Hi, everyone. In the previous content, we shared some basic knowledge of gas chromatographic column, mainly including the type of stationary phase, the parameters of column length, inner diameter and liquid film thickness of chromatographic column, as well as the service temperature limit of chromatographic column. Today, we will explore its stationary phase!
Whether it is gas chromatography or liquid chromatography, the adsorption retention of the components of the sample to be tested mainly depends on the stationary phase. The basic separation principle is to realize the separation of components through the difference of force type and action intensity between sample molecules and stationary phase. The polarity and intermolecular force of stationary phases with different structures are also different. As for gas chromatography, the gas-liquid distribution mode is the most used at present. The gas-liquid chromatographic stationary phase is also liquid at the conventional analysis temperature, so it is often called stationary liquid. The common stationary liquid mainly includes the following types:
1. Methyl Polysiloxane Stationary Liquid
The structure diagram of methyl polysiloxane stationary liquid is as follows:
From its structure diagram, it can be seen that it is composed of multiple siloxane polymerization, and each silicon atom on the skeleton can be connected with two functional groups. When its functional groups are methyl, it is called 100% dimethyl polysiloxane; “Two” means that two specific substituent groups are connected to the silicon atom. When the substituent groups are completely the same, this name can also be omitted, that is, 100% dimethylpolysiloxane is also called 100% methylpolysiloxane. In the structure diagram, the morphology of the stationary liquid formed will be different with the difference of polymerization degree N value. When the polymerization degree n value is small, the fixed liquid molecular weight is small, called dimethyl silicone oil, viscous liquid, such as THE OV-101 fixed phase developed by OhioValley (OV Company) in the United States; When the molecular weight is relatively large, it can be called dimethyl silicone grease and rubber, such as the SE-30 produced by GeneralElectric of the United States.
Methylpolysiloxane stationary liquids are non-polar stationary phases with a wide range of boiling points, suitable for the analysis of hydrocarbons and compounds containing other functional groups, ideal for the analysis of unknown samples.
2. Substituted by other different groups Polysiloxane stationary liquids
The number and type of substituents on the silicon atom of siloxane skeleton affect the polarity and thermal stability of the stationary phase. Generally speaking, the higher the content of polar substituents, the stronger the polarity of the stationary solution and the lower the temperature limit. The common substituents are shown in the following figure:
The content of substituted groups is usually described in terms of percentage. The following figure shows the structure of 5% diphenyl 95% dimethyl polysiloxane and 50% trifluoropropyl 50% methyl polysiloxane (or 100% trifluoropropyl methyl polysiloxane).
For the expression of the percentage of different groups replaced, 14% cyanopropyl phenyl 86% dimethyl polysiloxane is taken as an example, which means that it contains 7% cyanopropyl, 7% phenyl and 86% methyl. Because silicon atoms are connected with cyanopropyl and phenyl at the same time, 14% is a summation method (as shown in the figure below).
Role of different substituents:
● The introduction of phenyl into methylpolysiloxane can enhance the adsorption and retention of aromatic hydrocarbons due to structural similarity.
● The introduction of cyano group can make the fixed liquid have medium polarity or strong polarity. This kind of fixation has strong retention effect compared with compounds containing aryl and alkenyl groups, and is suitable for the separation of unsaturated hydrocarbons, aromatics and unsaturated fatty acids.
● Trifluoropyl has strong proton giving ability and is suitable for adsorption and retention of carbonyl compounds.
● The introduction of arylene into polysiloxane skeleton can enhance the thermal stability of stationary phase and reduce column loss.
3. Polyethylene glycol stationary liquids
It is a strongly polar stationary phase, mainly forming hydrogen bonds, and has strong retention of alcohols, acids, phenols, primary/secondary amines, etc. In the use of this type of fixed liquid chromatographic column, need to pay attention to the analysis of temperature, purity of carrier gas and other related issues, because polyethylene glycol polarity is strong, can withstand a low temperature limit, high temperature conditions in the gas oxygen, water will cause the decomposition of the stationary phase.
The structure of conventional polyethylene glycol stationary solution is shown as follows:
Polyethylene glycol (PEG), different polymerization degree n value, its molecular weight is different; Peg-20m and INOWAX are commonly used as polyethylene glycol with a molecular weight of 20,000. In order to analyze different types of compounds, different properties of compounds can be separated by modifying the surface layer of the column and the stationary solution.
It mainly includes the following:
● Alkali modified polyethylene glycol fixative: in the pharmaceutical industry, drug analysis is usually mainly alkaline, in the analysis of these substances, often appear steamed peak or peak tail phenomenon. In order to improve the peak shape of these compounds, the surface of the column can be treated with KOH into an alkaline surface, and then coated with polyethylene glycol fixed solution to realize the analysis of alkaline compounds.
● Acid modified polyethylene glycol fixator: ester fixator made of polyethylene glycol and different acids. The most widely used is FFAP (nitroterephthalic acid modified polyethylene glycol), which is mainly used to analyze small organic acids, volatile fatty acids and phenolic compounds.
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