a uniform response factor.  As such, it facilitates the analysis of complex and/or unknown samples.  For example, in gas chromatography the Flame Ionization Detector (FID) is widely used in organic analysis.  The FID has an excellent sensitivity, linear response range, and uniform response towards organic compounds proportional to the analyte carbon content.  However, conventional Liquid Chromatography (LC) and Supercritical Fluid Chromatography (SFC) are challenged in this area.  For example, since LC elution is largely based upon organic solvents, this prevents FID usage since the solvents create enormous background levels that destroy the signal.  Conversely, in SFC elution is commonly based on supercritical carbon dioxide (SC-CO₂), which does not respond in the FID and does allow certain SFC-FID applications.  However, because SC-CO₂ is non-polar, organic modifiers such as methanol must be added to the fluid to support most modern SFC polar analyte separations.  As a result, the FID cannot be used in such SFC methods either.  As a result, LC and SFC mainly rely upon ultraviolet-visible absorbance, refractive index, or evaporative light scatter detection methods.  While each has distinct advantages, none offers an entirely universal response.  For example, UV-vis requires an appropriate analyte chromophore, while RID and ELSD have variable response factors dependant upon analyte structure and volatility.  Thus, there is a continued need for the development of universal detection methods that can operate with organic solvent-based mobile phases in LC and SFC, and which can detect organic compounds independent of their volatility and optical properties.

     We have been developing a novel universal detector for use in such LC and SFC applications that involves measuring the frequency of acoustic emissions from an oscillating hydrogen/oxygen flame.  The "Acoustic Flame Detector (AFD)" is based upon the inner cone of a pre-mixed flame repeatedly retreating into a capillary and quenching at the burner walls.  This cycling produces an audible acoustic pitch from the detector.  When organic analytes reach the flame, they decrease this cycle time and increase the detector pitch frequency proportional to analyte carbon content.  Thus, the operator can hear and measure peaks moving through the AFD.  In this way, the AFD provides uniform hydrocarbon response qualitatively similar to an FID, but quantitatively like an ELSD.

     Most significantly though, unlike an FID, AFD response is not obscured by organic solvents in the mobile phase.  This significant distinction between the two devices results in a dramatic difference in the AFD and FID chromatograms produced under typical SFC and LC operating conditions.  Thus, although less sensitive than an FID, the AFD can be a potentially useful universal detector for LC and SFC applications that require organic mobile phases and are unable to use an FID.  Another advantage of the AFD is its simple inexpensive design (resembling an FID), which can be readily adapted to a conventional FID detector base.  We are continuing work to advance this detection method and explore its application in LC and SFC.