From Tiny Traces to Big Cases: How Gas Chromatography solves crimes

Author – Agin P, Sem IV, M. Sc Forensic Science, Banaras Hindu University, Varanasi 

Chromatography is a separation method used to separate the components of a mixture in gaseous or liquid state and for their identification and quantification. Solid mixtures can also be separated by this method by dissolving it in a suitable solvent. The basic principle of chromatography is the concentration equilibrium of the compound of interest between the two immiscible phases. 

Both phases are totally immiscible with each other and the first is called the stationary phase while the other is called the mobile phase. The stationary phase is fixed on a support or loaded in a column while the mobile phase is forced to move through the stationary phase. The phases are chosen so that the components of the mixture have different affinity to each phase. The mixture is separated by the differential migration of its constituent compounds between the two phases. 

Gas chromatography is an analytical method used for separation, isolation, identification, purity determination and quantitative analysis of thermally stable components present in a mixture. The mobile phase used is an inert gas while the stationary phase may be solid or liquid. When a powdered solid stationary phase is used, the separation of components occurs due to their different adsorption rates on the stationary phase. When a liquid stationary phase is used by coating it on a solid support, the separation of components occurs due to their different partition coefficients between the two phases. 

PRINCIPLE- The sample mixture is injected into the mobile phase or carrier gas which is forced to move through a column coated with the stationary phase. The components of the mixture distribute themselves between the two phases at different rates. The components with more affinity to the stationary phase are retained for a longer time which leads to different rate of movement of components and thus, different time of emergence from the column. As the components leave the column, they are detected by instrumental means. The detector also records the peak for each component which is separated, and the area under this peak indicates the amount of that particular component in the mixture. These peaks are called the chromatogram. 

Common gases used as carrier gas are He gas and N2 gas which are inert and do not interfere with the detector. Packed columns or capillary columns are used and they are made using glass, steel, copper etc. Detectors are kept at the end of the column to detect and measure the amount of the separated components in the carrier gas. The commonly used detectors include: 

• THERMAL CONDUCTIVITY DETECTOR 
• FLAME IONIZATION DETECTOR 
• NITROGEN-PHOSPOHORUS DETECTOR 
• ELECTRON CAPTURE DETECTOR 
• PHOTO IONIZATION DETECTOR

ADVANTAGES- 

• High Sensitivity: GC has a high sensitivity and can detect even trace amounts of compounds in the mixture.
• High Resolution: GC can separate complex mixtures with high resolution producing well distinct peaks for each separated component.
• Rapid Analysis: GC is a fast method in comparison to other methods and can separate complex mixtures easily.
• Quantitative Analysis: GC chromatogram can be used for quantitative analysis of the separated compounds.
• Versatility: GC can be paired with a variety of instruments like Mass Spectrometer to expand its application range.
• Reusability: The GC columns can be cleaned and reused a few times before they have to be replaced 

DISADVANTAGES-

• Volatility: GC can only be used to separate volatile compounds.
• Thermal Stability: GC can only be used for separation of thermally stable mixtures at the high temperature at which the GC column is kept 

CONCLUSION-

Gas chromatography is an invaluable technique in modern day laboratories for analysis of a wide variety of compounds. It finds a wide range of application in extraction and analysis. Its ability to work in conjunction with other instruments further add to its versatility of usage. 

REFERENCES- 

Rouessac, F & Rouessac, A. (1994).'Gas Chromatography'. Chemical Analysis: Modern Instrumentation Methods and Techniques.2nd edn. Sussex: John Wiley and Sons Ltd. 

Moore, Sarah (2020). 'Applications of Gas Chromatography'. New Medical Life Sciences 

Raja, Pavan M V & Barron, Andrew R (2021). Principles of Gas Chromatography. Chemistry LibreTexts

Image -https://www.chromatographytoday.com/assets/file_store/pr_files/31164/thumbnails/images/391w_300px-gc_oven_inside.jpg