Gas chromatograph is an instrument for qualitative and quantitative analysis of complex mixtures of multiple components using chromatographic separation and detection techniques. It can usually be used to analyze organic substances in soil that are thermally stable and have a boiling point not exceeding 500°C, such as volatile organic compounds, organochlorines, organophosphates, polycyclic aromatic hydrocarbons, phthalates and so on.

Gas Chromatograph Working Principle:

A gas chromatograph 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 attached to the back of the column based on the physicochemical properties of the components.

The signal given by the detector for each component is shown on the recorder as a peak, which is called a chromatographic peak. The maximum value on the chromatographic peak is the basis for qualitative analysis, while the area encompassed by the chromatographic peak depends on the content of the corresponding component, so the peak area is the basis for quantitative analysis. The curve obtained by the recorder after injection of a mixture sample is called the chromatogram. By analyzing the chromatogram, qualitative and quantitative results can be obtained.

Basic construction of a gas chromatograph:

There are many types of gas chromatographs with different functions, but their basic structures are similar. A gas chromatograph generally consists of a gas circuit system, a sample injection system, a separation system (column system), a detection and temperature control system, and a recording system.

Gas circuit system

The gas line system consists of a gas source, a purification and drying tube and a carrier gas flow control and gasification unit. It is a carrier gas continuous operation of the closed pipeline system, through which pure and stable flow rate of carrier gas can be obtained. Its gas tightness, accuracy of flow measurement and stability of carrier gas flow rate are important factors affecting the performance of gas chromatograph.
Commonly used carrier gases in gas chromatography are hydrogen, nitrogen, argon, and the purity is required to be more than 99%. Carrier gas selection in addition to the requirement to consider the impact of the column efficiency, but also with the analysis of the object and the detector used to match.

Sample feeding system

  1. Injector:
    Different injectors are used depending on the state of the sample. Liquid samples are usually injected with a micro-syringe. For gas samples, the push-pull six-way valve or rotary six-way valve configured in the chromatograph itself is commonly used. Solid samples are generally dissolved in the appropriate reagent, and then injected with a micro-syringe.
  2. Gasification chamber:
    The gasification chamber is generally made of a stainless steel tube with a heating wire wrapped around the outside of the tube, which serves to instantly gasify the liquid or solid sample into vapor. In order to allow the sample in the gasification chamber instantaneous gasification without decomposition, so the requirements of the gasification chamber heat capacity, no catalytic effect.
  3. Heating system:
    Used to ensure that the sample gasification, its role is to the liquid or solid sample into the column before the instantaneous gasification, and then quickly and quantitatively transferred to the column.

Separation system

The separation system is the heart of the chromatograph. Its function is to separate the components in the sample. The separation system consists of a column chamber, a chromatographic column, and temperature control components. The column is the core component of the chromatograph.

There are two main types of columns: Packed columns and capillary columns (open-tube columns). Column materials include metal, glass, fused silica, polytetrafluoroethylene and so on. The separation effect of the chromatographic column is not only related to the column length, column diameter and column shape, but also related to the selected stationary phase and the preparation technology of the column packing as well as many other factors such as operating conditions.

Detection System

A detector is a device that converts the concentration or mass (content) of each component separated by a chromatographic column into electrical signals (e.g., voltages, currents, etc.) that can be easily measured, and performs signal processing. It usually consists of three parts: detection element, amplifier and digital-to-analog converter. After being separated by the chromatographic column components sequentially into the detector, according to its concentration or quality changes over time, converted into the corresponding electrical signals, amplified by the record and display, drawing the chromatogram. The performance of the detector will directly affect the accuracy of the final analysis results of the chromatographic instrument.

According to the response principle of the detector, it can be categorized into concentration-type detector and mass-type detector.

  1. Concentration-type detector: Measurement is the instantaneous change of the concentration of components in the carrier gas, i.e., the response value of the detector is proportional to the concentration of components. Such as thermal conductivity detector, electron capture detector.
  2. Mass-type detector: The measurement is the carrier gas carried in the sample into the detector speed change, that is, the detector’s response signal is proportional to the mass of the component into the detector per unit time. Such as hydrogen flame ionization detector and flame photometric detector.

Temperature control system

In gas chromatography, temperature control is an important index, which directly affects the separation efficiency of the column and the sensitivity and stability of the detector. The temperature control system mainly refers to the temperature control of the gasification chamber, column and detector.

Recording system

The recording system is to record the detection signal of the detector for quantitative data processing. An automatic balanced electronic potential difference meter is generally used for recording and plotting the chromatogram. Some chromatographs are equipped with integrators to measure the area of the chromatographic peaks and directly provide accurate data for quantitative analysis. Advanced gas chromatograph is also equipped with an electronic computer, which can automatically process the chromatographic analysis data.