Separation mechanism of the hottest gas chromatogr

The separation mechanism of gas chromatography

gas chromatography is a chromatographic method that uses gas as the mobile phase (carrier gas) to flow through the chromatographic column containing filler for separation and determination. After the substance or its derivatives are gasified, they are brought into the chromatographic column by the carrier gas for separation. Each component enters the detector successively, and the chromatographic signal is recorded by the recorder, integrator or data processing system

the separation mechanism of gas chromatography mainly includes adsorption, distribution, etc

1. General requirements for instruments

the instrument used is a gas chromatograph, which is composed of carrier gas source, sample injection part, chromatographic column, detector and data acquisition system. The temperature of the injection part, chromatographic column and detector are under control. Since the temperature fluctuation of the column temperature box will affect the reproducibility of chromatographic analysis results, the temperature control accuracy of the column temperature box should be within ± 1 ℃, and the temperature fluctuation should be less than 0.1 ℃ per hour. The temperature control system is divided into constant temperature and programmed temperature

the mobile phase of carrier gas chromatography is gas, which is called carrier gas. Helium, nitrogen and hydrogen can be used as carrier gas, which can be provided by high-pressure steel cylinder or high-purity gas generator, passing through the injector and chromatographic column at a certain flow rate after passing through an appropriate decompression device; The carrier gas is selected according to the nature of the test article and the type of detector. Unless otherwise specified, the common carrier gas is nitrogen

generally, the injection method of the sampler can be solution direct injection or headspace injection

direct injection of solution adopts micro syringe, micro injection valve or gasification chamber with shunt device; When the solution is directly injected, the temperature of the injection port should be 30 ~ 50 ℃ higher than the column temperature; The injection volume is generally not more than a few microliters; The smaller the column diameter is, the less the injection volume should be. When using a capillary column, it should generally be shunted to avoid overload

headspace sampling is suitable for solid and liquid. Please consult with electrical engineers or people with equivalent knowledge about the separation and determination of volatile components in the test sample. Place the solid or liquid test sample in a closed vial, heat it in a thermostatically controlled heating chamber until the volatile components in the test sample reach equilibrium in the non gaseous and gaseous states, and then automatically suck a certain volume of top air into the chromatographic column by the sampler

chromatographic column the chromatographic column is a packed column or capillary column. The packed column is made of stainless steel or glass, with an inner diameter of 2 ~ 4mm and a column length of 2 ~ 4m. It is filled with adsorbent, polymer porous pellets or carriers coated with fixed liquid, and the particle sizes are 0.25 ~ 0.18mm, 0.18 ~ 0.15mm and 0.15 ~ 0.125mm. The commonly used carriers are diatomite or polymer porous pellets after acid pickling and silylation, and the commonly used stationary solutions are methyl polysiloxane, polyethylene glycol, etc. The capillary column is made of glass or quartz, the inner wall or carrier is coated with or cross-linked fixed liquid, the inner diameter is generally 0.25mm, 0.32mm and 0.53mm, the column length is 5 ~ 60m, and the fixed liquid film thickness is 0.1 ~ 5.0 μ m. Common stationary solutions include methyl polysiloxane, phenyl methyl polysiloxane with different proportions, polyethylene glycol, etc

new packed columns and capillary columns should be aged before use to remove residual solvents and low molecular weight polymers. If chromatographic columns are not used for a long time, they should be aged before use to stabilize the baseline

detectors suitable for gas chromatography include flame ionization detector (FID), thermal conductivity detector (TCD), nitrogen and phosphorus detector (NPD), electron capture detector (ECD), mass spectrometry detector (MS), etc. Flame ionization detector has good response to hydrocarbons and is suitable for the analysis of most drugs; The nitrogen and phosphorus detector has high sensitivity to compounds containing nitrogen and phosphorus elements; Flame photometric detector (FPD) has high sensitivity to compounds containing phosphorus and sulfur; The electron capture detector is suitable for compounds containing halogen atoms; The MS detector can also give the corresponding structural information of a component of the test sample, which can be used for structural confirmation. Unless otherwise specified, flame ionization detector is generally used, with hydrogen as fuel gas and air as auxiliary fuel gas. When using flame ionization detector, the detector temperature should generally be higher than the column temperature, and should not be lower than 150 ℃ to avoid water vapor condensation, usually 250 ~ 350 ℃

data processing system can be divided into recorder, integrator and computer workstation

except that the type of detector, the type of fixed liquid and the specially designated chromatographic column material shall not be changed arbitrarily, the chromatographic conditions specified under each variety in the text, such as the inner diameter of chromatographic column, length, carrier brand, particle size, coating concentration of fixed liquid, carrier gas flow rate, column temperature, injection volume, sensitivity of detector, etc., can be changed appropriately to adapt to specific varieties and meet the requirements of system applicability test. Generally, the chromatogram is recorded in about 30 minutes

2. System suitability test

unless otherwise specified, it shall be the same as that under high performance liquid chromatography

3. determination method

(1) internal standard method plus correction factor to determine the content of an impurity or principal component in the test sample

(2) determine the content of an impurity or principal component in the test sample by external standard method

(3) area normalization method

(4) determine the content of an impurity or principal component in the test sample by standard solution addition method

the specific contents of the above methods (1) to (3) are the same as those specified under high performance liquid chromatography

accurately weigh (measure) take an appropriate amount of the reference substance of an impurity or component to be tested, prepare a reference solution of appropriate concentration, take a certain amount, accurately add it to the test solution, measure the content of the impurity or principal component according to the external standard method or internal standard method, and then deduct the content of the added reference solution to obtain the content of an impurity and principal component in the test solution

can also be calculated according to the following formula. The correction factor should be the same before and after adding the reference solution, that is:

ais/ax=cx+ Δ Cxcx

then the concentration CX of the component to be tested can be calculated by the following formula:

cx= Δ CX (ais/ax) -1

where CX is the concentration of component X in the test sample

ax is the chromatographic peak area of component X in the test sample

Δ CX is the added component reference substance with known concentration to be tested

ais is the chromatographic peak area of component X after adding the reference substance

for quantitative analysis by gas chromatography, when manual injection is used, it is difficult to accurately control the injection volume due to the influence of needle retention time and room temperature, so it is best to use internal standard method for quantitative analysis; When using the automatic sampler, the external standard method can also be used to quantify on the premise of ensuring the injection error due to the improvement of injection repeatability. When headspace injection technology is used, because the test sample and the reference sample are in different matrices, the standard solution plus some electronic parts and circuit boards can be used to eliminate the influence of matrix effect by 10 points sensitive to static electricity; When the results of standard solution addition method are inconsistent with those of other quantitative methods, the results of standard addition method shall prevail

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