Recently, the Ministry of ecology and environment issued announcement No. 45 of 2019 on the determination of ozone depleting substances such as HCFC-22, CFC-11 and HCFC-141b in HJ 1057-2019 composite polyether by headspace / gas chromatography-mass spectrometry and HJ 1058-2019 the determination of ozone depleting substances such as CFC-12, HCFC-22, CFC-11 and HCFC-141b in rigid polyurethane foam and polyether polyether. Two portable national headspace / gas chromatography mass spectrometry (NHS / GC-MS), two national environmental protection standards. Among them, "HJ 1057-2019" analysis method referred to the use of fixed phase bonded silica gel porous layer column.
So, what is a porous open string? What's special about the porous open tubular column with bonded silica gel as the stationary phase?
1. Classification and composition of gas chromatography column
In gas chromatography, chromatographic column is the key component of sample separation and component analysis. In terms of the structure and separation efficiency, chromatographic columns are generally divided into capillary columns and packed columns.
Generally speaking, from the appearance, the length of the packed column is (1-10) m, the inner diameter is (2-4) mm, the outer diameter is (3-7) mm, and the outer diameter is mostly 3mm (or 1 / 8 inch). The length of the capillary column is more than 10 m, and the common length is 30 m, 50 m, 60 m and 100 m; the common inner diameter is 0.25 mm, 0.32 mm and 0.53 mm, and there are also columns with finer inner diameter, and the outer diameter is within 1 mm. The materials of packed column are stainless steel, glass and polytetrafluoroethylene, etc., and most of them are stainless steel; the materials of capillary column are glass, fused quartz and stainless steel, most of them are fused quartz.
1.1 composition of filling column
The packed column consists of a column tube and a stationary phase. The fixed liquid (i.e. fixed phase) with good stability and liquid state under operating conditions is mixed with carrier (carrier) with chemical inertia and porosity, and then it is put into the packed column tube, or solid adsorbent (i.e. fixed phase) is put into the packed column tube, and then quartz cotton or stainless steel mesh is used to fix the two ports. The following figure is an example of a packed column support and a fixing fluid:
1.2 composition of capillary column
The common capillary column mainly consists of three parts: the tube made of fused quartz; the polyimide coating on the outer wall of the tube; and the stationary phase coated on the inner wall of the tube.
The polyurethane coating on the outer wall of the tube can prevent the column from breaking and make the outer surface of the column dark amber. The stationary phase coated on the inner wall of the tube has the function of separating and mixing sample components, which has various types. The commonly used stationary phases include polysiloxane, polyethylene glycol (PEG) and solid adsorbent.
2 porous layer open string (plot)
For capillary column, because the stationary phase (fixed liquid or solid adsorbent) is coated on the inner wall, the center of the column is hollow, which is generally called open tubular column or hollow column. The stationary phase of the porous layer open tubular column (PLOT column) is very small porous particles, which are attached to the inner wall of the capillary column by adhesive or similar means. Based on the different adsorption properties of stationary phase, plot column can separate solute. Because particles are porous, there will be differences in particle size and shape. It is often used to separate volatile solutes (mainly gases).
Porous open tubular column with silica gel as the stationary phase
3.1 classification of stationary phases
According to the aggregation state of stationary phase, gas chromatography can be divided into gas-liquid chromatography (GLC) and gas-solid chromatography (GSC). Gas-liquid chromatography uses gas as mobile phase, high boiling organic compounds coated (or bonded) on the surface of inert carrier (i.e. fixed liquid, in working state as liquid) as stationary phase; gas-solid chromatography uses gas as mobile phase, and solid adsorbent as stationary phase.
There are two kinds of common solid stationary phases: inorganic adsorbents (including those prepared by chemical bonding method based on them) and porous polymers (adsorbents for organic compounds). In recent years, a variety of new materials have emerged, such as nano materials (graphene, carbon nitride, etc.), metal organic framework materials (MOF), cucurbit urea, etc.
3.2 silica gel stationary phase and its application in chromatographic column
Adsorbents made of inorganic materials are used in chromatography, such as molecular sieve, alumina, silica gel and carbon. For silica stationary phase, silica gel is made from silica gel and its chemical composition is SiH2O. NH2O, which can be used to analyze C1~C4 alkanes and SO2, H2S, COS, SF6 and other gas sulfides. In addition, CO2 can also be used to separate CO2 and other permanent gases because it flows out after C2H6. The new generation silica gel matrix stationary phase is made into porous spheres. The common ones are spherosil, porasil and chromosil. The application in packed column analysis can be seen in the following figure:
Due to the strong adsorption performance and limited application of silica gel, porous silica gel spheres can be used as the matrix, and the surface of silica gel can be modified with organic substances (chemical bonding) to form silica gel bonding phase. By changing the type and content of modifiers, the selectivity of separation can be adjusted in a large range (Analytical Chemistry Manual (Third Edition): gas chromatography).
In addition to filling silica gel stationary phase into packed columns, a uniform nano silica porous layer can be synthesized on the inner wall of the capillary by in-situ sol-gel process. The porous silica layer of the bonded silica gel column (PLOT) is prepared (Reference: nano silica porous layer capillary gas chromatographic column in-situ sol-gel method and its application, Zhao Guohong et al.) The common capillary column (PLOT column) with silica gel as stationary phase includes GS gaspro, CP silicaplot, SH RT? - silica bond, etc., which can be used to analyze CO2, light hydrocarbon and gas sulfide. The application in capillary column analysis can refer to the following:
(1) Inorganic gas
(2) C1 and C2 halogenated hydrocarbons (Freon)
(3) Sulfur compounds
At present, in addition to the determination of ozone depleting substances such as HCFC-22, CFC-11 and HCFC-141b in hj1057-2019 combined polyether by headspace / gas chromatography-mass spectrometry, a porous open tubular column with the fixed phase as the bonded silica gel is recommended, there are also such requirements in GB / T 7375-2006 gas chromatography for the determination of the purity of fluoromethane for industrial use.
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Factors affecting the repeatability of GC analysis 1: injection pad
Factors affecting the repeatability of GC analysis 2: liner
Factors affecting the repeatability of GC analysis 3: sample solvent
Factors affecting the repeatability of GC analysis 4: column installation
Factors influencing the repeatability of GC analysis 5: injection needle
Factors affecting the repeatability of GC analysis 6: detector
Part 25 flow calibration of electronic flow control device
Part 26 electronic flow control of capillary injection port (I)
Part 27 electronic flow control of capillary injection port (2)
Part 28: split injection and non split injection of capillary injection port
Part 29 overview of sample introduction device in gas chromatography