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Structural application of the chromatograph

Time:2019/01/09   Pageviews:0    Share:
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2.1 injection septum
Generally made of silicone rubber material, generally can be divided into three types: ordinary type, high quality type and high temperature type. Normal type is beige, not high temperature resistant, generally used below 200 °C; high quality type can withstand temperature up to 300 °C; high temperature type In green, the temperature can be above 350 ° C to 400 ° C for the highest column temperature. Because the injection septum is mostly made of silicone rubber material, it inevitably contains some residual solvents and/or low molecular oligomers. In addition, due to the high temperature of the vaporization chamber, the silicone rubber will partially degrade. These residual solvents If the degradation product enters the column, a "ghost peak" (ie, not the peak of the sample itself) may appear, affecting the analysis.
The solutions are as follows: one is to perform "separator purge" and the other is to replace the injection septum. Generally, the cycle of replacing the injection septum is subject to the following three conditions:
(1) The appearance of "ghost peaks";
(2) Retention time and peak area reproducibility are poor;
(3) The number of manual injections is 70 times, or the number of automatic injections is 50 times.
2.2 Glass liner
Gas chromatographic liners are mostly made of glass or quartz materials, mainly divided into three types: component flow liner, splitless liner, and packed glass liner. The liner protects the column and, during split/splitless injection, non-volatile sample components are retained in the liner without entering the column. If these contaminants accumulate in the liner a certain amount, it will have a direct impact on the analysis. For example, it will adsorb polar sample components and cause peak tailing, even peak splitting, and there will be “ghost peaks”, so be sure to keep the liner clean and pay attention to cleaning and replacement in time.
Principles and methods for cleaning glass liners:
When the following phenomena occur: (1) "ghost peaks" appear; (2) when the retention time and peak area reproducibility are poor, the liner should be considered for cleaning. The cleaning method and steps are as follows: (1) removing the glass liner; (2) removing the quartz glass wool; and (3) cleaning the inner wall of the liner with a gauze impregnated with a solvent such as acetone. Pay attention to the filling of glass wool when replacing the glass liner: the filling amount is 3~6mg, and the height is 5~10mm. It is required to fill evenly and evenly.
2.3 Gas filter
The color-changing silica gel can judge its performance according to the change of color, but the filter for adsorbing organic substances such as molecular sieve cannot be judged by the naked eye, so it must be replaced regularly, and usually replaced or regenerated once every three months. Since the molecular sieve filter in the split gas path is saturated or heavily contaminated, a large baseline drift occurs, and the filter must be replaced or regenerated at this time. The method of regeneration is: (1) Remove the filter and connect it to the original column position in the opposite direction. (2) Regeneration conditions: carrier gas flow rate 40 ~ 50ml / min, temperature 340 ° C, time 5h.
2.4 detector
If the column is the heart of the chromatographic separation, then the detector is the eye of the chromatograph. No matter how good the chromatographic separation is, if there is no good detector, it will “see” the separation effect. Therefore, high sensitivity and high selectivity detectors have always been the key technology for the development of chromatographs. At present, there are many kinds of detectors used in GC. Among them, the commonly used detectors mainly include flame ionization detector (FID), flame thermionic detector (FTD), flame photometric detector (FPD), and thermal conductivity detector ( TCD), electron capture detector (ECD), etc.
The following is a brief discussion of the routine maintenance of the detector:
2.4.1 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 permanent gases, halosilanes, H2O, NH3, CO, CO2, CS2, CCl4. and many more. 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. Therefore, attention should be paid to optimization. Generally, the ratio of the three should be close to or equal to 1:10:1.
(3) FID is a kind of flame generated by the combustion of hydrogen in air to ionize the substance to be tested, so attention should be paid to safety. Do not open the hydrogen valve when the column is not connected to prevent hydrogen from entering the oven. When measuring the flow rate, the hydrogen and air must not be mixed. When the hydrogen is measured, the air should be turned off, and vice versa. Whenever the flame is extinguished, the hydrogen valve should be closed as much as possible until the fault is re-ignited and the hydrogen valve is opened.
(4) To prevent contamination of the detector, the detector temperature setting should not be lower than the maximum temperature at which the column actually operates. If the detector is affected by the pollution, the sensitivity will be significantly reduced or the noise will increase, and the light will not catch fire. The way to eliminate pollution is to clean the nozzles and air lines. The specific method is: disconnect the column, pull out the signal collector; use a thin steel wire to insert the nozzle to dredge, and soak with acetone, ethanol and other solvents.
2.4.2 Flameless Thermal Ion Detector (FTD)
FTD use precautions:
(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%. Generally, the sensitivity of He is high;
(3) Air: It is best to choose cylinder air, no oil;
(4) Hydrogen: The required purity is 99.999%.
It is also important to note that when using FTD, columns containing cyano-based fixatives such as OV-1701 cannot be used.
2.4.3 Flame Photometric Detector (FPD)
FPD use precautions:
(1) FPD also uses a hydrogen flame, so the safety problem is the same as FID;
(2) The top temperature switch is normally open (250 ° C) 1.5ml chromatography vials;
(3) The flow rate of hydrogen, air and makeup gas of FPD is different from FID. Generally, hydrogen is 60-80 ml/min, air is 100-120 ml/min, and the sum of tail gas and column flow is 20-25 ml/min. Analysis of strongly adsorbed samples such as pesticides, the central temperature should be higher than the bottom temperature of about 20 ° C;
(4) When replacing the filter or ignition, first turn off the power of the photomultiplier tube;
(5) The flame detector, including FID and FPD, must be ignited after the temperature rises; when it is turned off, it should be turned off first and then cooled down.
2.4.4 Thermal Conductivity Detector (TCD)
TCD usage notes:
(1) Make sure that the hot wire is not blown. Before the detector is energized, make sure that the carrier gas has passed the detector. Otherwise, the hot wire may be blown, causing the detector to be scrapped. When shutting down, be sure to turn off the detector power supply and then turn off the carrier gas. Turn off the detector power supply at any time to perform an operation that may cut off the carrier gas flow through the TCD;
(2) When the carrier gas contains oxygen, the life of the hot wire will be shortened, so the carrier gas must be completely deaerated;
(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:
- Is the double column the same, and the flow rate of the double column gas is the same;
- whether it is leaking; replace the column to the graphite gasket of the detector
- pool body pollution; cleaning measures: n-hexane soaking and rinsing.
2.4.5 Electron Capture Detector (ECD)
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ECD usage notes:
(1) Gas path installation gas filter and oxygen trap; Oxygen trap regeneration:
(2) When using the packed column, it is also necessary to supply makeup gas (2~3ml/min);
(3) Operating temperature is 250 to 350 °C. Regardless of the temperature of the column, the temperature of the ECD should not be lower than 250 °C, otherwise the detector is difficult to balance.
(4) After closing the carrier gas and the makeup gas, seal the ECD outlet with a plug to prevent air from entering.
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