Cryofocusing

Cryogenic trapping or cryofocusing is a technique commonly used in gas chromatography to overcome peak distortion and problems with resolution of chromatographic peaks. Trapping does not directly occur; rather the migration time of the sample is slowed down by the inclusion of a cold trap is incorporated into a GC system (Hagman and Jacobsson 1988). The utility of these cryogenic traps has been reviewed clearly by Brettel and Grob (Brettell and Grob, 1985a, Brettell and Grob, 1985b, Grob and Barry, 2004).

This technique has been extensively used to improve separation during the analysis of VOCs in the real environment (Simmonds, 1984, Wylie, 1986, Kolb et al., 1986, Shimoda and Shibamoto, 1990, Buser et al., 1982, Hagman and Jacobsson, 1988, Pankow, 1983, Werkhoff and Bretschneider, 1987, Mehran et al., 1990), with majority of reported studies using a “conventional” GC-FID instrument similar to that used for experiments in HIRAC.

Cryofocusing can be carried out by either cooling the entire column to sub-ambient temperatures during sample injection, or more conveniently by using the first coil of the capillary column (Kolb et al., 1986). The former has a few shortcomings as it takes a longer time to cool down the entire column and will also consume copious amounts of coolant, generally liquid nitrogen or carbon dioxide. Cooling the whole column is a still regarded as a viable “simpler” method as no trapping loop connections is necessary and this method has been shown to provide good chromatographic efficiency (Pankow, 1983). According to the literature, it is thought that cryofocusing occurs primarily by slowing down the migration rates of the sample components by increasing their capacity factors, which can be expressed as:

RF = 1 / (1 + k) = us / uc (Eq 2.8)

where RF is the migration rate, k is the capacity factor of the solute, and us and uc are the respective velocities of the sample vapour and the carrier gas respectively (Kolb et al., 1986).

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Figure 2.18: Cold trap installation in sampling loop; Adapted from (Borgerding and Wilkerson, 1996).

The most common method of cryofocusing makes use of a high-pressure liquid that can reduce the column temperature to as low as ~230 K within seconds, as is shown in Figure 2.18. This is similar to a recently installed liquid carbon dioxide cryogenic cold trap in the HIRAC GC-FID system (Figure 2.19). This section will describe the function and purpose of this recently installed cold trap, and discuss recent tests using n-alkanes.

The efficiency of a cold trap, measured on its ability to retain compounds, is dependent upon a number of factors, including the lowest temperature achievable.

This is the reason liquid nitrogen and liquid carbon dioxide are used since they are inexpensive and can cool a trap to low enough temperatures to retain the majority of VOCs. Other factors include the temperature of the GC oven, the amount of analyte loaded onto column, timing of cooling and temperature gradients in the column.

Compounds with very low capacity factors have also been found to exhibit problems as a short portion of the column may not be sufficient for the trapping of these analytes. Increasing the sampling time and trapping time can make this problem less pronounced, but if the capacity factor is too small, some of the compound could break through the trap.

HIRAC: Instrumentation

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Figure 2.19: Liquid CO2 system installed on the wall of the HIRAC GC-FID instrument (Taken from: http://www.sge.com/).

The aim of this cold trap was to improve the separation of peaks in GC-FID chromatograms, which would be beneficial for more elaborate product studies in HIRAC. This setup aims at improving the quality of chromatographic peaks observed during runs by giving ideal peak shapes through focusing of components into a narrow band resulting in reduced peak tailing and band broadening, depending on the cause of the poor peak shape in the chromatography.

This SGE CO2 cold trap system installed works by controlling the flow of liquid CO2 to a low thermal mass trap through a pneumatically actuated valve.

Focusing takes place at the head of the column through cooling the first section of the column using liquid CO2. The cold trap is designed to rapidly cool and efficiently reheat afterwards. The cold trap setup consists of a tee that is mounted in the GC-FID oven and connected to the cooling CO2 liquid via insulated transfer lines. The flows are controlled by a pneumatic head switching valve that requires an actuating gas (nitrogen) pressure of ~4 bar. A three-way solenoid valve has been installed in order to automate this system by switching on and off the pneumatic nitrogen gas flow which in turn controls the switching valve for the cold trap which switches on and off the flow of CO2 into the tee. This is controlled using an in-house National Instruments LabView software added on to the already existing solenoid program used to control the GC solenoids described earlier.

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Initial tests of the cold trap were performed on a series of n-alkanes in HIRAC.

The results presented in Table 2.1 were obtained in HIRAC at 1000 mbar and 292 K for n-pentane, hexane and n-octane.

Table 2.1: Chromatographic data (retention time, width, area and height) for a series of n-alkanes with the cold trap turned on and off. The ratio reflects the change due to

the cold trap being on.

These results demonstrate the potential for using a cold trap in the effect it has on the width of the peaks, particularly for the smaller alkanes used. This seems to point toward a trade-off between the time the cold trap is “holding” the analytes in the column with their retention times (Hagman and Jacobsson, 1988). As can be noted the cold trap is having a more pronounced effect on the n-pentane peaks; this is due to a higher vapour pressure compared to n-octane. The timings used for the cold trap were the same for every consecutive injection and the peak parameters shown in Table 3.1 are taken from the average of 7 injections. In fact the width decreases by a factor of 2.67 for n-pentane, while the height increases by the same factor. For the hexane the width roughly halves while the height roughly doubles, and for n-octane the change noted is much less evident, with a decrease by 20% for the width and an increase by a factor of 42% for the height.

Small changes in the peak areas (5%) are also noted, and it is not clear what could be the reason for this, possibly due to retardation of elution for any possible contaminants present in HIRAC or in sample line that may overlap with the hydrocarbons. A thermocouple was attached to the cold trap during these experiments and an average temperature of ~223 – 233 K was reached within a few seconds of the cold trap being turned on. Other authors report lower temperatures would be more suitable to focus all VOCs, since if the temperature is not low enough, the focusing is not as pronounced, and hence the efficiency of the cold trap is minimised this way.

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2.3.10 Chromatographic techniques in other