How does a gas chromatograph work?

Gas chromatograph is an instrument for qualitative and quantitative analysis of multi-component complex mixtures by using chromatographic separation technology and detection technology.

Gas chromatography (GC) is a separation technique. The samples to be analyzed in practical work are often multi-component mixtures in complex matrices. For samples containing unknown components, professional separation is required before further analysis of related components. Separation of mixtures is based on differences in the physical and chemical properties of the components. Gas chromatography mainly uses the difference in boiling point, polarity and adsorption properties of substances to achieve the separation of mixtures.

After the sample to be analyzed is vaporized in the vaporization chamber, the inert gas (i.e. carrier gas, generally N2, He, etc.) is brought into the chromatographic column. The columns contain liquid or solid stationary phases. Due to the different boiling point, polarity or adsorption performance of each component in the sample, each component often forms a distribution or adsorption equilibrium between the mobile phase and the stationary phase. However, since the carrier gas is flowing, this balance is difficult to establish in practice. It is also due to the flow of the carrier gas that the sample components are repeatedly distributed or adsorbed/desorbed during the movement. In this way, the components with higher concentration in the carrier gas will flow through the column first,

When components flow well off the column, they immediately enter 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 component to be measured. When zoomed in and recorded, it contains all the original information of the chromatogram. When no components elute, the chromatogram record is the background signal from the Detector, the baseline of the chromatogram.

If the column is the heart of the gas chromatograph, then the Detector is the eyes of the gas chromatograph. No matter how good the chromatographic separation effect is, if there is no good Detector, the separation effect will be "invisible". Therefore, Detectors with high sensitivity and high selectivity have always been the key technologies for the development of gas chromatographs. At present, there are many Detectors used in gas chromatography, and the commonly used Detectors mainly include flame ionization Detector (FID), flame thermionic Detector (FTD), flame photometric Detector (FPD), thermal conductivity Detector and so on. Detector (TCD), electron capture Detector (ECD), etc.

1. Flame ionization Detector (FID)

(1) Although the FID is a quasi-multifunctional Detector, some substances have little or no response to the Detector. These substances include gases, halosilanes, H2O, NH3, CO, CO2, CS2, CCl4, etc. Therefore, the FID should not be used when testing these substances.

(2) The sensitivity of the FID is directly related to the ratio of hydrogen, air, and nitrogen, and 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 safety issues should be paid attention to. Do not open the hydrogen valve when the column is not connected to prevent hydrogen from entering the column oven. When measuring flow, hydrogen and air must not be mixed, that is, when measuring hydrogen, the air needs to be turned off, and vice versa. When the flame goes out for any reason, the hydrogen valve should be closed as much as possible, and the hydrogen valve should be reopened to re-ignite after the fault is eliminated.

(4) In order to prevent the Detector from being polluted, the temperature setting of the gas chromatographic 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 contamination is to clean the nozzles and gas lines. The specific method is: disconnect the chromatographic column, pull out the signal collector; insert a thin steel wire into the nozzle to dredge it, and soak it in solvents such as acetone and ethanol.

2. Flame Thermionic Detector (FTD)

Precautions for using FTD:

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

(2) Carrier gas: N2 or He, the required purity is 99.999%. In general, He has high sensitivity;

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

(4) Hydrogen: The required purity is 99.999%.

In addition, Beichao reminds you: when using FTD, you cannot use the chromatographic column containing cyano-based fixative.

3. Flame Photometric Detector (FPD)

Precautions for FPD use:

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

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

(3) The flow rates of hydrogen, air, and makeup gas are different for the FPD than for the 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 the analysis of strong adsorption samples such as pesticides, the middle temperature should be about 20°C higher than the bottom temperature;

(4) When replacing the filter or ignition, turn off the photomultiplier tube power supply;

(5) Flame Detectors, including FID and FPD, need to be ignited after the temperature rises; when closed, they should be closed before cooling down.

4. Thermal Conductivity Detector (TCD)

Precautions for using TCD:

(1) Make sure that the heating wire is not burned. Before powering on the Detector of the gas chromatograph, please ensure that the carrier gas has passed the Detector, otherwise the hot wire may be burnt and the Detector may be scrapped; when shutting down, please be sure to turn off the power of the Detector first, and then turn off the carrier gas. Power off the Detector whenever performing an operation that could cut off the flow of carrier gas through the TCD;

(2) When the carrier gas contains oxygen, it will shorten the life of the hot wire, so it is necessary to completely deoxidize the carrier gas;

( 3) When hydrogen is used as the carrier gas, the gas is discharged to the outside;

(4) When the baseline drift is large, the following issues should be considered: whether the two columns are the same, whether the gas flow rates of the two columns are the same; whether there is an air leak; replace the graphite gasket from the chromatographic column to the Detector. Swimming pool pollution; cleaning measures: soak and rinse with n-hexane.

5. Electron capture Detector (ECD)

Precautions for using ECD:

(1) Install gas filters and oxygen traps in the gas circuit;

(2) When using a packed column, it is necessary to supplement the gas (2 ~ 3ml/min);

(3) The working temperature is 250-350°C. No matter how cold the column is, the temperature of the ECD cannot be lower than 250°C or the Detector will struggle to equilibrate.

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