Exploring the Mechanisms of Gas Chromatography

Gas chromatography is an indispensable technique in analytical chemistry that allows for the accurate measurement and identification of chemical substances, even in trace amounts. In this article, we will explore the concept of gas chromatography and various aspects of its fundamentals.

What is a gas chromatograph

Gas Chromatography (GC) is an analytical technique for gases, liquids and solid samples that is used to separate and detect chemical components in a sample mixture. These chemical components are usually organic molecules or gases. In order to be successfully analyzed by GC, these components need to be volatile, typically have a molecular weight below 1250 Da, and be thermally stable.

Principle of Gas Chromatograph

Gas chromatography uses a gas as the mobile phase (carrier gas). When the sample is “injected” into the injector by a micro-syringe, it is carried by the carrier gas into a packed column or capillary column. Due to the differences in the distribution or adsorption coefficients of the components of the sample between the mobile phase (gas phase) and the stationary phase (liquid or solid phase) in the column. Under the flushing of the carrier gas, each component is repeatedly distributed between the two phases so that each component is separated in the column. The components are then sequentially detected by a detector connected to the back of the column based on their physicochemical properties.

How to read chromatograms

Source: Anthias Consulting.

A typical chromatogram is shown here. The horizontal axis indicates the time at which the component reaches the detector. The vertical axis indicates the signal intensity. The portion where no component is detected is called the baseline and the portion where the component is detected is called the peak. The time from when the sample is injected into the system until the peak appears is called the retention time. Since each component has a different elution time, each component can be separated and detected.

Basic structure of gas chromatograph

Gas chromatograph is mainly composed of carrier gas system, sample injection system, separation system, detection system, temperature control system and recording system.

Whether the components can be separated or not, the key lies in the chromatographic column, whether the components can be identified after separation lies in the detector, so the separation system and detection system are the core of gas chromatograph.

1. Carrier gas system: Including gas purification, gas source, gas flow rate control and flow. The accuracy of gas flow rate and flow rate control affects the stability of gas chromatography. Common carrier gases are hydrogen, nitrogen and helium.

2. Injector system: Including injector, vaporization chamber (instantaneous vaporization of liquid sample into vapor), the gas or liquid sample is quickly and quantitatively added to the upper end of the chromatographic column.

3. Separation system: The core of the separation system is the chromatographic column, which serves to separate the components of the sample to be tested.

   Classification of chromatographic columns: packed columns and capillary columns.

   Packed columns: Suitable for quantitative analysis of routine samples, especially high-purity samples.

   Capillary columns: Suitable for separation of complex or trace samples and residue analysis, etc. The separation efficiency is much higher than that of packed columns.

4. Detection system: The performance of the detector will directly affect the accuracy of the final analysis result of the chromatograph.

5. Temperature control system: In gas chromatograph, temperature control is extremely important, temperature control directly affects the separation efficiency of the column, the sensitivity and stability of the detector.

6. Recording system: The recording system of gas chromatograph is mainly used to record the detection signal of the detector and carry out quantitative data processing and recording.

Compounds suitable for analysis by gas chromatography

Components that can be analyzed by GC have the following 3 main characteristics.

1. Compounds with boiling points up to 400 °C

2. Compounds that do not decompose at the vaporization temperature

3. Compounds decompose at the vaporization temperature, but the amount of decomposition is always the same. 

Compounds that cannot be analyzed by GC or are difficult to analyze by GC.

Compounds that cannot be analyzed

1. Compounds that do not vaporize (Inorganic metals, ions, and salts)

2. Highly reactive compounds and chemically unstable compounds (Strong acids such as hydrofluoric acid, highly reactive compounds such as ozone and NOx)

Compounds that are difficult to analyze

1. Highly adsorbable compounds (Compounds containing carboxyl, hydroxyl, amino or sulfur)

2. Compounds for which it is difficult to obtain standard samples (Qualitative and quantitative analysis is difficult.)

Conclusion 

In summary, gas chromatography is an important analytical technique widely used to separate and analyze complex mixtures. Its applications are wide-ranging, transcending the boundaries of the laboratory and permeating many industries. Its high accuracy, sensitivity and ability to handle a wide range of sample types make it invaluable in fields such as chemistry, environmental science and quality control.