The use of gas chromatograph and precautions

First, the composition of the instrument Gas chromatograph by the carrier gas source, injection part, column, column thermostat, detector and data processing system. The temperature of the injection section, column, and detector are all under control.

Second, the basic requirements of the instrument 1. The general requirements of the instrument (1) carrier gas source gas helium, nitrogen and hydrogen can be used as the mobile phase of gas chromatography, according to the nature of the test sample and the choice of carrier gas carrier Unless otherwise specified, the commonly used carrier gas is nitrogen.

(2) The injection mode of the injection part can generally be directly injected into the solution or injected into the headspace. When using the direct injection of the solution, the injection port temperature should be higher than the column temperature 30 ~ 50 °C. Headspace sampling is suitable for the separation and determination of volatile components in solids and liquids.

(3) Columns are selected as needed. The new packed and capillary columns need to be aged prior to use to remove residual solvents and low molecular weight polymers. Columns that have not been used for a long time should be aged prior to use to stabilize the baseline.

(4) Column oven temperature The fluctuation of the oven temperature will affect the reproducibility of the chromatographic results. Therefore, the temperature accuracy of the column oven should be ±1°C and the temperature fluctuation is less than 0.1°C per hour.

(5) Detectors Suitable gas chromatography detectors include flame ionization detector (FID), thermal conductivity detector (TCD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD), electron capture detector (ECD), Mass Spectrometry Detector (MS), etc. The flame ionization detector responds well to hydrocarbons and is suitable for the detection of most drugs; nitrogen and phosphorus detectors are highly sensitive to compounds containing nitrogen and phosphorus; flame photometric detectors are highly sensitive to compounds containing phosphorus and sulfur; The capture detector is suitable for halogen-containing compounds; the mass spectrometry detector can also give the corresponding structural information for a component of the test article, which can be used for structural confirmation. Unless otherwise specified, flame ionization detectors generally use hydrogen as the fuel gas and air as the oxidizer gas. When using flame ionization detectors, the detector temperature should generally be higher than the column temperature, and should not be less than 150 °C, so as not to condensation, usually 250 ~ 350 °C.

(6) Data processing systems currently use computer workstations. The Pharmacopoeia stipulates that the chromatographic conditions stipulated under each species, except for carrier gas, detectors, types of fixative liquids, and specially designated chromatographic column materials, shall not be changed. Others include the column diameter, length, carrier grade, particle size, and fixed liquid coating. Concentration, carrier gas flow rate, column temperature, sample volume, detector sensitivity, etc., can be appropriately changed to suit specific species and meet system suitability test requirements. The general chromatogram was recorded in about 30 minutes.

2. System suitability test Unless otherwise specified, according to the provisions of high-performance liquid chromatography.

Third, the determination method (1) internal standard method plus a correction factor to determine the content of a certain impurity in the test or the main component (2) external standard method for the determination of an impurity in the test crystal or the main component content (3) area normalization The specific contents of the above methods 1 to 3 are the same as the corresponding provisions of high performance liquid chromatography.

IV. Precautions 1. General requirements The instrument must work under the specified conditions. The operation should be carried out according to the regulations. Otherwise, the result is inaccurate and meaningless.

2. Air System (1) The most common problem with air systems is leakage. The result of the leak is that the normal operation of the instrument is affected, and accidents are caused when the result is heavy. Leak detection must always be carried out. The leak detection method is to use a brush or a brush to pick up the soapy water, and the EPC system can automatically detect leaks to some extent.

(2) Regulating valve and needle valve should be adjusted slowly. When not in use, the regulator valve should loosen the handle (turn clockwise) to prevent the ripple tube from fatigue failure; the needle valve, on the other hand, should be in the “open” state (counterclockwise rotation) to prevent the valve seal ring Stick on the valve and damage it.

(3) When the pressure of the carrier gas cylinder is lower than 1.5×10 6 Pa, the new cylinder should be replaced because the pressure of the flow path is unstable and the purity of the mobile phase is reduced.

3. The main requirements for manual sample introduction are:

(1) The injection speed is faster. After sampling, hold the syringe with one hand, the other hand to protect the needle tip (to prevent bending during insertion), carefully insert the needle through the septum, then insert the syringe into the vaporizer as quickly as possible, and quickly inject the test sample into the vaporization chamber. Pull out the syringe quickly.

(2) The sample volume should be accurate.

(3) To avoid mutual interference between the test products. Before sampling, use the sample solvent to wash the needle at least 3 times (full 2/3 of the needle, and then discharge), and then use the sample to be analyzed to wash the needle at least 3 times.

(4) Select the right syringe. GC analysis is the most commonly used 10μl microsyringe, the injection volume is generally not less than 1μl. When the injection volume is controlled below 1μl, a 5μl or 1μl syringe should be used. Since this type of syringe is often used to draw the sample inside the needle tip, the needle should be pushed and pulled repeatedly to ensure that there are no air bubbles in the needle tip.

(5) The micro syringe must be cleaned with an organic solvent such as methanol after use.

4. Column system (1) The new column should generally be tested for its performance before analysis. The method is to check according to the factory test conditions. Avoid unnecessary economic losses.

(2) If the column is not used in a short time, it should be unloaded from the instrument. Both ends of the column should be plugged with a piece of silicone rubber (available for waste injection) and placed in the corresponding column packing box to prevent the stigma from being contaminated.

(3) The oven temperature should be reduced to less than 50°C before shutting down each time before shutting off the power supply and carrier gas. When the temperature is high, the carrier gas is cut off, and the diffusion of air into the column tube will cause the oxidative degradation of the fixed solution.

(4) Each time a new column is installed, the oven protection temperature must be reset before use to ensure that the temperature does not exceed the maximum operating temperature of the column.

(5) When the column is used for a period of time, the baseline may fluctuate or ghost peaks may occur due to the retention of high-boiling components in the column. At this point, the column should be aged.

(6) When aging the column, the post-column tubing should be disconnected from the detector to avoid contamination of the instrument tubing and detector.

5. The detector must be loaded with carrier gas before starting the instrument.

(1) TCD

1 Try to use a high-purity gas source (purity of 99.99%).

2 After ventilating for 0.5h or longer, the air in the air circuit can be driven off to prevent the oxidation of the hot wire components. After the first carrier gas is added, the bridge flow is added and the rated value must not be exceeded.

3 When the thermal conductivity cell is used for high temperature analysis, if it needs to stop, first cut off the bridge current, and then the temperature of the detection chamber is lower than 100°C, and then shut off the gas source.

(2) FID

1 Try to use a high-purity gas source (such as 99.99% purity of N2 or H2), the air must be fully purified through a 0.5nm molecular sieve.

2 Operate at optimum N2/H2 ratio and optimum air flow rate. The sensitivity of the FID is directly related to the ratio of the three. This ratio is close to or equal to 1:10:1.

3 The column must undergo a rigorous aging process.

4 The ion chamber should pay attention to external interference and ensure that it is in a shielded, dry and clean environment.

5 should pay attention to safety issues. When measuring flow, when measuring hydrogen, turn off the air and vice versa. Whenever the flame is extinguished for any reason, the hydrogen valve should be closed as soon as possible until the fault is rectified and the gas valve is re-ignited. The high-grade instrument has the function of automatic detection and protection. When the flame goes out, it can automatically shut off the hydrogen.

6 To prevent contamination, the detector temperature setting should be higher than the maximum working temperature of the column. The main way to eliminate pollution is to clean the nozzle surface and gas pipelines. The specific method is to remove the nozzle, followed by soaking with solvents of different polarities (such as acetone, chloroform, and ethanol) and soaking in an ultrasonic bath for more than 10 min. Fine stainless steel wire can also be used through the hole in the middle of the nozzle, or use an alcohol lamp to burn off the oil in the nozzle. When using silanized or silylated carriers and similar test articles, long-term use can cause nozzle clogging. The baseline is not good and the correction factor is not repeated. The nozzles and collectors should be carefully scrubbed with fine sandpaper and washed with methanol, acetone, etc.

6. Instrument connection (1) The host and recorder should be grounded. Instrument stability is directly related to grounding. Grounding method: Use a copper wire with an inner diameter of 1 to 1.5mm to weld on a 200mm x 200mm copper plate, and then bury the copper plate in a 0.5m deep wetland. Do not connect the ground directly to water pipes or radiators, nor can the center line of the power supply replace the grounding point.

(2) All grounding points of the instrument are connected together to make them equipotential so as not to introduce mutual interference signals.