Questions and Answers on Gas Chromatography

1. What are the characteristics of gas chromatography?
Answer: Gas chromatography is a type of chromatography, that is, chromatography using gas as the mobile phase. In terms of separation and analysis, it has the following characteristics:
1. High sensitivity: It can detect 10-10 grams of material, which can be used for trace impurities analysis of ultra-pure gas and polymer monomers and trace poisons in the air.
2. High selectivity: It can effectively separate various isomers and various isotopes with extremely similar properties.
3. High efficiency: It can separate the complex components into single components.
4. Fast speed: General analysis can be completed in just a few minutes, which is conducive to guiding and controlling production.
5. Wide application range: it can analyze low-content gas and liquid, and can also analyze high-content gas and liquid, without being limited by the content of components.
6. The required sample volume is small: a few milliliters for general gas samples and a few microliters or tens of microliters for liquid samples.
7. The equipment and operation are relatively simple. The instrument is cheap.

2. What is the separation principle of gas chromatography?
Answer: Gas chromatography is a physical separation method. Using the small difference in the distribution coefficient (solubility) of each component of the test substance between different two phases, when the two phases move relative to each other, these substances are repeatedly distributed between the two phases, so that there is only a small difference in properties The effect is to separate the different components.

3. What is gas chromatography? How many types does it have?
Answer: Any chromatography technique that uses gas phase as the mobile phase is commonly known as gas chromatography. Generally can be classified according to the following aspects:

1. Classification according to the aggregation state of the stationary phase:
(1) Gas-solid chromatography: The stationary phase is a solid adsorbent,
(2) Gas-liquid chromatography: The stationary phase is a liquid coated on the surface of the support.
2. Classification according to the principles of process physical chemistry:
(1) Adsorption chromatography: The difference in physical adsorption performance of different components on the solid adsorption surface is used to achieve separation chromatography.
(2) Partition chromatography: use different components to have different partition coefficients in the two phases to achieve separated chromatography.
(3) Others: ion exchange chromatography using the principle of ion exchange: electrochromatography established using the electrokinetic effect of colloids; thermochromatography developed using temperature changes and so on.
3. Classification according to the type of stationary phase:
(1) Column chromatography: The stationary phase is installed in the chromatographic column. Packed columns, hollow columns, and capillary columns belong to this category.
(2) Paper chromatography: using filter paper as a carrier,
(3) Thin film chromatography: the stationary phase is indifferent pressed by powder.
4. Classification according to the principle of kinetic process: it can be divided into three types: flushing method, replacement method and head-up method.

4. What is the flow of a simple analysis device for gas chromatography?
Answer: The flow of a simple analytical device for gas chromatography is basically composed of four parts:
1. Gas source part 2, sampling device 3, chromatographic column 4, identifier and recorder

5. Explanation of some common terms and basic concepts of gas chromatography?
Answer: 1. Phase, stationary phase and mobile phase: a uniform part of a system is called a phase; during chromatographic separation, a stationary phase is called a stationary phase; a fluid that passes or moves along a stationary phase is called It is the mobile phase.
2. Chromatographic peak: After the substance enters the identifier through the chromatographic column, the curves appearing on the recorder are called chromatographic peaks.
3. Baseline: Under chromatographic operating conditions, when no measured component passes the identifier, the graph of the detector noise recorded with the recorder over time is called the baseline.
4. Peak height and half-peak width: the height between the maximum point of the concentration of the chromatographic peak to the time coordinate and the intersection point between the vertical line and the baseline is called the peak height, which is generally expressed by h. The width at half the height of the chromatographic peak is the half-peak width, which is generally expressed as x1 / 2.
5. Peak area: the area formed by the elution curve (chromatographic peak) and the baseline is called the peak area, which is denoted by A.
6. Dead time, retention time and calibration retention time: the time from injection to the maximum value of the inert gas peak is called the dead time, expressed in td. The time required from injection to the highest value of the chromatographic peak is called the retention time, expressed in tr. The difference between the retention time and the dead time is called the corrected retention time. Expressed in Vd.
7. Dead volume, retained volume and corrected retained volume: the product of the dead time and the average flow rate of the carrier gas is called the dead volume, expressed as Vd, and the average carrier gas flow rate is expressed as Fc, Vd = tdxFc. The product of the retention time and the average flow rate of the carrier gas is called the retention volume, expressed in Vr, and Vr = trxFc.
8. Retention value and relative retention value: The retention value is a value indicating the residence time of each component in the sample in the chromatographic column, usually expressed in time or the volume of the carrier gas required to bring the component out of the chromatographic column. With a substance as a standard, the ratio of the retention values ​​of other substances to this standard is called the relative retention value.
9. Instrument noise: The instability of the baseline is called noise.
10. Base flow: hydrogen flame chromatography, when there is no sample injection, the base current (bottom current) existing in the instrument itself, referred to as base flow

6. What is the general basis for choosing carrier gas? What are the common carrier gases for gas chromatography?
Answer: As a gas carrier gas for gas chromatography, it is required to have good chemical stability; high purity; cheap and easy to obtain; suitable for the detector used. Commonly used carrier gases are hydrogen, nitrogen, argon, helium, carbon dioxide, etc.

7. Why should carrier gas be purified? How should it be purified?
Answer: Purification is to remove some organic substances, trace oxygen, moisture and other impurities in the carrier gas to improve the purity of the carrier gas. Impure gas used as carrier gas can cause column failure, sample changes, hydrogen flame chromatography can cause increased base flow noise, and thermal conductivity chromatography can cause the linearity of the discriminator to deteriorate, so the carrier gas must be purified. Generally, chemical treatment is used to remove oxygen, such as active copper to remove oxygen; molecular sieve, activated carbon and other adsorbents are used to remove organic impurities; and silica gel and molecular sieve are used to remove moisture.

8. What are the sample injection methods?
Answer: Chromatographic separation requires a certain amount of sample to be injected in the form of a "plug" in the shortest time. The injection methods can be divided into:
1. Gas sample: There are four general sampling methods:
(1) Syringe injection (2) Measuring tube injection (3) Constant volume injection (4) Gas automatic injection.
Commonly used syringe injection and automatic gas injection. The advantage of syringe injection is that it is flexible to use and the method is simple, but the repeatability of injection volume is poor. The automatic gas sampling is performed with a quantitative valve, which has good repeatability and can be operated automatically.
2. Liquid sample: generally use micro-syringe to inject sample. The method is simple and the sample is injected quickly. Quantitative automatic sampling can also be used, which has good repeatability.
3. Solid sample: The sample is usually dissolved with a solvent, and then the sample is injected in the same way as the liquid sample. It can also be injected with a solid sampler.

9. Briefly describe the effects of operating conditions such as column length, column inner diameter, column temperature, carrier gas flow rate, stationary phase, and sample injection on gas chromatography analysis?
Answer: The operating conditions have a great influence on the chromatographic separation.
1. Column length and column inner diameter: Generally speaking, the growth of the column tube can improve the separation ability. The shorter the component is, the faster the distillation is. The smaller the column inner diameter is, the better the separation effect is. As a result, the support cannot be evenly distributed in the chromatography column. The column tube for analysis generally has an internal diameter of 3-6 mm and a column length of 1-4 meters.
2. Column temperature: It is an important operation variable that directly affects separation efficiency and analysis speed. The choice of column temperature is based on the boiling point range of the mixture, the ratio of the fixed solution and the sensitivity of the identifier. Increasing the column temperature can shorten the analysis time; lowering the column temperature can increase the chromatographic column selectivity, which is beneficial to the separation of components and the stability of the chromatographic column, and the column life is extended. Generally, it is more appropriate to use a column temperature equal to or higher than tens of degrees above the average boiling point of the sample. Use a low column temperature for volatile samples and a high column temperature for non-volatile samples.
3. Carrier gas flow rate: Carrier gas flow rate is one of the important reasons for chromatographic separation. Generally speaking, the chromatographic peaks with high flow rate are narrow, and vice versa, but the flow rate is too high or too low will have an adverse effect on the separation. The flow rate should be stable, and the commonly used flow rate range is between 10-100 liters per minute.
4. Stationary phase: The stationary phase is composed of a solid adsorbent or a support coated with a fixing liquid.
(1) Thickness of solid adsorbent or carrier: generally 40-60 mesh, 60-80 mesh, 80-100 mesh. When columns of the same length are used, the separation efficiency of fine particles is better than that of coarse ones.
(2) Fixed liquid content: The fixed liquid content has a great influence on the separation efficiency, and the weight ratio between it and the support is generally 15% to 25%. Too large a ratio detracts from the separation. Too small a ratio will cause peak tailing.
5. Injection: Generally speaking, the injection is fast, the injection volume is small, and the injection temperature is high, and the separation effect is good. For liquid samples, the speed should be fast, and the vaporization temperature should be higher than the boiling point of the high-boiling components in the sample. Once vaporized, the peak shape of the chromatogram will not be widened and the column efficiency will be high. When the injection volume is within a certain limit, the half width of the chromatographic peak is unchanged. Too much injection volume will overload the column. Generally speaking, the length of the column is increased four times, and the allowable amount of the sample is doubled. For routine analysis, the liquid injection volume is 1-20 microliters; the gas injection volume is 0, 1-5 ml.

10. What principles should be used to select the material of the chromatography column tube? What material is the commonly used column tube made of?
Answer: The material of the chromatographic column tube should be selected according to the following requirements:
1. It shall not react with stationary phase, sample and carrier gas.
2. It should be easy to process and shape.
3. The inner wall of the pipe should be smooth and the cross section should be uniformly round. Generally, the shape of the chromatographic column tube is U-shaped or spiral, mostly made of copper, stainless steel, glass and other materials.


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