18. What is a solid stationary phase? Can it be roughly divided into several categories?
A: Refers to an active porous solid material that is directly loaded into the column as a stationary phase. Solid stationary phases can be broadly classified into three categories:
The first type is the adsorbent. Such as: molecular sieve, silica gel, activated carbon, alumina, etc.;
The second type is a high molecular polymer. Such as domestic GDX-type polymer porous microspheres, foreign porapak series, etc.;
The third type is a chemically bonded stationary phase. In gas chromatography, a fixing solution is usually applied to the surface of a carrier. The use of chemically bonded stationary phases for the analysis of polar or non-polar materials generally yields symmetrical peaks with high column efficiency and improved thermal stability of the stationary phase.
Nineteen, what is a fixed solution? What are the requirements for fixatives?
Answer: Generally, it is a liquid film of high-boiling organic matter. The components are separated in the column by the action of different molecules of different components. For fixed liquids for gas chromatography, there are generally the following requirements:
1. Low vapor pressure at operating temperature, good thermal stability, and no irreversible reaction with the analyzed physical or carrier gas;
2. It is liquid at the operating temperature, and the lower the viscosity, the better. The mass transfer rate of the substance in the high viscosity fixed solution is slow, and the column efficiency is thus lowered. This determines the minimum use temperature of the fixative;
3, can be firmly attached to the carrier, and form a uniform and structurally stable thin layer;
4. The separated material must have a certain solubility in it, otherwise it will be quickly carried away by the carrier gas and cannot be distributed between the two phases;
5. Separation of substances with similar boiling points and different types, that is, the ability to retain one type of compound is greater than the other. This separation ability is the selectivity of the fixative.
20. What are the principles for the selection of fixatives?
A: According to the interaction between the separated components and the molecules of the fixing liquid, the selection of the fixing solution is generally based on the so-called "similarity principle", that is, the similarity between the properties of the fixing solution and the separated components, such as Functional groups, chemical bonds, polarities, certain chemical properties, etc. When the properties are similar, the interaction between the two molecules is strong, the solubility of the separated components in the fixing solution is large, the partition coefficient is large, and the retention time is long; On the contrary, the solubility is small and the partition coefficient is small, so that it can quickly flow out of the column. Let's discuss the different situations below:
a, separation of polar compounds, using a polar fixative. At this time, the interaction between the components of the sample and the immobilized liquid molecules is mainly the orientation force and the inducing force. The order of the peaks of each component is in the order of polarity, and the first peak with a small polarity, the greater the polarity, the slower the peak is;
b. Separation of non-polar compounds, application of non-polar fixative, the interaction between the components of the sample and the molecules of the fixative is dispersive force, no special selectivity, then the components are peaked in the order of boiling point, and the boiling point is low. Out of the peak. For the separation of isomers with similar boiling points, the efficiency is very low;
c. When separating the mixture of non-polar and polar compounds, a polar fixing solution may be used, in which case the non-polar component is first distilled off, and the stronger the polarity of the fixing solution, the more easily the non-polar component flows out;
d. For samples capable of forming hydrogen bonds. For the separation of alcohol, phenol, amine and water, a polar or hydrogen-bonded fixing solution is generally selected, and the hydrogen bonding ability between the components and the fixing liquid molecules is separated. The "similar compatibility principle" is the general principle for selecting a fixed solution. Sometimes, when the existing fixing solution cannot achieve a satisfactory separation result, a "mixed fixing solution" is often used, and two or more properties are different. The fixed liquid mixed in a suitable ratio allows the separation to have a satisfactory selectivity without prolonging the analysis time. However, the choice of fixative in actual work is often the reference material or the examples described in the literature to select the fixative.
What are the treatment methods for mixed fixatives?
A: There are three ways to handle mixed fixatives:
1. After being coated on the support, mix separately;
2. Mix the fixing solution and then apply it. Note that the fixing solution used at this time should be dissolved in the same solvent;
3. Spread the stains separately, fill in the columns with the length and length, and then connect them in series. The results of the above three treatment methods are basically the same, but there are some differences for special separations.
廿 Second, how much is the usual fixed solution?
Answer: Because the fixed liquid content has a great influence on the separation efficiency. Therefore, the ratio of its weight to the weight of the support is 5%, generally 15%-25%. If the liquid ratio is large, the sample to be analyzed will have a diffusion phenomenon on the relatively thick liquid film, which will damage the separation; when the liquid ratio is too low, the residual adsorption capacity on the surface of the support will be revealed because the liquid film is too thin. Tail the peaks. Since a low ratio promotes the establishment of a balance, a higher carrier gas flow rate can be used, so a low liquid ratio, plus a small amount of sample, can shorten the analysis time. The content of the diatomaceous earth support liquid can be 15-30% larger; since the surface area of ​​the fluorine support is small, it can only be up to 10%; as for the surface area of ​​the glass microspheres, the fixed liquid content can only be maintained at about 0.25%. .
廿Three, what are the fixed liquid solvents commonly used in the column? What is the principle of choosing a solvent?
Answer: Commonly used solvents are: methanol, ethanol, ether, acetone, n-butanol, n-hexane, petroleum ether, benzene, toluene and chloroform. The selection principle is: 1. Good solubility, 2. No chemical reaction with fixed solution, 3. Low boiling point, 4. Low toxicity.
廿 Fourth, what is the conventional method of applying fixed liquid on the support when the column is matched?
A: Generally, the commonly used columns are mostly “conventional†coating method. The brief operation is as follows: take the required amount of fixing solution, dissolve it with a proper amount of solvent (which can be immersed in the carrier), and slowly pour the carrier into it. Then, after stirring, and then irradiated with an infrared lamp (or evaporation in a water bath) to drive off the solvent, the fixing solution adheres to the support.
廿5. What are the common filling methods for columns?
Answer: The quality of the stationary phase filling will directly affect the efficiency of the column. Usually, the pumping filling method is used, that is, one end of the column is stuffed with glass wool, connected to a vacuum pump, and the other end is connected to a funnel, and the stationary phase is added under suction. Tap the column while loading until the stationary phase is no longer in place. After it is installed, stuff the glass wool. Packing is required to be filled evenly and tightly, and there should be no gaps.
Sixth, why should the newly packed column be aged for a while before it can be used?
Answer: After the packed column is connected to the instrument, it should be pressure tested, leak tested, and then subjected to analysis after being purged with carrier gas for several hours at a constant temperature. This is generally referred to as the aging process of the column. The purpose of aging is to drive away the residual solvent of the stationary phase, low-boiling impurities, low-molecular-weight fixing liquid, etc., so that the baseline of the recorder is straight, and the fixing liquid has a redistribution process on the surface of the support at the aging temperature, thereby coating More even and firm. The packed column is stable after a period of aging, so that it can be used.
廿7. What are the performances after the column fails? What is the reason for its failure?
A: Column failure is mainly characterized by poor chromatographic separation and significantly shorter component retention times. The main reason for column failure is that the activity or adsorption performance of the stationary phase is reduced for gas-solid chromatography. For gas-liquid chromatography, it is caused by the gradual loss of the fixative during use.
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