Temperature Programming of the Second Dimension in

Letter pubs.acs.org/ac

Temperature Programming of the Second Dimension in Comprehensive Two-Dimensional Gas Chromatography Hei-Yin J. Chow and Tadeusz Górecki* Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada S Supporting Information *

ABSTRACT: Comprehensive two-dimensional gas chromatography (GC × GC) provides a significant increase in selectivity and peak capacity for the separation of complex mixtures. Optimization of the system is often complicated, with many interconnected parameters between the two dimensions and additional problems like peak wraparound that need to be eliminated or minimized. Wraparound peaks are compounds with retention times in the second dimension that are longer than the modulation period. This results in broad peaks that elute in subsequent modulation cycles, potentially coeluting with separated compounds. The use of a secondary oven is often the solution to the problem. By applying a constant positive temperature offset from the main oven temperature, the retention of all analytes can be reduced so that they elute within their respective modulation periods. However, this reduces the separation of less retained compounds, a classical consequence of the general elution problem due to the isothermal conditions during the limited separation time in the second dimension. To overcome this problem, the second dimension was temperature-programmed by resistively heating an electrically conductive secondary column using constant current. The column was cooled through forced convection inside the GC oven within the time frame of a single modulation period. Temperature programming in the second dimension of GC × GC was able to improve separation while eliminating wraparound peaks and reducing peak widths, leading to significantly increased second dimension peak capacity.

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mentioned that highly polar phases tend to retain polar compounds so much that they wrap around to subsequent modulation periods.15 The current solution to alleviate this problem is the use of a secondary oven providing a constant positive temperature offset from the main GC oven temperature, to reduce the retention of all compounds separated in the secondary column. Although this can reduce the retention of highly retained compounds and eliminate wraparound peaks, it also results in the loss of resolution for less retained compounds. With or without a secondary oven, the second dimension separation is still nearly isothermal due to the short duration (several seconds) of the modulation period. Ever since the introduction of gas chromatography (GC),16 it was recognized that complex mixtures with a wide range of boiling points would require temperature programming for efficient separation within a reasonable timespan.17 However, to the best of our knowledge, temperature programming of the second dimension of GC × GC has not been described in the literature. In this letter, we demonstrate the proof of concept that temperature programming of the second dimension in GC × GC helps eliminate wraparound peaks while improving the

omprehensive two-dimensional gas chromatography (GC × GC) is a powerful separation technique used in the analysis of highly complex mixtures in many sectors of research and industry.1−5 As the success of a GC × GC separation hinges on the effective transfer of the effluent from the primary to the secondary column while maintaining the first-dimension separation, much of the research in GC × GC has been focused on the modulators.6−12 Since the first GC × GC separation was carried out by Liu and Phillips in 1991 using thermal desorption modulator,13 many different modulator types have been developed. Examples include cryogenic jet-based modulators, flow modulators, and consumable-free modulators.6,7 Both flow and consumable-free modulators are areas of continued interest in research, in an attempt to reduce the long-term cost of operating a cryogenic modulator (due to the large consumption of expensive cryogens) while reproducing its performance.10−12 However, in spite of the advancements in modulator design, improvements to second dimension separation have been largely ignored. In GC × GC optimization, many user-adjustable parameters affect the separation in both dimensions, due to the coupling of the primary and secondary columns and the shared GC oven. The intricacy of GC × GC optimization was illustrated in the “spaghetti diagram” showing the interplay of GC × GC parameters.14 Through a careful selection of columns and experimental conditions, it has been demonstrated that an order-of-magnitude gain in peak capacity was possible.15 In the choice of the second dimension stationary phase, it was © XXXX American Chemical Society

Received: June 5, 2017 Accepted: July 28, 2017 Published: July 30, 2017 A

DOI: 10.1021/acs.analchem.7b02134 Anal. Chem. XXXX, XXX, XXX−XXX

Letter

Analytical Chemistry

consumable-free modulator developed at the University of Waterloo.9 Analytes were trapped and focused using an actively cooled stainless-steel coated capillary trap and injected into the secondary column by rapid resistive heating using capacitive discharge. Voltage and timing of the discharge were controlled and set manually on the capacitive discharge power supply. The power supply was equipped with two channels for discharge, either of which could be used with the modulator. A dc power supply (QW-MS305D, Wuxi Qiaowei Electronics Co., Ltd., China) was used to provide constant current to an electrically conductive secondary column for resistive heating. The heating rate and temperature increase were a function of the current supplied. A remotely controlled switch was connected to the capacitive discharge power supply to turn the current from the dc power supply on and off. The secondary column was mounted directly in front of the GC oven fan using a modified GC column cage that was lined with a glass fiber sleeving for electrical insulation. Fused-silica guard columns were connected before and after the secondary column as electrical insulators. Second Dimension Temperature Programming. Reinjection of trapped analytes into the secondary column (the beginning of the modulation period) and heating of the secondary column were controlled by the timing of the discharge from channel 1 and channel 2 of the capacitive discharge power supply. Discharge from channel 1 turned the remote switch on, allowing current from the dc power supply to pass to the secondary column. Discharge from channel 2 turned the switch off and allowed the secondary column to cool back to the oven temperature through the forced convection from the GC oven fan. With the modulator connected to channel 2, the general current vs time profile for the secondary column looked as shown in Figure 2A. The secondary column was rapidly cooled to oven temperature at the start of each modulation period, followed by heating until the end of the modulation period after a preset delay. Second Dimension Constant Temperature Offset. By connecting the modulator to channel 1 on the capacitive discharge power supply and turning channel 2 off, the switch remained in the “on” state throughout the GC × GC run. This provided constant current to the secondary column, which kept it at a constant positive temperature offset similar to the use of a secondary oven. The magnitude of the current supplied to the secondary column determined the temperature offset.

separation of early eluting compounds compared to the use of secondary oven. The secondary column was heated and cooled back to oven temperature within the span of a modulation period for each modulation in a GC × GC run.

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EXPERIMENTAL SECTION A diagram of the GC × GC setup is presented in Figure 1. An Agilent 6890A GC was equipped with a single-stage

Figure 1. GC × GC setup used for temperature programming of the second dimension. The capacitive discharge power supply periodically heats the modulator and controls the timing of the dc power supply providing constant current for the secondary column.

Figure 2. Current vs time profile for the secondary column in relation to the modulation period and channel discharge of the capacitive discharge power supply (A). The corresponding temperature vs time profile (B).

Figure 3. Heating and cooling times for an MXT-WAX (0.49 m × 0.25 mm) and MXT-65 (0.50 m × 0.18 mm) column. B


Letter

Analytical Chemistry Secondary Column Temperature Measurement. A 50 μm K-type thermocouple (OMEGA Engineering, Quebec, Canada) was spot-welded to the secondary column and connected to a HAMEG HM208 oscilloscope (HAMEG Instruments, Germany) through a 100× gain amplifier. Signal changes over time were measured with carrier gas flowing through the column. Experimental Conditions. A diesel sample was analyzed using an Rxi-5 ms (30 m × 0.25 mm × 0.25 μm) primary column and with either an MXT-WAX (0.49 m × 0.25 mm × 0.50 μm) or MXT-65 (0.50 m × 0.18 mm × 0.20 μm) secondary column. Column connections were made using SilTite mini unions (SGE, TX). A volume of 1 μL of diesel fuel was injected with a 300:1 split ratio. Hydrogen was used as the carrier gas at a constant flow of 1.2 mL/min with the MXTWAX column set and 2.0 mL/min for the MXT-65 column set. A modulation period of 5 s was used with the MXT-WAX column set and 4 s for the MXT-65 column set. A flame ionization detector (FID) was used with nitrogen as the makeup gas. The GC oven temperature program for the MXT-WAX column set was 40 °C, 5 °C/min to 200 °C (10 min hold) for a total runtime of 42 min. The temperature program for the MXT-65 column set was 50 °C, 3 °C/min to 250 °C for a total runtime of 66.67 min. GC × GC separation with a constant temperature offset was achieved by applying a constant current of 1.45 A (40 °C) or 0.98 A (30 °C) throughout the duration of the run to the MXT-WAX and the MXT-65 columns, respectively. The GC × GC separation with second dimension temperature programming for the MXT-WAX column set consisted of a 1.5 s cooling period at the start of the modulation period followed by a 3.5 s heating period to 40 °C (1.45 A) above the GC oven temperature. For the MXT-65 column set, it utilized a 1.5 s cooling period followed by a 2.5 s heating period to 30 °C (0.98 A) above the GC oven temperature.

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RESULTS AND DISCUSSION The time required to heat and cool the secondary column was measured for various temperature increases (20 °C, 30 °C, 40 °C) and at different oven temperatures (50 °C, 100 °C, 150 °C, 200 °C, 250 °C (MXT-65 only)). For the MXT-WAX column, measurements were taken up to an oven temperature of 200 °C due to the upper-temperature programming limit of 250 °C for the stationary phase. For the MXT-65 column, the temperature range for the stationary phase was 50 to 300 °C. Measurements were taken up to an oven temperature of 250 °C. The general temperature profiles obtained are represented in Figure 2B. Cooling followed a typical exponential profile, being very rapid in the initial portion of the cooling period and becoming slower as the column temperature approached the oven temperature. Similarly, the temperature increase was very rapid for the initial duration of the heating, followed by a plateau in temperature as the column reached a steady-state between heat generation and heat dissipation. Heating and cooling durations were measured from the start of the heating or cooling, respectively, to the start of the plateau. Figure 3 illustrates the heating and cooling times for the MXT-WAX and MXT-65 columns. Overall, the MXT-WAX column took around 2 s to heat and cool for the various temperature increases and oven temperatures. For the MXT-65 column, it took around 1.5 s to heat and cool under the same conditions. The reduced diameter and wall thickness of the

Figure 4. GC × GC separation of diesel using a 1D Rxi-5 ms (30 m × 0.25 mm × 0.25 μm) and 2D MXT-WAX (0.49 m × 0.25 mm × 0.50 μm) column set. (A) Standard GC × GC separation; (B) constant temperature offset of 40 °C; (C) second dimension temperature programming with 1.5 s of cooling followed by 3.5 s of heating to 40 °C above the oven temperature.

Figure 5. Raw GC data for the GC × GC separation of diesel using an Rxi-5 ms (30m × 0.25 mm × 0.25 μm) and MXT-WAX (0.49 m × 0.25 mm × 0.50 μm).

MXT-65 column resulted in lower thermal mass, which led to shorter heating and cooling times, and less current was required to achieve the same temperature increase. On the basis of the C


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Figure 6. GC × GC separation of diesel using an 1D Rxi-5 ms (30m × 0.25 mm × 0.25 μm) and 2D MXT-65 (0.50 m × 0.18 mm × 0.20 μm) column set. (A) Standard GC × GC separation; (B) constant temperature offset of 30 °C; and (C) second dimension temperature programming with 1.5 s of cooling followed by 2.5 s of heating to 30 °C above the oven temperature.

0.0% RSD in the first dimension, and below 2% RSD in the second dimension for temperature programming (see the Supporting Information). For the standard GC × GC separation, average RSDs were below 0.5% in the first dimension and 1.4% in the second dimension. The differences were not statistically significant at the 95% probability level. The resolution for five peak pairs across the chromatogram was calculated for each method using the MXT-65 column set. Compared to the constant temperature offset separation, temperature programming and standard GC × GC separation resulted in an average resolution increase of 32% and 69%, respectively (see the Supporting Information for raw data). For the MXT-65 column set, the lower polarity stationary phase compared to the wax column resulted in shorter retention times in the second dimension. This allowed for a slower temperature program as well as a shorter modulation period for this setup. The standard GC × GC separation, shown in Figure 6A, resulted in wraparound peaks, but they had very little overlap with peaks in the following modulation period. A constant temperature offset of 30 °C resulted in a significant loss of separation in the second dimension, with most of the peaks eluting between 0.9 and 2.0 s as a single band (Figure 6B). With temperature programming, all wraparound peaks were eliminated, while most of the separation in the band of early eluting compounds was maintained (Figure 6C). While the cooling duration remained the same, the heating duration (2.5 s vs 3.5 s) and temperature increase (30 °C vs 40 °C) were reduced to reflect the reduced retention of polar compounds on this stationary phase.

cooling time required for each column, the appropriate cooling duration was estimated for different modulation period durations. The cooling duration was determined in consideration of the heating duration needed to eliminate or minimize the impact of wraparound peaks. Diesel fuel was analyzed to evaluate the effect of temperature programming of the second dimension. Using the MXT-WAX secondary column, significant wraparound of peaks was observed for the standard GC × GC separation with both columns at the same temperature (Figure 4A). When a temperature offset of 40 °C was applied, all wraparound peaks were eliminated (Figure 4B). However, the reduced second dimension retention of all compounds led to dramatic reduction of the separation of the nonpolar compounds, the width of the early eluting band of aliphatic alkanes being reduced from ∼0.9 s to ∼0.3 s. When the second dimension was temperature-programmed, the separation of the nonpolar compounds was affected much less (∼0.6 s width of the band), while a majority of the wraparound peaks were eliminated (Figure 4C). Compared to the constant temperature offset separation, the available separation space was utilized more effectively. By allowing the secondary column to cool in the initial 1.5 s of the modulation period, it provided a pseudo hold time at an average temperature close to the oven temperature. This allowed the nonpolar compounds to be better separated. The temperature-programmed increase following the cooling period was then able to reduce the retention enough for some of the most polar compounds in diesel to elute by the end of the modulation period or early in the following modulation period. Any wraparound peaks observed with the second dimension temperature programming separation did not overlap with peaks in the following modulation period. When comparing the raw GC data in Figure 5, it was observed that average second dimension peak widths were reduced for the temperature-programmed separation compared to the standard GC × GC separation; thus, temperature programming provided higher peak capacity in the second dimension. In addition, the number of distinguishable peaks between the two separations remained the same. In particular, the separation of the three early eluting peaks was comparable between the standard GC × GC and temperature programming separation. For the GC × GC separation with a constant temperature offset, only 3 distinct peaks could be observed compared to the 5 peaks in the standard GC × GC and temperature-programmed separation. The average relative standard deviations (RSDs) of the retention times for run-to-run reproducibility of 10 peaks for five replicate injections using the MXT-65 column set were

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CONCLUSIONS

It was demonstrated that temperature programming of the second dimension in GC × GC could successfully eliminate (or at least significantly reduce) wraparound peaks, while maintaining better separation of early eluting compounds compared to constant temperature offset. Even with this simple design and suboptimal heating profile, temperature programming significantly improved the second dimension separation compared to a constant temperature offset. Thus far, the potential of temperature programming of the second dimension has gone unexplored. Improvements of the design can be made to gain finer control over the heating rate to further improve the second dimension separation. D


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.7b02134. Raw data for RSD and resolution calculations (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tadeusz Górecki: 0000-0001-7727-7516 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Jacek Szubra, Hiruy Haile, and Harmen Heide from the Science Technical Services group for their assistance in electronics and machining. Financial support for this research was provided by NSERC.

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Temperature Programming of the Second Dimension in

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