Attention should be paid to the injection of gas chromatograph

Install the column

1. Installation and removal of the column must be at room temperature.

2. The packed column has ferrule seal and gasket seal. The card sleeve is divided into three types, metal ferrule, plastic ferrule, and graphite ferrule. It is not easy to screw when installed. The gasket seal is replaced with a new gasket each time the column is installed (Shimadzu chromatography is a gasket seal).

3. Whether the two ends of the column are packed with glass wool. Prevent the glass wool and filler from being blown into the detector by the carrier gas.

4. The length of the capillary column installation and insertion depends on the instrument manual. Different chromatographic vaporization chambers have different structures, so the length of insertion is also different. It should be noted that if you use a capillary column without splitting, the vaporization chamber uses a packed column interface. At this time, the capillary column connected to the vaporization chamber cannot be probed too much, slightly beyond the ferrule.

Effect of hydrogen to air ratio on FID detector

The ratio of hydrogen to air should be 1:10. When the proportion of hydrogen is too large, the sensitivity of the FID detector drops sharply. When the other conditions are the same when using the chromatogram, the sensitivity is lowered to check the hydrogen and air flow rates. Hydrogen and air have a kind of gas that emits a "beep" when it is ignited, and then extinguishes it. Generally, when you ignite it, it will be extinguished, and then it will be turned off and then extinguished.

Use TCD detector

1. When hydrogen is used as a carrier gas, the exhaust gas must be discharged to the outside.

2. Nitrogen can not be set up as a carrier gas bridge, which is much smaller than when using hydrogen.

3. If the carrier gas is not passed, the bridge flow cannot be given. The bridge flow should be given before the sample temperature is stabilized.

How to determine if the FID detector is on fire

Different instruments have different judgment methods. The base flow shows the size of the base flow. Without the base flow display, the wrench with the polished surface is close to the detector outlet, and the surface is observed to have no condensation.

How to determine if the inlet seal should be replaced

It feels particularly easy when injecting. When the TCD detector is not injecting, there are regular small peaks on the recorder, indicating that the gasket is leaking. Replace the gasket without tightening too tightly. Generally, it will be at normal temperature when it is replaced. It will be tighter after the temperature rises. If the gasket is tightened too tightly, it will cause difficulty in injection, and the syringe needle will often be bent.

How to choose the right gasket

The gasket is divided into a general gasket and a high temperature resistant gasket. When the temperature of the vaporization chamber exceeds 300 ° C, a high temperature gasket is used, and one side of the high temperature gasket has a film, and the surface of the membrane is facing downward when used.

How to prevent the needle from bending

Many newcomers to chromatographic work often bend the needle and syringe shaft of a syringe for:

1. The injection port is too tight, and the screw is too tight at room temperature. When the temperature of the vaporization chamber rises, the silicone gasket will expand more tightly, and the syringe is difficult to get stuck.

2. Position is not good to find the needle stuck in the metal part of the inlet.

3. The syringe shaft bend is too strong when injecting, and the inlet chromatograph has an injector frame. When the sample is loaded with the injector frame, the syringe rod will not be bent.

4. Because the inner wall of the syringe is contaminated, the needle bar is pushed at the time of injection. When the syringe is used for a while, it is found that there is a small black piece near the top of the needle tube. At this time, the injection of the sample is difficult. The cleaning method pulls out the needle bar, injects a little water, inserts the needle bar into the polluted position and pushes and pulls it repeatedly. Once again, it can not inject water until the contaminants are removed. At this time, you will see the water in the syringe become cloudy. Pull the needle bar out and wipe it with filter paper, then wash it several times with alcohol. When the sample to be analyzed is a solvent-dissolved solid sample, the syringe should be washed with a solvent in time.

5. Be sure to be steady when injecting. If you are eager to hurry, you will bend the syringe. As long as you are proficient in the injection, it will be faster.

Gas chromatography common fault diagnosis

There are many types of gas chromatographs and their performances vary. It mainly consists of two systems. That is, the pneumatic system and the circuit system. The pneumatic system mainly includes pressure gauges, purifiers, pressure regulators, steady flow valves, rotameters, six-way injection valves, injectors, columns, detectors, etc.; the electronic system includes voltage regulators for various electrical components. Electronics such as power supplies, temperature control devices, amplifier lines, auto-injection and collection devices, data processors and recorders.

To analyze and judge the fault of the chromatograph, you must be familiar with the gas chromatography process and the two systems of gas and circuit, especially the structure and function of the two system components. The malfunction of the chromatograph is various, and the cause of a fault is also multi-faceted. It is necessary to adopt a partial inspection method, that is, the exclusion method, to narrow the scope of the fault. For the failure of the pneumatic system, it is nothing more than the leakage of various gases (especially the carrier gas), the poor gas, the stable flow of the gas, and so on.

For example, if the baseline drifts downward all the time, that is, the “level” value gradually becomes smaller to a negative number, which is most likely a carrier gas leak, then it is necessary to find out whether each joint component has a leak phenomenon, and if the leak does not leak, the baseline still drifts. , it may be a malfunction of the circuit system. The fault on the chromatographic gas path can be found and eliminated by the analyst, but it is not easy to eliminate the fault on the circuit. It is necessary for the analyst to have some knowledge of the electronic circuit, and to figure out the wiring diagram of the host and each The electrical schematic of the system (especially the wiring diagram). The relationship between the control unit and the controlled object is clearly drawn on these figures. The number and direction of each connector lead are specifically indicated. It is very convenient to check the circuit and find the fault according to the figure.

The failure of the chromatographic circuit system is generally the failure of the temperature control system and the failure of the detection amplification system, and of course the failure of the power supply to each system is not excluded. The main circuit of the temperature control system (including column temperature, detector temperature control, injector temperature control) is composed of thyristor and heating wire. The change of conduction angle of thyristor makes the heating power change and the temperature changes. (constant or not constant). The auxiliary circuit (or temperature control circuit), including platinum resistance (thermal element) and linear integrated circuit, is controlled to control the conduction angle change of the thyristor.

It can be seen from the above that if the temperature control system is faulty, it should first check whether the thyristor is bad, whether the heating wire is broken (broken or short-circuited), whether the platinum resistance is broken (broken or short-circuited) or whether the contact is poor. Next, check the other electronic components of the auxiliary circuit. Common faults in the amplification system are that the ion signal line is wet or disconnected, the high-impedance switch (ie, sensitivity selection) is damp, and the performance of the integrated operational amplifier (such as AD515JH, OP07, etc.) is worse or worse, indicating that the fault is not in the amplifier and processor ( Or recorder) while in the gas path section or temperature.

The elimination of chromatographic faults must be done locally and must be considered as a whole. If there is a “fruit”, there must be a “cause” to clarify the direction of the line and gradually eliminate the “cause” of the “fruit” (fault) and narrow the fault range.

For example, if there is constant jitter in the baseline or the baseline noise is high, you can first disconnect the signal input line of the amplifier and observe the baseline condition. If the positive control unit is restored, the fault will occur in the amplifier and recorder (or Processor) and other units. This partially excluded check failure method is very useful in practice.

How to age the column

Newly filled columns are not ready for use and require aging. There are two purposes for aging, one is to completely remove the residual solvent in the filler, and some volatile impurities, and the other purpose is to promote uniform and firm distribution of the fixing solution on the surface of the monomer.

The method of aging: connect the column to the vaporization chamber, and disconnect it from the detector. With nitrogen as the carrier gas, the flow rate is half of normal. The temperature is the highest temperature for the fixed solution. The aging time is about 20 hours. The aging is completed. After the instrument temperature is lowered to near room temperature, turn off the chromatograph, wait until the temperature of the instrument returns to room temperature, and then connect the column to the detector (the end of the vaporization chamber is preferably connected to the detector when aging), turn on the machine, and look at the baseline at the use temperature. Whether it is stable, if the column is aging, it will continue to age.

Aging temperature of several common columns

Squalane (iso-heptane) 140

Apisson L 300

Apisson K, M 250

PEG 20M 200

PEG 6000 175

Ucon HB 2000 225

SE 30 300

OV 1 300

E 301 300

OV 17 300

SE 52 300

DC FS-1265 (formerly known as QF-1) 250

DC 703 250

SE 54 300

Porapak-Q 250

Porapak-T 200

GDX-101, 102, 103, 104, 201 270

GDX-301, 401, 403 250

GDX-601 200

TDX-01 400

Carbon molecular sieve 400