1. INTRODUCTION:

J. Calvin Giddings was the creator of the idea of separating the components of the mixture using the GC×GC technique, proposed in 1984.^[1][2] The professor assumed that the best results of GC×GC analysis would be obtained when the separation mechanisms in both dimensions are completely different from each other and strictly orthogonal. The complete GC×GC technique was intended to solve the problem of limited peak capacity and the difficulty of separating certain compounds in the classical GC. It was essential to find a suitable way to divide the components of the mixture into the first column by dividing the whole separation process into the second column by using another separation mechanism while still separating the components obtained in the first column.

In the 1990s, J. Phillips proposed the implementation of the GC×GC modulator. Officially recognized as the year 1991, the GC×GC is equipped with a modulator as the year of the launch of the complete two-dimensional gas chromatography technique.^[3] Further development of GC×GC consisted mainly in the construction of new types of modulators, as well as appropriate software that would enable the automation of the processing of large amounts of data.^[4]

What is GCxGC?

GCxGC is a truly hyphenated technique employs a pair of GC column each containing a chemical phase that is orthogonal to the other, connected in series via a modulator. Effluent from the first columns is trapped in the modulator for a given period (modulation time) before focused and released into the second column. The chromatogram obtained through repeated trapping and injection is rendered in two dimensions using specialized software, with the first and second dimensions on respective axes.

2. Principle:

The principle of separation of 2D-GC is based on the interaction of analytes with the stationary phase and the mobile phase. As a mobile phase, inert gas mostly helium or nitrogen, is used. Introduced into the chromatography system, the sample is evaporated in the injector and entrained through the carrier gas stream. Separation of chemicals is possible through the use of two serially-bound, stationary polarization columns with different polarity. The heart of the system is a modulator, which combines both chromatographic columns and the transfer of individual components from one column to another.^[5]

Figure 1: Two- dimensional gas chromatography.

3. Working:

The working of 2D GC is shown in (fig 1).

3.1. Injector:

It is a component of the chromatographic system used to entry the sample into the chromatographic column. In the two-dimensional gas chromatography the same dispenser types are used, as in GC.

3.2. Column:

Capillary type of columns are used in two-dimensional gas chromatography as shown in (fig 2). Generally 2D-GC employs two columns of different sensitivity (such as boiling point versus polarity). Column sets are mainly poly dimethyl siloxane in the first dimension and polyethylene glycol in the second dimension.

Figure 2: Capillary column.

3.2.1. Chromatographic column 1:

The first dimension chromatography column is typically 15 to 30 m in length and has an internal diameter of 0.25 mm. Contains a stationary film with a thickness of 0.25 to 1.0 μm. In this case 100% polydimethylsiloxane or 95/5% phenyl/methyl siloxane is the non-polar stationary phase. The chromatographic column allows generation of peaks of approximately 10-20 s width.

3.2.2. Chromatographic column 2:

A second dimension chromatographic column filled with another stationary phase than the first column. Its length is usually from 0.5 to 1.5 m and the inside diameter is 0.1 mm. Due to its higher efficiency compared to the first column, the stationary film layer is also thinner and ranges from 0.1 to 0.25 mm. The most commonly used stationary phases are 50/50% phenyl / methyl siloxane or Carbowax. Chemical compounds must leave this chromatographic column in a very short time, shorter than the modulation period.

3.3. Modulator:

In between the two columns is a high-frequency modulator. The Modulator is designed to quickly collect and inject fractions from the first column onto the second column.^[6] The rapid process of modulation (every 2-5sec) preserves the peak separation of the primary column and enables the secondary column. Their are two general categories of modulators, they are thermal modulator and flow modulator. Irrespective of the type of modulator, the modulator function is:

a. of small amounts of the analyte from the first column.

b. Adjusting their retention time and dispensing frequency.

c. Introducing fractions into the second-dimension column.^[7]

3.3.1. Thermal modulator:

A thermal modulator alternates cold and hot jets to trap and inject the effluent onto the secondary column. This type of modulator diverts the entire sample onto the secondary column and is best for trace analysis. Thermal modulator is shown in(fig 3).

Figure 3: Thermal modulator.

3.3.2. Flow modulator:

A flow modulator uses valve switching to send small bands of the primary effluent onto the secondary column. This type of modulator is best for low cost applications where ultimate sensitivity is not required. Flow modulator is shown in (fig 4).

Figure 4: Flow modulator.

3.4. Chromatographic oven:

A gas chromatograph element responsible for maintaining the chromatographic system at a suitable temperature to ensure effective separation of the analyzed components of the mixture.

3.5. Detectors:

A device for detecting components. The most commonly used is flame ionisation detector. Sometimes a nitrogen phosphorus detector are also used.

3.5.1. Flame ionisation detector:

The ionisation detectors are based upon the electrical conductivity of carrier gases. At normal temperature and pressure, gases act as insulators, but become conductive if ions are present.

The carrier gas used with this type of detector can be hydrogen. If the carrier gas is either nitrogen or argon, it can be mixed with hydrogen and reach the burner tip made up of platinum capillary, which acts as one electrode(cathode). The anode is silver gauze placed little above the burner tip. When pure carrier gas alone passes, their is no ionisation and no current flows. When a component emerges from the column number of ions are produced because of ionisation by the thermal energy of the flame. This causes a potential difference and causes a flow of current which is amplified and recorded as signal.

The FID measurements are often labelled “total hydrocarbon”^[8] or “total hydrocarbon content”, although a more accurate name would be “total volatile hydrocarbon content”^[9], as hydrocarbons which have condensed out are not detected, even though they are important. The (fig 5) explain the working of flame ionisation detector.

Figure 5: Flame ionisation detector.

4. Criteria for compounds to be analysed:

A. Volatility:

Unless a compound is volatile, it cannot be mixed with mobile phase. Hence volatility is important.

B. Thermostability:

All the compounds will not be in the form of vapour. There will be solid as well as liquid samples. Hence to convert them to a vapour form they have to be heated to a higher temperature. At that temperature the compounds have to be thermostable. If they are not thermostable the compounds cannot be analysed.

5. Advantages:

A. GCxGC separates more chemical compounds than 1D GC with no run time penalty, improves sensitivity, and generates chromatograms far easier to interpret than those generated by 1D GC.

B. GCxGC can reveal information on to composition of a sample in a way that cannot be done by 1DGC.

C. The use of GCxGC in environmental forensic investigation is beneficial to provide a detailed and comprehensive finger print of the chemical contaminants and potential source.

D. GCxGC include increased peak capacity, improved resolution, and unique selectivity compared to 1D GC.

E. Allows analysis of samples with complex matrix composition.^[10]

F. Allows identification of trace or ultra-trace analyte.^[11]

G. Allows for more detailed quantitative and qualitative analysis.

H. Great distribution capacity and high peak capacity.^[12]

6. Applications:

A. Two-dimensional gas chromatography is a technique often used in many different areas of chemistry.

B. It was originally used to analyse petroleum samples, other complex matrices.

C. Increasingly, the GCxGC technique is used for forensic, environmental, clinical and food Analysis.^[13]

7. Case studies:

A. Gas chromatography is an effective solution in the analysis of fruits and fruit products.

B. Analysis of fresh and aged tea tree essential oils.

C. Analysis of perfume allergens by using comprehensive 2D GC and a rapid scanning quadrupole mass spectrometer.

D. Analysis of human plasma fatty acids.

E. Analysis of roasted coffee beans aroma by using comprehensive 2D GC combined with quadrupole mass spectrometer

8. CONCLUSION:

In contrast, the use of 2D GC allows more sensitive analysis and provides more detailed quantitative and qualitative analysis. Despite this, the high price of the apparatus and the problem involved in carrying out the quantitative analysis makes it impossible to put it into industrial laboratories.¹⁴

9. ACKNOWLEDGEMENT:

I want to acknowledge our beloved principal Prof. M. Sumakanth RBVRR women’s college of pharmacy for giving me the opportunity for writing this review paper.

10. CONFLICT OF INTEREST:

Declared none.

11. REFERENCES:

1. J.C. Giddings, Anal. Chem., 56, 1258A–1270A (1984).

2. J.C. Giddings, J. Chromatogr. A, 703, 3–15 (1995).

3. Z. Liu and J.B. Phillips, “Phillips_Comprehensive 2-dimensional gas chromatography using an on-column thermal modulator interface”, Oxford University Press, (1991), pp.227–231.

4. T. Górecki, J. Harynuk, and O. Panić, Comprehensive two-dimensional gas chromatography (GCxGC), in: Nowe Horyzonty I Wyzwania W Anal. I Monit. Środowiskowym, rozdzielanie mieszaniny jest oparte na oddziaływaniu rozdzielanych składników z fazą ruchomą i stacjonarną, Gdańsk (2003), pp. 61–83.

5. P. Marriott and R. Shellie, TrAC - Trends Anal. Chem., 21, 573–583 (2002).

6. M. Adahchour, J. Beens, and U.A.T. Brinkman, J. Chromatogr. A, 1186, 67–108 (2008).

7. D. Gąsior, Pr. Inst. Ceram. I Mater. Bud., R. 6, nr 13, (2013).

8. ^ASTM D7675-2015: Standard Test Method for Determination of Total Hydrocarbons in Hydrogen by FID-Based Total Hydrocarbon(THC) Analyzer. ASTM. December 2015. Doi:10.1520/D7675-15.

9. ^“ Total Hydrocarbons”. Analytical chemists, Inc. Retrieved 23 January 2017.

10. X. Lamani, S. Horst, T. Zimmermann, and T.C. Schmidt, Anal. Bioanal. Chem., 407, 241–252 (2015).

11. P.Q. Tranchida, P. Dugo, G. Dugo, L. Mondello, P. Quinto, P. Dugo, G. Dugo, and L. Mondello, J. Chromatogr. A, 1054, 3–16 (2004).

12. P.Q. Tranchida, F.A. Franchina, P. Dugo, and L. Mondello, Mass Spectrom. Rev., 35, 524–534 (2016).

13. M. Adahchour, J. Beens, R.J.J. Vreuls, and U.A.T. Brinkman, TrAC - Trends Anal. Chem., 25, 438–454 (2006).

Received on 28.06.2021 Modified on 24.07.2021

Asian J. Pharm. Ana. 2021; 11(4):293-296.

DOI:10.52711/2231-5675.2021.00050