Comprehensive two-dimensional gas chromatography
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Comprehensive Two-dimensional gas chromatography, or GCxGC was originally described in 1991 by Professor Phillips and his student Liu. Since then the GC × GC has been extensively applied to solve complex problems of separations. Started in the Oil and Gas Industry the technology was mainly used for the complex oil samples to determine the many different types of Hydrocarbons and its isomers. Nowadays in these types of samples it has been reported that over 30000 different compounds could be identified in a crude oil with this Comprehensive Chromatography Technology (CCT). The CCT evolved from a technology only used in academic R&D laboratories, into a more robust technology used in many different industrial labs. Comprehensive Chromatography is used in forensics, food and flavor, environmental, metabolomics, biomarkers and clinical applications. And almost daily new applications are being developed in laboratories all over the world. Some of the most well-established research groups in the world that are found in Australia, Italy, Holland, Canada, United States, and Brazil use this analytical technique.
Modulation: The process
In GC × GC two columns are connected sequentially, typically the first dimension is a conventional column and the second dimension is a short fast GC type, with a modulator positioned between them. The function of the modulator can be divided into basically three processes:
- continuously collect small fractions of the effluent from 1D, ensuring that the separation is maintained in this dimension;
- focus or refocus the effluent of a narrow band;
- to quickly transfer the 2D fraction collected and focused as a narrow pulse. Taken together, these three steps is called modulation cycle, which is repeated throughout the chromatographic run.
The most frequently used type of modulation is the thermal modulation (patent holder is ZOEX Corporation), where liquid nitrogen is used to (cryogenic) trap (immobilize) all the components eluting from the first dimension. After a fixed time interval a hot stream pulse is mobilizing a part of the compounds again. This hot pulse can be considered as the injection starting point into the second dimension column. The latest version is called a loop-type Thermal modulator where the released compounds are trapped and refocused for a second time (and released again) to have perfect peak shapes and maximum resolution in the second dimension. With the thermal modulator also very volatile compounds can be modulated. The thermal modulation in practice is a liquid nitrogen cooled loop system that provides the lowest temperature for thermal modulation, and modulates the widest range (C2 to C55) of organic compounds. The temperature at the jet is -189 °C. The maximum temperature of the hot jet is 475 °C. Even methane has been modulated with liquid nitrogen cooled gas jets like those in this type of modulator. The second important modulator type is the so-called Closed Cycle Refrigerated Loop Modulation. This loop modulation system eliminates the need for liquid nitrogen for thermal modulation. The system employs a closed cycle refrigerator/heat exchanger to produce -90 °C at the jet. The cooling is done by indirect cooling of gaseous nitrogen and therefore this type modulates volatile and semi volatile compounds over the C6+ range. Another type of modulation is flow modulation. This is a valve-based approach, where differential flows are used to ‘fill’ and ‘flush’ a sample loop. Flow modulation does not suffer from the same volatility restrictions as thermal modulation, as it does not rely on trapping analytes using a cool jet - meaning volatiles <C5 can be efficiently modulated.
The time required to complete a cycle is called the period of modulation (modulation time) and is actually the time in between two hot pulses, which typically lasts between 2 and 10 seconds is related to the time needed for the compounds to eluted in 2D.
Another key aspect of GC x GC that can be highlighted is that the result from the refocusing in the 1D, which occurs during the modulation, causes a significant increase in sensitivity. The modulation process causes the chromatographic bands in GC × GC systems are 10-50 times closer than in 1D-GC, resulting in values for much better peak widths (FWHM Full Width Half Mass) between 50 ms to 500 ms, which requires detectors with fast response and small internal volumes.
Regarding the set of columns, can be combined in various types. The Original column sets in the beginning were mainly poly(dimethylsiloxane) in the first dimension and poly(ethyleneglycol) in the second dimension. These so-called straight phase column sets are very suitable for Hydrocarbon analysis. Therefore, these are still used most frequent in Oil and Gas industry. If the application is to determine more polar compounds in a non-polar matrix. The reverse phase column set gives more resolution for the polar compounds. The first dimension column in this situation is a polar column, followed by a mid-polar second dimension column. For each application there can be an optimal column set. In this respective you can use also chiral columns for optical isomer separation or for example PLOT columns for volatiles and gas samples. To optimize the application is a bit more complex compared to 1D separations, as there are more parameters involved. Flow and oven temperature program are still very important but also hot jet pulse duration, length of the second dimension column and modulation time have a big influence on the final results. The outcome is also different. No longer a traditional chromatogram is produced, but now a 3 dimensional plot is produced by specially designed software packages. It is a different way of evaluate data, but you get much more information. With modern software, you can perform group-type separation as well as automated peak identification (with Mass Spectrometry).
Due to the small width of the peak in the second dimension suitable detectors are needed. For example, flame ionization detector (FID), (micro) electron capture detector (µECD) and mass spectrometry analyzers such as fast time of flight (TOF). Several authors have published work using quadrupole Mass Spectrometry (qMS), though some trade-offs have to be accepted as these are much slower.
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