An Introduction to Different Types of Gas Chromatography

Gas Chromatography was introduced traditionally for the first time by James and Martin in 1952. As years went on, gas chromatography was further developed by combining various detectors and was used for different purposes. In this journal, 4 different types of detectors coupled with Gas Chromatography were discussed: Gas Chromatography-Mass Spectrometry (GC-MS), Gas Chromatography Olfactometry (GC-O), Gas Chromatography Flame Ionization (GC-FID), Gas Chromatography Time of Flight (GC-TOF), and also a new column technology: Multidimensional Gas Chromatography (MDGC). Each type is unique and utilized differently since they differ in their end detector. These detectors may further be combined to obtain better identification of volatile compounds.

It was claimed that the installation of the device was large enough and operated well.
Despite being published, the article was completely forgotten. In the article, it was briefly mentioned that the founder of Beckman Instruments, Arnold Beckman, was interested in the device and going to produce it the idea was then abandoned as the device was deemed to be too complex and gas chromatography was a dead end (Kolomnikov et al, 2018).

HOW GC WORKS
Gas Chromatography is an instrument that works to separate, identify and quantatively determine volatile compounds with boiling points up to 350 o C or 400 o C (Keulemans and Verver, 1959). GC is a partition chromatography where the liquid film is held in a solid support that acts as a stationary phase, and controlled gas flows in the surface of the liquid surface acting as the moving phase. The temperature used in GC column is adjusted according to the mixture pf components undergoing the partitioned separation (Horning et al., 1964). Some limitations of this method include the necessity of the compounds being analayzed to be a stable volatile to some extent; and the dependency of temperature limit on the column. Horning et al. (1964) stated that even though molecular weight limitation is yet to be known, it is predicted that it will be around C75 for hydrocarbons (a region where carboncarbon bond dissociation energy is equal to dispersion forces overcoming energy to provide vapor phase). Non-volatile polar substances must be derived to a less polar form before an analysis. GC also involves high temperature in the process, thus compounds decomposition cannot be avoided.
Before the process begins, there are a few things that need to be considered, such as the form of the sample, choices of column, as well as the carrier gas, oven temperature and evaporation during injections. (Sparkman et al., 2011). A sample injected to the instrument will be converted to a gaseous state and carried by a carrier gas. The separation of the components will take place at the column and will be identified by the detector. GC provides both qualitative and quantitative very important and valuable analytical data (Horning et al., 1964)

TYPES OF GC
GC is differentiated to a few types, where the difference is in its rear detector. This section will discuss a few types of GC.  A column selected in an appropriate manner will produce an accurate and reliable analysis; but if the column was selected improperly, it will generate inadequate, inaccurate, poor and unreliable separations which ultimately will lead to invalid or complex results. (Al-Bukhaiti et al., 2017). Gas Chromatography (GC) can analyse more than 10,000 compounds with more than 400 GC capillary columns (Singh et al., 2013). The choices of column and supporting instruments have a keen influence on the final result of separation optimization.

Gas Chromatography-Mass
The modification of the column parameters (stationary phase, length, inner diameter and film thickness) enhance chromatographers control for its column efficiency, resolution and speed of analysis (Grob and Barry, 1977).
The common carrier gases used are helium, nitrogen and hydrogen (Al-Bukhaiti et al., 2017). Inertness, absence of oxygen, dryness, safety, as well as the cost and availability of gas carriers should be considered for using (Fowlis, 1995).

Application of GC-MS in food analysis involves
food composition, such as food additives, flavor and aroma components, and also its contaminants, for example natural toxins, pesticides, fumigants, environmental pollutants, veterinary drugs, or packaging materials (Lehotay and Hajslova, 2002).  vocabularies and it will last days-weeks (Cain, 1979). There are two ways to evaluate the data, aroma extract dilution analysis (AEDA) which and combined hedonic aroma response method (CHARM) analysis. AEDA measures the maximum dilution of an extract that an odor is perceived in (Delahunty et al., 2006), it expresses the raw data as a flavor dilution values (Friedrich and Acree, 1998).
Both data are comparable because it expresses a relative number of dilutions until the odor elutes from the column and become undetectable by the sniffer (Friedrich and Acree, 1998). The results of GC/O analysis could be expressed by odor activity values which can be classified as a chromatogram.
The chromatogram describes the intensity and pattern of the odor-active compounds because an odor activity value is the ratio of the concentration of an odorant to its odor intensity (Friedrich and Acree, 1998). Charm values are obtained using a certain algorithm so they become proportional both to the amount of compound in the sample extract and inversely to the odor detection threshold (Delahunty et al., 2006). An easier interpretation of chromatogram is made by counting the ratio of the total amount of odoractive compounds eluting at a particular index to the threshold amount for that same mixture of compounds (Acree et al., 1984). AEDA reports the maximum dilution value, which is equivalent to the height of the Charm peak (Delahunty et al., 2006). AEDA is limited to the availability of statistical data manipulation, non-consideration of odor loss during the process of isolation and synergistic/ suppressive/ distinct compounds in flavor mixture (Zellner et al., 2008). Factor that also will affect the odor results is the dilution. This relevancy was shown by demonstrating the similarity of standard solution, a mixture of identified potent aroma compounds, to an actual food product (Gut and Gsroch, 1994;Grosch et al, 1995).

Drawbacks of GC/O analysis is directly related
to the use of human detector. Abbott et al. (1993) mentioned that it is hard for panellists to detect the end of odor region. GC/O is timeintensive and typically done with 1-2 panellists. Panellists need to be carefully prescreened for sensitivity and specific anosmia (Friedrich and Acree, 1998). It also has been shown that individual's olfactory sensitivity changes throughout the day, where ovulatory cycle occurs (Koster, 1968), which became a concern because series of dilution analysis often takes weeks to perform. A GC-TOF scheme can be seen in Figure 4.

Gas Chromatography-Flame
Samples injected into the machine needs to be either gas or liquid. If the sample is solid, then the sample needs to be dissolved beforehand.
Liquid samples are going to be vaporized first.
After the sample is vaporized, gaseous ions will form from the analyte.    (2012) A H/C-MDGC instrument (shown in Figure 6) uses two approaches of effluent switching (Marriott et al., 2012). Sequential transfer of compounds from the first to second column is usually done using an on-line heart cut, only allowing the transport of some key analytes (Herrero et al., 2009). Herrero et al. (2009) highlighted that the system is based on longitudinally modulated cryogenic system  co-elute each other and separate chirals so odor intensity, potency and enantiomers can be evaluated (Delahunty et al., 2006;Begnaud & Chaintreau, 2005).

CONCLUSIONS
It is concluded that GC has a very broad utilization in food analysis. Because the types of detectors are very specific, it is usually used for different analysis, but it can also be combined to obtain better data analysis. GC/MS is used to determine wide range of samples as it has mass spectra database which can be used to interpret and compared to the sample. GC/O is used to determine volatiles and aromas present in food sample, by using human as a sniffer.
GC/FID is usually used in hydrocarbons and oils analysis, because the principle is to detect conductivity of the burned sample. GC/TOF is combined with a MS detector, to identify unknown sample by calculating ion's time of flight to a detector. MDGC is combination of multiple columns (two or more) to obtain higher peak capacity, since complex compounds can be separated.