Biomedical subjects

Kevin B Thurbide

Publications and source records attributed to Kevin B Thurbide.

10 recordsLinked to original sources

PubMed2006

Rapid column heating method for subcritical water chromatography.

A novel resistive heating method is presented for subcritical water chromatography (SWC) that provides higher column heating rates than those conventionally obtained from temperature-programmed gas chromatography (GC) convection ovens. Since the polarity of water reduces dramatically with increasing temperature, SWC employs column heating to achieve gradient elution. As such, the rate at which the mobile phase is heated directly impacts the magnitude of such gradients applied in SWC. Data from the current study demonstrate that the maximum column heating rate attainable in a typical SWC apparatus (i.e. using a GC convection oven) is around 10 degrees C/min, even at instrument oven settings of over three times this value. Conversely, by wrapping the separation column with ceramic insulation and a resistively heated wire, the column heating rates are increased five-fold. As a result, elution times can be greatly decreased in SWC employing gradients. Separations of standard alcohol test mixtures demonstrate that the retention time of the latest eluting component decreases by 35 to 50% using the prototype method. Additionally, solute retention times in this mode deviate by less than 1% RSD over several trials, which compares very well to those obtained using a conventional GC convection oven. Results suggest that the developed method can be a useful alternative heating technique in SWC.

Chromatography, Supercritical Fluid↗

PubMed2006

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.

Acoustics↗

PubMed2006

Subcritical solvent regeneration of contaminated HPLC columns used for separating biomolecules.

The regeneration of contaminated chromatography columns using subcritical mobile phases is presented. The method employs minimal amounts of organic solvents (pressurized and heated above their boiling point) to clean strongly retained sample components from hydrophobic stationary phase particles. The process lasts approximately 1 h and is demonstrated to restore the performance of polymeric and bonded-phase columns. The technique is simple to apply and widely accessible, requiring only common laboratory equipment. Results indicate that this could be a beneficial alternative approach for cleaning contaminated columns either directly or in cases in which conventional methods have been unsuccessful.

Chromatography, High Pressure Liquid↗

PubMed2005

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.

Acoustics↗

PubMed2005

Direct analysis of gramicidin double helices using packed column supercritical fluid chromatography.

Direct analysis of the monomeric and four double helical dimeric conformations of gramicidin has been achieved using packed column supercritical fluid chromatography (pSFC). Using a PRP-1 polymeric column and typical conditions of 40 degrees C column temperature, 25 MPa column pressure, and 35% n-pentanol modifier addition, all of the gramicidin conformers were readily separated. To evaluate the method, the dynamic characteristics of the monomer and dimer species were monitored as a function of solvent type, incubation time, solvent temperature, and initial concentration. The findings agree with those previously obtained by other methods but also yield new information about the relative amounts of two closely related dimers (species 1 and 2) as well as the simultaneous changes in the full dimer/monomer distribution. Results indicate that the developed pSFC method can be an informative complimentary tool for readily monitoring changes in the full profile of gramicidin species present in different environments.

Chromatography, Supercritical Fluid↗

PubMed2005

Characteristics of sulfur response in a micro-flame photometric detector.

A recently reported micro-flame photometric detector (microFPD) has been examined in greater detail for its sulfur response characteristics. While supporting an "upside down" flame on a stainless steel capillary burner (delivering oxygen) in a counter flowing stream of premixed hydrogen and oxygen, the extremely small flame of the muFPD (30 nL) was observed to produce linear sulfur emission as HSO(*). In this mode, linear sulfur response was obtained over four orders of magnitude with a minimum detectable flow of 2 x 10(-10) g S/s. Additionally, a broad series of sulfur compounds ranging in chemical structure were examined in the microFPD in order to determine the extent of equimolarity and reproducibility of response toward this element. Results of exploring both the linear (HSO(*)) and quadratic (S(2)(*)) modes indicate that the %RSD and equimolarity of sulfur response are comparable between that of the microFPD and a conventional flame photometric detector (FPD).

Microchemistry↗

PubMed2005

Separation of linear gramicidins using carbon dioxide-containing mobile phases.

Packed-column supercritical-fluid chromatography (pSFC) is presented as a novel method for separating and analyzing gramicidin samples. By use of methanol-modified carbon dioxide as a mobile phase the pentadecapeptides gramicidin A (gA), gramicidin B (gB), and gramicidin C (gC) are readily separated and eluted from a PRP-1 poly(styrene-divinylbenzene) column. Although optimum separation conditions are typically achieved near a column temperature of 40 degrees C, a column pressure of 11 MPa, and 30% methanol modifier, pressure and modifier gradients around these values are also found to improve the overall separation time. Measurements indicate that the mobile phase solubility of gramicidin under these conditions is 5.0+/-0.4 microg mL(-1). Collection of individual peaks during chromatography achieved analytical-scale isolation of 2 microg refined gC from 20 microg injected gramicidin D. Further, supercritical-fluid extraction of 200 microg gramicidin D from a Chromosorb 102 support packed into the vessel produced 57 microg gA in 90% purity. The results establish that carbon dioxide-based mobile phases can be successfully used for the separation of individual gramicidin species.

Carbon Dioxide↗

PubMed2004

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.

Journal Article↗

PubMed2004

Novel flame photometric detector for gas chromatography based on counter-current gas flows.

A novel analytical device has been developed for gas chromatography. It is based on optical emission from a counter-current (i.e. counter-flowing) air or oxygen flame, which burns in an opposing stream of hydrogen and column effluent. The flame is typically positioned "upside down" on the upper (air) jet, which faces the lower (hydrogen + effluent) jet. It can also be positioned on the lower jet, be connected to both jets, or be suspended in the gap between them. Excellent stability can be obtained in any of these modes. Overall, this new "counter-current flame photometric detector" (ccFPD) responds to analytes in the manner of a conventional flame photometric detector (FPD); however, it can be operated over a much wider range of gas flows. For instance, the same physical ccFPD burner easily supports stable flames of air flows between 5 and 200 ml/min and corresponding hydrogen flows between 5 and 10,000 ml/min. Visual observation of the counter-current flame, in the presence of sulfur and phosphorus as test analytes, reveals intense, steady luminescence under a wide variety of conditions. Additionally, and in contrast to the commercial FPD, flame conductivity signals can be obtained that are similar in quality to those produced by a conventional flame ionization detector (FID). Thus the ccFPD is a flexible, easily optimized photometric detector. The exceptional flow stability of the ccFPD was used to explore the earlier reported phenomenon of strong signal/noise (S/N) ratios, which had been obtained for hetero-elements of the iron group from a conventional FPD with a small, stoichiometric flame. Results using the ccFPD, which also exhibits this unusual response, indicate that these high S/N ratios are only partly due to the predictable decrease in flame noise with decreasing flame size. Contrary to expectations, the absolute analyte signal often increases as the flame size decreases to the point of extinction. The signal intensity and the magnitude of the observed changes depend to some degree on the flame composition (H2/O2 ratio).

Chromatography, Gas↗

PubMed2003

Flame photometric detection inside of a capillary gas chromatography column.

A novel micro-flame photometric detector (FPD) employing a miniature counter-current flame is described. The micro-FPD flame, encompassing a volume of about 30 nL, is operated inside the end of a capillary gas chromatography column (i.e. on-column) or inside of a quartz capillary after the column (i.e. post-column). Either air or oxygen can support a hydrogen flame in the device, although oxygen is far preferable. The detector can be operated for several hours without any observed degradation in performance or flame stability. The optimal gas flows established for the detection of sulfur and phosphorus are in the range of 4 mL min(-1) of oxygen and 9 to 13 mL min(-1) of hydrogen. The fuel-rich micro-FPD flame generates chemiluminescent blue S2* emission for sulfur and green HPO* emission for phosphorus, similar to a conventional FPD. Sulfur response in the micro-FPD is quadratic over nearly 3 orders of magnitude while that of phosphorus is linear over nearly 5 orders of magnitude. The micro-FPD detection limit for sulfur is 1 x 10(-9) g S s(-1), and that of phosphorus is 2 x 10(-10) g P s(-1). The properties established for the initial prototype of the micro-FPD make this counter-current flame method potentially suitable for integration with on-chip gas chromatography or other micro-analytical devices where flame-based detection methods are desirable.

Journal Article↗