A procedure is described for the preparation of high-performance etched silicon columns for gas chromatography. Rectangular channels, 150 μm wide by 240 μm deep are fabricated in silicon substrates ...by gas-phase reactive ion etching. A 0.1−0.2-μm-thick film of dimethyl polysiloxane stationary phase is deposited on the channel walls by filling the channel with a dilute solution in 1:1 n-pentane and dichloromethane and pumping away the solvent. A thermally activated cross-linking agent is used for in situ cross-linking. A 3-m-long microfabricated column generated ∼12 500 theoretical plates at optimal operating conditions using air as carrier gas. A kinetic model for the efficiency of rectangular cross-section columns is used to evaluate column performance. Results indicate an additional source of gas-phase dispersion beyond longitudinal diffusion and nonequilibrium effects, probably resulting from numerous turns in the gas flow path through the channel. The columns are thermally stable to at least 180 °C using air carrier gas. Temperature programming is demonstrated for the boiling point range from n-C5 to n-C12. A 3.0-m-long column heated at 10 °C/min obtains a peak capacity of over 100 peaks with a resolution of 1.18 and a separation time of ∼500 s. With a 0.25-m-long column heated at 30 °C/min, a peak capacity of 28 peaks is obtained with a separation time of 150 s. Applications are shown for the analysis of air-phase petroleum hydrocarbons and the high-speed analysis of chemical warfare agent and explosive markers.
A method for the determination of volatile organic compounds (VOCs) at subtrace levels in breath samples based on a multibed sorption trap for the collection and concentration of VOCs, a ...comprehensive multidimensional gas chromatograph (GC×GC) for the separation of complex mixtures, and a time-of-flight mass spectrometer detector is designed and developed. The good performance of the trap tube device developed for the concentration together with the high sensitivity and separation power of the GC×GC results in a powerful system. In the analysis of samples, more than 100 different compounds are detected of which between 65 and 85 are clearly identified. A total of ∼250 different compounds are observed in all the samples evaluated of which 142 are identified. A preliminary study to evaluate breath biomarkers for active smoking is performed. The levels of previously described biomarkers are found to be strongly time-dependent with amounts found ∼1 h after smoking returning to the levels found in nonsmoking volunteers. However, 2,5-dimethylfuran, 2-methylfuran, and furan are found to be effective biomarkers given that they were only found in samples taken from smokers and could still be detected more than 2 h after smoking.
The fabrication, assembly, and initial testing of a hybrid microfabricated gas chromatograph (microGC) is described. The microGC incorporates capabilities for on-board calibration, sample ...preconcentration and focused thermal desorption, temperature-programmed separations, and "spectral" detection with an integrated array of microsensors, and is designed for rapid determinations of complex mixtures of environmental contaminants at trace concentrations. Ambient air is used as the carrier gas to avoid the need for on-board gas supplies. The microsystem is plumbed through an etched-Si/glass microfluidic interconnection substrate with fused silica capillaries and employs a miniature commercial pump and valve subsystem for directing sample flow. The latest performance data on each system component are presented followed by first analytical results from the working microsystem. Tradeoffs in system performance as a function of volumetric flow rate are explored. The determination of an 11-vapor mixture of typical indoor air contaminants in less than 90 s is demonstrated with projected detection limits in the low part-per-billion concentration range for a preconcentrated air-sample volume of 0.25 L.
The design, fabrication, and performance of gas chromatography columns etched in silicon substrates are described. Deep reactive-ion etching formed the 3-m-long, 150-μm-wide, 240-μm-deep rectangular ...cross section channels. A glass cover plate was anodically bonded to the remaining surface of the substrate forming the gastight channel. For some of the columns, the silicon channels were oxidized before the channels were sealed with the glass plates. Fused-silica capillary connecting tubes were sealed into ports on the edge of the 3.2-cm × 3.2-cm substrate chips. Dynamic coating was used to deposit a film of nonpolar dimethyl polysiloxane or moderately polar trifluoropropylmethyl polysiloxane stationary phase. The columns were evaluated in a conventional benchtop GC instrument with split injection and flame ionization detection. Column efficiency was evaluated by the use of plots of height equivalent to a theoretical plate versus average carrier gas velocity using both hydrogen and air as carrier gases. The number of theoretical plates measured at the average carrier gas velocity giving the minimum plate height ranged from 4600 to 8200 plates for the dimethyl polysiloxane columns and from 3500 to 5500 plates for the trifluoropropylmethyl polysiloxane columns. Minimum plate height was significantly smaller with air as carrier gas. For the nonpolar phase, the nonoxidized surface gave ∼1500 plates more than the oxidized surface for both carrier gases. For the polar phase, the oxidized surface gave ∼200 plates more than the nonoxidized surface. Isothermal chromatograms of a 20-component multifunctional mixture and temperature-programmed chromatograms of a normal alkane mixture are presented.
This paper presents the theory, fabrication, and experimental results for a high-performance low-power micro gas chromatography column. The suspended-dielectric 1-m-long column is split into two ...sections, permitting independent pressure and temperature programming. Integrated column heaters have a mean resistance of 16.8 kOmega and a temperature coefficient of resistance of 431 ppm/degC. The suspended column requires 11 mW to raise its temperature by 100degC in vacuum (1 mtorr). The column separates ten volatile organic compounds in 52 s and four chemical warfare agent simulants and an explosive simulant in 60 s.
The combination of a tandem column ensemble and an on-line microsorption trap is used for the analysis of organic compounds in human breath samples. The four-bed sorption trap uses a series of ...discreet sorption beds containing three grades of graphitized carbon and a carbon molecular sieve to quantitatively remove most organic compounds from 0.8-L breath samples. The trap is then heated to 300 °C in ∼1.5 s and maintained at this temperature for 10 s. The resulting vapor plug width is in the range 0.7−1.3 s for the compounds found in the breath samples. The separation is performed with a 15-m-long, 0.25-mm-i.d. capillary using a 0.5-μm-thick film of nonpolar dimethyl polysiloxane coupled in series to a polar column, either trifluoropropylmethyl polysiloxane or poly(ethylene glycol). Both column combinations are successful in separating the early-eluting compounds acetone, isoprene, pentane, methyl alcohol, and ethyl alcohol, which are all common in breath samples. The poly(ethylene glycol) combination gave better separation but showed relatively fast deterioration for repeated analysis of wet samples. Breath samples were obtained under different conditions (smoker, nonsmoker, gum chewer), and 25 compounds were identified in the various samples. Many additional peaks are observed but not identified. Analytical curves (log−log) of peak area versus sample volume for test compounds are linear in the range 80−800 cm3. Detection limits (3σ) for several volatile compounds in 800-cm3 samples are in the 1−5 ppb range.
The reddish brown haze that surrounds Titan, Saturn's largest moon, is thought to consist of tholin-like organic aerosols. Tholins are complex materials of largely unknown structure. The very high ...peak capacity and structured chromatograms obtained from comprehensive two-dimensional GC (GC
×
GC) are attractive attributes for the characterization of tholin pyrolysis products. In this report, GC
×
GC with time-of-flight MS detection and a flash pyrolysis inlet is used to characterize tholin pyrolysis products. Identified pyrolysis products include low-molecular-weight nitriles, alkyl substituted pyrroles, linear and branched hydrocarbons, alkyl-substituted benzenes and PAH compounds. The pyrolysis of standards found in tholin pyrolysate showed that little alteration occurred and thus these structures are likely present in the tholin material.
Chronic thromboembolic pulmonary hypertension (CTEPH) is characterized by a fibrotic thrombus persisting and obliterating the lumen of pulmonary arteries; its pathogenesis remains poorly defined. ...This study investigates a potential contribution for progenitor cell types in the development of vascular obliteration and remodeling in CTEPH patients. Endarterectomized tissue from patients undergoing pulmonary thromboendarterectomy was collected and examined for the structure and cellular composition. Our data show an organized fibrin network structure in unresolved thromboemboli and intimal remodeling in vascular wall tissues, characterized by smooth muscle alpha-actin (SM-alphaA)-positive cell proliferation in proximal regions (adjacent to thromboemboli) and neoangiogenesis/recanalization in distal regions (downstream from thromboemboli). Cells that are positively stained with CD34 and fetal liver kinase-1 (Flk-1) (CD34(+)Flk-1(+)) were identified in both the proximal and distal vascular tissues; a subpopulation of CD34(+)Flk-1(+)CD133(+) cells were further identified by immunostaining. Triple-positive cells are indicative of a population of putative endothelial progenitor cells or potential colony-forming units of endothelial cells. In addition, inflammatory cells (CD45(+)) and collagen-secreting cells (procollagen-1(+)) were detected in the proximal vascular wall. Some of the CD34(+) cells in CTEPH cells isolated from proximal regions were also positive for SM-alphaA. Our data indicate that putative progenitor cell types are present in the neointima of occluded vessels of CTEPH patients. It is possible that the microenvironment provided by thromboemboli may promote these putative progenitor cells to differentiate and enhance intimal remodeling.
A multi-bed sorption trap designed to quantitatively collect volatile organic compounds from large-volume vapor samples and inject them into a gas chromatograph is combined with a comprehensive ...two-dimensional gas chromatograph (GC
×
GC) for the analysis of organic compounds in human breath samples. The first-column effluent of the GC
×
GC is modulated by a single-stage, resistively-heated and air-cooled segment of 0.18-mm i.d. stainless steel column using the same stationary phase as the first column. Cooling gas is provided by a two-stage conventional refrigeration system, and thus no consumables other than carrier gas and electric power are required. The sorption trap uses four discreet beds, three containing different grades of graphitized carbon and one containing a carbon molecular sieve. The ordering of the beds in the trap tube is from the weakest to strongest adsorbent during sample collection. Breath samples are collected in gas sampling bags, and samples are passed through the trap at a flow rate of about 50
cm
3/min. After sample collection, hydrogen carrier gas flow is initiated in the direction opposite to the sample collection flow, and the metal trap tube is resistively heated to inject a sample plug into the GC
×
GC. Performance data for the combined GC
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GC/sorption-trap instrument is described, and human breath-sample chromatograms are presented.
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