Q and A for chromatography

What low-level operating errors are easy to make when learning gas chromatography at the beginning?

First, the injection speed is slow

Slow injection speed, easy to cause peak tail. In the manual injection of gas chromatography, the injection needle should be fast in, fast in and out, so as to ensure the peak shape standard of the spectrum and good repeatability.

Two, can not maintain the instrument in time

1. Failure to replace the sample pad in time will cause air leakage in the septa, which will affect the analysis results in light cases and damage the instrument in heavy cases.

2. Failure to replace the gasified liner in time will cause inaccurate analysis results, poor repeatability of analysis results, and serious pollution of chromatographic column.

Three, carrier gas or combustion gas operation error

1. When opening the instrument, the carrier gas valve is not opened in advance and the carrier gas is passed into the instrument. If the carrier gas is not passed at high temperature, the gas phase column will be damaged.

2. The gas column will be damaged if the correct shutdown steps are not performed when the instrument is maintained by the sampling port, replaced by the column and shut down.

3. Forget to turn off the combustion gas after the instrument is shut down, and the temperature of the detector will lead to water in the detector and damage the instrument.

How to ensure the safety of gas chromatography?

The impurities in the gas are mainly some permanent gas, low molecular organic compounds and water vapor, analysis, mainly will affect the analysis object, chromatographic column, detector and chromatogram, affect the sensitivity and stability, as well as the result of the analysis, so all the gas before entering the instrument to strictly purify, ensure the purity of the instrument work.

The exhaust gas (gasket scavenging gas, shunt vent, detector vent and carrier gas) in the working process of the instrument should be connected to the outside, depending on its impact on the indoor environment and safety, as well as the use of combustible gas, safety consideration should be the first.

In order to ensure the purification effect and safety of the gas used, it is necessary to consider the air source, use process and exhaust emission.

What kind of gas source is used, whether by high-pressure cylinders or gas generators, to be specific analysis. General gas plant can ensure the quality of gas supply, the cost is relatively low, and the laboratory is convenient to replace the cylinder, the best use of cylinder. If hydrogen cylinders are used, they should be placed outdoors or in a separate gas room to ensure safety. If the laboratory ventilation is frequent or inconvenient to store, it is recommended to use a gas generator.

Hydrogen generator and nitrogen generator, we should pay attention to the purity of the reagent used and whether the pipeline leakage, water vapor is the main impurity.

To remove impurities from the gas, a purification device is connected between the air source and the instrument. Such as molecular sieve or activated carbon adsorption to remove low molecular organic compounds, with color-changing silica gel to remove water vapor. Purification of molecular sieve, activated carbon and silica gel, after a certain time, should pay attention to check and replace, replace the filler can be reactivated to continue to use, but pay attention to remove the powder in the filler, to avoid blocking the gas path.

In order to ensure the safety of gas use, it is also necessary to consider the emission safety of exhaust gas (vent gas). For example, gasket purging, shunt vent, detector vent and other pipelines should be connected to the outdoor as far as possible (when hydrogen is used as carrier gas, it must be connected to the outdoor) to avoid toxic and harmful substances polluting the indoor air and endangering the health of operators. Meanwhile, the safety issues of hydrogen as carrier gas should also be considered

How to check gas chromatography system contamination?

You can check from the following aspects:

For each analysis sample mentioned in the following processing steps, samples with normal peak output before pollution are used as analysis objects, and no less than two parallel samples are analyzed each time, so as to truly reflect the effect of the screening steps. If the problem abates when the sample is analyzed, repeat the sample several times for final confirmation.

(1) Carrier gas pollution or insufficient gas source pressure

Check carrier gas pressure; Confirm the timing of contamination, whether it occurred after the carrier gas replacement; Check whether the molecular sieve, activated carbon and silica gel used for purification are discolored. If no, go to the next step.

(2) Maintain the sample inlet

Replace the injection pad and gasified liner, and increase the inlet temperature by 30 ~ 50 ℃. If the same miscellaneous peak still appears after analyzing the sample, proceed to the next step.

(3) column regeneration

If the column is capillary, cut off about 15 cm from the injector end. If it is a stuffed column, remove the inlet end and check the quartz cotton inside the stuffed column for obvious traces of contamination.

The column is aged at high temperature after inspection. The temperature is set at 30 ~ 50 ℃ above the service temperature, or 10 ℃ below the maximum service temperature if it exceeds the permissible service temperature. Set the appropriate program heating method, aging 3 ~ 5 hours.

If the problem cannot be solved after aging, it is necessary to clean the inlet, and the shunt inlet also needs to clean the shunt flow path. Because the cleaning work needs to disassemble the injector and chromatographic column and clean with a large amount of solvent, it is best to consult the manufacturer's engineer.

Solvent cleaning can be used to clean the column. Because solvent cleaning chromatographic column needs special tools, the operation is not simple, if conditions to replace a normal column is also a quick way to eliminate whether the chromatographic column pollution.

The following is attached to the solvent cleaning chromatographic column part of the knowledge for learning reference.

Solvent cleaning of a capillary column is the method by which the column to be cleaned is removed from the GC and a few ml of solvent is injected into the column. The solvent cleaning unit is connected to a pressurized gas source (N2 or He) that forces the solvent to flow through the column. Contaminants dissolve in the solvent and bounce off the column with the solvent. The solvent is then purged out of the column and the column is properly aged.

Any residue soluble in cleaning solvents is removed from the column. If the column is not removed by instrumental analysis, injecting large amounts of solvent will not provide a thorough cleaning of the column, nor will it remove all contaminants from the column. Capillary GC columns must have bonded and crosslinked stationary phases to be cleaned with solvents. The column is seriously damaged by solvent cleaning of unbonded stationary phase.

Methanol, methylene chloride, and n-hexane are recommended, and they work well in most cases. Acetone can be used instead of methylene chloride to avoid the use of chlorinated solvents.

What causes GC column box temperature not ready?

Gas chromatograph column box temperature can not balance or balance time is long, generally can be investigated from three aspects:

First, look at the temperature difference between the set temperature of the column box and the ambient temperature

General chromatograph requires column box temperature lower limit of room temperature +5 ℃, generally in use, the column box program temperature rise or use temperature lower limit is set to room temperature above 10 ℃, so as to ensure normal temperature control. When the temperature of the column box is set close to room temperature, the temperature of the column box takes a long time to balance or even cannot balance because of the thermal balance.

For this reason, it can be eliminated and solved by modifying the temperature setting of column box.

Two, column box temperature control problem

In the column box, there is generally a platinum resistance at the back of the column and in front of the cooling fan to detect the temperature of the column box. When there is dust accumulation or failure on the platinum resistance, the measurement may be inaccurate, and then the temperature control of the column box is not good. However, it is less likely that the temperature of the column box cannot be ready due to this problem.

Three, the chromatography column box back door control failure

The back of the column box has a heat dissipation door, through the motor adjustment after the opening of the door and column box heating control system, to complete the heat balance in the column box. When the motor adjustment of the rear door fails, or the rear door of the column box is stuck, it will cause the failure of the execution of the temperature control program or the execution is not in place, and then lead to the fault that the temperature of the column box can not be ready.

If the rear opening motor is faulty, it needs to be replaced. Most chromatographic manufacturers need engineers to deal with this problem

Q: Does high inlet temperature affect the column?

The inlet temperature does have a little effect on the column end, but has little effect on the column as a whole. Substances with boiling point of more than 300 degrees should be diluted with solvent before sampling, and the inlet temperature should be within the range of 280-320 degrees Celsius.

The maximum service temperature stated by the column is the stationary phase tolerance temperature. When the temperature of the injector is too high, the stationary phase of the column installed in the injector will be lost, that is, a few centimeters to a dozen centimeters in length. This section will not have any effect on the separation effect of the column as a whole.

How to choose the right column chamber temperature?

A: Column temperature is an important factor affecting the retention time of compounds in gas chromatography. In use, attention should be paid to the selection of column temperature, because column temperature is related to: first, if the column temperature of the column fixed liquid life is higher than the maximum use temperature of the fixed liquid, it will cause the loss of the fixed liquid with the carrier gas, not only affect the life of the column, but also the fixed liquid with the carrier gas into the detector, will pollute the detector, affect the analysis results. Second, separation efficiency and analysis time if the column temperature is too high, the distribution coefficient of each component will be reduced, the separation degree will be reduced; However, if the column temperature is too low, the mass transfer rate will be significantly reduced, the column efficiency will be decreased, and the analysis time will be prolonged. The higher the column temperature of compound retention time, the faster the peak, the smaller the retention time. The reproducibility of retention time is not good due to the variation of column temperature, which affects the qualitative results of sample components. Generally, the column temperature changes by 1 ℃, and the retention time of components changes by 5%. If the column temperature changes by 5%, the retention time of components changes by 20%. 4. The temperature rise of chromatographic peak-peak column will normally lead to narrowing of half-peak width, increasing peak height, and constant peak area. However, as the peak height of components increases, the analysis results may change when the peak height is used for quantitative analysis. If column temperature decreases, the opposite is true. Therefore, the column temperature selection needs to start from the following aspects: first, it should ensure that the column temperature is not higher than the maximum use temperature of the fixed liquid (that is, the maximum tolerance temperature of the column), to avoid the loss of fixed liquid and affect the column efficiency and service life of the column; Secondly, select the appropriate column temperature, which should make the two components difficult to separate achieve the expected separation effect. It is appropriate that the peak shape is normal and the analysis time is not long. Generally, the column temperature should be 20-30 ℃ lower than the average boiling point of each component in the sample, which is determined by the test. For the samples with a wide range of boiling points, programmed temperature should be adopted, and the temperature should be increased linearly or nonlinearly with time according to the predetermined heating rate. The general rate of warming is linear. Finally, especially to ensure the stability and uniformity of the instrument column temperature control, as well as the consistency between the actual temperature and the preset temperature. General gas chromatograph column temperature control accuracy of ±0.1 ℃, some manufacturers can reach ±0.01 ℃.