Application of a high surface area solid-phase microextraction air sampling device: collection and analysis of chemical warfare agent surrogate and degradation compounds.

This work examines a recently improved, dynamic air sampling technique, high surface area solid-phase microextraction (HSA-SPME), developed for time-critical, high-volume sampling and analysis scenarios. The previously reported HSA-SPME sampling device, which provides 10-fold greater surface area compared to commercially available SPME fibers, allowed for an increased analyte uptake per unit time relative to exhaustive sampling through a standard sorbent tube. This sampling device has been improved with the addition of a type-K thermocouple and a custom heater control circuit for direct heating, providing precise (relative standard deviation ∼1%) temperature control of the desorption process for trapped analytes. Power requirements for the HSA-SPME desorption process were 30-fold lower than those for conventional sorbent-bed-based desorption devices, an important quality for a device that could be used for field analysis. Comparisons of the HSA-SPME device when using fixed sampling times for the chemical warfare agent (CWA) surrogate compound, diisopropyl methylphosphonate (DIMP), demonstrated that the HSA-SPME device yielded a greater chromatographic response (up to 50%) relative to a sorbent-bed method. Another HSA-SPME air sampling approach, in which two devices are joined in tandem, was also evaluated for very rapid, low-level, and representative analysis when using discrete sampling times for the compounds of interest. The results indicated that subparts per billion by volume concentration levels of DIMP were detectable with short sampling times (∼15 s). Finally, the tandem HSA-SPME device was employed for the headspace sampling of a CWA degradation compound, 2-(diisopropylaminoethyl) ethyl sulfide, present on cloth material, which demonstrated the capability to detect trace amounts of a CWA degradation product that is estimated to be less volatile than sarin. The rapid and highly sensitive detection features of this device may be beneficial in decision making for law enforcement, military, and civilian emergency organizations and responders, providing critical information in a contaminated environment scenario when time is of the essence.

Directly heated high surface area solid phase microextraction sampler for rapid field forensic analyses.

A high-surface area solid phase microextraction (HSA-SPME) sampler is described for dynamic sampling at high air velocities (up to several hundred centimeters per second). The sampling device consists of a thin wire coated with carboxen/polydimethylsiloxane (carboxen/PDMS) material, wound in the annular space between two concentric glass tubes, providing a large trapping surface from which analytes may then be thermally desorbed with little power consumption upon resistive heating of the wire. Desorbed analytes are focused and reconcentrated on a microtrap that is subsequently resistively heated to introduce analytes for GC or GC/MS analysis. Benzene, toluene, ethylbenzene, and xylenes (BTEX) included in a 39-component toxic organics (TO-14) gas mixture were used to evaluate the efficiency of the HSA-SPME sampler. Quantitation of trace-level BTEX compounds present during weapons cleaning was completed using stepwise calibration. Detection limits of 0.2-6.9 pptr(v) were observed for these analytes using single ion monitoring GC/MS analysis, and an improvement in sensitivity of several orders of magnitude was achieved when compared to standard dynamic flow SPME with a commercially available 10 mm carboxen/PDMS fiber. The potential for rapid analyte uptake and improved sensitivity using the HSA-SPME design will make it possible to rapidly collect and analyze VOC samples in field settings using a portable hand-held pump and a small, low power GC/MS instrument. This system will be especially useful for situations involving forensics, public safety, and military defensive or intelligence needs where rapid, sensitive detection of airborne analytes is required.

Low thermal mass liquid chromatography.

A novel technique, low thermal mass liquid chromatography (LTMLC), is introduced in this study. The use of an LTM assembly that utilizes the principle of resistive wire heating and a temperature sensor to accurately deliver unprecedented heating (up to 1800 degrees C/min) or cooling (100 to approximately 200 degrees C/min) rates is reported. With the use of packed microcolumns (<0.5 mm i.d.), essentially instantaneous heat transfer from the assembly to the mobile phase was obtained. A systematic investigation was conducted to study the performance of the LTMLC technique. Both isocratic and gradient mobile phase conditions were used. For temperature control, isothermal, temperature-increasing, and temperature-decreasing gradients were applied. Three model mixtures, two of which containing neutral and acidic analytes and the other containing neutral, acidic, and basic analytes, were used to study the effect of temperature on elution time, resolution, column efficiency, and selectivity. It was found that the LTMLC experimental setup delivered reliable temperature control, as evidenced by linear van't Hoff plots for neutral and acidic compounds. The effect of temperature on the elution of basic analytes yielded nonlinear van't Hoff plots, explaining the dramatic selectivity changes observed for bases with changes in column temperature. Column efficiency generally increased with the increase in column temperature in the range of 25 to approximately 75 degrees C and decreased in the range of 75 to approximately 150 degrees C at a fixed column flow rate (3 microL/min), when extra column band broadening was taken into account. The increase in efficiency upon the increase in column temperature in the low temperature range was mainly due to the decreased mass transfer term resulting from increased analyte diffusivity. However, under even higher temperatures, the longitudinal diffusion dominated band broadening, explaining the decrease in column efficiency upon a further increase in column temperature. Resolution and selectivity decreased at elevated temperature for neutral and acidic compounds. For mixtures that contain bases, improved resolution was obtained by simultaneously tuning temperature and solvent programming. In addition to heating ability, LTMLC also demonstrated reliable cooling capability, allowing performance of oscillated or cycled temperature programming for fine-tuning the separation of critical band pairs for the first time. Finally, ultrafast reproducible LTMLC was also demonstrated, showing the potential of utilization of this technology for fast and ultrafast separations.

Multidimensional gas chromatography with capillary flow technology and LTM-GC.

2-D GC is a logical and cost effective extension to 1-D GC for improving the separation resolution, selectivity, and peak capacity of an analytical system. The advent of electronic pressure control systems that are accurate to the third decimal place, combined with recently innovated chromatographic devices such as capillary flow technology, has eliminated many deficiencies encountered in current conventional 2-D GC by making the technique reliable and simple to implement in both production and research analytical facilities. Low thermal mass GC (LTM-GC) was successfully integrated with capillary flow technology to further enhance overall 2-D GC chromatographic system performance by providing not only faster throughput via rapid heating and cooling, but independent temperature control for each dimension to maximize separation power. As an example, despite the enhanced peak capacity obtained from conventional 2-D GC, alkyl naphthalene isomers such as 2,3-dimethyl and 1,4-dimethyl naphthalene coeluted. These two critical compounds were well resolved (R = 5.2) using 2-D GC with LTM-GC with a similar analytical time. This paper demonstrates the benefits of combining capillary flow technology with LTM-GC to provide major enhancements to conventional 2-D GC. The synergy of these techniques is highlighted with practical industrial applications.

Low thermal mass gas chromatography: principles and applications.

In gas chromatography (GC), temperature programming is often considered to be the second most important parameter to control, the first being column selectivity. A radically new GC technology to achieve ultrafast temperature programming with an unprecedented cool down time and low power consumption has recently become available. This technology is referred to as low thermal mass GC (LTMGC). Though the technology has its roots in resistive heating, which forms the basis of principle and design concept, the approach taken to achieve ultrafast heating and cool down time by LTMGC represents a significant break-through in GC. Despite some rectifiable shortcomings, LTMGC has proven to be an ideal methodology to deliver near/real time GC data, high precision, and high throughput applications. It is a new approach for modern high-speed GC. This paper documents the fundamental design principles behind LTMGC, performance data, and examples of applications investigated.

Stacked injection with low thermal mass gas chromatography for PPB level detection of oxygenated compounds in hydrocarbons.

The presence of oxygenated compounds in light hydrocarbons can have a negative impact in manufacturing processes and on the quality of products produced. The development of an analytical technique termed "stacked injection" has been reported earlier. With this technique, sensitivity in the parts-per-billion (ppb) range for oxygenated compounds can be achieved, even with a flame ionization detector; however, there are drawbacks for this approach that limit its overall effectiveness. A new, improved analytical technique has been developed that not only addresses the shortcomings encountered, but offers markedly higher analytical performance. The new concept employs multidimensional gas chromatography (GC) with low thermal mass GC. With this new approach, throughput improvements of up to 5 times, range extension of solutes amenable for this analysis of up to nC16 alcohol, and ppb levels of detection for oxygenated compounds are achieved. Apart from alcohols, this technique is successfully employed for the ppb level analysis of other classes of oxygenated compounds, such as ethers, aldehydes, and aromatics.