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How does a gas chromatograph work? December 16,2020.

Gas chromatography (GC)


Gas chromatograph is an instrument that uses chromatographic separation technology and detection technology to perform qualitative and quantitative analysis of multi-component complex mixtures.

Gas chromatography (GC) is a separation technique. The samples to be analyzed in actual work are often multi-component mixtures in a complex matrix. For samples containing unknown components, they must be separated first, and then the relevant components can be further analyzed. The separation of the mixture is based on the difference in the physical and chemical properties of the components. The gas chromatograph mainly uses the difference in the boiling point, polarity and adsorption properties of the substance to realize the separation of the mixture.

After the sample to be analyzed is vaporized in the vaporization chamber, the inert gas (ie carrier gas, generally N2, He, etc.) is brought into the chromatographic column. The column contains liquid or solid stationary phase. Due to the boiling point, polarity or adsorption of each component in the sample Different in performance, each component tends to form a distribution or adsorption equilibrium between the mobile phase and the stationary phase. However, because the carrier gas is flowing, this equilibrium is actually difficult to establish. It is also because of the flow of the carrier gas that the sample components are repeatedly distributed or adsorbed/desorbed during the movement. As a result, the carrier gas The components with high medium concentration will flow out of the column first, and the components with high concentration will flow out after the stationary phase.

When the component flows out of the chromatographic column, it immediately enters the detector. The detector can convert the presence or absence of the sample component into an electrical signal, and the magnitude of the electrical signal is proportional to the amount or concentration of the measured component. When zoomed in and recorded, it contains all the original information of the chromatogram. When there is no component flowing out, the record of the chromatogram is the background signal of the detector, that is, the baseline of the chromatogram.


Detector

If the column is the heart of the gas chromatograph, then the detector is the eye of the gas chromatograph. No matter how good the chromatographic separation effect is, if there is no good detector, the separation effect will not be "see". Therefore, high-sensitivity and high-selectivity detectors have always been the key technology for the development of gas chromatographs. At present, there are many detectors used in gas chromatographs, among which the commonly used detectors mainly include flame ionization detector (FID), flame thermionic detector (FTD), flame photometric detector (FPD), and thermal conductivity detector. Detector (TCD), electron capture detector (ECD), etc. The following is a brief discussion on the routine maintenance of the

detector:

1  flame ionization detector (FID)

(1) Although FID is a quasi-universal detector, some substances have little or no response on the detector. These substances include gas, halogenated silane, H2O, NH3, CO, CO2, CS2, CCl4, etc. Wait. Therefore, FID should not be used when testing these substances.

(2) The sensitivity of FID is directly related to the ratio of hydrogen, air, and nitrogen, so attention should be paid to optimization. Generally, the ratio of the three should be close to or equal to 1:10:1.

(3) FID uses the flame generated by the combustion of hydrogen in the air to ionize the measured substance, so attention should be paid to safety issues. When the column is not connected, do not open the hydrogen valve to prevent hydrogen from entering the column oven. When measuring the flow rate, hydrogen and air must not be mixed, that is, when measuring hydrogen, the air must be turned off, and vice versa. When the flame goes out for whatever reason, the hydrogen valve should be closed as much as possible, and the hydrogen valve should be opened again when the fault is eliminated and the ignition is restarted.

(4) To prevent the detector from being contaminated, the temperature setting of the gas chromatograph detector should not be lower than the actual working Zgao temperature of the chromatographic column. If the detector is contaminated, the sensitivity will decrease significantly or the noise will increase, or the detector will not catch fire. The way to eliminate pollution is to clean the nozzle and gas pipeline. The specific method is: disconnect the chromatographic column, pull out the signal collector; insert a thin steel wire into the nozzle to dredge, and soak it with acetone, ethanol and other solvents.


2  flame thermionic detector (FTD)

FTD use precautions:

(1) Beads: avoid water in the sample, the service life is about 600~700h;

(2) Carrier gas: N2 or He, with a purity of 99.999% required. Generally, He has high sensitivity;

(3) Air: Z is better to choose cylinder air, no oil;

(4) Hydrogen: 99.999% purity is required.

   Another thing to note is that when using FTD, you cannot use chromatographic columns containing cyano-based stationary liquid.


3  flame photometric detector (FPD)

FPD use precautions:

(1) FPD also uses hydrogen flame, so the safety issues are the same as FID;

(2) The top temperature switch is normally open (250℃);

(3) The flow of hydrogen, air and makeup gas of FPD is different from FID. Generally, hydrogen is 60~80ml/min, air is 100~120ml/min, and the sum of makeup gas and column flow is 20~25ml/min. For analysis of strongly adsorbing samples such as pesticides, the temperature in the middle part should be about 20℃ higher than the temperature at the bottom;

(4) When replacing the filter or igniting, turn off the power of the photomultiplier tube;

(5) Flame detectors, including FID and FPD, must be ignited after the temperature rises; when they are turned off, they should be turned off before cooling down.


4  thermal conductivity detector (TCD)

TCD use precautions:

(1) Ensure that the heating wire is not burnt. Before energizing the gas chromatograph detector, make sure that the carrier gas has passed the detector, otherwise, the hot wire may be burned out, causing the detector to be scrapped; when shutting down, be sure to turn off the detector power first, and then turn off the carrier gas. Whenever performing operations that may cut off the carrier gas flow through the TCD, turn off the detector power;

(2) When the carrier gas contains oxygen, the life of the hot wire will be shortened, so the carrier gas must be completely deoxygenated;

(3) When hydrogen is used as a carrier gas, the gas is discharged outdoors;

(4) When the baseline drift is large, the following questions should be considered: whether the two columns are the same, whether the gas flow rate of the two columns are the same; whether there is gas leakage; replace the graphite gasket from the chromatographic column to the detector. Pollution of the pool; cleaning measures: soak and rinse in n-hexane.


5 electron capture detector (ECD)

ECD use precautions:

(1) Install gas filter and oxygen trap in gas path;

(2) Makeup gas (2~3ml/min) is also required when using a packed column;

(3) The operating temperature is 250~350℃. No matter how low the chromatographic column temperature is, the temperature of the ECD should not be lower than 250°C, otherwise the detector will be difficult to equilibrate.

(4) After turning off the carrier gas and makeup gas, seal the ECD outlet with a plug to prevent air from entering.


Guangzhou Biaoji Packaging Equipment Co., Ltd. was established in 2002, has obtained IOS 9001 certification, and is recognized as a high-tech enterprise and software enterprise by the national authority. It is specialized in laboratory testing equipment , mask testing equipment , and standard materials. For more than 20 years, proficiency-tested enterprise companies have provided complete sets of solutions and technical services for production enterprises, institutions, scientific research institutes, and many well-known domestic and foreign brands.


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