Process of Gas Chromatography

The gas chromatography (GC) process is carried out in the following steps:

Sample injection and Vaporization

Sample injection is a crucial step in gas chromatography (GC) that involves introducing the sample into the instrument and vaporizing it. There are several injection techniques available, including splitless injection, split injection, direct injection, and on-column injection. The choice of injection technique depends on the sample type, concentration, and desired sensitivity.

Carrier Gas Flow

Carrier gas flow is a critical component of gas chromatography (GC) that involves using an inert gas, such as helium or nitrogen, to carry the sample through the column. The flow rate of the carrier gas is carefully controlled to ensure consistent separation of the components in the sample. The carrier gas should be dry, oxygen-free, and inert chemicals are present. The choice of carrier gas depends on the application, and helium is preferred for thermal conductivity detectors due to its high thermal conductivity. The flow rate can be controlled by adjusting the pressure, column flow rate, or linear velocity.

Chromatographic Separation

Chromatographic separation is a technique used to separate a mixture of chemical substances into its individual components. The separation is based on the differential partitioning between the mobile and stationary phases. The components in the mixture are distributed between the two phases. Because of the differences in structures and properties of each component, the affinity and size of each interaction with the stationary phase are not identical.

Thus, under the same driving force, the retention time of different components differs in the column, moving out of the column in different orders. The components are separated inside the column, and the detector measures the quantity of the components that exit the column.

Detection and Analysis

In gas chromatography (GC), detecting separated components is essential. Several types of detectors are used in GC, which can be categorized as destructive or non-destructive.

Destructive detectors

• Flame Ionization Detector (FID): Detects organic compounds containing carbon atoms.
• Flame Photometric Detector (FPD): Detects compounds containing phosphorus or sulfur.
• Nitrogen Phosphorus Detector (NPD): Detects nitrogen and phosphorus-containing compounds.
• Atomic Emission Detector (AED): Detects elements based on their emission spectra.
• Mass Spectrometer (MS): Identifies compounds based on their mass spectrum.

Non-destructive detectors

• Thermal Conductivity Detector (TCD): Detects changes in thermal conductivity.
• Electron Capture Detector (ECD): Detects compounds that capture electrons, such as halogenated compounds.
• Photoionization Detector (PID): Detects volatile organic compounds.
• Olfactometric Detector: Uses human smell to detect specific odors.

Data Analysis

Data Analysis is a critical step in gas chromatography (GC) that involves processing and interpreting the data obtained from the detector. The data is typically displayed as a chromatogram, which is a graph of the detector response versus time. The chromatogram provides information about the retention time, peak shape, and peak area of each component in the sample.

The retention time is the time it takes for an element to travel through the column and reach the detector. The peak shape provides information about the efficiency of the separation, while the peak area is proportional to the amount of the component in the sample. The data can be analyzed qualitatively and quantitatively, and various methods are available for data analysis, including pattern recognition, classification, and calibration curves.

The components in the sample are identified based on their retention times and compared to known standards or databases. It is a highly sensitive, efficient, and selective technique that can separate volatile compounds based on their retention times and can be hyphenated with mass spectrometry for identification purposes.

Gas Chromatography (GC) is a technique used in analytical chemistry for separating and analyzing volatile compounds in a mixture. This technique uses a mobile phase to carry the sample through the stationary phase. The compounds interact with the stationary phase at different rates, resulting in distinct retention times and separation of the components. Mikhail Semenovich Tsvett discovered it in the early 1900s to separate compounds. It can analyze volatile mixtures in human breath, blood, saliva, and air samples.

In this article, we look into what gas chromatography is, its definition, principle, separation mechanism, types, etc.