Electrically Heated, Air-Cooled Thermal Modulator and at-Column

This was accomplished with an automated syringe pump (Cole-Parmer 74900 Series, Cole-Parmer, Vernon Hills, IL) connected to capillary plastic tubing a...
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Anal. Chem. 2005, 77, 2786-2794

Electrically Heated, Air-Cooled Thermal Modulator and at-Column Heating for Comprehensive Two-Dimensional Gas Chromatography Mark Libardoni,† J. Hunter Waite,‡ and Richard Sacks*,†

Department of Chemistry and Department of Atmospheric, Oceanic, and Space Science, University of Michigan, Ann Arbor, Michigan 48109

An instrument for comprehensive two-dimensional gas chromatography (GC×GC) 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 °C. The modulator tube is silicalined stainless steel with an internal film of dimethylpolysiloxane. The modulator tube is 0.18 mm i.d. × 8 cm in length. The modulator produces an injection plug width as small as 15 ms. Comprehensive two-dimensional gas chromatography (GC×GC) has emerged as a powerful method for the separation of complex mixtures of volatile and semivolatile organic compounds.1-7 The method has been used for petroleum-product analysis,8-12 essential oils separations,13,14 environmental analysis,15-17 and pyrolysis GC * Corresponding author. Phone: 734-763-6148. Fax: 734-647-4865. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Atmospheric, Oceanic, and Space Science. (1) Phillips, J. B.; Xu, J. J. Chromatogr. A 1995, 730, 327-334. (2) Van Deursen, M.; Beens, J.; Reijenga, J.; Lipman, P.; Cramers, C. J. High Resolut. Chromatogr. 2000, 23 (7/8), 507-510. (3) Ledford, E. B.; Billesbach, C. A. J. High Resolut. Chromatogr. 2000, 23, 205-207. (4) Beens, J.; Blomberg, J.; Schoenmakers, P. J. J. High Resolut. Chromatogr. 2000, 23, 182-188. (5) Seeley, J. V.; Kramp, F. J.; Sharpe, K. S. J. Sep. Sci. 2001, 24, 444-450. (6) Seeley, J. V.; Kramp, F. J.; Sharpe, K. S.; Seeley, S. K. J. Sep. Sci. 2002, 25, 53-59. (7) Schoenmakers, P. J.; Oomen, J. L. M. M.; Blomberg, J.; Genuit, W.; van Velzen, G. J. Chromatogr. A. 2000, 892, 29-46. (8) Phillips, J. B.; Beens, J. J. Chromatogr. 1999, 856, 331-347. (9) Bertsch, W. J. High Resolut. Chromatogr. 2000, 23, 167-181. (10) Frysinger, G. S.; Gaines, R. B. J. High Resolut. Chromatogr. 2000, 23, 197201. (11) Dimandja, J. M. Am. Laboratory 2003, 2, 42-53. (12) Frysinger, G. S.; Gaines, R. B.; Ledford, E. B. J. High Resolut. Chromatogr. 1999, 22, 195-200. (13) Dimandja, J. M.; Stanfill, S. B.; Grainger, J.; Patterson, D. G. J. High Resolut. Chromatogr. 2000, 23, 208-214. (14) Marriott, P. J.; Shellie, R.; Fergeus, J.; Ong, R.; Morrison, P. Flavour Fragr. J. 2000, 15, 225-239.

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of solid samples.18 Several attributes have led to the increasing popularity of GC×GC. These include the enhanced powers of detection relative to conventional GC, very high peak capacity, and in some cases highly structured chromatograms. In 1991, Phillips and Lui reported the first two-dimensional system.19 Since its inception more than a decade ago, several reviews have appeared by different authors.8,9,20-24,40 With GC×GC, chromatographic data (detector signal) are presented on a two-dimensional retention plane, and every separated component of a mixture produces a feature (cone) with unique coordinates on the retention plane and a volume proportional to the amount of component injected. Two-dimensional separations are obtained by connecting two columns by means of a chemical modulator interface. The two columns use different stationary phases. Often the first column is relatively long and nonpolar. This column is operated under conditions that elute relatively broad peaks. The modulator collects sequential portions of the first-column effluent and introduces them as narrow (time compressed) plugs into the second column for very fast separations. The second column usually is polar, quite short, and operated at a higher flow velocity than the first column so that the second-column chromatogram is complete before the next sample plug is introduced from the modulator. The result is a large number (often hundreds) of second-column chromatograms producing a one-dimensional data stream of detector signal versus time. Software is then used to generate the two-dimensional chromatograms by converting data from the second column separations into a matrix by use of the modulation period and plotting these data as a function of time in both dimensions. The heart of the GC×GC system is the device used to modulate the concentration of the solute eluting from the first column. Modulation is necessary for the collection of effluent from (15) Dalluge, J.; van Stee, L. L. P.; Xu, X.; Williams, J.; Beens, J.; Vreuls, R. J. J.; Brinkman, U. A. Th. J. Chromatogr. A 2002, 974, 169-184. (16) De Geus, H. J.; Aidos, I.; de Boer, J.; Luten, J. B.; Brinkman, U. A. Th. J. Chromatogr. A 2001, 910, 95-103. (17) Lui, Z.; Sirimanne, S. R.; Patterson, D. G.; Needhan, L. L.; Phillips, J. B. Anal. Chem. 1994, 66, 3086-3092. (18) Frysinger, G. S.; Gaines, R. B. J. Forens. Sci. 2002, 47, 471-482. (19) Lui, Z.; Phillips, J. B. J. Chromatogr. Sci. 1991, 29, 227-231. (20) Pursch, M.; Sun, K.; Winniford, B.; Cortes, H.; Weber, A.; McCabe, T.; Loung, J. Anal. Bioanal. Chem. 2002, 373, 356-367. (21) Phillips, J. B.; Beens, J. J. Chromatogr. A 1999, 856, 331-347. (22) Marriott, P.; Shellie, R. Trends Anal. Chem. 2002, 21, 573-583. (23) Marriott, P.; Ong, R. C. Y. J. Chromatogr. Sci. 2002, 40, 276-291. (24) Blumberg, L. M. J. Chromatogr. A 2003, 985, 29-38. 10.1021/ac040161b CCC: $30.25

© 2005 American Chemical Society Published on Web 04/05/2005

the first column and its injection into the second column as a series of narrow plugs. Therefore, the modulator acts as a continuous injector for the second column throughout the entire analysis. The thermal modulator was initially developed as a single-stage resistively heated device.25 Limitations in its design and cooling capability led to the development of more robust and efficient designs. Several very effective modulator designs have been described,26-32 and some are available commercially.27-29 Complete reviews of modulators have been discussed by various authors.20,41 Most modulator designs use either pneumatic or thermal means for collecting sample from the first column and generating the narrow injection plugs needed for the second column. Pneumatic devices31 usually use a sample loop and valve to collect the first column effluent and inject it into the second column. The plug injected into the second column is sharpened by operating the second column inlet at a higher pressure than the outlet from the first column. An important advantage of pneumatic devices is that no consumables are required other than electric power, carrier gas, and detector gas. Disadvantages include some loss of sample, limited vapor plug compression, and problems associated with valves operated at elevated temperatures,20 although recent work by Seeley and his group has overcome the temperature restrictions and sample loss by using a novel flow switching valve.42 Thermal modulators use a segment of capillary column that is cooled for sample collection and rapidly heated for injection of a sample vapor plug into the second column.26-29,32 A commonly used configuration involves a microbore (0.10 mm i.d.) second column to achieve higher carrier gas velocity than in the first column. The upstream end of the second column is alternately cooled and heated to modulate the effluent concentrations from the first column. Commercial devices use two-stage modulation, where analyte material collected in the first stage is thermally released into a second stage for additional focusing prior to injection into the second column for separation. Thermal modulators generally eliminate sample loss (two-stage modulators), provide for greater peak concentration ratios (secondcolumn peak height/first-column peak height), and are more (25) Phillips, J. B.; Luu, D.; Pawliszyn, J. B. Anal. Chem. 1985, 57, 2779-2787. (26) Phillips, J. B.; Ledford, E. B. Field Anal. Chem. Technol. 1996, 1, 23-29. (27) Marriott, P. J.; Kinghorn, R. M. Anal. Chem. 1997, 69, 2582-2588. (28) Ledford, E. B.; Billesbach, C. J. High Resolut. Chromatogr. 2000, 23, 202204. (29) Beens, J.; Adahchour, M.; Vreuls, R. J. J.; van Altena, K.; Brinkman, U. A. Th. J. Chromatogr. A 2001, 919, 127-132. (30) Bruckner, C. A.; Prazen, C. A.; Synovec, R. E. Anal. Chem. 1998, 70, 27962804. (31) Seeley, J. V.; Kramp, F.; Hicks, C. J. Anal. Chem. 2000, 72, 4326-4352. (32) Lee, A. L.; Lewis, A. C.; Bartle, K. D.; McQuaid, J. B.; Marriott, P. J. J. Microcolumn Sep. 2000, 12, 187-193. (33) Gaines, R. B.; Frysinger, G. S.; Hendrick-Smith, M. S.; Stuart, J. D. Environ. Sci. Technol. 1999, 33, 2106-2112. (34) Frysinger, G. S.; Gaines, R. B. J. High Resolut. Chromatogr. 2000, 23, 197201. (35) Frysinger, G. S.; Gaines, R. B. J. Sep. Sci. 2001, 24, 87-96. (36) Frysinger, G. S.; Gaines, R. B. J. High Resolut. Chromatogr. 1999, 22, 251255. (37) Rooney, T. A.; Hartigan, M. J. HRC CC 1980, 3, 416. (38) Jennings, W.; Yabumoto, K. HRC CC 1980, 3, 177. (39) Grall, A.; Leonard, C.; Sacks, R. Anal. Chem. 2000, 72, 591-598. (40) Dalluge, J.; Beens, J.; Brinkman, U. A. Th. J. Chromatogr. A 2003, 1000, 69-108. (41) Lee, A. L.; Lewis, A. C.; Bartle, K. D.; McQuaid, J. B.; Marriott, P. J. J. Microcolumn Sep. 2000, 12 (4), 187-193. (42) Bueno, P. A.; Seeley, J. V. J. Chromatogr. A 2004, 1027, 3-10.

robust for compounds with higher boiling point relative to pneumatic devices.20 The most significant limitation of thermal modulators is their reliance on cryogenic materials and in some cases compressed gases for their operation. Consumption rates of these materials may be quite large, making these instruments very resource intensive. These resource requirements represent a serious limitation to the more widespread use of GC×GC, particularly for on-site analysis. This report describes experiments aimed at the development of a high-performance thermal modulator that requires only 110 V ac power for its operation. The single-stage modulator uses a segment of commercially available stainless steel capillary tubing that is lined with silica and coated with a thin film of nonpolar stationary phase. Modulator cooling is provided by cold air from a conventional refrigeration unit coupled to a heat exchanger and a recirculating pump. Resistive heating of the modulator is provided by current through the stainless steel tubing from a lowvoltage dc power supply. Both columns use at-column heating, where a collinear ensemble of the fused silica capillary column, a heater wire, and a sensor wire are wrapped with fiber insulation. This obviates the need for a convection oven for column heating and requires only about 1% of the power required to heat a conventional GC oven. Design features and performance data for the modulator and the complete GC×GC instrument are presented. EXPERIMENTAL SECTION Apparatus. Figure 1 shows a diagram of the experimental system. An HP 5890 GC (Agilent Technologies, Palo Alto, CA) was used as a platform. The HP split inlet and flame ionization detector (FID) were used without change. The FID is used with a fast (5 ms time constant) electrometer (Chromatofast, Inc., Ann Arbor, MI). The GC oven was used only to house the thermal modulator and connecting lines and to provide a convenient means of temperature-programmed heating of these components. This allows the modulator environmental temperature to be controlled independent of the column temperatures. The two column modules prepared for at-column heating are mounted outside the GC oven and are independently temperature controlled by means of controllers and power supplies provided by the manufacturer (RVM Scientific, Santa Barbara, CA). Heated transfer lines, provided by the manufacturer, pass through holes in the GC wall and into the oven. The ends of the transfer lines are connected to the split inlet, the FID, and the modulator by means of deactivated fused silica tubing. The first column is a 30 m long, 0.25 mm i.d. fused silica capillary with a 0.25 µm thick stationary phase of nonpolar dimethyl polysiloxane (Rtx-1, Restek Corp., Bellefonte, PA). The second column is a 1.0 m long, 0.10 mm i.d. fused silica capillary with a 0.10 µm thick stationary phase of polar poly(ethylene glycol) (Rtx-Wax, Restek). Modulator Design. Details of the modulator design are shown in the inset in Figure 1. Three modulators were constructed that varied in the length of tubing that was cooled for sample collection. Devices with cooled lengths of 4.5, 5.5, and 11.5 cm were made from segments of 0.18 mm i.d. stainless steel tubing with a silica lining and a 0.20 µm thick stationary phase of nonpolar dimethylpolysiloxane (MXT-1, Restek). The actual tube lengths are 6.0, 8.0, and 15.0 cm. Analytical Chemistry, Vol. 77, No. 9, May 1, 2005

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Figure 1. GC×GC instrument using a single-stage air-cooled and electrically heated thermal modulator and an at-column-heated column. The inset shows details of the modulator design. See the text for details.

The device shown in the inset of Figure 1 uses an 8.0 cm long segment of stainless steel tubing with the center 5.5 cm long portion cooled for sample collection. The modulator is housed in a machined aluminum block containing a 1.6 cm o.d., 1.0 cm i.d., 5.0 cm long ceramic tube (Macore, McMaster-Carr, Atlanta, GA). Holes are drilled in the ceramic tube to accommodate the cooling air flow. The holes in the ends of the aluminum block are sealed with standard injection-port septa. The modulator tube passes through the septa, which center the modulator tube along the axis of the ceramic tube and provide gastight seals. A septum retainer ring, which seals against an elastomer O-ring, is locked in position with a set screw through the metal block. The modulator tube is heated by passing a current through it from an adjustable-voltage dc power supply (Model DS-304M, Zurich MPJA, Lake Park, FL). Heating pulse timing and duration are controlled by a PC through a solid-state relay (RSDC-DC-120000, Continental industries, Inc., Mesa, AZ). Modulator cooling is provided by a continuous flow of cold air. The cold air at a flow rate of 35 L/min is obtained from a conventional two-stage refrigeration unit (Model CC-100 Cryocool Immersion Cooler, Neslab Instruments, Portmouth, NH). This device uses a 63.5 cm long, 2.6 cm diameter flexible cold probe that can provide 100 W of cooling power at a temperature of -90 °C. The cold probe is located along the axis of a 72 cm long, 5.1 cm i.d. metal heat exchanger built in-house. The air exiting the heat exchanger has a temperature of -45 °C. The cooling air leaving the modulator is returned to the heat exchanger by means of a recirculating pump (KNF NO26.1.2.P, Trenton, NJ). By 2788

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recirculating the air, ice accumulation in the heat exchanger is eliminated, and maintenance is minimal. Materials and Procedures. Materials used to construct the modulator limited the HP oven temperature to 250 °C. The modulator tube, however, could be heated to 350 °C, which is the limit imposed by the thermal stability of the dimethylpolysiloxane stationary phase. To minimize the risk of cold spots in the transfer lines, the HP oven temperature was always maintained 10 °C above the temperature of the first column. The voltage applied to the modulator tube for electrical heating was set to a value just below that required to observe a detector signal from stationary-phase bleed when the heating pulse was applied. This adjustment was made with the HP oven at its maximum operating temperature of 250 °C. For all studies, the heating pulse duration was 100 ms (nominal). Data from the FID are logged at a sampling rate of 100-200 Hz by means of a 16-bit A/D board (PC1-DAS1602/16, Measurement Computing, Middleboro, MA) and a PC. Data are initially stored as a one-dimensional text file and then processed into a matrix based on the modulation period using MatLab software (The Math Works, Natick, MA). Peak areas and widths were measured from individual modulated peaks by Grams Spectral Notebook software (Thermo Galactic, Salem, NH). Chromatograms were obtained for liquid injections of weathered gasoline and the 40-component mixture described in Table 1. For studies of modulator performance, the second column was replaced by a 40 cm long segment of 0.10 mm i.d. deactivated fused silica tubing having a holdup time of about 100 ms for a column flow of 1.75 cm3/min. For measurements of modulator breakthrough time, a steady-state vapor concentration in the carrier gas stream was delivered to the modulator. This was accomplished with an automated syringe pump (Cole-Parmer 74900 Series, Cole-Parmer, Vernon Hills, IL) connected to capillary plastic tubing and a needle to pierce the septum on the HP split inlet. This allowed for the controlled addition of analyte vapor into the carrier stream before the stream is split. Atmospheric-pressure vapor samples, typically in the 100-1000 ppm range, were prepared in gas sampling bags by injecting microliter quantities of the pure liquids or liquid mixtures into the bags and diluting with filtered dry air. Series dilutions, again using clean, dry air, were used to obtain lower concentrations. Aliquots of these vapor samples were used for syringe-pump injections. RESULTS AND DISCUSSION A common configuration for GC×GC consists of a 0.25 mm i.d. capillary for the first column and a 0.10 mm i.d. second column, and modulation occurs at the first few centimeters of the secondary column. The cooling method used in the present study is not capable of achieving temperatures as low as for devices cooled with cryogenic materials. Thus, to reduce breakthrough for volatile compounds, the modulator i.d. was chosen to be between that of the first column and the second column. This reduces the carrier gas velocity in the modulator relative to the case where the upstream end of the microbore second column is used as the modulator. In addition, the 0.20 µm thick stationary phase film in the modulator provides more sample capacity than the 0.1 µm thick film often used when the end of the second column is the modulator. Most GC×GC work has utilized a

Table 1. Components in Test Mixture peak no.

component

bp, °C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

methanol ethanol acetone 2-propanol pentane butanal 2-butanol hexane ethyl acetate 1,2-dichloroethane 1,1,1-trichloroethane benzene 1-butanol 2-pentanone pentanal trichloroethylene 2,5-dimethylfuran heptane 2,5-dimethylhexane toluene 2-methylheptane hexanal tetrachloroethylene octane butyl acetate chlorobenzene ethylbenzene m-xylene cyclohexanone styrene nonane cumene R-pinene mesitylene β-pinene 3-octanol 1,2,3-trimethylbenzene butylbenzene nonanal undecane

64.6 78.3 56.2 82.4 35-36 76 99.5 69 77.1 83.5 74.1 80.1 117.6 100-110 103 86.7 92-94 98.4 109 110.6 118 131 121.1 126 126.1 130 136.2 139.1 155.6 145.2 150.8 151 155 165 167 174-176 175 183 190 195.9

midpolar second column, such as a DB-1701 (14% cyanopropyl) for petrochemical samples or a DB-50 (50% phenyl) for pesticide analysis, and these columns limit the peak modulation temperature to about 300 °C.40 The modulator described here uses a dimethylpolysiloxane stationary phase, which can be heated to 350 °C. Modulator Performance. A thermocouple in the modulator housing indicated that a quiescent trapping temperature of -32 °C is obtained for a cooling air flow rate of 35 L/min and a 40 °C external temperature (HP oven temperature). Higher flow rates resulted in higher temperatures due to the reduced efficiency of the heat exchanger, and lower flow rates also resulted in higher temperatures because of increased heat transfer in the line connecting the heat exchanger to the modulator. As the HP oven temperature is increased during temperature-programmed operation, the quiescent trapping temperature gradually increases, reaching a maximum value of -20 °C for an HP oven temperature of 250 °C. In preliminary studies, several modulator lengths were investigated. Figure 2 shows modulated peaks for n-octane injections. The peak width (4σ) from the first column was about 10 s, and typically two or three modulated peaks are observed for the 5.0 s modulation period. Both columns were operated isothermally at 100 °C. Chromatogram a was obtained with a 6 cm long modulator

Figure 2. Modulated peaks for 5 ppm n-octane using modulator tube lengths of 6 cm (a), 8 cm (b), and 15 cm (c). The modulation period was 5.0 s. The modulated peak widths are 72, 61, and 137 ms, respectively.

using a 5 ppm n-octane sample. Because of the relatively high concentration and the low sample capacity of the short modulator tube, sample vapor continues to exit the modulator after the first modulated peak, producing an unacceptable artifact. The full width at half-height of the larger modulated peak is 72 ms. Chromatogram b was obtained under the same conditions, except the modulator tube length was increased to 8 cm. The longer modulator tube completely eliminates the breakthrough between the modulated peaks. The width of the first modulated peak is 61 ms. For chromatogram c, a modulator length of 15 cm was used. No breakthrough is observed, but the width of the first modulated peak is increased to 137 ms. Figure 3 shows plots of the log of the integrated peak area (a) and peak width at half-height (W1/2) (b) versus the log of the n-octane concentration using modulator tube lengths of 15 cm (A), 8 cm (B), and 6 cm (C). The modulation period was 5.0 s, and the first column was operated isothermally at 100 °C. No second column was used, and the modulator was connected directly to the FID with 40 cm of 0.10 mm i.d. deactivated fused silica capillary tubing. The integrated peak area was computed by summing the areas of all modulated peaks for the analyte peak eluting from the first column. These integrated areas were averaged over three replicate injections at each concentration. The linear regression lines in Figure 3a are for the 15-cm modulator (slope 0.93) and the 6-cm modulator (slope 0.91). The 8-cm modulator plot (not shown) has a slope of 0.92. Correlation coefficients for the plots are in the range 0.998-0.999. Regression line statistics for these plots are for the concentration range 10 ppb to 2 ppm, but the plots are extrapolated to show the large deviations as the concentration exceeds about 10 ppm. Regression line slopes for the concentration range 10-200 ppb are in the range 0.96-0.97. Extrapolation of these data to a signal-to-noise ratio of 3.0 gives an estimated detection limit for the modulated n-octane peak of approximately 150 ppt. Analytical Chemistry, Vol. 77, No. 9, May 1, 2005

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Figure 3. Plots of log integrated peak area (a) and full width at half-height (b) versus log concentration of n-octane vapor entering the first column. Plots labeled C, B, and A are for modulator tube lengths of 6, 8, and 15 cm, respectively.

Figure 4. Modulated peaks for 52 ppm of n-hexane (a) and n-heptane (b) illustrating solute breakthrough. Sample vapor was continuously delivered to the modulator by means of a syringe pump.

These data show that at low concentration, the quantitative performance of the three modulator lengths is nearly identical, and a dynamic range exceeding four decades of concentration with reasonably good linearity can be achieved with these singlestage, electrically heated modulators. While some breakthrough is inevitable with a single-stage modulator, no breakthrough is detected above the noise floor for concentrations less than about 2 ppm. At higher concentrations, the modulators become saturated and no further increase in modulated peak areas can occur. This occurs at higher concentration for the longer modulator because of its greater analyte capacity. Solute breakthrough is detected in this concentration range. The peak-width data in Figure 3b show that the widths of the modulated peaks increase gradually with increasing analyte concentration from 10 ppb up to a few parts per million, where the peak widths begin to increase rapidly with increasing concentration. The modulated peaks from the 15-cm modulator are more than twice as wide as those from the shorter modulators for all concentrations less than 1 ppm. Peak widths from n-octane for concentrations in the low and sub-parts per billion range are less than 20 ms. This compares very favorably with reported data for two-stage modulators.29,32 For the higher concentration values, the intermediate modulator length (8 cm) gives the narrowest modulated peaks. For this reason, this modulator was used for all further studies. Solute Breakthrough. Since the breakthrough signal during trapping does not benefit from the signal enhancement of modulation, the breakthrough solute usually goes undetected for solute concentrations less than 1 ppm. Therefore, solute breakthrough time studies were limited to steady state concentrations above 1 ppm. Figure 4 shows modulated peaks from n-hexane (a) and n-heptane (b). In both cases, the solute concentration in

the carrier gas entering the first column was 52 ppm. Sample introduction was by means of the syringe pump, resulting in a steady-state vapor concentration entering the modulator. The first column was operated isothermally at 100 °C, and the HP oven temperature was 110 °C. No second column was used. For n-hexane, significant detector signal is observed both before and after the modulated peak, corresponding to two different breakthrough processes that occur at this very high concentration. Prior to each modulated peak, the detector signal gradually increases due to the transport of analyte through the trap at the quiescent trapping temperature of about -30 °C. After the modulator is heated to over 300 °C, producing the modulated peak, the modulator does not cool rapidly enough to prevent breakthrough at the elevated temperature. Eventually, the modulator cools to the point where the hexane vapor is again efficiently trapped, and the detector signal returns to the baseline only to begin increasing again as the modulator becomes saturated and breakthrough commences at the quiescent trapping temperature preceding the next modulated peak. The full width at half-height of the modulated peak is 410 ms. For n-heptane at the same concentration, there is no breakthrough at the quiescent trapping temperature of -30 °C using a 5-s modulation interval. This is because of the reduced migration rate of the higher boiling point compound. Breakthrough after the modulation peak during modulator cooling is observed. However, its duration is slightly less than for n-hexane. Again, this is the result of the higher boiling point of n-heptane, which allows for quantitative trapping at a higher temperature than for n-hexane. The full width at half-height of the modulated peak is 350 ms, which is substantially less than for n-hexane. Trapping breakthrough time is important, particularly for more volatile compounds, because it determines the maximum modula-

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Figure 5. Solute breakthrough profiles for n-octane (a) and nheptane (b) deliverd continuously to the modulator. Concentrations in the carrier gas stream were 89 ppm (A), 38 ppm (B), 28 ppm (C), 21 ppm (D), and 16 ppm (E).

tion period before sample loss from the modulator is detected. Because of the relatively low retention factors (k′ values) associated with low molecular weight components, the amount of breakthrough for these components can be an important issue for quantitative measurements. Figure 5 shows trapping breakthrough profiles (breakthrough signal versus time) for n-octane (a) and n-heptane (b) for concentrations of 89 ppm (A), 38 ppm (B), 28 ppm (C), 21 ppm (D), and 16 ppm (E). Breakthrough time is defined here as the time after the previous modulation pulse that a signal from the analyte is first detected. For these studies, several modulator-heating pulses were used to completely purge the modulator of sample, and the breakthrough time was measured after the last of these heating pulses. The breakthrough profiles are relatively complex and show several patterns. For both compounds, at higher concentrations, both the breakthrough time and the initial rate of increase in the breakthrough signal are independent of concentration. For lower concentrations, the breakthrough time becomes longer and the initial rate of increase of the breakthrough signal becomes lower with decreasing concentration. For any specified concentration considered in this study, the breakthrough time for n-heptane is less than for n-octane, due to the greater retention of the n-octane in the modulator. In addition, the initial rate of increase in the breakthrough signal for any specified concentration is greater for n-heptane. Figure 6 shows plots of breakthrough time versus concentration of the sample injected continuously into the carrier gas stream for n-octane (A), n-heptane (B), n-hexane (C), and n-pentane (D). For higher concentrations, the breakthrough time is independent of concentration. This trend continues up to at least 85 ppm for

Figure 6. Plots of breakthrough time versus the concentration of solute entering the first column for n-octane (A), n-heptane (B), n-hexane (C), and n-pentane (D).

all compounds evaluated. At lower concentrations, the breakthrough time increases rapidly with decreasing concentration. The concentration at which this increase begins decreases with decreasing solute boiling point. For n-pentane, breakthrough begins almost immediately for concentrations greater than about 15 ppm. For a pentane concentration of 7 ppm, which is the lowest value investigated, the breakthrough time is 3.0 s. Injection Characteristics. For this study, mixtures at various concentrations were prepared containing all normal alkanes from C5 through C20. Acetone was used as solvent, and samples were injected manually with a microsyringe. A combination of serial dilutions and changes in split ratio were used to adjust the vapor concentration in the carrier-gas stream entering the column. The column was temperature-programmed at 3 °C/min with a starting temperature of 30 °C. Note that the oven containing the transfer lines and the modulator housing was also programmed at 3 °C/ min, starting at 40 °C. The temperature program was ended at 250 °C for both the first column and the HP oven, and isothermal operation continued until n-C20 eluted. No second column was used, and the modulator was connected directly to the FID by 40 cm of 0.10 mm i.d. deactivated fused silica capillary tubing. Figure 7 shows plots of the modulated peak width (W1/2) versus solute boiling point for the alkane mixture. The peak widths are the averages from all detected modulations of the indicated alkane, and this value was averaged over three replicate injections. Plots A-F are for concentrations in the carrier gas stream entering the first column, 10, 2, and 1 ppm and 100, 50, and 25 ppb, respectively. The temperature program ended before the elution of C19 and C20, and these components eluted under isothermal conditions at 250 °C. For the higher concentrations, there is a steady increase in injection plug width with increasing solute boiling point through C18. Trapping of the higher boiling point compounds is very Analytical Chemistry, Vol. 77, No. 9, May 1, 2005

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Figure 7. Modulated peak widths (full width at half-height) versus solute boiling point for injections of a mixture containing C5 through C20 normal alkanes in acetone at concentrations of 10 ppm (A), 2 ppm (B), 1 ppm (C), 100 ppb (D), 50 ppb (E), and 25 ppb (F).

efficient, and thus, the increased injection plug width is probably the result of the limited heating rate of the modulator. The smaller injection plug widths for C19 and C20 are the result of these components eluting from the first column under isothermal conditions with substantially broader peaks than the other alkanes. This effectively reduces the vapor concentration entering the modulator with a corresponding reduction in injection plug width. For the lower concentrations, injection plug widths are substantially smaller and become increasingly less dependent on concentration, particularly for concentrations less than 100 ppb. For the 25 ppb case (plot F), peak widths are less than 90 ms for all the alkanes. For the lower concentrations, a dip in the peakwidth plots is observed with a minimum injection plug peak width of 17 ms for C8. For compounds more volatile than C8, migration through the modulator at the quiescent trapping temperature is more rapid, and while no breakthrough was observed, the sample plug occupies a larger portion of the modulator tube, with the result of a wider modulated peak. Thus, the trapping process contributes to wider injection plugs. For compounds less volatile than C8, injection is slower during the heating pulse with the result of wider injection plugs. All studies reported here used a 100 ms wide (nominal) current pulse for modulator heating. Preliminary studies with 300 ms wide heating pulses at the same applied voltage (5.0 V) showed that injection plug widths for the higher molecular weight alkanes were substantially reduced. For 1 ppm n-C18, the plug width was reduced from 99 ms with a 100 ms current pulse to 72 ms for a 300 ms wide current pulse. No breakthrough was detected with the wider current pulse. However, for lower molecular weight alkanes, the wider current pulse resulted in greater breakthrough at the higher concentration values. This suggests the use of a programmable 2792 Analytical Chemistry, Vol. 77, No. 9, May 1, 2005

Figure 8. GC×GC chromatogram of a 1-µL injection of weathered gasoline. The second column was a 1 m long, 0.10 mm i.d. capillary with a 0.10 µm thick film of poly(ethylene glycol) stationary phase: (top) three-dimensional presentation; (bottom) two-dimensional projection.

power supply for modulator heating, where the applied voltage and/or the heating pulse width can be changed during the course of an analysis. Studies using this approach are in progress. Example Chromatograms. Figure 8 shows a GC×GC chromatogram of a weathered gasoline sample obtained with the electrically heated modulator. The top portion of the figure shows a three-dimensional presentation, while the lower portion shows the two-dimensional projection. The horizontal axis in the twodimensional projection shows the first-column retention times, and the vertical axis shows the second-column retention times. The total run time is 30 min. A 1.0 m long, 0.10 mm i.d. fused silica capillary with a 0.10 µm thick film of poly(ethylene glycol) was used as the second column. The modulation period was 5.0 s. The temperature of the first column was programmed from 30 to 250 °C at 3 °C/min with a 3 min isothermal delay at the beginning of the run. The second column was also programmed at 3 °C/ min from a starting temperature of 40 °C with a 3 min isothermal delay at the beginning of the run. Gasoline has been studied extensively by GC×GC, and numerous chromatograms have been published.10,12,33-35 Thus, gasoline is a useful sample by which to compare instrument performance with that of other instrument designs. One useful feature of GC×GC is the patterns of peaks often observed in the chromatograms. The box labeled A in Figure 8 (bottom) contains peaks for both linear and branched alkanes. These alkanes cover a range from C5 to C12. Several series of peaks with second-column retention times in the 1-2 s range (box labeled B) are from homologous series of alkyl-substituted benzenes. The group of

The total separation number (SNt) was then calculated using eq 2.39 j

SNt )

∑(SN + 1)

(2)

i

Figure 9. GC×GC chromatogram of a 1-µL injection of a 40component test mixture in methyl alcohol using a 100:1 split ratio.

peaks with second-column retention times in the range 3.5-4.5 s is from naphthalene and alkyl-substituted naphthalenes (box labeled C). In general, good peak shapes are observed with some tailing of the larger peaks for the alkyl-substituted benzenes due to their high concentrations in the weathered gasoline. This is also observed in some GC×GC chromatograms using thermal modulators.12,36 Peak widths at half-height range from less than 100 ms (alkanes) to greater than 200 ms (high concentration aromatics) due to the wide concentration range of components in the sample. Figure 9 shows the GC×GC chromatogram from a 40component mixture (Table 1). The mixture covers a boiling point range from 35 °C (pentane) to 195 °C (undecane). This range of volatility is well-suited for the performance characteristics of the modulator used in this study. All components were present at low parts per million in methanol. The split ratio was 100:1. The modulation period was 5.0 s. The temperature of the first column was programmed from 20 to 175 °C at 3 °C/min with a 3 min isothermal delay at the beginning of the run. The second column was also programmed at 3 °C/min from a starting temperature of 30 °C with a 3 min isothermal delay at the beginning of the run. Components eluting from the primary column had a base width (4 σ) of 20-40 s. This allowed for a minimum of four modulations per component. For clarity of presentation, the solvent peak was removed from the display. Peak identification was based on individual runs of each component. All modulated peaks have full width at half-height of less than 115 ms. Peak capacity for the first dimension in Figure 9 was determined by calculating the total separation number37 (SNt) based on a manually injected n-alkane standard containing C5-C12. The separation number is defined as the number of perfectly spaced peaks that will fit between the peaks from an adjacent pair of n-alkanes with a resolution of 1.18. Separation number is the preferred measure for temperature programmed GC.38 Using eq 1, the separation number (SN) was calculated for the homologous series starting from C5 to C6 and continuing through C11 to C12.

SN )

(

)

∆tr

(W1/2)1 + (W1/2)2

-1

(1)

The total separation number (SNt) was determined to be 44 for the first dimension using a resolution of 1.18. This relatively low total separation number, as compared to an optimized temperatureprogrammed separation, is a result of the operating conditions used to generate wide eluting bands from the first column in GC×GC. Because of the short duration of the second-column separation and the relatively slow temperature-programming rate, each second column separation is nearly isothermal. For this case, peak capacity (np) for the second dimension in Figure 9 was estimated from the measured peak widths for a resolution (Rs) of 1.18 and using eq 3.

np ) 1 +

xL/H ln(tp/tm) 4Rs

(3)

where L is the length of the column, H is the height equivalent to a theoretical plate, tp is the modulation period, and tm is the column hold up time. For the second dimension, the peak capacity varied with the volatility range due to the variations in injection plug width as described in Figure 7. For the n-C8 region, which gives the narrowest injection plugs (peak width at half-height of 20 ms), the second-dimension peak capacity is 55 peaks for the 5.0 s analysis time and a resolution of 1.18. For the n-C12 region, which gives the widest injection plugs for this mixture (peak width at half-height of 112 ms), the second dimension peak capacity is 29 peaks for a resolution of 1.18. Thus, the overall peak capacity is in the range of 1270-2420. Note that a one-dimensional separation using a 30-m long × 0.25-mm i.d. column generating about 120 000 plates (4000/m) with a run time of 30 min and a holdup time of 1 min (average carrier gas velocity is 50 cm/s) will generate a peak capacity of about 250 peaks. CONCLUSION The modulator described in this report is very simple and very robust. The modulator used in this study has logged 150 000 heating/cooling cycles without a failure. This corresponds to over 400 GC×GC chromatograms with 30-min run times and 5-s modulation period. The only mechanical parts in the system are the recirculating air pump and the conventional refrigeration unit, both of which are very low maintenance. Operating costs also are much lower than for instruments requiring cryogenic materials for modulator cooling. Two highly significant attributes of the GC×GC system described in this report are low resource requirements (carrier gas, detector gas, and line voltage), due to the use of a conventional refrigeration unit for modulator cooling and a dc current source for modulator heating, and at-column heating, which greatly reduces column heating power requirements and provides the potential for a relatively small and lightweight Analytical Chemistry, Vol. 77, No. 9, May 1, 2005

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instrument. The use of at-column heating also provides for completely independent temperature control of the two columns. The instrument described here is not portable, due to the relatively large refrigeration unit and the convection oven used to control the temperature of the modulator housing and the transfer lines. Work is in progress to remedy both limitations by the use of solid-state cooling and independently heated transfer lines. This will also allow the transfer lines to be heated to temperatures greater than 250 °C, thus extending the boiling point range of sample components that can be eluted from the first column under temperature-programmed conditions. The modulator described here produces injection plugs with widths comparable to two-stage modulators. The use of electrical heating also allows for the straightforward adjustment of the heating profile, and preliminary studies have shown that programmable modulator heating may be effective in reducing injectionplug widths for higher boiling point compounds while breakthrough of more volatile compounds is minimized. The principal limitations of the single-stage modulator described in this report are the relatively short breakthrough times for volatile components due to the higher quiescent trapping

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temperature relative to cryogenically cooled devices and the inevitability of some breakthrough after the modulated sample plug is injected due to the finite cooling rate following a heating pulse. However, plots of modulated peak area versus concentration indicate that this breakthrough is relatively minor, particularly at lower concentrations, and a dynamic concentration range of about four decades can be obtained with good precision and relatively good linearity. ACKNOWLEDGMENT The authors would like to thank the Jet Propulsion Laboratory (JPL), Director’s Discretionary Funding for financial support; Dr. E F. Hasselbrink, Jr., Department of Mechanical Engineering, the University of Michigan for technical discussions; Bruce Block, Department of Atmospheric, Oceanic and Space Science, the University of Michigan for technical discussions and shop services; and RVM Scientific, Santa Barbara, CA for the LTM columns. Received for review September 14, 2004. Accepted February 22, 2005. AC040161B