Gas Chromatography with State-of-the-Art Micromachined Differential ...

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Journal of Chromatographic Science, Vol. 44, May/June 2006

Gas Chromatography with State-of-the-Art Micromachined Differential Mobility Detection: Operation and Industrial Applications Jim Luong1,*, Ronda Gras1, Rony Van Meulebroeck2, Frances Sutherland1,†, and Hernan Cortes3 1Dow

Chemical Canada, P.O. BAG 16, Highway 15, Fort Saskatchewan, Alberta, Canada T8L 2P4; 2Dow Chemical Benelux B.V., Herbert Dowweg 5, P.O. BOX 48, Terneuzen, the Netherlands; and 3Dow Chemical USA, Midland, MI

Abstract Ion mobility spectrometry (IMS) has potential analytical applications in very diverse fields such as chemical, petrochemical, environmental, and, more recently, in drug, chemical warfare agent, and explosives detection. Commercially available IMS instruments are based on time-of-flight (TOF) mass spectrometry. IMS is inherently suitable for field operation as it uses relatively simple microfluidic devices and operates at atmospheric pressure. It is portable, highly sensitive with tunable selectivity, yet can be produced at relatively low cost. Key limitations of this analytical detection technique are low duty cycle, ion cluster formation, short linear dynamic range, and restriction to only positive or negative ion collection in a single analysis. Microelectromechanical system, radio frequency modulated IMS (MEMS RF-IMS), also known as differential mobility spectrometry, has recently been developed and commercialized. The technology is based on IMS, and MEMS RFIMS offers substantially better performance. In this study, the strengths and limitations of the recently introduced differential mobility detector when used with gas chromatography in trace analyses are discussed and illustrated with applications of industrial significance.

Introduction There are several ambient pressure ionization detection options for capillary column gas chromatography (GC). For the non-selective detection of organic compounds, flame ionization detection (FID) is often the method of choice. When higher sensitivity is needed, as in the case of analysis of ultra-trace part-perbillion (ppb) levels of halogenated compounds, the employment of electron capture detection (ECD) (1) can be a good solution. Those solutes that are not detectable by FID, such as trace levels of oxygen, nitrogen, carbon monoxide, carbon dioxide, water, formaldehyde, hydrogen sulfide, and ammonia, can be detected by pulsed discharge helium ionization (PDD) (2,3) or dielectric barrier discharge detection (DBD) (4). In the examples cited, * Author to whom correspondence should be addressed: email [email protected]. † On sabbatical from Northern Alberta Institute Technology (NAIT), Edmonton, Alberta, Canada.

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however, these detectors rely on one single process to yield analytical information. By combining ionization chemistry with ion mobility measurement, as in the case of ion mobility spectrometry (IMS), greater flexibility for chromatographic detection can be attained. Although IMS offers advantages in terms of speed and detectability, commercialization has been hindered by the limitations of the technology in tuneable selectivity, dynamic range, low duty cycle, ion cluster formation, and restriction to only positive or negative ion detection in a single analysis (5–9). Based on the IMS technique, a microelectromechanical system that is radio frequency modulated (MEMS RF-IMS), also known as differential mobility detection, has recently been innovated and commercialized (10–12). In contrast to conventional time-offlight (TOF) IMS, which operates in the low field regime where the applied field strength is less than 1000 V/cm and the ion mobility is relatively constant, MEMS RF-IMS uses the nonlinear mobility dependence in strong RF electric fields for ion filtering. Another significant advantage of the differential mobility approach is that it does not require ion pulses for operation, and the resolution is not dictated by the width of the ion pulse. Instead, the ions are introduced continuously into the ion filter and practically all of the tuned ions are passed through the filter, maintaining the high sensitivity of the device. In addition, the simultaneous detection of both positive and negative ions is feasible with differential mobility detection. Detectors based on this technology should, therefore, offer substantially higher performance than TOF-IMS, especially when used not only as a sensor but as a detector in GC. Through our collaboration with the manufacturers, two detectors were made available for application developments. In this study, the strengths and limitations of the detector when used with both benchtop and micromachined GC are discussed and illustrated with applications of industrial significance.

Experimental The differential mobility detector, designated as SVAC (small

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value added component), model SVAC-V, was innovated and commercialized by Sionex Corporation (Waltham, MA). The same model, when licensed to Varian Inc. (Middleburg, the Netherlands) for use with the CP-4900 micromachined GC, is called the DMD. The functionality of the DMD and the SVAC model are actually identical, and both are based on Sionex patented microDMx chip technology. In this paper, for simplicity, the detector is referred to as the SVAC–DMD. For the research work described, hardware configurations are as follow: An Agilent HP-6890A series GC (Agilent Technologies, Little Falls, DE) was used as an analytical platform for the Sionex SVAC. The GC was equipped with two split/splitless injectors and two FIDs. The outlet of the analytical column was interfaced directly into the SVAC. A Varian VF-1MS column (10 m × 0.25-mm i.d., 0.25 µm) was used for the separation of solutes. The exposed capillary tubing was insulated with high temperature Kevlar tape but not heated. System control and data collection were conducted with Sionex Expert 2.04 software using a Hewlett-Packard 600 NC notebook computer (HP, Palo Alto, CA). A Varian CP-4900 micromachined GC equipped with two detectors in series—the first being a micromachined thermal conductivity detector (µTCD) and the second being a DMD—was also used in this project for application developments. The outlet of the µTCD was connected directly to the DMD via a heated, insulated, and deactivated transfer line. The CP-4900 was equipped with a CP-Sil 5CB capillary column (8 m × 0.25-mm i.d., 0.25 µm). System control and data collection were conducted with Varian Maitre Elite software in combination with Sionex Expert 2.04 software. Transport gases, such as nitrogen and air, were obtained from BOC (Nisku, Alberta, Canada), and chemicals were obtained from either local manufacturing sites or Aldrich Chemicals (Oakville, Ontario, Canada), Methyl isocyanate was obtained as a permeation tube from VICI (Houston, TX). GC conditions

Acrylonitrile application A Varian CP-4900 µGC–µTCD–DMD was used as the GC. The injection device was a micromachined valve (200 ms). The injection temperature was 120°C. The column used was a CP-Sil 5CB (8 m × 0.25-mm i.d., 0.25 µm). The oven temperature was 110°C, and the carrier gas was helium. The average linear velocity was 80 cm/s, the detector was a µTCD–DMD. The transfer line temperature was 95°C, and the detector temperature was set at 110°C. Air at 500 mL/min was used as the transport gas. The RF voltage was 800 V, positive channel. Epichlorohydrin application A Varian CP-4900 µGC–µTCD–DMD was used as the GC. The injection device was a micromachined valve (250 ms), and the injection temperature was 120°C. The column used was a CP-Sil 5CB (8 m ¥ 0.25-mm i.d., 0.25 µm). The oven temperature was 110°C, and the carrier gas was helium. The average linear velocity was 80 cm/s, and the detector used was a µTCD–DMD. The transfer line temperature was 95°C, and the detector temperature was set at 90°C. Air at 500 mL/min was used as the transport gas. The RF voltage was set at 800 V, positive channel.

Sulfur compounds in hydrocarbons application A Varian CP-4900 µGC–µTCD–DMD was used as the GC. The injection device was a micromachined valve (200 ms). The injection temperature was 120°C. The column used was a CP-Sil 5CB (8 m × 0.25-mm i.d., 0.25 µm). The oven temperature was 40°C, and the carrier gas was helium. The average linear velocity was 80 cm/s. The detector used was a µTCD–DMD. The transfer line temperature was 95°C, and the detector temperature was set at 50°C. Air at 350 mL/min was used as the transport gas. The RF voltage was set at 700 V, negative channel. Methanol in hydrocarbons application An Agilent HP-6890A GC was used. The injection device was a split/splitless injector in split mode (10:1), injection size 100 µL. The injection temperature was 200°C. The column was a VF-1 column (10 m × 0.25-mm i.d., 0.25 µm) VF-1 column. The oven temperature was 50°C, and the carrier gas was helium. The average linear velocity was 100 cm/s. The detector used was an SVAC. The transfer line temperature was 60°C, and the detector temperature was set at 90°C. Air at 500 mL/min was used as the transport gas. The RF voltage was set at 1200 V, positive channel. Methyl isocyanate application A Varian CP-4900 µGC–µTCD–DMD was used as the GC. The injection device was a micromachined valve (300 ms). The injection temperature was 120°C. The column was a CP-Sil 5CB (8 m × 0.25-mm i.d., 0.25 µm). The oven temperature was 80°C, and the carrier gas was helium. The average linear velocity was 80 cm/s. The detector was a µTCD–DMD. The transfer line temperature was 95°C, and the detector temperature was set at 80°C. Air at 550 mL/min was used as the transport gas. The RF voltage was set at 1200 V, negative channel. Ethylene oxide application A Varian CP-4900 µGC–µTCD–DMD was used as the GC. The injection device was a micromachined valve (200 ms). The injection temperature was 120°C. The column was a CP-Sil 5CB (8 m × 0.25-mm i.d., 0.25 µm). The oven temperature was set at 50°C, and helium was used as the carrier gas. The average linear velocity was 80 cm/s. The detector used as a µTCD–DMD. The transfer line temperature was 95°C, and the detector temperature was set at 80°C. Air at 550 mL/min was used as the transport gas. The RF voltage was set at 700 V, negative channel.

Results and Discussion The difference between IMS and RF-IMS

IMS has been used in very diverse fields in analytical chemistry and more recently in the detection of explosives (9,10,13). Invariably, these devices are operated in the TOF configuration. In this configuration, ions produced via atmospheric-pressure chemical ionization (10,14) are electronically pulsed into a region containing a linear electric field, and the time of their flights (hence time-of-flight) to an ion collector is measured. Normally, a large gas flow, such as air or nitrogen, opposite in direction to the ion flight path is used. The need for a shutter gate that can add

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complexity to the design, short linear dynamic range, low duty cycle, ion cluster formation, and restriction to only positive or negative ion detection are significant limitations. RF-IMS, although based on the IMS technique, offers some significant differences and advantages. In contrast to conventional TOF-IMS, which operates in the low RF field, RF-IMS uses the nonlinear mobility dependence in strong RF electric fields for ion filtering. Another significant advantage of the RF-IMS approach is that it does not require ion pulses for operation and, therefore, the resolution is not influenced by the width of the ion pulse. The ions are introduced continuously into the ion filter and practically all of the tuned ions are passed through the filter, maintaining the high sensitivity of the device. This offers improved sensitivity over the already sensitive IMS. SVAC–DMD MEMS-RF-IMS

In comparison with contemporary detectors such as the FID, DBD, and PDD, the SVAC–DMD is physically much smaller and consists of two planar electrodes separated by an analytical gap. The electrode dimensions are 15 × 1.5 mm, and the analytical gap is a mere 0.5 mm. Ionization is achieved by the use of a 185 mbq (5 mC) 63Ni radioactive source. Alternatively, UV light and Corona Discharge (10,11) could be used as ionization sources. The ion trajectory through the detector is dependent on the difference in ion mobility when the field oscillates between the low and high field values. Ions experiencing the asymmetric field of approximately 1.2 MHz RF with an associated controllable voltage variable from 500 to 1500 V will move with a “zigzag” motion as the field is applied. Only ions whose high field mobility and low field mobility are equal make it through to the detector. Any ions whose net vector allows them to touch the plates are neutralized and are not detected. As the appropriate ions pass through the detector, a pair of biased collector electrodes collects both positive and negative ions simultaneously. In order to make the detector tuneable (i.e., able to select the ions of interest), a perpendicular DC tuning field, known as the compensation voltage (Vc), is also applied. This DC field is superimposed on the oscillating asymmetrical field. The compensation voltage can be adjusted, allowing only specific ions to pass through the sensor to the detector. This compensating voltage can be scanned from –40 to +15 V, allowing ions with a range of mobility to be detected. The principle of separation and detection are depicted in Figure 1 (see page 6A). The detector contains no moving parts and is easy to operate. Detector control and a limited chromatographic data handling capability are handled by Sionex Expert version 2.04, a software suite provided by the manufacturer. De-convolution of threedimensional data to the familiar two-dimensional data for quantitative analysis is handled by Expert integrated with the Varian Maitre Elite chromatographic data system. Optimization of the detector involved first selecting an appropriate detector temperature. For the version tested, the detector could only be operated up to 150°C because of the viton o-rings used in the detector seals. Other customized detectors constructed of materials such as ceramics can operate up to 600°C. Three-dimensional data obtained from the detector is handled using a topographic chart that plots Vc on the x-axis, retention time (s) on the y-axis and concentration of the solute (color) on

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the z-axis for both positive and negative channels. The topographic chart also documents operating conditions of the detector, such as temperature, transport gas flow rate, and RF voltage. Figure 2 (see page 6A) shows a topographic chart of the detector at 90°C and 140°C, respectively. At 140°C, the ridge of the reactant ion product (RIP) is much lower in intensity, and the detector encountered more noise, seen as spurious ridges. In contrast, the RIP of the detector at 90°C is much more intense, and the background is much cleaner. Ideally, the detector should be operated at the lowest temperature possible, but still high enough to avoid the condensation of the matrix or the solute of interest. For the analysis of airborne organics, temperatures ranging from 80°C to 110°C can be used with good results. The impact of RF voltage on the intensity of the RIP is shown in Figure 3 (see page 7A). Clearly, RF voltage is inversely proportional to RIP intensity. Though this might indicate that lower RF voltage offers better sensitivity, a higher asymmetric field can help suppress ion cluster formation. This provides better selectivity and, in some cases, better overall signal-to-noise ratio (as shown in Figure 4, see page 7A) with ethylene oxide as an analyte of interest. As a result, each application must be individually optimized for an appropriate RF voltage and its corresponding Vc once the operating temperature of the detector is selected. A display of RF voltage and the corresponding Vc is called a dispersion plot. Figure 5 (see page 7A) shows dispersion plots of background air blank and of 1-chloro-1,1-difluoroethane, also known as Freon142B, a common blowing agent used in the manufacture of Styrofoam. In the case of Freon-142, excellent response and selectivity of the detector can be obtained at an RF frequency higher than 900 V on both positive and negative channels. The choice of monitoring either the positive or the negative ion should be considered based on ion product formation, sensitivity, selectivity, and spectral interference. Figures 6 and 7 demonstrate the unique high selectivity and sensitivity of the detector. In Figure 6, a sample of hexane vapor was analyzed. Air, water, and hexane were detected by the µTCD

A

B

Figure 6. Chromatograms of hexanes by µGC–TCD (A) and µGC–SVAC–DMD (B).

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in publications by Eiceman et al. (15) and those for negative ionization in publications by Karasek et al. (16,17). The following are some applications of industrial significance to illustrate the performance of the detector.

A

B

Figure 7. Chromatograms of 50 ppb (v/v) of methyl mercaptan in hexanes by µGC–TCD (A) and µGC–SVAC–DMD (B).

but not by the SVAC–DMD. Figure 7 shows a chromatogram of the same sample spiked with 50 ppb (v/v) of methyl mercaptan. The lack of chromatographic separation of the column used resulted in a perfect coelution of methyl mercaptan with hexanes, but because of the selectivity of the SVAC–DMD, methyl mercaptan was accurately detected without any interference. As in the case of other highly selective detectors, separation requirement of the column can be alleviated with the use of the SVAC–DMD to render fast(er) chromatography. Applications of industrial significance

Not all compounds are suitable for analysis by this detection technique. The key factor is the successful generation of stable product ions. The formation of product ions occurs in the ionization region predominantly by collisions between the reactant ions and the sample molecules. These equations describe a simplified scheme for product ion formation. Positive ion products are formed through proton-transfer reactions: M + H+(H2O) ➝ MH+ + nH2O

Eq. 1

Acrylonitrile Acrylonitrile (CAS 107-13-1) is produced commercially by propylene ammoxidation in which propylene, ammonia, and air are reacted in a fluidized bed. Acrylonitrile–butadiene rubber (NBR), also known as nitrile rubber, is often considered the workhorse of the industrial and automotive rubber products industries. NBR is actually a complex family of unsaturated copolymers of acrylonitrile and butadiene. In manufacturing facilities, acrylonitrile is monitored for industrial hygiene purposes. It was found that an increased incidence of lung cancer has been associated with long term exposure to acrylonitrile (18). Figures 8 (see page 7A) and 9 show a topographic chart of acrylonitrile with RF voltages of 700 and 800 V and a chromatogram of 100 ppb (v/v) of acrylonitrile in air. Note the excellent sensitivity and selectivity of the detector for acrylonitrile under the conditions used, and the analysis can be conducted in less than 30 s. Epichlorohydrin Epichlorohydrin (CAS 106-89-8) has been used in the production of various synthetic materials, including epoxy resins, synthetic glycerin, and elastomers. Other uses include insect fumigation and a chemical intermediate for the formation of glycidyl acrylate derivatives. Epichlorohydrin is a reactive epoxide and a known mutagen (19). Figures 10 (see page 8A) and 11 show a topographic chart of epichlorohydrin with RF voltage at 800 V and a chromatogram of 100 ppb (v/v) of epichlorohydrin in air. Both acrylonitrile and epichlorohydrin can be monitored in less than 45 s. Sulfur compounds in hydrocarbons The presence of volatile sulfur-containing compounds such as mercaptans in hydrocarbons can have a negative impact on both

The formation of cluster ions also occurs: MH+ + nL ➝ MH+ . nL

Eq. 2

Dimerization is also possible: MH+ + M ➝ M2H+

Eq. 3

Figure 9. Chromatogram 100 ppb (v/v) of acrylonitrile in air.

Negative ion products are formed by processes similar to those in an electron capture detector. e– + M ➝ M– (Associative electron attachment)

Eq. 4

e– + MX ➝ M + X– (Dissociative electron attachment)

Eq. 5

where M is the molecule of interest, L is the ligand molecule (also known as complex), and nL is the complex of ligand molecules. Details of positive ionization mechanisms have been reported

Figure 11. Chromatogram 100 ppb (v/v) of epichlorohydrin in air.

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the manufacturing processes as well as on the products themselves. At low ppm levels, sulfur compounds can cause odor and have a negative impact on the quality of products. At high levels, sulfur compounds contribute to plant corrosion. Figures 12 (see page 8A) and 13 show a topographic chart of common sulfur compounds and a chromatogram of 10 ppm of the sulfur compounds cited in hexanes. The lack of detector response towards light hydrocarbons and the excellent sensitivity for sulfur compounds make this detector highly suitable for use in the monitoring of sulfur compounds in hydrocarbons and an excellent complemen-

Figure 13. Chromatogram of mercaptans as stated in Figure 12 (see page 8A).

Figure 15. A chromatogram of 100 ppb of methanol in ethylene.

Figure 17. Overlay of 1 ppm methyl isocyanate. Note the high sensitivity and selectivity of the SVAC–DMD technology.

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tary technique to other selective sulfur detectors such as the dualplasma sulfur chemiluminescence detector or the pulsed-flame photometric detector. Methanol in hydrocarbons Apart from being an undesirable byproduct in the process of manufacturing hydrocarbons, methanol is also being added to the hydrocarbon product pipeline to prevent hydration or for subzero degree environment operations. But, the presence of oxygenated compounds in light hydrocarbon products can have a negative impact on the quality of these products as well as downstream final products such as polyethylene. Currently, there is a lack of a sensitive and selective GC detector for alcohols. In order to detect low levels of oxygenated compounds in hydrocarbons, sample enrichment techniques are required (20). Figures 14 (see page 8A) and 15 show a topographic chart of methanol, ethanol, and 2-propanol and a chromatogram of 100 ppb of methanol in ethylene. The lack of selectivity of the detector towards light hydrocarbons is quite advantageous for the application described. Methyl isocyanate Methyl isocyanate (CAS 624-83-9) is used to produce carbamate pesticides, polyurethane foam, and plastics. Methyl isocyanate is extremely toxic to humans from acute, short-term exposure (21). Figures 16 (see page 8A) and 17 show a topographic chart and an overlay of 1 ppm of methyl isocyanate in air using a µTCD and SVAC–DMD, respectively. Based on 10 replicate injections of a 1 ppm (v/v) methyl isocyanate in air, a respectable standard deviation of 2.3% was obtained using the SVAC–DMD. Ethylene oxide Ethylene oxide (EO) (CAS 75-21-8) is used to produce antifreeze, solvents, textiles, detergents, adhesives, polyurethane foam, pharmaceuticals, and for sterilization of surgical equipment. Industrial hygiene time weighted average maximum exposure for EO is 1 ppm. Because ethylene oxide can not be trapped effectively by commercially available adsorbents, indirect measurement such as converting ethylene oxide to bromoethanol has been the method of choice (22). Although this is adequate, it is not ideal and can be time consuming. Figures 18 (see page 8A) and 19 show a topographic chart and a chromatogram of 100 ppb ethylene oxide in air using the SVAC–DMD. Under the conditions used, listed in the Experimental section, ultratrace ppb level of ethylene oxide can be analyzed in less than 30 s without any sample enrichment.

Figure 19. Chromatogram of 120 ppb of ethylene oxide in air.

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Limitations

Although the detector has many advantages, it also has some limitations. Some of these limitations are rectifiable. Noteworthy limitations include: Ionization source The detector was commercialized with 63Ni as an ionization source. 63Ni is also used in ECD. Though this is a reliable and stable ionization source, it is also radioactive. For the possession and operation of a radioactive device, a special license is needed from the local authority. The whereabouts, operations, and storage of the ionization source will be tracked and subjected to audit by the appropriate agency. This restriction will have a negative impact on the ability of the detector to attain widespread use by practitioners in chromatography. High consumption of transport gas The detector requires clean (preferably CO2 and water free) transport gas. At a rate of 500 mL/min, a standard 1A cylinder of gas (nitrogen or air) will be consumed in 10 days. Measures to conserve transport gas can be taken by turning the gas down to a minimum flow rate of 200 mL/min when the system is not in use. The commercialization of a recirculated transport gas system will be very helpful. Limited linear dynamic range Like IMS, MEMS-RF-IMS also has a very short linear dynamic range. Figure 20 shows the linear range of epichlorohydrin from 80 ppb to 20 parts per million (ppm) (v/v). Note the short linear dynamic range obtained. At high concentration, detector saturation is an issue. At low levels, heat management is important to reduce system noise. Limited maximum operating temperature The detector tested has a maximum operating temperature of 150°C. Although this is more than adequate for applications involving volatile and semivolatile solutes, this is not suitable for applications with high-boiling-point solutes. Carry over from condensation and severe peak tailing has been observed, as in the case of tertiary dodecyl mercaptan.

Conclusion A highly reliable and easy to operate micromachined differential mobility detector has recently been successfully developed and commercialized for use with GC. The advent of the SVAC–DMD addressed unmet detector needs in GC in crucial applications. When properly optimized, the detector offers very high sensitivity and selectivity. The high degree of selectivity of the detector allows the redistribution of separation power from the analytical column to the detector. High-speed GC analyses can be achieved in seconds with the synergy generated by combining two orthogonal separation mechanisms: temporal and spatial. In certain applications, the intrinsically high sensitivity of the detector can eliminate the need for sample enrichment devices, such as purgeand-trap or headspace analyzers. The detector has the potential for use as a stand-alone sensor or for online applications.

Acknowledgments This paper is dedicated to Dr. Mary Fairhurst on the occasion of her retirement. We are grateful for all her support, teaching, and encouragement in the pursuit of excellence in science over the years. Dr. Jos Curvers (of Varian Inc.), Dr. Raanan Miller, and Dr. Erkinjon Nazarov (of Sionex Corporation) are acknowledged for the many fruitful discussions and for making available of the SVAC and the DMD for evaluation and applications developments. Prof. Dr. Pat Sandra of the Research Institute of Chromatography (Ghent University, Ghent, Belgium) is acknowledged for valuable advice. Lau Shreurs, Dr. Freddy Van Damme, Dr. Terry McCabe, The Global GC Steering Team, Vicki Carter, Michelle Baker, and Dr. Nicholas Darby of The Dow Chemical Company are acknowledged for their support and help. The authors would also like to express their gratitude to the editors and reviewers for their help in preparing the manuscript. This project was partially funded by the Dow Chemical Company, Analytical Sciences, Core Technologies, 2006 Corporate Innovation Funds.

References

Figure 20. Linearity study of epichlorohydrin in air. Note the short linear dynamic range of SVAC–DMD. Epichlorohydrin ( ), log (epichlorohydrin (–).

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chloride and other chlorinated and aromatic compounds in air samples. J. High Resolut. Chromatogr. 19: 201–311 (1996). C. Wu, H. Hill, U. Rasulev, and E. Nazarov. Surface ionization ion mobility spectrometry. Anal. Chem. 71: 273–78 (1999). G. Asbury and H. Hill. Using different drift gases to change separation factor in ion mobility spectrometry. Anal. Chem. 72: 580–84 (2000). H. Hill and D. McMinn. “Detectors for capillary chromatography”. In Chemical Analysis, Monographs on Analytical Chemistry, and Its Applications, vol. 20. Wiley-InterScience, New York, NY, 1992. G. Eiceman and A. Karpas. Ion Mobility Spectrometry. CRC Press, Boca Raton, FL, 1994. R. Miller, G. Eiceman, G. Nazarov, and T. King. A MEMS radio frequency ion mobility spectrometer for chemical agent detection. Solid State Sensor and Actuator Workshop, Hilton Head Island, SC, June 4–8, 2000. J. Curvers and H. Van Schaik. Differential mobility—an application specific, detection principle for gas chromatography. Am. Lab. June: 18–23 (2004). G. Lambertus, C. Fix, S. Reidy, R. Miller, D. Wheeler, E. Nazarov, and R. Sacks. Silicon microfabricated column with microfabricated differential mobility spectrometer for GC analysis of volatile organic compounds. Anal. Chem. 77(23): 7563–71 (2005). R. Scott. Chromatographic Detectors—Design, Function, and Operation. Chromatographic Science Series, vol. 73, Marcel Dekker, Inc., New York, NY, 1996. G. Eiceman and A. Karpas. Ion Mobility Spectrometry, 2nd ed. CRC Press, Boca Raton, FL, 2004. G. Eiceman. Ion mobility spectrometry as a fast monitor of chemical composition. Trends Anal. Chem. 21(4): 259–72 (2002).

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16. F. Karasek, H. Hill, S. Kim, and S. Rokushika. Gas chromatographic detection modes for the plasma chromatograph. J. Chromatogr. A 135(2): 329–39 (1977). 17. M. Dressler. Selective Gas Chromatographic Detectors. Journal of Chromatography Library, vol. 36. Elsevier Scientific Publisher, B.V., Amsterdam, the Netherlands, 1986. 18. Office of Environmental Health Hazard Assessment. Determination of NonCancer Chronic Reference Exposure Levels Batch 2B—acrylonitrile. California State Government, Sacramento, CA, 2001. http://www.oehha.org/air/chronic_rels/pdf/acrylonitrile.pdf (February 1, 2006). 19. Office of Environmental Health Hazard Assessment. Determination of Acute Reference of Exposure Levels of Airborne Toxicants— Epichlorohydrin. California State Government, Sacramento, CA, May, 1999. http://www.oehha.org/air/acute_rels/pdf/106898a.pdf (February 14, 2006). 20. J. Luong, R. Gras, H. Cortes, and R. Mustacich. Stacked injection for PPB level detection of oxygenated compounds in hydrocarbons by low thermal mass gas chromatography. J. Chromatogr. Sci. 44: 219–26 (2006). 21. United States Environmental Protection Agency. Air Toxic Site— Methyl isocyanate, Chemical 624-83-9. EPA, Cincinnati, OH, 2000. http://www.epa.gov/ttn/atw/hlthef/methylis.html (March 4, 2006). 22. United States Occupational Safety and Health Administration (OSHA). Methodology for Ethylene Oxide Analysis, Method Number 49. http://www.osha.gov/dts/sltc/methods/organic/org049/org049. html (March 4, 2006). Manuscript received March 23, 2006; revision received March 29, 2006.

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Gas Chromatography with State-of-the-Art Micromachined Differential Mobility Detection: Operation and Industrial Applications (see pp. 276–82) A

Figure 1. Principles of separation and detection of microDMx technology. Picture courtesy of Dr. Raanan Miller and Dr. ErkinJon Nazarov (Sionex Corporation).

B

Figure 2. Topographic chart of both positive and negative channels at 90°C (A). Topographic chart of both positive and negative channels at 140°C (B). Note the lower intensity of RIP and higher noise with the detector at 140°C.

Color figures continued on page 7A

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A

B

Figure 3. Impact of RF and transport gas flow rate on intensity of RIP. Impact of RF (500 to 1000 Volt) on sensitivity of RIP (A) . Note higher the voltage lower the intensity. RF increases in 100 V increments from right to left. Impact of reactant gas flow (from 200 mL/min to 500 mL/min) on intensity of RIP flow increases in 50 mL/min increment from top to bottom (B). Figure 5. Optimization of detector sensitivity and selectivity, dispersion plot. Dispersion plot of system background, air as transport gas (A). Dispersion plot of 1-chloro-1,1-difluoroethane (B). 900 V

1000 V

1100 V

1200 V

1300 V

Figure 4. Impact of RF (voltage ranging from 900 to 1300 V) on selectivity of ethylene oxide. Note the change of Vc of ethylene oxide and the reduction of cluster formation at higher RF voltage.

Figure 8. Topographic chart of acrylonitrile in air, RF at 700 and 800 V, respectively (concentration ca. 50 ppm).

Color figures continued on page 8A

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Figure 10. Topographic chart of epichlorohydrin in air, RF at 700 V (concentration ca. 13 ppm). Figure 16. Topographic chart of methyl isocyanate at RF 1100 and 1200 V.

Figure 12. Topographic chart of 10 ppm each of iso-propyl mercaptan, tertbutyl mercaptan, sec-butyl mercaptan, n-propyl mercaptan, iso-butyl mercaptan, n-butyl mercaptan, and tert-amyl mercaptan in hexanes.

Figure 14. Topographic chart of methanol (10 ppm), ethanol, and 2-propanol in ethylene. The black line is marked Vc of methanol.

8A

Figure 18. Topographic chart of 10 ppm of ethylene oxide in air.

Journal of Chromatographic Science, Vol. 35, July 1997

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