Gas Chromatography-Infrared Spectroscopy (GC-IR)

Chromatography is one of the most active and practical research fields in analytical chemistry. Its high sensitivity and high separation efficiency make chromatography an ideal separation and quantitative analysis tool. However, chromatography is relatively inefficient for qualitative analysis. The main basis of chromatographic method for qualitative analysis is retention value, so it is difficult to make qualitative judgments on unknown components.

Infrared spectroscopy can provide extremely rich molecular structure information, has strong structure identification capabilities, and is an ideal qualitative analysis tool. Almost no two different substances have exactly the same infrared spectrum. However, infrared spectroscopy does not have the ability to separate. In principle, infrared spectroscopy can only be used for the qualitative analysis of pure compounds, and it is often useless for mixtures.

Combining these two methods, the chromatograph is equivalent to the pre-separation tool of the infrared spectrometer, and the infrared spectrometer is equivalent to the qualitative detector of the chromatogram. Gas Chromatography-Infrared Spectroscopy (GC-IR) combines the high-efficiency separation ability of gas chromatography and the molecular structure identification ability of infrared spectroscopy, and is an effective method for the analysis of complex mixtures.

Development of GC- FTIR

With the development and maturity of the interference fourier transform infrared spectrometer (FTIR), GC-FTIR technology has made breakthrough progress. FTIR has high detection sensitivity and can detect trace components. At the same time, FTIR has a fast scanning speed, which can simultaneously track and scan gas chromatographic fractions, which overcomes the biggest obstacle of slow scanning speed when chromatogram and infrared spectroscopy are combined. These developments make the combination of GC and FTIR successful and enter the stage of commercial production. GC-FTIR has been widely used in biomedicine, chemistry and chemical industry, metallurgy, energy, environmental protection and other fields.

Principle of GC- FTIR

The working principle of GC-FTIR is: After the sample is separated by GC, each fraction enters the interface in order of retention time. At the same time, the interference light modulated by the interferometer converges to the interface. Each fraction in the interface selectively absorbs the interference light, and then the interference signal generated is detected by the detector. The computer system stores the collected interferogram information from the detector, and obtains the gaseous infrared spectra of the components through fast Fourier transform. Finally, the molecular information of each component can be retrieved through the library.

The GC-FTIR system consists of four parts: gas chromatograph, interface, fourier transform infrared spectrometer, and computer data system.

• Gas Chromatograph

The gas chromatograph in GC-FTIR mostly uses capillary gas chromatograph, which can be equipped with thermal conductivity detector (TCD), hydrogen flame ionization detector (FID), etc., whose function is to separate the components in the mixture.

• Interface

The interface in GC-FTIR is a key part of the combined system. At present, there are two types of commercial interfaces: light pipe interface and frozen trap interface. The frozen capture interface has the advantages of high signal-to-noise ratio and low detection limit, but it is expensive and cumbersome to operate. The light pipe interface has the advantages of real-time recording, relatively cheap, and easy to operate, and is still widely used.

• Fourier transform infrared spectrometer

In GC-FTIR, single optical path FTIR is generally used.

• Computer data system

Control online operation and collect and process data.

GC- FTIR instrument structure diagram

Data Information of GC- FTIR

GC-FTIR's data processing system can provide the following information:

• Chromatographic retention value

The chromatographic retention value can be used as a qualitative auxiliary basis. The chromatographic retention value is very important for compounds with different numbers of repeating units in the molecular structure such as homologs. Because these compounds have similar infrared spectral characteristics but different chromatographic retention values.

• Chromatogram

The chromatogram obtained by GC-FTIR is called the reconstructed chromatogram, which is the result of computer processing the interferogram recorded by the infrared detector.

• Infrared spectra

The infrared spectrum characterizes the absorption frequency and intensity of each group in the compound molecule. Infrared spectroscopy can be used to identify the molecular structure of the compound.

• GC-FTIR library search

Comparing the gaseous infrared spectrum obtained by GC-FTIR with the gaseous infrared standard spectrum stored in the computer can realize the confirmation of unknown components.

Application of GC-FTIR

With the continuous development and improvement of GC-FTIR technology, GC-FTIR technology has become an effective means for qualitative and quantitative analysis of complex organic mixtures. GC-FTIR has incomparable advantages in many aspects, especially the separation and identification of isomers. GC-FTIR is widely used in natural product volatile oil (such as medicinal volatile oil analysis), flavor and fragrance analysis, petrochemical analysis, environmental pollution analysis (such as wastewater analysis, air pollutant analysis, pesticide residue detection, toxic substance detection), fuel analysis (coal and petroleum distillate analysis), etc.