Gas Chromatography (GC) Principle || Instrumentation and Working of GC

Unveiling the Science Behind Gas Chromatography: Principle, Instrumentation, and Operational Mechanics

Introduction:

Gas Chromatography is a powerful modern-day analytical technique that is used in several chemical and pharmaceutical industries. It is used to separate and identify the components in a mixture or individual analysis of any ingredient. In this process, an injection of sample is induced into a heated column through which an inner carrier gas, such as helium flows. 

The sample components interact with the stationary phase of the column which causes the separation. The separated components of the sample then pass through a detector where they generate a signal which is used to identify and quantify them as a component. 

Image-source-taawon.com

Gas Chromatography principle:

Gas chromatography is a distinct technique of analysis of complex small-size molecules. Basically, it works on the principle of differential partitioning between the mobile phase and the stationary phase. It is a powerful system to separate and analyze volatile components. 

Mobile Phase is an inert gas carrier where helium and nitrogen are commonly used. On the other hand, the stationary phase is a column's inner surface that is a liquid coating on the inner surface.

Here are some steps, on how it works;

1. Sample Injection:  A prepared sample that is volatile or vaporizable in nature, is injected into the heated port of the system. It vaporizes the sample and introduces it into the mobile phase.
2. Separation of components: The vaporized components present in the sample travel with the mobile phase (gas) through the column. Here, different components of the sample interact with the stationary phase to varying degrees, leading to differential migration. During this process, components with higher volatility or lower polarity spend less interaction time with the stationary phase and travel faster through the column. Conversely, components with lower volatility or higher polarity spend more time interacting with the stationary phase and travel slower. This differential partitioning leads to the separation of the components.
3. Detection: Finally, the separated component elutes from the column, and then it passes through a detector. The detector generates a signal that is proportional to the amount of each component present. The data generated in the detector is sent to the attached software for data analysis.
4. Data Analysis: The signal generated by the detector is recorded and analyzed using a computer program by software provided by the GC system. The resulting chromatogram shows peaks corresponding to each component in the mixture. The retention time (time it takes for a component to reach the detector) and peak area can be used to identify and quantify each component.

History and Development of Gas Chromatography:

History of Gas Chromatography found back in the 20th century. However, tactful details were found from the mid-1950s. Here is a brief history and development process of Gas Chromatography:

• 1906: Russian Botanist Mikhail Tsvet invented liquid column chromatography. It was invented for plant pigments but soon it was found that this technique can be used for analyzing and separating many homogenous complex mixtures.
• 1930: Nobel Prize laureate Archer Martin, Richard Synge, and Antony James started the use of this technique in the early 1930s, however, there are some questions about the exact dates of this technique used by them. They experimented with this technique for the separation of amino acid mixtures. 
• 1947: German physicists Erika Cremer and Fritz Prior (Austria) developed a column consisting of silica gel. It is considered to be the first GC column and a thermal conductivity detector.
• 1951-1952: Archer Martin and Anthony James developed a gas-liquid chromatography (GLC) that is called GC these days. The liquid stationary phase with a solid support concept was introduced at that time. It improved the separation efficiency significantly. 
• 1960: Until 1960 commercial gas chromatography instruments were available. There is widespread acceptance of technology in various fields. Pharmaceuticals and chemical industries adopted the technique in the first phase. 

Advancements and Innovations in GC:

• 1970: In the decade of 1970 capillary columns were arrived to thrill the industry with accuracy and higher resolution guarantee. 
• 1980 to 1990: In this decade, mass spectrometer detectors (MS) were introduced and coupled with gas chromatography. It enhanced the capabilities of identification of the molecules.
• 2000: After 2000, new stationary phases developed that provided a new level of confidence in this technology. Some new and advanced detectors were developed. Data analysis became automated with the help of new software. These advancements improve the performance and capabilities of GC. 
• 2010s: In this period, two-dimensional gas chromatography (GCxGC) developed that is effectively able to analyze complex mixtures. It is one of the most demanding and dependable techniques in the modern pharmaceutical industry.

Understanding Gas Chromatography Instrumentation

 Types of Gas Chromatographs:

There are two main types of Gas chromatography; Gas-Liquid Chromatography and Gas-Solid Chromatography.

1. Gas-Liquid Chromatography (GLC): In Gas-Liquid Chromatography, the stationary phase is used as a non-volatile liquid that is held on an inert support. While the mobile phase remains a carrier gas (helium or nitrogen). Then sample is injected into the injection port, then it vaporizes and gets mixed in the carrier gas. The resulting mixture of sample and gas passed through the column, here components of the sample are separated by reacting with the stationary phase. It is the most used technique of gas chromatography. It is most effective can analyzing volatile compounds.
2. Gas-Solid Chromatography (GSC): In Gas-solid Chromatography, the stationary phase remains a solid adsorbent (silica gel or activated alumina). However, the mobile phase is similar to GLC, a carrier gas (helium or nitrogen). The next process of sample injection and passing through the column is similar to in GLC system. GSC is less common than GLC in the pharmaceutical and chemical industry. It is typically used in the analysis of gases and other non-polar compounds.  
   Here is a table that summarizes the key differences between GLC and GSC:

   Feature	GLC	GSC
   Stationary phase	Liquid	Solid
   Mobile phase	Carrier gas	Carrier gas
   Sample type	Volatile	Gases and non-polar compounds
   Applications	Wide range of applications	Analysis of gases and non-polar compounds

Instrumentation Setup and Configuration:

1. Carrier Gas: The carrier gas is usually helium or nitrogen. The carrier ought to be of high purity. 
• Gas Cylinder and regulator: The carrier gas is stored in a high-pressure cylinder as other gases are kept. The role of the regulator remains to control the flow of gas. It should be a constant flow so that optimum accuracy in analysis can be achieved. 
• Flow Meter: A high-quality flow meter is used to measure the flow of the carrier gas. It is a systematic requirement.
2. Sample Injection System: The injector is used to sample to the GC system through a column. There are two types of injectors split/splitless injectors and on-column injectors. It depends on the system requirement and the type of sample to analyze.
• Injection Port: The injection port is a part of the GC system where the sample is injected into the instrument. It remains heated to get the sample vaporized immediately after injecting.
• Septum: Septum is an important part of the injection port, it is a rubber seal that prevents the carrier gas from leaking out of the injection port. It is replaced with a certain time interval to avoid and loss of analysis accuracy. 
3. Column and column Oven: Column is a main part of the Gas Chromatography instrumentation. It is a long narrow tube that contains the stationary phase. It is the part where the sample is separated by interacting with the material filled in the stationary phase.
• Column Oven: It is an oven that makes the column heated at a specific temperature. The role of the column oven maintaining a constant temperature till a period of analysis. 
4. Detector: The detector is a system where data is sent after separation of the sample in the column. It measures the amount of each component present in the sample. There are two types of column Flame Ionization detectors(FID) and Mass spectrometers (MS).
5. Data Acquisition System: The data acquisition system is a software-based system that collects the data from the detector and analyzes it with its preloaded algorithms. The output of this analyzed data is visible on a computer screen and can be printed out for analysis records. 

Configuration of the Gas Chromatography system:

Configuration of GC refers to the process when a GC is ready to perform an analysis of a particular sample, it requires some specific instructions to analyze the sample within the conditions that include;

• Carrier gas flow rate: The carrier gas flow rate should be set according to the manufacturer's specifications or the sample's instructions in a particular monograph of a pharmacopeia.
• Column temperature: The column sample is set to a temperature that is mentioned in the STP of the sample or in the directive SOP. 
• Detector temperature: The detector temperature should be set to a temperature that is compatible with the detector.
• Injection parameters: The injection parameters, such as the injection volume and split ratio, should be set according to the type of sample and the desired analysis.

Pros of Gas Chromatography:

Gas chromatography is a high-efficiency technique of analyzing the components. here are some Pros of GC over other techniques:

1. High separation power
2. Wide range of applications
3. Sensitivity
4. Quantitative analysis 
5. Robustness

Cons of Gas Chromatography:

Here are some Cons of Gas Chromatography;

• Limited sample types
• High initial cost
• Technical expertise required
• Time-consuming
• Safety concerns

Summary:

Gas Chromatography is a strong and evergreen analytical technology for the separation and identification of volatile components in a complex mixture where High-Performance Liquid chromatography(HPLC) does not work. GC technology provides the confidence of high separation efficiency, sensitivity, and quantitative analysis capabilities. However, it is a high-cost technology. It is not so easy to operate a GC with a low-rank or new chemist in the laboratories. It works only with volatile samples. It is a great technology for the pharmaceutical industry where complex mixtures remain challenges to analyze. 

References:

1. Gas chromatography-[Wikipedia(.org)]
2. Chromatography: Basic Principles, Sample Preparations and Related Methods, 2013, by Elsa Lundanes & Léon Reubsaet & Tyge Greibrokk
3. Chromatography Resources; Technique, Theory, and Instrumentation-[University of Toronto(Site)]
4. Early stages in the history of gas chromatography-By: Ivan G. Kolomnikov, Alexander M. Efremov, Tatyana I. Tikhomirova, Nadezhda M. Sorokina, Yury A. Zolotov-[Science Direct(.com)]
5. Monolithic column in gas chromatography; A. Kurganov-Analytica Chimica Acta, Volume 775, 2 May 2013, Pages 25-40-[ScienceDirect(.com)]
6. The Top 10 Milestones in MS, Highlighting 50 years of MS developments: Rick Yost-11/05/2021-[The Analytical Scientist (Website)]
7. Gas-Solid Chromatography-Comprehensive Analytical Chemistry
Volume 28, 1991, Pages 223-462-[ScienceDirect(.com)]
8. Gas-Liquid Chromatography-Jim Clark:-[LibreTexts Chemistry(.com)]