Biomedical subjects

Richard Sacks

Publications and source records attributed to Richard Sacks.

12 recordsLinked to original sources

PubMed2006

Analysis of human breath samples with a multi-bed sorption trap and comprehensive two-dimensional gas chromatography (GCxGC).

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 (GCxGC) for the analysis of organic compounds in human breath samples. The first-column effluent of the GCxGC 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 cm3/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 GCxGC. Performance data for the combined GCxGC/sorption-trap instrument is described, and human breath-sample chromatograms are presented.

Breath Tests↗

PubMed2006

High-performance, static-coated silicon microfabricated columns for gas chromatography.

A procedure is described for the preparation of high-performance etched silicon columns for gas chromatography. Rectangular channels, 150 mum wide by 240 mum deep are fabricated in silicon substrates by gas-phase reactive ion etching. A 0.1-0.2-mum-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 approximately 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 degrees 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 degrees C/min obtains a peak capacity of over 100 peaks with a resolution of 1.18 and a separation time of approximately 500 s. With a 0.25-m-long column heated at 30 degrees 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.

Air Pollutants↗

PubMed2006

At-column heating and a resistively heated, liquid-cooled thermal modulator for a low-resource bench-top GC x GC.

A transportable GC x GC instrument is under development for on-site applications that would benefit from the enhanced resolution and powers of detection, which can be achieved by this method. In the present study, a low-resource GC x GC instrument using an electrically heated and liquid-cooled single-stage thermal modulator that requires no cryogenic materials is evaluated. The instrument also uses at-column heating, thus eliminating the need for a convection oven to house the two columns. The stainless-steel modulator tube is coated with PDMS, which can be heated to 350 degrees C for sample injection into the second-dimension column. The modulator is cooled to -30 degrees C by a 100 mL/min flow of PEG by means of a commercial liquid chiller and a small recirculating pump. Resistive heating of the modulator tube is provided by a programmable power supply, which uses a voltage program that results in increasing modulator temperature during an analysis. This, together with more rapid cooling by the use of a liquid cooling medium, results in reduced solute breakthrough following each heating cycle as the modulator cools to a temperature where quantitative trapping resumes. As a result, modulated peak widths at half-height of less than 40 ms are observed. Design and performance details are presented along with chromatograms of gasoline and an essential oil sample.

Journal Article↗

PubMed2005

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.

Chromatography, Gas↗

PubMed2005

Band acceleration device for enhanced selectivity with tandem-column gas chromatography.

An electrically heated and air cooled metal sheath surrounding the first 50 cm of the second column in a series-coupled, capillary-column ensemble of a non-polar and a polar column is used to obtain enhanced isothermal separation of component pairs that are separated by the first column in the ensemble but co-elute from the ensemble by virtue of the different selectivity of the two columns. As the first of the two components passes into the second column, a current pulsed through the metal sheath rapidly heats the first 50 cm of the second column thus accelerating the band for the first component. Ensemble retention-time shifts of several seconds are easily obtained. The device is then rapidly cooled to quiescent oven temperature by a flow of pressured air through the space between the metal sheath and the fused silica capillary column and an additional flow through a larger, co-axial plastic tube. Both heating and cooling require only a few seconds. If substantial cooling of the device occurs before the band for the second component enters the device, the band experiences less thermally-induced acceleration with the result that the separation of the two targeted components is enhanced in the ensemble chromatogram with no significant change in the pattern of peaks for the other mixture components. If the device is cooled to a temperature below oven temperature before the arrival of the band for the second component, this band will be slowed, and further enhancement of separation is achieved in the ensemble chromatogram. A band trajectory model, based on retention factor versus temperature data for the two components in the two columns, is used to predict peak separation and to aid in the selection of temperature-pulse initiation times.

Chromatography, Gas↗

PubMed2005

Electrically heated, air-cooled thermal modulator and at-column heating for comprehensive two-dimensional gas chromatography.

An instrument for comprehensive two-dimensional gas chromatography (GCxGC) is described using an electrically heated and air-cooled thermal modulator requiring no cryogenic materials or compressed gas for modulator operation. In addition, at-column heating is used to eliminate the need for a convection oven and to greatly reduce the power requirements for column heating. The single-stage modulator is heated by current pulses from a dc power supply and cooled by a conventional two-stage refrigeration unit. The refrigeration unit, together with a heat exchanger and a recirculating pump, cools the modulator to about -30 degrees C. The modulator tube is silica-lined stainless steel with an internal film of dimethylpolysiloxane. The modulator tube is 0.18 mm i.d. x 8 cm in length. The modulator produces an injection plug width as small as 15 ms.

Journal Article↗

PubMed2005

Stop-flow programmable selectivity with a dual-column ensemble of microfabricated etched silicon columns and air as carrier gas.

A series-coupled ensemble of microfabricated GC columns made by dry reactive ion etching of silicon substrates is evaluated for use with pneumatic selectivity enhancement techniques for targeted pairs of volatile organic compounds. Each column is 3.0 m long with a 150 miceom wide by 240 microm deep cross section. Dynamic coating was used to prepare a nonpolar column with a dimethyl polysiloxane stationary phase and a moderately polar column with a trifluoropropylmethyl polysiloxane stationary phase. Each column generates 5000-6000 theoretical plates. The columns are operated in series with the nonpolar column connected to a split inlet, the polar column connected to a flame ionization detector, and a valve connected between the column junction point and the inlet to the first column. When the valve is closed, the effluent from the first column passes directly into the second column. When the valve is open, both ends of the first column are at the inlet pressure, and flow stops in this column while increased flow is obtained in the second column. For analyte pairs that are separated by the first column but coelute from the column ensemble, the valve is opened for a few seconds after the first component of the pair has passed into the second column but the second component is still in the first column. The result is enhanced separation of the pair in the ensemble chromatogram. Relatively thick cross-linked stationary-phase films are used to increase retention for volatile compounds. The combination of air carrier gas and stationary-phase film thickness in the range 1-2 microm requires the use of relatively low average carrier gas velocities (typically less than 10 cm/s) for adequate resolving power of the column ensemble. Selectivity enhancement under isothermal conditions for a 14-component mixture of volatile organic compounds is demonstrated where neither of the columns alone nor the column ensemble without selectivity enhancement could obtain a complete separation.

Air↗

PubMed2004

Design, fabrication, and evaluation of microfabricated columns for gas chromatography.

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-microm-wide, 240-microm-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 x 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 approximately 1500 plates more than the oxidized surface for both carrier gases. For the polar phase, the oxidized surface gave approximately 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.

Journal Article↗

PubMed2003

Characterization and quantitative analysis with GC/TOFMS comparing enhanced separation with tandem-column stop-flow GC and spectral deconvolution of overlapping peaks.

Time-of-flight mass spectrometry is unique in that ion abundance ratios are constant over the chromatographic peak profile provided that the peak contains only one component. This provides the means for the automated finding and spectral deconvolution of overlapping chromatographic peaks from completely unknown mixtures if the mass spectra for the overlapping components are sufficiently unique. This can greatly reduce the chromatographic resolution requirements, which allows for very rapid quantitative analysis as well as for high-speed mixture characterization. High-speed GC with stop-flow operation of a series-coupled column ensemble can be used to completely separate some component pairs that coelute from the column ensemble, thus eliminating the need for spectral deconvolution of those mixture components. This provides two options for high-speed qualitative and quantitative analysis, using either the mass spectra from deconvoluted overlapping peaks or the mass spectra from the completely separated peaks obtained with stop-flow operation of the tandem column ensemble. These options are compared with respect to the similarity for spectral matching with a library and to peak area linearity with concentration, calibration plot correlation coefficients, and shot-to-shot reproducibility.

Journal Article↗

PubMed2003

Programmable selectivity for GC with series-coupled columns using pulsed heating of the second column.

A series-coupled ensemble of a nonpolar dimethyl polysiloxane column and a polar trifluoropropylmethyl polysiloxane column with independent at-column heating is used to obtain pulsed heating of the second column. For mixture component bands that are separated by the first column but coelute from the column ensemble, a temperature pulse is initiated after the first of the two components has crossed the column junction point and is in the second column, while the other component is still in the first column. This accelerates the band for the first component. If the second column cools sufficiently prior to the second component band crossing the junction, the second band experiences less acceleration, and increased separation is observed for the corresponding peaks in the ensemble chromatogram. High-speed at-column heating is obtained by wrapping the fused-silica capillary column with resistance heater wire and sensor wire. Rapid heating for a temperature pulse is obtained with a short-duration linear heating ramp of 1000 degrees C/min. During a pulse, the second-column temperature increases by 20-100 degrees C in a few seconds. Using a cold gas environment, cooling to a quiescent temperature of 30 degrees C can be obtained in approximately 25 s. The effects of temperature pulse initiation time and amplitude on ensemble peak separation and resolution are described. A series of appropriately timed temperature pulses is used to separate three coeluting pairs of components in a 13-component mixture.

Algorithms↗

PubMed2002

High-speed characterization and analysis of orange oils with tandem-column stop-flow GC and time-of-flight MS.

High-speed GC with time-of-flight (TOF) MS detection is used for the characterization and analysis of oils rendered from the peel of five diverse species of orange including bergemot orange, bitter orange, tangerine, mandarin orange, and sweet orange. With a user-defined signal-to-noise threshold of 100, 44 peaks were found and 36 compounds identified in the various oils. Some major constituent components show large concentration ranges over the five species. A 14-m-long, 0. 18-mm-i.d. column ensemble consisting of 7.0-m lengths of a trifluoropropylmethyl polysiloxane and a 5% phenyl dimethyl polysiloxane column was temperature-programmed at 50 degrees C/min starting at the time of injection to achieve analysis times under 140 s. The TOFMS was operated with a spectral acquisition rate of 25 spectra/s, and automated peak finding software successfully found all of the components, with the exception of one severely overlapping peak pair in bitter-orange oil. Of the 44 peaks, 25 were identified by use of a TOFMS library created for this study; another 11 were identified with a commercial terpene library, and 8 were not identified. A quantitative comparison (percent of total peak area) is presented for 16 components, which comprise 98.8-99.5% of the total peak area for the five orange species. Stop-flow operation of the column ensemble is used to enhance selectivity for targeted component pairs to facilitate single-channel detection for QA/ QC analysis of characterized samples and to enhance column selectivity for TOFMS characterization in cases in which peak overlap is so severe that only a single peak is observed.

Chromatography, Gas↗

PubMed2002

Selectivity enhancement for high-speed GC analysis of volatile organic compounds with portable instruments designed for vacuum-outlet and atmospheric-pressure inlet operation using air as the carrier gas.

A tandem ensemble of two 4.5-m-long x 0.25-mm-i.d. capillary columns with the first using a 0.50-microm film of nonpolar dimethyl polysiloxane and the second using a 0.25-microm film of polar trifluoropropylmethyl polysiloxane is operated with atmospheric pressure air as the carrier gas and an outlet pressure of 50.5 kPa established using a small vacuum pump. A thicker stationary-phase film is used in the first column to increase retention for very volatile compounds. This significantly increases the resolution of these compounds. The thicker film in the first (nonpolar) column decreases the polarity of the tandem column ensemble and, thus, changes its selectivity. A low-dead-volume valve, connected between the column junction point and a source of atmospheric pressure air, is used to obtain pulsed modulation of the carrier gas flow through the column ensemble. When the valve is open, the ensemble inlet pressure and the junction-point pressure are nearly the same, and carrier gas flow nearly stops in the first column, and flow in the second column increases. Enhanced resolution of a component pair that is separated by the first column but coelutes from the column ensemble can be obtained if the valve is opened for a few seconds after one of the components has crossed the junction and is in the second column, but the other component is still in the first column. A sequence of appropriately timed pulses is used to obtain enhanced resolution of several pairs of components that coelute from the column ensemble. These methods enabled the complete separation of an 18-component vapor mixture of common solvents in air in 3.5 min.

Air↗