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Gas Chromatograph System - Detector Operation Points
Gas Chromatograph System - Detector Operation Points
First, the use of tail gas
The makeup gas is a gas that enters the detector directly from the outlet of the column, and is also called supplemental gas or auxiliary gas. The packed column does not use a tail gas, and the capillary is mostly blown. This is because the carrier gas flow rate in the capillary column is too low (normally 1-3 mL/min) and does not meet the optimal operating conditions of the detector (typical detector requires a carrier gas flow of 20 mL/min). Adding a carrier gas directly to the detector after the column ensures that the detector operates in a highly sensitive state. Another important role of the makeup gas is to eliminate the extra-column effect of the dead volume of the detector. After the separated compound flows out of the column, volume expansion may occur due to an increase in the volume of the pipe, resulting in a slow flow rate, which causes band broadening. This phenomenon is eliminated by the addition of makeup gas.
So, how much is the tail gas flow right? This depends on the size of the detector and column used. For example, when using a 0.53mm large diameter column, the flow rate in the column can reach 15mL/min, which is already large enough for the micro TCD and the monofilament TCD, so there is no need to add a tail gas. For FID, NPD, FPD, a flow rate of at least 10 mL/min of makeup gas is required, and for ECD, 20 mL/min of tail gas is required (ECD generally requires a total carrier gas flow rate of more than 25 mL/min). When using conventional or micro-diameter columns, the makeup gas flow should be increased accordingly. The empirical reference values ​​are: FID, NPD, FPD requires the sum of the flow rate of the carrier gas and the makeup gas in the column to be about 30 ml/min, and the ECD requires 40-60 mL/min. When working at the highest sensitivity, the makeup gas flow and other gas flows should be optimized for the specific sample. In general, the type of gas used for the makeup gas should be the same as the carrier gas.
The makeup gas flow rate is measured by a soap film flow meter at the outlet of the detector after the column is installed. Note that other gases should be turned off when measuring the flow of the makeup gas (if the FID is used to shut off the air and hydrogen). When using a column with an inner diameter of 0.32 or less, the carrier gas in the column may not be closed. The sum of the gas and makeup gas flow.
Second, FID use precautions
1. Although the FID is a general-purpose detector, some substances have little or no response on this detector. These materials include permanent gases, halosilanes, H 2 O, NH 3 , CO, CO 2 , CS 2 , CCl 4 and the like. Therefore, FID should not be used when testing these substances.
2. FID is the use of a flame generated by the combustion of hydrogen and 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. If the flame is extinguished for whatever reason, the hydrogen valve should be closed as soon as possible until the fault is removed and the hydrogen valve is opened again after re-ignition. High-end instruments have automatic detection and protection functions, and the hydrogen can be automatically turned off when the flame is extinguished.
3. The sensitivity of FID has a direct relationship with the ratio of hydrogen, air and nitrogen, so pay attention to optimization. Generally, the ratio of the three is close to or equal to 1:10:1, such as hydrogen 30-40mL/min, air 300-400mL/min, and nitrogen 30-40mL/min. In addition, some instruments are designed with different nozzles for filling columns and capillary columns, and you should check the instructions when using them.
4. To prevent contamination of the detector, the detector temperature setting should not be below the maximum temperature at which the column actually operates. Once the detector is contaminated, the sensitivity is reduced or the noise is increased, and the point is not ignited. The way to eliminate pollution is to clean, mainly to clean the nozzle surface and the gas pipeline. The specific method is to remove the nozzle, sequentially soak with different solvents (acetone, chloroform and ethanol), and ultrasonically in the ultrasonic water bath for more than 10min. It is also possible to use a fine stainless steel wire to pass through the hole in the middle of the nozzle, or to burn the oil in the nozzle with an alcohol lamp for thorough cleaning. Sometimes it takes a long time, and the surface of the nozzle will deposit carbon (a layer of black deposits), which will affect the sensitivity. Gently sand the surface with a fine gauze paper to remove. After the cleaning, the nozzle is dried and then placed in the detector for measurement.
Third, TCD use precautions
1. Make sure 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! At the same time, be sure to turn off the detector power before shutting down, then turn off the carrier gas. Turn off the detector power whenever it is possible to shut off the flow of TCD carrier gas. This is the rule that TCD operations must follow!
2. When the carrier gas contains oxygen, the life of the hot wire will be shortened, so the carrier gas must be completely deaerated when there is TCD. Also, do not use Teflon as a carrier gas delivery tube because it will penetrate oxygen.
3. The type of carrier gas has a greater impact on the sensitivity of TCD. The principle is that the greater the difference between the heat transfer coefficient of the carrier gas and the measured object, the better the sensitivity of hydrogen or helium as the carrier gas than when the carrier gas is used as the carrier gas. Of course, nitrogen must be used as a carrier gas to determine hydrogen.
Fourth, NPD use precautions
1. NPD is developed on the basis of FID. It differs from FID in that it adds a thermal ion source (consisting of strontium salt beads) using a micro-hydrogen flame. Under the condition that the thermionic source is heated by electricity, the ionization efficiency of the nitrogen-containing and phosphorus-containing compounds is greatly improved, so that the two types of compounds can be selectively detected. Due to the use of hydrogen, the safety of NPD is the same as for FID.
2. The temperature change of the thermionic source has a great influence on the sensitivity of the detector.
The temperature is high and the sensitivity is high, but the life of the salt bead is shortened. Increasing the voltage of the thermionic source and increasing the hydrogen flow rate can increase the sensitivity. However, it must be noted that the air flow rate is too low and the balance time of the detector is too long; the hydrogen flow rate is too high, and a flame like FID is formed, which greatly reduces the service life of the beryllium salt beads and destroys the nitrogen and phosphorus. The choice of a trusted response. The gas flow rate is generally set to 3-4 mL/min of hydrogen and 100-120 mL/min of air. The packed column and the large-diameter column are used. The carrier gas flow rate is about 20 mL/min, and the tail gas is not used. When the conventional capillary column is used, the tail is used. The blowing is set to about 30 mL/min.
3. When adjusting and setting the voltage of the thermionic source, remember to turn off the detector power to avoid accidentally burning the salt beads.
4. The active element (onium salt) of the hot ion source is easily contaminated and shortens its service life.
To extend its service life, it should be noted; first, to avoid SiO 2 entering the detector, the column should be well aging, especially the silicone type fixing liquid, and the liquid film should be thin. Also avoid SiO 2 residues in the sample after derivatization into the column. Second, before turning off the carrier gas (such as changing the cylinder or changing the column), the voltage of the thermionic source should be adjusted to 0. Otherwise, no carrier gas will pass through, and the salt beads will burn out within a few minutes. Third, use a low thermal ion source voltage as much as possible while meeting the sensitivity requirements. Fourth, the instrument should be stored to avoid moisture. When the instrument is not in use, it is best to keep the detector temperature above 100 (thermionic source voltage should be turned off). Fifth, if you do not inject the analysis for a period of time (such as overnight), you should lower the thermionic source voltage, but do not turn it off. Because the salt beads are still hot after the voltage is reduced, the voltage is quickly stabilized when the sample is injected again. If the voltage is turned off after the shutdown, the detector requires a few hours of equilibration time.
Five, ECD use precautions
1. Prevent radioactive contamination. ECD has a radioactive source (generally 63Ni), so the detector outlet must be connected to the outside, preferably to the ventilation outlet. Do not disassemble the ECD yourself without special training. Follow the laboratory's regulations on radioactivity. For example, at least 6 months should be tested for radioactive leaks.
2. The operating temperature of the ECD is generally higher, and the usual temperature range is 250-300 °C. Regardless of the low column temperature, the ECD temperature should not be lower than 250 °C. This is because the detector is difficult to balance when the temperature is low.
3. There are two options for carrier gas when using ECD. One is nitrogen, and the other is argon containing 5% methane. The former has higher sensitivity, but the noise is also higher; when the latter is used, the detection limit is basically the same as the former, but the linear range is wider. Hydrogen can also be used as a carrier gas, but nitrogen is used as a tail gas. The sum of the flow rates of the carrier gas and the makeup gas is generally 60 mL/min. Too small a flow will make the peak tailing severe, and too much flow will reduce sensitivity.
4. ECD should avoid contact with oxygen or moisture, otherwise the noise will increase significantly. Therefore, the carrier gas and the makeup gas require good purification. In addition, the detector contamination test and leak test must be carried out in strict accordance with the instrument operating procedures.
Six, FPD use precautions
1. FPD can also use hydrogen flame, so the safety problem is the same as FID (see above FID usage precautions).
2. The hydrogen, air and makeup gas of FPD are different from FID, generally 60-80mL/min for hydrogen, 100-120mL/min for air, and 20-25mL/min for tail gas and column flow.
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