Progressive thermal desorption of vapor mixtures from a preconcentrator with a porous metal foam internal architecture and variable thermal ramp rates

A vapor preconcentrator has been designed with the porous polymer (Tenax) packed into a highly porous metal foam to facilitate thermal conductivity and temperature uniformity throughout the bed of the preconcentrator during heating. Vapors were desorbed using linear temperature programming from room temperature to a maximum temperature of 170 or 200 degrees C; the programmed duration of the thermal ramp was varied from 10 to 180 s. The partial separation of vapor mixtures that are thermally desorbed from the preconcentrator has been examined in terms of a metric for resolution, using methyl ethyl ketone, toluene, and dimethyl methylphosphonate as a test mixture. Vapors desorbed as a sequence of partially separated overlapping peaks, as observed with a polymer-coated flexural plate wave sensor. It was shown that vapor mixture resolution improved as the total time of the thermal ramp was extended from 30 to 120 s. In this way, the preconcentrator serves to act as a preseparator in addition to its usual functions for sampling, signal modulation, and improving sensitivity. Overlapping peaks were modeled, and peak areas were extracted using an exponentially modified Gaussian model. Peak areas were independent of the thermal ramp rate. Uses of such preconcentrators with multivariate detectors, such as sensor arrays, are discussed.     

Comparisons of polymer/gas partition coefficients calculated from responses of thickness shear mode and surface acoustic wave vapor sensors

Apparent partition coefficients, K, for the sorption of toluene by four different polymer thin films on thickness shear mode (TSM) and surface acoustic wave (SAW) devices are compared. The polymers examined were poly(isobutylene) (PIB), poly(epichlorohydrin) (PECH), poly(butadiene) (PBD), and poly(dimethylsiloxane) (PDMS). Independent data on partition coefficients for toluene in these polymers were compiled for comparison, and TSM sensor measurements were made using both oscillator and impedance analysis methods. K values from SAW sensor measurements were about twice those calculated from TSM sensor measurements when the polymers were PIB and PECH, and they were also at least twice the values of the independent partition coefficient data, which is interpreted as indicating that the SAW sensor responds to polymer modulus changes as well as to mass changes. K values from SAW and TSM measurements were in agreement with each other and with independent data when the polymer was PBD. Similarly, K values from the PDMS-coated SAW sensor were not much larger than values from independent measurements. These results indicate that modulus effects were not contributing to the SAW sensor responses in the cases of PBD and PDMS. However, K values from the PDMS-coated TSM device were larger than the values from the SAW device or independent measurements, and the impedance analyzer results indicated that this sensor using our sample of PDMS at the applied thickness did not behave as a simple mass sensor. Differences in behavior among the test polymers on SAW devices are interpreted in terms of their differing viscoelastic properties.     

Development and calibration of field-effect transistor-based sensor array for measurement of hydrogen and ammonia gas mixtures in humid air

A sensor array for analyzing hydrogen and ammonia gas mixtures in humid air has been developed, built into a rugged system, and calibrated for laboratory testing. The sensor array is comprised of four chemically sensitive field-effect transistors (CHEMFETs). Chemically sensitive layers for the sensors were developed and tested using a Kelvin probe. A combination of catalytic and noncatalytic thin layers (palladium and polyaniline) was selected for the four-sensor array. The work function responses of the CHEMFET sensor array to mixtures of hydrogen, ammonia, and humid air were measured. Chemometric multivariate methods, linear and nonlinear partial least squares, were used for the calibration of the sensor array using gas mixtures in the concentration range from 0 to 10?000 ppm hydrogen and ammonia in humid air. The sensor array for ammonia showed good sensitivity, selectivity, response time, and stability and is recommended for field deployment. In contrast, the sensor array for hydrogen, though highly sensitive to hydrogen, demonstrated inadequate stability, requiring further development before deployment is recommended.     

Highly Sorbent Films Derived from Ni(SCN)(2)(4-picoline)(4) for the Detection of Chlorinated and Aromatic Hydrocarbons with Quartz Crystal Microbalance Sensors

A quartz crystal microbalance (QCM) spray-coated with a Ni(SCN)(2)(4-picoline)(4) film is a sensitive detector for small aromatic (benzene, toluene) and chlorinated (trichloroethylene, perchloroethylene) vapors with a planar molecular geometry. Frequency changes during transient exposures to these vapors are rapid and reversible. In contrast, frequency changes during transient exposures to carbon tetrachloride vapor exhibit a very slow rise and decay. Impedance studies demonstrate that the QCMs are responding only to mass changes in the film. Calibration curves exhibit both linear and near-saturation responses, depending on the vapor and vapor concentration. Partition coefficients obtained from the linear response regimes of the calibration curves are in the 10?000-100?000 range, more than an order of magnitude larger than the partition coefficients for a prototypical soft polymer, poly(isobutylene). Despite the absence of evidence for crystallinity by optical or X-ray diffraction methods, the spray-coated films appear to be forming clathrates with the organic vapors. The Ni(SCN)(2)(4-picoline)(4) film is promising for the development of very sensitive and partially selective piezoelectric sensors for nonpolar or weakly polar organic vapors in air.     

Parvovirus enteritis in vaccinated juvenile bush dogs

Parvovirus enteritis developed in 10 of 17 vaccinated juvenile bush dogs (Speothos venaticus) from 4 litters in a 5-month period. Nine dogs died. The first outbreak involved 6 of 9 bush dogs from 2 litters. Each had been vaccinated with a killed feline-origin parvovirus vaccine at 11 and 14 weeks of age. The 6 affected dogs became ill at 29 weeks of age and died. The second outbreak involved a litter of 6 bush dogs. Each had been vaccinated every 2 weeks starting at 5 weeks of age. Two were isolated from the colony at 16 weeks of age for treatment of foot sores. Three of the 4 nonisolated dogs developed parvovirus enteritis at 20 weeks of age; 2 died at 6 and 8 days, respectively, after onset of signs. The 3rd outbreak involved a litter of 2 bush dogs. Both had been vaccinated every 2 to 3 weeks, starting at 6 weeks of age. One of these dogs became ill at 17 weeks and died 13 days later. A litter of 6 maned wolves (Chrysocyon brachyurus) and a litter of 3 bush dogs were isolated from their parent colonies at 13 and 15 weeks of age, respectively. Each animal had been vaccinated weekly, beginning at 8 weeks of age, using an inactivated canine-origin parvovirus vaccine. None of the isolated animals developed the disease. Serologic testing during isolation did not reveal protective titers (greater than or equal to 1:80) against canine parvovirus in the bush dogs until they were 23 weeks old, whereas protective titers developed in the maned wolves when they were 14 to 18 weeks old. One hand-raised bush dog was vaccinated weekly, beginning at 8 weeks of age, and a protective titer developed by 21 weeks of age. It was concluded that the juvenile bush dogs went through a period during which maternal antibodies interfered with immunization, yet did not protect against the disease. When the pups were isolated from the colony during this period, then vaccinated repeatedly until protective titers developed, the disease was prevented.     

High-speed gas chromatography using synchronized dual-valve injection

A novel injection technique for high-speed gas chromatography is demonstrated. Synchronized dual-valve injection is shown to provide peak widths as low as 1.5 ms (width at half-height) for an unretained analyte. This was achieved using a 0.5-m DB-5 column with an internal diameter of 100 microm and a film thickness of 0.4 microm operated at a temperature of 150 degrees C with a column absolute head pressure of 85 psi, resulting in a dead time of only t(o) = 26 ms ( approximately 1900 cm/s, 26 mL/min). Using the DB-5 column in a 1-m length under the same instrumental parameters, with a resulting linear flow velocity of 935 cm/s (12.7 mL/min, t(o) = 117 ms), a minimum peak width of 3.3 ms was obtained. During an isothermal separation, 10 analytes were separated in a time window of 400 ms. A rigorous comparison of experimental and theoretical band-broadening data based on the Golay equation showed that band broadening is limited almost entirely by the chromatographic band broadening terms expressed by the Golay equation and not by extra column band broadening due to the injection process. Synchronized dual-valve injection offers a rugged and inexpensive design, providing extremely reproducible injections with peak height precision of 2.4% (RSD) and low run-to-run variation in retention times, with an average standard deviation less than 0.1 ms. Herein, synchronized dual-valve injection is demonstrated as a proof of principle using high-speed diaphragm valves. It is foreseen that the injection technique could be readily implemented using a combination of thermal modulation and high-speed valve hardware, thus optimizing the mass transfer and not significantly sacrificing the limit of detection performance for high-speed GC. Further implications are that, if properly implemented, high-speed temperature programming coupled with this new technology should lead to very large peak capacities for approximately 1-s separations.     

Radionuclide sensors based on chemically selective scintillating microspheres: renewable column sensor for analysis of (99)tc in water

A method for chemically selective radiometric sensing of non-γ-emitting radionuclides in solution is described. Using scintillating microspheres with selective radionuclide uptake properties, radiochemical separation and radiometric detection steps are integrated within a sensor device. These microspheres are loaded into a renewable minicolumn that serves to capture, preconcentrate, and separate radionuclides. The preconcentrating minicolumn also localizes and retains radionuclides within a detector of well-defined geometry and emits a photometric signal. The sensor material in the column can either be regenerated with eluent chemistries or be renewed by fluidic replacement of the beads. The latter method allows the use of materials that bind analytes irreversibly or are unstable under regeneration conditions. Radionuclide-selective scintillating microspheres were prepared by coimmobilization of scintillating fluors and selective organic extractants within the pores of an inert polymeric support. Preparation and characterization of microspheres, and their use for selective quantitative sensing of (99)Tc(VII), is described in detail. A sensor-based procedure for (99)Tc(VII) analysis was developed and successfully applied toward the determination of (99)Tc(VII) in groundwater samples from the Hanford site, using standard addition techniques for quantification. Using a 50-mL sample volume and signal accumulation time of 30 min, the detection limit for (99)Tc(VII) was 0.37 dpm/mL (9.8 pg/mL).