Acoustic Flame Detector for Gas Chromatography

Increased flow rate compatibility for universal acoustic flame detection in liquid chromatography

The liquid chromatography (LC) flow rate tolerance of the universal Acoustic Flame Detector (AFD) is characterized and significantly expanded through using larger bore burners. For example, increasing the burner i.d. from 1.00 to 4.00 mm increases the AFD upper flow rate limit from 20 to 100 μL/min. While signal and noise each reduce as the burner i.d. widens, the best current performance is obtained with a 2.30 mm i.d. burner. This approach also allows AFD operation over a broader range of mobile phase temperatures. As a result, the overall increased flow rate compatibility of the detector can facilitate improved chromatography and further development of LC-AFD applications.

Acoustic methods of detection in gas chromatography

A brief review of the use of acoustic detection methods in GC is presented. While a number of methods (some quite similar) have been developed for use as gas-phase sensors in various applications, this article focuses specifically on those techniques that have been used to detect analytes following their separation by GC. Overall, a number of "active" acoustic methods (which measure analytes through their interaction with a controlled external acoustic wave source) were reportedly used as GC detectors. These include ultrasonic, thickness shear mode, surface acoustic wave (SAW), and flexural plate wave methods. Conversely, "passive" acoustic methods (those that produce an acoustic signal through some chemical reaction with the analyte) have also been used as GC detectors. These include photoacoustic and acoustic flame methods of detection. Of the two major classifications, reports of active methods are far more prevalent. In particular, the usage of SAW techniques with GC is an area of research that has seen accelerated growth in recent years.

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/sigma = 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-CO(2)) 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-CO(2) 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.

Detector for Liquid Chromatography Based on Acoustic Emissions from an Oscillating Flame

The acoustic flame detector (AFD) is examined as a novel detector for liquid chromatography (LC). It is based upon the acoustic emission frequency of an oscillating hydrogen/oxygen premixed flame and produces a universal response toward organic molecules. A stable frequency near 1000 Hz, which further depends on mobile-phase composition, is achieved for flow rates in the microliter per minute range. The mass flow sensitivity of the AFD demonstrates a linear response over 3 orders of magnitude and a detection limit (S/sigma = 3) of approximately 15 ng of C/s for a series of alcohols. For cyclopentanol, this amounts to an injected mass of approximately 77 ng based on a 0.5-microL injection of a 196 ppm solution in methanol (flow rate 20 microL/min methanol; peak width 30 s). Similar sensitivity is observed using a water mobile phase. Low-frequency (1/f ) noise contributions are dominant with or without mobile phase present. The AFD demonstrates a uniform molar sensitivity toward carbon compounds independent of their optical properties or volatility. Results suggest the device might serve as a simple, inexpensive universal LC detector.

Gas chromatography

This review of the fundamental developments in gas chromatography (GC) includes articles published from 1996 and 1997 and an occasional citation prior to 1996. The literature was reviewed principally using CA Selects for Gas Chromatography from Chemical Abstracts Service, and some significant articles from late 1997 may be missing from the review. In addition, the online SciSearch Database (Institute for Scientific Information) capability was used to abstract review articles or books. As with the prior recent reviews, emphasis has been given to the identification and discussion of selected developments, rather than a presentation of a comprehensive literature search, now available widely through computer-based resources. During the last two years, several themes emerged from a review of the literature. Multidimensional gas chromatography has undergone transformation encompassing a broad range of activity, including attempts to establish methods using chromatographic principles rather than a totally empirical approach. Another trend noted was a comparatively large effort in chromatographic theory through modeling efforts; these presumably became resurgent with inexpensive and powerful computing tools. Finally, an impressive level of activity was noted through the themes highlighted in this review, and this was particularly true with detectors and field instruments.