First, the separation principle
1 gas: gas chromatography is a physical separation method. Using the measured material components in a slight difference in the different distribution coefficients of the two phases, when the relative motion of the two phase, these substances in two phases are repeated allocation, make originally only minor differences in the nature of the produced great effect, and the different components are separated.
2. Liquid phase high performance liquid chromatography (HPLC) method is on the basis of classical chromatography, quoting the theory of gas chromatography (GC), in mobile phase change for conveying high pressure; at the same time column is connected with a high sensitivity of the detector, continuous monitoring of effluents.
Two, application scope
1. The gas phase: gas chromatography method has good separation ability, high sensitivity, analysis speed, easy to operate, and other advantages, but by the technical conditions of the restrictions, substances with high boiling point substance or thermal stability is poor are difficult to use gas chromatography were analyzed. In general, the material part which is not easy to be decomposed or heated to be decomposed under 500 is used to derive the derivative method or the cracking method.
2 liquid phase: high performance liquid chromatography method, only the sample can be made into solution, and do not need to gasification, therefore not subject to the sample volatile. High boiling point, thermal stability and relative molecular weight can be applied to the separation and analysis of high boiling point.
Three, their respective advantages
Gas chromatography is not suitable for non - volatile matter and the heat labile substance, while the liquid chromatography is not limited by the volatility and thermal stability of the sample. Some samples can not be passed through the column, and the heat of the material is not stable. This allows the use of gas chromatography to be limited by the scope of use. According to statistics, the organic matter which can be analyzed by gas chromatography is only 15% to 20% of all organic matter. On the other hand, liquid chromatography is not limited by the sample's volatility and thermal stability. Therefore, liquid chromatography is suitable for the separation of biological and pharmaceutical related macromolecular and ionic compounds, unstable natural products, a wide variety of other polymers and unstable compounds.
For very hard to separate the sample, using liquid chromatography is often easier than using gas chromatography to complete separation, the main reasons for the following three aspects:
In liquid chromatography, the separation process is also affected by the flow phase, which provides additional factors for the control and improvement of the separation. Gas phase chromatography carrier gas generally do not affect the distribution, that is to say, in liquid chromatography, the interactions of the two phase and sample molecules selectively.
There are many kinds of columns packed with special efficiency in liquid chromatography, which makes the choice of the stationary phase is greater, which increases the possibility of separation.
Liquid chromatography is used to separate the temperature and the interaction between molecules is more effective at low temperature, so the temperature can be decreased by increasing the separation efficiency of gas chromatography. Compared with gas chromatography, liquid chromatography is easy to recover the sample, and it is quantitative, and the sample can be separated easily. Therefore, in many cases, liquid chromatography is not only an analytical method, but also as a means of separation, which can be used to purify and prepare a single substance with moderate purity. In gas chromatography, the samples can be recovered, but they are not convenient, and the quantitative difference is not very convenient. Liquid chromatography has been widely used in many fields due to its advantages, which are not available.