Design and performance evaluation of a two-stage resistively-heated thermal modulator for GC × GC

Abstract

The design and performance evaluation of a two-stage resistively-heated thermal modulator that does not use any cryogenic consumables is described. A commercially available piece of stainless-steel wall coated capillary column is used as the modulator. Cooling of the modulator is provided by a two-stage refrigeration unit with an external heat exchanger and a closed-loop recirculating air system. The modulator is resistively heated by sending a current pulse through each stage of the modulator tube. Studies evaluating the modulator trapping efficiency, cooling efficiency, desorption efficiency, and resultant peak shape are presented and discussed. Performance evaluation of this modulator is compared to similar single-stage modulators, both qualitatively and quantitatively, and has exceeded the previous data reported for the single stage versions. GC × GC chromatograms of a C₆-C₂₀ n-alkane mixture, a petroleum sample, and a 38-component mixture of aliphatic, aromatic, and halogenated volatile organic compounds are provided. This two-stage modulator proves to be a viable alternative to commercially available modulators which require large amounts of cryogenic fluids.

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Introduction

Since its invention just over a decade ago, the novel technique of comprehensive two-dimensional gas chromatography (GC × GC) has been developed to separate and analyze complex samples such as petroleum,^1–5 flavors,⁶ environmental^7–9 and even human breath samples.¹⁰ The technique of GC × GC employs two coupled columns of different selectivity and subjects the entire sample to a two-dimensional separation. Effluent from the primary column is modulated to produce sharp chemical pulses, which are rapidly separated on the second column. A separation plane is produced by the two orthogonal retention time axes for the columns.¹¹ Usually, the first column contains a non-polar stationary phase and the second column a polar stationary phase. Although reverse-phase and other application-specific column combinations are used as well, the traditional combination allows components to be independently separated according to their volatility and then according to their polarity. In comparison to conventional single-column gas chromatography, GC × GC has much higher peak capacity because the available peak capacity is the product of the peak capacity of both dimensions. Other advantages of GC × GC include enhanced detectability due to analyte refocussing, true background around resolved peaks, more reliable identification due to two retention times, and well ordered bands of compound groups.¹²

The key element in a GC × GC system is the modulator, which focuses segments of the effluent from the primary column and re-injects them onto the secondary column. Different types of modulators have been designed and shown to be capable of producing GC × GC chromatograms.^13–19 In their pioneer work, Phillips and Lui used an on-column two-stage thermal modulator which was heated by a resistive gold film painted onto the capillary column and cooled by ambient air.¹¹ This modulator was cumbersome to operate and had a short lifetime. Advances in the group led to a similar thermal modulator using a wrapped wire instead of the painted film, which had higher durability but sluggish thermal response and limited cooling.²⁰ A more robust thermal modulator using a heated sweeper showed good performance, however, the operation of the GC oven had to be about 100 °C lower than the maximum allowed temperature of the stationary phase in the modulation capillary.²¹

Instead of using heating, Marriott and Kinghorn developed a modulator using cooling.²² This design regularly traps and releases solutes from the first column by moving a cryogenic trap back and forth along the second column. While achieving good performance, this modulator showed problems with ice build-up, which were eventually overcome by design modifications.^23,24 Both the heated sweeper technique and the moving cryogenic trap technique have a common drawback; frequent breakage of capillaries by moving parts in the system. A more recent development of the modulation technique is the jet-cooled modulator, which uses no moving parts. Adapted from the jet-cooled thermal modulator designed by Ledford and Billesbach,²⁵ a jet-cooled and heated modulator was reported by Ledford.²⁶ The modulator employs two nitrogen jets that are pulsed to alternately cool and heat two spots at the front end of the second column for focussing and remobilizing analytes eluting from the primary column. Most commercial GC × GC systems employ a jet-cooled and heated thermal modulator. While this type of modulator allows excellent modulation of compounds even as volatile as butane, the use of liquid nitrogen to cryogenically cool the cold gas is a major limitation.

Previous work from the authors used a single-stage resistively-heated gas or liquid-cooled modulator that required no consumables other than line voltage and carrier gas.^27,28 The inherent drawback to a single-stage modulator is sample loss through the modulator during a heating and cooling cycle. Two-stage modulators overcome this limitation by operating each stage of the modulator with independent heating and cooling cycles. Components that are trapped in the primary stage are released into the second stage by a heating pulse sent to the primary stage. The second stage is not heated until the primary stage has cooled to a temperature sufficient for quantitative trapping. This alternating heating and cooling of each stage results in quantitative performance with minimal sample loss.

This paper describes the design and performance evaluation of a two-stage resistively-heated thermal modulator that does not use any cryogenic consumables. A commercially available piece of stainless-steel wall coated capillary column is used as the modulator. Cooling of the modulator is provided by a two-stage refrigeration unit with an external heat exchanger and a closed-loop recirculating air system. The modulator is resistively heated by sending a sequence-timed current pulse through each stage of the modulator tube. In addition to the performance of the resistively-heated two-stage modulator, a side-by-side comparison of previously studied single-stage modulators will be presented.

Experimental

Apparatus

Fig. 1 shows the schematic of the experimental system. An HP 5890 GC (Agilent Tech., Palo Alto, CA) equipped with a manual split/splitless injector and a flame ionization detector (FID) is used as the GC × GC platform. The primary column is a 30 m × 0.25 mm i.d. × 0.25 μm film thickness, polydimethyl siloxane (RTX-1, Restek Corp, Bellefonte, PA) fused silica capillary that is independently heated and located outside the GC oven. The secondary column in the GC × GC system is a fused silica column measuring 1.0 m × 0.10 mm i.d. × 0.10 μm film thickness of polyethylene glycol (RTX-Wax, Restek Corp.). The independently heated fused silica columns use at-column heating, where the co-linear ensemble of the column, heating wire and temperature sensor are wounded into a 4-inch coil and covered with an insulating sheath of aluminium foil. Electrical leads for the heating elements and the temperature sensor exit the insulation and attach to the power supply and electronic controls. Both columns were prepared at RVM Scientific, Santa Barbara, CA. Electronic controls for heating the columns were also provided by RVM Scientific.

Fig. 1 Schematic of the GC × GC system. A HP-5890 GC is used as the experimental platform. Primary column (C-1) and secondary column (C-2) are located outside the GC oven. Expanded view of the two-stage modulator is shown with electrical contact points and center ground. Inset (B) shows the voltage profile for each stage as a function of time.

Located inside the GC oven is the two-stage thermal modulator. The modulator is a stainless steel piece of capillary tubing measuring 10 cm × 0.18 mm i.d. × 0.20 μm film thickness, polydimethyl siloxane (MXT-1, Restek Corp, Bellefonte, PA). The modulator is divided into two stages, a 5.5 cm primary stage and a 2.2 cm secondary stage. Although the modulator is one continuous piece of stainless steel capillary, a soldered junction point serves as the electrical contact between the two stages. Fig. 1 and 2 show the design of the modulator and the modulator housing. Fig. 2 displays component pictures of the modulator housing and modulator cartridge. The modulator cartridge is 0.5 inches in diameter and 3.2 inches long, not including the end caps. A machined aluminium block, mounted to the GC oven wall, houses the modulator cartridge [Fig. 2(B)]. The aluminium block is 3.4 inches long, 1.7 inches high and 0.8 inches wide. A center ground ring constructed from brass is used as the stage divider and electrical contact. The center ground ring is soldered directly onto the metal column. The modulator is placed inside the cartridge and the end-cap of the cartridge is threaded into place. The engineering of the cartridge provides electrical contact throughout the brass ring as well as the ends of the modulator tube. Once assembled, the modulator cartridge is inserted inside the aluminium block. Set screws are used to hold the modulator cartridge in place and provide electrical contact. Prior to mounting on the GC oven wall, the entire block is wrapped in insulating Kevlar.

Fig. 2 Conceptual drawing of the two-stage thermal modulator cartridge (A). Figure B is the modulator housing and modulator cartridge with size reference to a U.S. quarter. Inset C is an exploded view of the modulator cartridge. Photos and modulator drawing courtesy of Bruce Block, University of Michigan Space Physics Research Laboratory.

Materials and procedures

Modulator cooling is provided by cold air from a conventional refrigeration unit (Model CC-100 Cryocool Immersion Cooler, Neslab Instruments, Portsmouth, NH) by means of a heat exchanger built in house and a re-circulating pump to prevent ice accumulation in the heat exchanger. The device is very low maintenance and requires only line voltage for its operation.

To determine the optimal cooling-gas flow rate, a thermocouple was placed inside the modulator cartridge. The thermocouple was routed through one-side of the centering septa and data were recorded via computer and a 16-bit A/D board (PC1-DAS1602/16, Measurement Computing, Middleboro, MA).

The modulator tube is resistively heated by the current from two adjustable-voltage dc power supplies (Model DS-304M, Zurich MPJA, Lake Park, FL). A 100 ms long heating pulse is applied to each stage of the modulator tube every 5.0 s. The pulse voltages are 14.26 V and 8.58 V applied to the first and second stage, respectively. Initial studies used a 1.0 s offset between the first and second stage heating pulses. This voltage delay allows the primary stage to cool and begin trapping the components that are eluting from the primary column. Once trapping is quantitative, the second stage is heated and a narrow injection plug is released from the second stage into the second column. Efficient operation of a two-stage modulator eliminates breakthrough as previously reported with single-stage modulators. Heating pulse timing is controlled by a PC through two solid-state relays (RSDC-DC-120–000, Continental Industries, Inc., Mesa, AZ).

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).

Quantitative evaluation of the modulator was carried out by injections of a liquid mixture of normal alkanes ranging from C₅ to C₂₀ and selected polar components. In order to reduce band broadening in the second dimension, the second column was replaced with a piece of 30 cm × 0.10 mm i.d. deactivated fused silica column. This transfer line was kept inside the oven and connected the modulator tube directly to the FID. Calibration plots, dynamic range and RSD values were calculated over three orders of magnitude. LODs for the alkane series were determined by extrapolation of the plotted data to three times the S/N ratio. Log integrated area vs. log concentration trend lines were inserted to cover a range from 2–1000 parts-per-billion (ppb) and from 2–5000 ppb.

Qualitative evaluation of the two-stage modulator was carried out by analyzing a liquid petroleum sample and a liquid 38-component mixture. The chromatograms obtained from the sample analysis were compared to previously collected data using a single-stage resistively heated modulator.

Results and discussion

Modulator cooling

Experimental studies showed that the optimal cold-air flow rate was 35 L min⁻¹, and the air exiting the heat exchanger had a temperature of −45 °C. Temperature inside the modulator was −33 °C. For flow rates below 35 L min⁻¹, the recirculating air temperature was increased due to transport time from the heat exchanger to the modulator. For flow rates above 35 L min⁻¹, the cooling-gas resident time inside the heat exchanger was decreased and therefore the exiting temperature was above −45 °C. This results in a higher temperature inside the modulator cartridge. Cooling-gas flow rates and temperature measurements were conducted under isothermal conditions with an oven temperature of 30 °C. When the temperature of the GC oven is elevated, the temperature inside the modulator is also elevated. Experiments showed that an oven temperature of 250 °C produced a temperature inside the modulator cartridge of 15 °C. Although this temperature is approximately 45 °C higher, this temperature is adequate for trapping high molecular weight components that would elute from the primary column under such conditions.

Modulator heating

The primary and secondary stages of the modulator are heated with individual power supplies, and a waveform file was written to control the firing time of each modulator with respect to the modulation period. Fig. 1(B) displays the voltage profile as a function of time for each stage. Experimental studies resulted in a 1.0 s stage delay for efficient operation. This 1.0 s offset allows the primary stage enough time to cool to a quantitative trapping temperature. Experimental data showed that a stage offset less than 0.5 s resulted in sample loss. A stage delay greater than 2.0 s showed adequate performance, however, the capacity of the second stage becomes an issue. The ability to retain highly volatile components in the short second-stage for times longer than 2.0 s is decreased when a longer offset time is used. Based upon experimental data for the single-stage modulator and preliminary studies with the two-stage modulator, a stage offset of 1.0 s was used in this work.

Modulator performance

Fig. 3 shows the chromatogram for a 1 ppm solution of n-C₆ through n-C₂₀ alkane mixture. The sample was analyzed with the primary column at 30 °C followed by an immediate ramping of 5 °C min⁻¹ to 250 °C. The temperature of the oven, which houses the modulator and the transfer lines, was set at a +10 °C offset and followed the same temperature program. A modulation period of 5.0 s was used with a second-stage delay of 1.0 s. The modulated peaks are well defined and there is no presence of breakthrough at this concentration. The inset shows an expanded view of the one-dimensional modulation of octane with an arbitrary time axis. The peak width at half-height for the center peak is 32 ms.

Fig. 3 Chromatogram of a n-C₆ through n-C₂₀ alkane standard mixture. The inset shows the one-dimensional modulation of n-C₈ with center peak width at half-height of 32 ms. A 5.0 s modulation period was used with a 1.0 s stage offset.

Dilutions of stock solution that contained the n-C₅ through n-C₂₀ alkane mixture were used to generate calibration curves that covered a concentration range of 2–5000 ppb. Replicate runs (n = 3) were performed at each concentration and the acquired data are displayed in Table 1. The slopes of the log-log fitted trend lines show that a linear response is achieved from low ppb to approximately 1 ppm. When concentrations increase above 1 ppm the data becomes non-linear. The log integrated area vs. log concentration plots have slopes ranging from 0.9215 (n-C₅) to 0.9999 (n-C₈), in the 2–1000 ppb concentration range. The reported slopes are measured against an ideal slope of 1.000. Slopes below 1.000, as reported in this work, represent a small fraction of sample loss. Experimental results show sample loss to be in the range of 1.6% for the highly volatile components such as pentane to about 0.8% for the low volatile components such as decane. The reported log-log slopes are better than previously reported data collected with the single-stage modulator.²⁷ Extrapolation of the fitted curves to 3 times the S/N ratio gave limit of detection (LOD) values in the range of 137 ppt (octane) to 201 ppt (pentane). These values are not as low as the single-stage liquid cooled modulator because of the noise floor associated with the detector on the GC.²⁸ However, in comparison with the single-stage aircooled modulator that was evaluated on the same GC platform, the observed LOD values are similar.

Table 1 Quantitative data of alkanes and selected polar components. Slopes are for log integrated area vs. log concentration linear fitted trend lines

#	Component	Log vs. Log Slope		R 2	RSD (%)	LOD (ppt)
		[x] = 2–5000 ppb	[x] = 2–1000 ppb
1	pentane	0.8483	0.9256	0.9964	1.09	201
2	hexane	0.8960	0.9648	0.9971	1.01	185
3	heptane	0.8970	0.9687	0.9918	0.94	148
4	octane	0.9115	0.9783	0.9989	0.81	137
5	nonane	0.9075	0.9891	0.9943	0.94	152
6	decane	0.9115	0.9825	0.9925	0.91	155
7	undecane	0.8964	0.9910	0.9918	0.85	167
8	dodecane	0.9095	0.9908	0.9967	0.83	159
9	tridecane	0.8936	0.9879	0.9967	0.82	143
10	tetradecane	0.9098	0.9896	0.9985	0.79	169
11	pentadecane	0.9033	0.9891	0.9935	0.76	158
12	hexadecane	0.9000	0.9893	0.9919	0.81	155
13	heptadecane	0.9030	0.9915	0.9983	0.83	162
14	benzene	0.9039	0.9894	0.9980	0.76	171
15	toluene	0.9012	0.9915	0.9949	0.74	164
16	2-pentanone	0.9094	0.9862	0.9949	0.81	173
17	benzaldehyde	0.9106	0.9826	0.9904	0.72	174
18	m-xylene	0.9000	0.9816	0.9986	0.93	164
19	2-heptanone	0.9022	0.9857	0.9961	0.85	175
20	1-pentanol	0.9151	0.9826	0.9973	0.81	181

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Fig. 4 shows plots of the modulated peak width at half-height versus solute boiling point for the alkane mixture. For these experiments, the second column was replaced with a 0.5 m × 0.10 mm uncoated transfer line. The temperature began at 30 °C and immediately ramped to 300 °C at a programming rate of 8 °C per minute. 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–E are for concentrations in the carrier gas stream entering the first column of 2000 ppb, 1000 ppb, 100 ppb, 50 ppb and 10 ppb respectively.

Fig. 4 Peak width at half-height vs. compound boiling point for n-C₅ through n-C₂₀ alkanes. Analysis performed with a two-stage air-cooled modulator. Data set (A) is 2000 ppb, data set (B) is 1000 ppb, data set (C) is 100 ppb, data set (D) is 50 ppb and data set (E) is 10 ppb.

For the higher concentrations, there is a steady increase in injection plug width with increasing solute boiling point through C₂₀. Trapping of the higher-boiling-point compounds is very efficient, and thus the increased injection plug width is probably the result of the limited temperature differential between the boiling point of the component and the maximum modulator temperature achieved.

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 10 ppb case (plot E), peak widths are less than 90 ms for all the alkanes. For the lower concentrations, a dip in the peak-width plots is observed with a minimum injection plug peak width of 18 ms for C₈. For compounds more volatile than C₈, migration through both stages of 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.

Example chromatograms

Fig. 5 is a chromatogram of a 38-component mixture. A list of the mixture components is displayed in Table 2. Component volatility ranged from 31.7 °C (1,1-dichloroethene) to 214 °C (1,2,4-trichlorobenzene). A 1 μl injection was used with a primary column temperature of 30 °C for 3 min and a ramp rate of 8 °C min⁻¹ to 200 °C. The second dimension wax column followed the same programming although it began with a 10 °C offset and ended at 210 °C. The chromatogram shows nice peak shape and minimal streaking, except for the solvent located at the beginning of the analysis. A peak capacity of approximately 1850 was calculated based upon the equations described below.

Fig. 5 GC × GC chromatogram of a 38-component mixture. A 5.0 s modulation period was used with a 1.0 s stage offset. The large streak on the left side is from the solvent. Peak capacity was calculated to be approximately 1850.

Table 2 Component listing of 38-component mixture used for qualitative analysis

Component	#	B.P./°C	mol wt.
benzene	1	80.1	78.1
bromodichloromethane	2	90.1	163.8
bromoform	3	149.5	252.7
carbon tetrachloride	4	76.7	153.8
chlorobenzene	5	130.0	112.5
chloroform	6	61.7	119.3
1,2-dibromo-3-chloropropane	7	195.0	236.3
cyclohexane	8	80.7	84.1
dibromochloromethane	9	120.0	208.2
1,2-dibromoethane	10	131.7	187.8
1,2-dichlorobenzene	11	180.5	147.0
1,3-dichlorobenzene	12	173.0	147.0
1,4-dichlorobenzene	13	173.4	147.0
1,1-dichloroethane	14	57.3	98.9
1,2-dichloroethane	15	83.5	98.9
1,1-dichloroethene	16	31.7	96.9
cis-1,2-dichlorothene	17	60.0	96.9
trans-1,2-dichloroethene	18	47.5	96.9
1,2-dichloropropane	19	96.8	112.9
cis-1,3-dichloropropene	20	104.3	110.9
trans-1,3-dichloropropene	21	112.0	111.0
ethylbenzene	22	136.2	106.1
isopropylbenzene	23	151.0	120.1
methyl acetate	24	56.9	74.0
methylcyclohexane	25	101.0	98.1
methylene chloride	26	39.8	84.9
styrene	27	145.2	104.1
methyl tert-butyl ether	28	55.2	88.1
1,1,2,2-tetrachloroethane	29	146.3	167.8
tetrachloroethene	30	121.1	165.8
1,2,4-trichlorobenzene	31	214.4	181.4
1,1,1-trichloroethane	32	74.1	133.4
1,1,2-trichloroethane	33	113.8	133.4
trichloroethylene	34	86.7	131.3
1,1,2-trichloro-1,2,2-trifluoroethane	35	47.6	187.3
m-xylene	36	139.1	106.1
o-xylene	37	144.0	106.1
p-xylene	38	138.3	106.1

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Peak capacity for the first dimension was determined by calculating the total separation number³³ (SN_t). 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 (SN) is the preferred measure for temperature programmed GC.³⁴ Using eqn (1), SN was calculated for the homologous series starting from C₅ to C₆ and continuing through C₁₁ to C₁₂.

The total separation number (SN_t) was then calculated using eqn (2).³⁵

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 (n_p) for the second dimension was estimated from the measured peak widths for a resolution (R_s) of 1.18 and using eqn (3).

where L is the length of the column, H is the height equivalent to a theoretical plate, t_p is the modulation period and t_m is the column hold up time.

Fig. 6 is a chromatogram of a liquid gasoline sample. After a 1 μl injection with a 100 mL min⁻¹ split flow, the primary column was held at 30 °C for 3 min then began a temperature ramp of 5 °C min⁻¹ to 200 °C. The secondary column and GC oven were also held isothermal for 3 min and ramped at 5 °C min⁻¹ but started with a +10 °C offset, ending at 210 °C. For the gasoline analysis, the second dimension wax column was used outside the GC oven. Gasoline has been studied extensively by GC × GC, and numerous chromatograms have been published.^5,29–32 Thus, gasoline is a useful sample by which to compare instrument performance with that of other instrument designs.

Fig. 6 GC × GC chromatogram of a petroleum sample. Ellipse labeled A refers to linear and branched alkanes. Ellipses labeled B are alkyl-substituted benzenes. Ellipses labeled C are naphthalene and alkyl-substituted naphthalenes. A 5.0 s modulation period was used with a 1.0 s stage offset.

One useful feature of GC × GC is the patterns of peaks often observed in the chromatograms. The ellipse labeled A in Fig. 6 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 (ellipses labeled B) are from homologous series of alkyl-substituted benzenes. The group of peaks with second-column retention times in the range 3.5–4.5 s is from naphthalene and alkyl-substituted naphthalenes (ellipses labeled C). In general, the chromatogram shows excellent peak shape. There are no signs of streaking or sample overloading. Peak widths at half-height range from 35 ms (highly volatile components) to 158 ms (low volatility components) are produced. Peak capacity calculations determined an approximate peak capacity of 3150 using the equations described above.

Modulator comparison

Commercially available GC × GC systems use two-stage thermal modulators. The main feature of a two-stage modulator is the ability to limit breakthrough and sample loss during the modulator heating and cooling processes. The performance of the two-stage resistively-heated modulator described here has shown to be as effective as most commercially available thermal modulators. Qualitative studies have shown similarity in generated chromatograms of known samples. Quantitative studies have shown the resistively-heated two-stage modulator to be linear over at least 3 orders of magnitude in the low to high ppb concentration range. In this research study, the two-stage resistively-heated modulator is compared to the previously described single-stage resistively-heated modulator.^27,28

Fig. 7 shows two GC × GC chromatograms for a 38-component mixture analyzed with two different resistively-heated modulators. Chromatogram 7(A) was obtained on the HP-GC × GC platform using a single-stage resistively-heated and air-cooled modulator. Chromatogram 7(B) was obtained on the same HP-GC × GC system but with a two-stage resistively-heated and air-cooled modulator. Both chromatograms were obtained under the same chromatographic conditions. A primary column temperature of 30 °C was followed by a 3 min isothermal period and then ramped at 8 °C min⁻¹ to 200 °C. The second column followed the same conditions with the exception of a +10 °C initial offset. Inlet pressure was set at 15 psig and the carrier-gas outlet flow was 1.75 mL min⁻¹. A manual liquid injection of 2 μL with a split flow of 100 mL min⁻¹ was used. Both chromatograms appear very similar with the exception of the highly volatile components that elute from the first dimension at approximately 5 min. Chromatogram 7(A), obtained with the single-stage modulator, has greater peak broadening and streaking, features previously documented with the single-stage modulator. The chromatogram obtained with the two-stage modulator, Chromatogram 7(B), has narrower peaks relative to the single-stage modulator. The peaks from the two-stage modulator show minimal tailing and good peak shape reproducibility.

Fig. 7 GC × GC chromatograms for a 38-component mixture. Inset (A) was analyzed with a single-stage air-cooled modulator. Inset (B) was analyzed with a two-stage air-cooled modulator.

Conclusions

The design and evaluation of a two-stage resistively-heated modulator has been described. Performance evaluation of this modulator has exceeded the previous data reported for comparable single-stage modulators. The engineering design is well suited for GC oven mounting. The robustness of this modulator is still under investigation. However, more than 30 000 modulations have occurred without performance loss or modulator breakage.

The use of a closed-loop recirculating air-cooled system proved to be an effective means for cooling the modulator. Trapping efficiency at the low end (highly volatile components) was improved, and minimal breakthrough was reported. Low volatility components up to n-C₂₀ were effectively modulated, but peak broadening was experienced due to the temperature/voltage profile. Using an adjustable power supply will improve peak widths for higher-boiling point compounds.

The gasoline chromatogram obtained with the two-stage modulator is one of the best reported throughout this work. It compares nicely with previously reported gasoline chromatograms that were generated with commercially available modulators. Further improvements could be made with respect to column choice and temperature programming in an effort to generate a similar separation in a shorter analysis time.

The two-stage modulator has proven to be a viable alternative to commercially available modulators which require large amounts of cryogenic gases. Eliminating this consumable moves this technology closer to a field portable instrument without significantly sacrificing performance.

Acknowledgements

The authors performed this work in the University of Michigan lab of the late Dr Richard Sacks, for whom this paper is dedicated. Design of the modulator was carried out in a collaborative effort with Bruce Block, University of Michigan Space Physics Research Laboratory, and Dr Ernest Hasselbrink, University of Michigan Department of Mechanical Engineering. This work was supported by the NSF through grant CHE0650647. In addition, the authors would like to thank Dr Josh Whiting and Dr Pete Stevens for technical support.

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