Programmable control of column selectivity for temperature-programmed GC

A computer-driven pressure controller connected to the junction point of a series-coupled ensemble of two capillary GC columns having different stationary-phase selectivity is used to obtain on-the-fly (programmable) changes in ensemble selectivity. Changes in the junction-point pressure result in differential changes in the local carrier gas velocity in the two columns, and this results in changes in the pattern of peaks eluting from the ensemble. When used with relatively fast temperature programming (30 degrees C/min), the pattern of eluting peaks can be very sensitive to the time at which a selectivity (junction-point pressure) change is implemented. These elution pattern changes are described for a set of six PCB congeners that elute with a small range of retention times. The components are considered as a group, and changes in their elution pattern are described for a single junction-point pressure change, which is implemented at various times after sample injection. If the pressure change is implemented after the components have migrated across the junction point, the final pressure has relatively little impact on the ensemble retention pattern. Pressure changes implemented prior to the components reaching the junction can have a large effect and usually result in a pattern of peaks similar to the pattern obtained when the final pressure is used for the entire separation. For pressure changes made when the group of components is near the junction point, the observed peak pattern may be very sensitive to the time of the pressure change. The time at which the junction-point pressure change occurs is varied in 1.0-s intervals. Artifacts such as peak doubling and peak focusing or broadening are observed if a migrating band is crossing the column junction point at the time of the programmed pressure change.     

High-speed GC and GC/MS with a series-coupled column ensemble using stop-flow operation

A pneumatically actuated valve is used to connect the junction point of a series-coupled column ensemble to a ballast chamber containing carrier gas at the ensemble inlet pressure in order to periodically stop the carrier gas flow in the first column. When the valve is opened, mixture components, which have migrated across the column junction, are accelerated toward a time-of-flight mass spectrometer that is used as an ensemble detector. Mixture components, which are still in the first column, are frozen in position. This allows for the insertion of time windows into the ensemble chromatogram that can aid in the separation of some overlapping component peaks. The capillary column ensemble (0.18-mm i.d. x 0.18-microm film thickness) consists of a 7.0-m length of polar, (trifluoropropyl)methyl polysiloxane column followed by a 7.0-m length of nonpolar dimethyl polysiloxane column. A flame ionization detector located at the column junction point is used to monitor a portion of the effluent from the first column in order to determine the valve timing sequence needed to enhance the separation of component pairs that are separated by the first column but coelute from the column ensemble. When one of the components of a targeted pair has crossed the junction but the other component is still in the first column, the valve is opened, typically for 1-5 s. The stop-flow system is used to enhance the separation of a mixture containing some common essential oil components and a mixture containing some common pesticides.     

Band-trajectory model for temperature-programmed series-coupled column ensembles with pressure-tunable selectivity

A model and a spreadsheet algorithm is described for the prediction of solute-band migration trajectories in a series-coupled combination of two capillary GC columns with pressure-tunable and -programmable selectivity and operated under temperature-programmed conditions. The model takes into account the acceleration of carrier gas in the two columns as a result of decompression effects, the deceleration of carrier gas as a result of the increase in viscosity during temperature programming, the decrease in solute retention factors with increasing temperature during the temperature program, the differences in retention factors for the two columns, and programmed changes in the carrier-gas flow rates in the two columns during selectivity programming. In the model, the 20-meter-long column ensemble is divided into 1-cm-long intervals, and the carrier-gas velocity and column temperature are assummed to be constant in any interval. Migration times for all of the mixture solutes are computed for each column interval, and the solute-band positons in the column ensemble are plotted versus the running sum of these migration times to obtain band trajectory plots. The sum of these migration times for all 2,000 intervals gives the ensemble retention times for the solutes. Isothermal retention factors (k) for all of the mixture components at various column temperatures (Tc) are used as imput to the algorithm. Slope and intercept values of In(k) vs 1/Tc plots are used in the algorithm. General features of the model are tested using a mixture of C12-C24 normal alkanes. A mixture of polar and nonpolar compounds is used to test the utility of the model for the predicition of peak separations and retention times with pressure-tunable and -programmable selectivity. Good agreement is observed in all cases.     

Pressure-tunable column selectivity for high-speed vacuum-outlet GC

A pressure-tunable ensemble of two series-coupled capillary columns operated at subambient outlet pressure is described. The ensemble consists of a 4.5-m length of nonpolar dimethyl polysiloxane column followed by a 7.5-m length of polar trifluoropropylmethyl polysiloxane column. Air at an inlet pressure of 1.0 atm is used as carrier gas, and a vacuum pump is used to pull the carrier gas and injected samples through the column ensemble. Detection is provided by a photoionization detector operated at a pressure of 0.3 psia. Ensemble selectivity is controlled by means of an electronic pressure controller located at the junction point between the columns. The minimum pressure step size is 0.1 psi, and 50 different set-point pressures can be used, each one producing a different pattern of peaks eluting from the column ensemble. Measured ensemble retention factors for a set of target compounds produce straight lines when plotted versus the ratio of the calculated holdup time of the first column in the ensemble to the total ensemble holdup time. A component band trajectory model is used to describe the effects of ensemble junction-point pressure on the elution patterns generated by the ensemble. Ensemble retention times predicted by the model are in good agreement with values obtained from chromatograms. The use of on-the-fly set-point pressure changes during a separation (selectivity programming) is demonstrated and used to improve the quality of the separation of a 19-component test mixture.     

GC analysis of human breath with a series-coupled column ensemble and a multibed sorption trap

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 degrees C in approximately 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-microm-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 (3sigma) for several volatile compounds in 800-cm3 samples are in the 1-5 ppb range.     

Temperature-programmed GC using silicon microfabricated columns with integrated heaters and temperature sensors

Columns were fabricated in silicon substrates by deep reactive-ion etching. The channels were sealed with a glass wafer anodically bonded to the silicon surface. Heaters and temperature sensors were fabricated on the back side of each column chip. A microcontroller-based temperature controller was used with a PC for temperature programming. Temperature programming, with channel lengths of 3.0 and 0.25 m, is described. The 3.0-m-long channel was fabricated on a 3.2 cmx3.2 cm chip. Four columns were fabricated on a standard 4-in. silicon wafer. The 0.25-m-long channel was fabricated on a 1.1 cmx1.1 cm chip, and approximately 40 columns could be fabricated on a 4-in. wafer. All columns were coated with a nonpolar poly(dimethylsiloxanes) stationary phase. A static coating procedure was employed. The 3.0-m-long column generated about 12000 theoretical plates, and the 0.25-m-long channel generated about 1000 plates at optimal carrier gas velocity. Linear temperature ramps as high as 1000 degrees C/min when temperature programmed from 30 to 200 degrees C were obtained with the shorter column. With the 0.25-m-long column, normal alkanes from n-C5 through n-C15 were eluted in less than 12 s using a temperature ramp rate of 1000 degrees C/min. Temperature uniformity over the column chip surface was measured with infrared imaging. A variation of about 2 degrees C was obtained for the 3.0-m-long channel. Retention time reproducibility with temperature programming typically ranged from +/-0.15% to +/-1.5%. Design of the columns and the temperature controller are discussed. Performance data are presented for the different columns lengths.     

Pulsed flow modulation for high-speed GC using a pressure-tunable column ensemble

A computer-driven pressure controller is used to deliver pressure pulses to the junction point of two series-coupled columns using different stationary-phase chemistries. The column ensemble consists of a trifluoropropylmethyl polysiloxane column followed by a dimethyl polysiloxane column. Each pressure pulse causes a differential change in the carrier gas velocities in the two columns, which lasts for the duration of the pulse. A pressure pulse is used to selectively increase the separation of a component pair that is separated by the first column but coelutes from the series-coupled ensemble. If both components are on the same column when the pulse is applied, a small change in the ensemble separation occurs. If one component of the pair is on the first column and the other component is on the second column, a pressure pulse can result in a much larger change in the ensemble separation for the component pair. A model with a spreadsheet algorithm is used to predict the effects of a pressure pulse on the trajectories of component bands on the column ensemble. The effect of the initiation time of a pressure pulse is investigated for a two-component mixture that coelutes from the column ensemble. For the case where the entire pressure pulse occurs when one of the components is on the first column and the other component is on the second column, the peak separation from the ensemble increases nearly linearly with the product of the pressure pulse amplitude and the pulse duration. Peak shape artifacts are observed if the pressure pulse occurs when a solute band is migrating across the column junction point.     

Determination of unbiased selectivity coefficients using pulsed chronopotentiometric polymeric membrane ion sensors

A new procedure for the determination of selectivity coefficients of neutral carriers using pulsed chronopotentiometric ion selective sensors (pulstrodes) is established. Pulstrode membrane which lacks an ion-exchanger suppresses the zero current ion flux, allowing a Nernstian response slope for even highly discriminated ions. Unlike previously developed methods, unbiased selectivity remains unaltered even with the exposure to the primary ion solution for prolonged time. Studies with potassium-, silver-, and calcium-selective electrodes reveal that pulstrodes yield the same or slightly favorable unbiased selectivity coefficients than reported earlier. In contrast to alternative methods for the determination of unbiased selectivity, this technique offers a unique simplicity and reliability. Therefore the new procedure promises to be a valuable additional tool for the characterization of unbiased selectivity coefficients for the ISEs.     

Micromachined GC columns for fast separation of organophosphonate and organosulfur compounds

This article demonstrates how to prepare microfabricated columns (microcolumns) for organophosphonate and organosulfur compound separation that rival the performance of commercial capillary columns. Approximately 16,500 theoretical plates were generated with a 3 m long OV-5-coated microcolumn with a 0.25 microm phase thickness using helium as the carrier gas at 20 cm/s. Key to the advance was the development of deactivation procedures appropriate for silicon microcolumns with Pyrex tops. Active sites in a silicon-Pyrex microcolumn cause peak tailing and unwanted adsorption. Experimentally, we found that organosilicon hydride deactivation lowers adsorption activity in microcolumns more than silazane and silane treatments. But without further treatment, the phosphonate peaks continue to tail after the coating process. We found that heat treatment with pinacolyl methylphosphonic acid (PMP) eliminated the phosphonate peak tailing. In contrast, conventional resilylation employing N, O-bis(trimethylsilyl)acetamide, hexamethyldisilazane, and 1-(trimethylsilyl)imidazole does not eliminate peak tailing. Column activity tests show that the PMP treatment also improves the peaks for 2,6-dimethyl aniline, 1-octanol, and 1-decanol implying a decrease in the column's hydrogen bonding sites with the PMP treatment. FT-IR analysis shows that exposure to PMP forms a bond to the stationary phase that deactivates the active sites responsible for organophosphonate peak tailing.     

Silicon microfabricated column with microfabricated differential mobility spectrometer for GC analysis of volatile organic compounds

A 3.0-m-long, 150-microm-wide, 240-microm-deep channel etched in a 3.2-cm-square silicon chip, covered with a Pyrex wafer, and coated with a dimethyl polysiloxane stationary phase is used for the GC separation of volatile organic compounds. The column, which generates approximately 5500 theoretical plates, is temperature-programmed in a conventional convection oven. The column is connected through a heated transfer line to a microfabricated differential mobility spectrometer. The spectrometer incorporates a 63Ni source for atmospheric-pressure chemical ionization of the analytes. Nitrogen or air transport gas (flow 300 cm(3)/min) drives the analyte ions through the cell. The spectrometer operates with an asymmetric radio frequency (RF) electric field between a pair of electrodes in the detector cell. During each radio frequency cycle, the ion mobility alternates between a high-field and a low-field value (differential mobility). Ions oscillate between the electrodes, and only ions with an appropriate differential mobility reach a pair of biased collectors at the downstream end of the cell. A compensation voltage applied to one of the RF electrodes is scanned to allow ions with different differential mobilities to pass through the cell without being annihilated at the RF electrodes. A unique feature of the device is that both positive and negative ions are detected from a single experiment. The combined microfabricated column and detector is evaluated for the analysis of volatile organic compounds with a variety of functionalities.     

Temperature measurements in the pulsed laser heating of metals

The results of an investigation of the temperature dynamics of certain metals, measured by a photoemission method with a time resolution of 1 μsec, when they are heated in air and in an argon medium by millisecond laser radiation (λ = 1.06 μm) with energy densities of 75–140 J/cm² are presented.     

Hihg-speed GC/MS of gasoline-range hydrocarbon compounds using a pressure-tunable column ensemble and time-of-flight detection

A pressure-tunable series-coupled ensemble of two capillary GC columns is combined with a time-of-flight MS detector for the high-speed characterization of mixtures containing hydrocarbon compounds. The column ensemble consists of a nonpolar 5% phenyl poly(dimethylsiloxane) column and a very polar poly(ethylene glycol) column. The TOFMS instrument uses time-array detection to obtain up to 500 complete electron mass spectra per second. Instrument software allows for automated peak finding and the spectral deconvolution of severely overlapping unknown chromatographic peaks, if their fragmentation patterns are significantly different and if at least two spectra can be recorded between the peak apexes. By adjusting the carrier-gas pressure at the column-junction point, the separations between adjacent peak pairs can be adjusted to enhance the capabilities of the TOFMS detector. The sensitivity of peak-pair separation to changes in junction-point pressure is studied for combinations of alkanes, olefins, and aromatic compounds. When complete separation is required, the use of pressure-tunable column ensembles cannot always provide sufficient control of peak-pair separation for structurally similar compounds. However, complete chromatographic separation typically is not required with the TOFMS detection, and a pressure-tunable column ensemble is very useful for the high-speed characterization of hydrocarbon mixtures.     

Thermal processes and stability of a superconductive dipole magnet with pulsed heating of the winding

Results of an experimental and numerical study of thermal processes occurring upon pulsed heating of the superconductive winding of a dipole magnet are presented.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 48, No. 2, pp. 301–305, February, 1985.     

Design and performance of a ZnSe tetra-prism for homogeneous substrate heating using a CO2 laser for pulsed laser deposition experiments

We report on the design and performance of a ZnSe tetra-prism for homogeneous substrate heating using a continuous wave CO(2) laser beam in pulsed laser deposition experiments. We discuss here three potential designs for homogenizing prisms and use ray-tracing modeling to compare their operation to an alternative square-tapered beam-pipe design. A square-pyramidal tetra-prism design was found to be optimal and was subjected to modeling and experimental testing to determine the influence of interference and diffraction effects on the homogeneity of the resultant intensity profile produced at the substrate surface. A heat diffusion model has been used to compare the temperature distributions produced when using various different source intensity profiles. The modeling work has revealed the importance of substrate thickness as a thermal diffuser in producing a resultant homogeneous substrate temperature distribution.     

A tandem column ensemble with an atmospheric pressure junction-point vent for high-speed GC with selective control of peak-pair separation

A series-coupled (tandem) ensemble of two capillary GC columns using different stationary phases and a pneumatically actuated low-volume valve connecting the column junction point to an atmospheric-pressure vent line is used to adjust the ensemble separation of selected pairs of target compounds. The valve is normally closed, and the pressure at the column junction point assumes the value that would occur in the absence of any other connections. The valve can be opened for brief periods of time, thus producing pulses of atmospheric pressure at the column junction point. If a component pair is separated by the first column but coelutes from the column ensemble, the ensemble separation can be increased if a pulse occurs when one of the components has migrated across the column junction but the second component is still on the first column. All of the mixture components that are on the same column during the time that the valve is open (pulse duration) will be shifted to either larger or smaller retention times, but the pattern of peaks (elution order) for these components from the column ensemble will be relatively unaffected by the pressure pulse. Multiple pulses can be used to enhance the separation of different component pairs, which sequentially reach the column junction point. Performance of the valve-operated system is described. Time-of-flight mass spectrometry with time-array detection is used to examine the effects of pulse duration on the separation achieved for different component pairs.     

Thermodynamic evaluation of distillation columns using exergy loss profiles: a case study on the crude oil atmospheric distillation column

This paper presents a case study on the crude oil atmospheric distillation column of Tabriz refinery plant to show the applicability of exergy loss profiles in thermodynamic examination of the different retrofit options. The atmospheric distillation column of Tabriz refinery has been revamped as a consequence of increase of the plant capacity to 100,000 bpd. To cover the deficit of feedstock of the revamped unit, a blend of the existing feedstock with imported crude oil is used as a feedstock. However, to investigate how the blend of these two different types of crudes as a feedstock has an influence on the operating conditions, the examination of the column is needed. Exergy as a comprehensive thermodynamic property which translates the temperature, pressure and composition change into a common unit has been chosen to evaluate the distillation column thermodynamically. Furthermore, the exergy loss profile of the base case serves as a scoping tool to pinpoint the source of inefficiencies. Then, the exergy loss profile as a screening tool has found the retrofit options which are likely to yield greatest energy saving from a list of retrofit options proposed by the industrial partner. In the presented case study, the exergy loss profile identifies the best retrofit option with 17.16% reduction in exergy losses, which finally lead to 3.6% reduction of primary fuel demand.     

薄圆柱体高频感应温度场的测量和有限元模拟

本文利用自制的NiCr-NiAl热电偶精确地测量了薄圆柱工件端面高频感应温度场的分布;应用有限元分析软件ANSYS/Thermal对高频感应加热过程的温度场进行了模拟.热电偶测量的数据直观可信,但测取工件温度场分布费时费力;有限元分析则可较为快速获知工件各处的温度,弥补了实验工作的不足.本文研究结果显示,两种方法得出的结果具有可比性,能够相互参照.     

Investigation of the thermophysical properties of zirconium by subsecond pulsed heating technique

The heat capacity, enthalpy, and spectral (for the wavelength of 0.65 μm) emissivity of zirconium are investigated in the temperature range from 1000 to 2100 K by the method of single subsecond resistance pulsed heating. The heat of the α-β phase transition is measured. A number of singularities are discovered in the variation of the properties of material in the region of α-β transition and in the β phase, which are associated with variations in the oxide film and with the specific features of volume heat release during heating in the two-phase region.