What Is Gas Chromatography?

Gas chromatograph is an instrument that uses chromatographic separation technology and detection technology to perform qualitative and quantitative analysis on complex mixtures of multiple components. It can be used to analyze the organic compounds in the soil that are thermally stable and have a boiling point not exceeding 500 ° C, such as volatile organic compounds, organic chlorine, organic phosphorus, polycyclic aromatic hydrocarbons, phthalates, and so on.

Chinese name: Gas Chromatograph

Foreign name: Gas Chromatograph
Category Name: medical instruments

Basic structure of gas chromatograph

There are many types of gas chromatographs with different functions, but their basic structures are similar. A gas chromatograph is generally composed of a gas path system, a sampling system, a separation system (chromatographic column system), a detection and temperature control system, and a recording system ^[1] .

Pneumatic system

The gas circuit system includes a gas source, a purification and drying pipe, a carrier gas flow rate control and a gasification device, and is a closed pipeline system for continuous operation of the carrier gas. Through this system, a pure, stable flow of carrier gas can be obtained. Its air tightness, accuracy of flow measurement and stability of carrier gas flow rate are all important factors affecting the performance of gas chromatographs.

Carrier gases commonly used in gas chromatography include hydrogen, nitrogen, and argon, with a purity of more than 99%. They are chemically inert and do not react with related substances. In addition to the choice of carrier gas, the effect on the column efficiency must be considered, and it must be matched with the analysis object and the detector used.

2. Injection system

(1) Sampler: Different samplers are used depending on the state of the sample. Microinjectors are generally used for the injection of liquid samples. Gas sample injection is usually a push-pull six-way valve or rotary six-way valve that is configured with the chromatograph itself. Solid samples are usually dissolved in appropriate reagents and then injected with a microsyringe

(2) Gasification chamber: The gasification chamber is generally made of a stainless steel tube with a heating wire wound around the tube. Its role is to instantly vaporize a liquid or solid sample into vapor. In order to make the sample instantaneously vaporize in the gasification chamber without decomposition, it is required that the gasification chamber has a large heat capacity and no catalytic effect.

(3) Heating system: It is used to ensure the gasification of the sample. Its role is to instantly vaporize the liquid or solid sample before entering the chromatographic column, and then quickly and quantitatively transfer to the chromatographic column.

3. Separation system

The separation system is the heart of the chromatograph. Its role is to separate the components of the sample. The separation system consists of a column chamber, a chromatographic column, and a temperature control component. The chromatographic column is the core component of the chromatograph. There are two main types of chromatographic columns: packed columns and capillary columns (open-tube columns). Post materials include metal, glass, fused quartz, polytetrafluoro, and the like. The separation effect of a chromatographic column is not only related to the column length, diameter, and shape, but also to many factors such as the preparation of the stationary phase and column packing technology used, and the operating conditions.

Detection system

A detector is a device that converts the concentration or mass (content) of each component separated by a chromatographic column into an easily measurable electrical signal (such as voltage, current, etc.) and performs signal processing. It is the eye of a chromatograph . Usually consists of a detection element, an amplifier, and a digital-to-analog converter. The components separated by the chromatographic column enter the detector in turn, and are converted into corresponding electrical signals according to the change of their concentration or mass with time. The amplified chromatograms are recorded and displayed. The performance of the detector will directly affect the accuracy of the final analysis result of the chromatographic instrument.

According to the response principle of the detector, it can be divided into a concentration type detector and a mass type detector.

(1) Concentration type detector: It measures the instantaneous change of the component concentration in the carrier gas, that is, the response value of the detector is proportional to the component concentration. Such as thermal conductivity detector, electron capture detector.

(2) Mass type detector: It measures the change of the speed of the sample carried in the carrier gas entering the detector, that is, the response signal of the detector is proportional to the mass of the component entering the detector per unit time. Such as hydrogen flame ionization detector and flame photometric detector.

5. Temperature control system

In gas chromatography, temperature control is an important indicator, which directly affects the separation performance of the column, the sensitivity and stability of the detector. The temperature control system mainly refers to the temperature control of the gasification chamber, the chromatography column and the detector. In the gasification chamber, instantaneous gasification of the liquid sample must be ensured; in the column chamber, the temperature required for separation must be accurately controlled; when the sample is complex, the temperature of the separation chamber needs to be controlled according to a certain procedure, and the components are at the optimal temperature Separation; the detector does not condense as the separated components pass through.

There are two types of temperature control methods: constant temperature and programmed temperature.

(1) Constant temperature: For simple samples whose boiling range is not too wide, the constant temperature mode can be used. General gas analysis and simple liquid sample analysis use the constant temperature mode.

(2) Program temperature rise: The so-called program temperature rise refers to the linear or non-linear change of the temperature of the chromatographic column from low temperature to high temperature in an analysis cycle. Flow, which improves separation and shortens analysis time. For complex samples with a wide boiling range, if it is difficult to achieve a good separation at a constant temperature, a programmed temperature method should be used.

6. Recording system

The recording system records the detection signal of the detector and performs quantitative data processing. Generally, an automatic balance electronic potentiometer is used for recording and the chromatogram is drawn. Some chromatographs are equipped with an integrator that measures the area of a chromatographic peak and directly provides accurate data for quantitative analysis. The advanced gas chromatograph is also equipped with an electronic computer, which can automatically process the chromatographic analysis data.

Gas chromatograph works

Gas chromatographs use gas as the mobile phase (carrier gas). After the sample is injected into the injector by a microsyringe, it is carried by the carrier gas into a packed or capillary column. Due to the difference in the distribution or adsorption coefficient of each component in the mobile phase (gas phase) and stationary phase (liquid phase or solid phase) in the chromatographic column, under the flushing of the carrier gas, each component repeatedly repeats between the two phases. Sub-distribution separates each component in the column, and then uses a detector behind the column to detect each component in sequence according to the physical and chemical characteristics of the component ^[1] .

The signal given by the detector for each component appears as individual peaks on the recorder, called chromatographic peaks. The maximum value on the chromatographic peak is the basis for qualitative analysis, and the area encompassed by the chromatographic peak depends on the content of the corresponding component, so the peak area is the basis for quantitative analysis. After a sample of the mixture is injected, the curve recorded by the recorder is called a chromatogram. Qualitative and quantitative analysis results can be obtained by analyzing the chromatogram.

Gas chromatograph applications

Gas chromatography is a chromatographic analysis method using gas as the mobile phase. It is mainly used to separate and analyze volatile substances. Gas chromatography has become one of the most important methods of separation and analysis, and has been widely used in medical and health, petrochemical, environmental monitoring, biochemical and other fields. The gas chromatograph has the advantages of high sensitivity, high efficiency, high selectivity, fast analysis speed, small amount of required samples, and wide application range.

A gas chromatograph, after the analysis sample is vaporized in the injection port, is carried by the carrier gas into the chromatographic column, and the components are separated by a chromatographic column with different retention properties for the components in the mixture to be detected, which are sequentially introduced into the detector. In order to obtain the detection signal of each component. According to the sequence of introduction of the detector, after comparison, what components can be distinguished, and the content of each component can be calculated based on the peak height or peak area. Commonly used detectors are: thermal conductivity detector, flame ionization detector, helium ionization detector, ultrasonic detector, photoionization detector, electron capture detector, flame photometric detector, electrochemical detector, mass spectrometer Detector, etc.

How to use gas chromatograph

General analysis process of gas chromatograph: The carrier gas flows from the high-pressure steel cylinder, and after the pressure is reduced to the required pressure by the pressure reducing valve, the carrier gas is purified through the purification and drying pipe, and then the pressure regulator and the rotor flow meter are used to stabilize The pressure and constant speed flow through the gasification chamber and mix with the gasified sample, and substitute the sample gas into the chromatographic column for separation. The separated components flow into the detector with the carrier gas, and then the carrier gas is vented. The detector converts the change in the concentration or mass of the substance into a certain electrical signal, records it on a recorder after amplification, and obtains a chromatographic outflow curve. According to the retention time of each peak on the chromatographic effluent curve, qualitative analysis can be performed, and quantitative analysis can be performed based on the peak area or peak height ^[2] .

Gas chromatograph routine maintenance

In order to ensure the normal operation of the gas chromatograph and ensure the accuracy and timeliness of the analysis data, regular maintenance of the gas chromatograph is required ^[2] .

Gas source inspection: Check whether the generator or gas cylinder is in a normal state; check the dehydration filter, activated carbon and deoxidation filter, and replace the fillers regularly.
Pipeline leakage: Check the pipeline regularly for leaks. You can use soap drops to check the interface.
Maintenance of the gasification chamber: The gasification chamber includes: a sampling chamber nut, a septum purge outlet, a carrier gas inlet, a split gas outlet, and a sample liner. Different parts have different maintenance methods:

The sampling chamber nut, septum purge outlet, carrier gas inlet, and split gas outlet need to be cleaned periodically according to the manufacturer's requirements: Remove these components from the gasification chamber and place them in a beaker containing acetone solution Soak and sonicate for 2 hours, use after drying; if damaged, replace it in time;
The sample liner must be cleaned regularly, first with a washing solution, then soaked with acetone solution, and then blow dried with a hair dryer for later use, and timely add quartz wool;
If damaged, it should be replaced in time.

Detector maintenance: The detector's collector, detector receiving tower, flame nozzle, detector base, chromatographic column nut, etc. must be cleaned with acetone solution, usually ultrasonically for 2 hours, until cleaned, use a hair dryer after cleaning Blow dry and set aside.
Maintenance of the column thermostat: The shell and volume of the column thermostat can be scrubbed with absorbent cotton dipped in ethanol.
Maintenance cycle: The maintenance cycle of the gas chromatograph is generally set to 3 months. In actual work, the maintenance period can be appropriately extended or shortened according to the workload and operation of the instrument.

Gas Chromatograph Instrument Maintenance

Purge and clean the inside of the instrument. After shutting down the gas chromatograph, open the side and rear panels of the instrument, and use the instrument air or nitrogen to purge the dust inside the instrument. Use a soft brush to clean the dusty or difficult places Work with it. After the purging is completed, scrub the water or organic solvent where the organic matter is contaminated inside the instrument. Wipe the water-soluble organic matter with water first, and then use the organic solvent to treat the areas that cannot be thoroughly cleaned. Organic compounds that react with water are cleaned with organic solvents that do not react with them, such as toluene, acetone, carbon tetrachloride, etc. Note that the surface or other parts of the instrument must not be corroded or re-contaminated during the cleaning process.

1. Maintenance and cleaning of circuit boards:

Before the gas chromatograph is ready for maintenance, cut off the power of the instrument, first use the instrument air or nitrogen to purge the circuit board and the circuit board slot, and use a soft brush to clean the dusty parts of the circuit board and the slot during the purge Clean up carefully. Wear gloves as much as possible during operation to prevent static electricity or sweat on your hands from affecting some components on the circuit board. After the purge is complete, you should carefully observe the use of the circuit board to see if the printed circuit board or electronic components are significantly corroded. Electronic components and printed circuits that are contaminated with organic matter on the circuit board are carefully wiped with absorbent cotton dipped in alcohol, and the circuit board interface and slot parts must also be wiped.

2. Cleaning of glass liner and diverter plate:

Carefully remove the glass liner from the instrument. Use tweezers or other small tools to carefully remove the glass wool and other impurities from the liner. Do not scratch the liner surface during the removal process. If conditions permit, the glass liner that has been cleaned up can be cleaned with ultrasonic in organic solvent and then used after drying. It can also be washed directly with organic solvents such as acetone and toluene. After cleaning, it can be used after drying.

The most ideal method for cleaning the splitter plate is to sonicate it in a solvent and use it after drying. You can also choose a suitable organic solvent for cleaning: after removing the splitter plate from the injection port, first wash it with an inert solvent such as toluene, then wash it with an alcohol solvent such as methanol, and use it after drying.

3. Cleaning of shunt lines:

When the gas chromatograph is used for the analysis of organics and polymer compounds, many organics have a low freezing point. During the process of venting the sample from the gasification chamber through the split line, some organics are solidified in the split line. After the gas chromatograph has been used for a long time, the inner diameter of the split line gradually becomes smaller, or even completely blocked. After the split line is blocked, the instrument inlet shows abnormal pressure, poor peak shape, and abnormal analysis results. During the maintenance process, regardless of whether the shunt pipeline can be judged in advance for clogging, the shunt pipeline needs to be cleaned. Generally, organic solvents such as acetone and toluene are used for the cleaning of the shunting pipeline. Sometimes, the shunting of the severely blocked shunting pipeline is difficult to clean with a simple cleaning method, and some other auxiliary mechanical methods are needed to complete it. You can select a suitable thickness of steel wire to simply clear the shunt line, and then clean it with organic solvents such as acetone and toluene. Because it is not easy to make an accurate judgment of the condition of the splitting part in advance, it is necessary for the manual separation of the gas chromatograph to clean the splitting line during the maintenance process.

3. Inlet cleaning:

For EPC-controlled gas chromatographs, due to long-term use, it is possible for some small sample pads to enter the interface between the EPC and the gas pipeline, which may cause blockage of the EPC part or cause changes in the inlet pressure at any time. Therefore, the EPC part of the instrument should be inspected as much as possible during each maintenance process, and it should be washed with organic solvents such as toluene and acetone, and then dried.

Cleaning the inlet During maintenance, it is necessary to clean the glass liner, split plate, split line, and EPC of the inlet of the gas chromatograph. Due to sample injection and other reasons, some organic compounds may form on the outside of the injection port at any time. You can use absorbent cotton dipped in organic materials such as acetone and toluene to wipe the injection port initially, and then mechanically remove the organic matter that cannot be wiped off. Caution must be taken during the removal of solidified organic matter, so as not to damage the instrument components. After the solidified organic matter is removed, the instrument parts are carefully wiped with an organic solvent.

4. Cleaning of TCD and FID detectors

The TCD detector may be contaminated by sediment from the column or other substances entrained in the sample during use. Once the TCD detector is contaminated, the baseline of the instrument jitters and noise increases. It is necessary to clean the detector. HP's TCD detector can be thermally cleaned. The specific method is as follows: Turn off the detector, remove the column from the detector connector, plug the detector connector in the oven with a dead plug, and set the flow rate of the reference gas. When the temperature is 20 30ml / min, set the detector temperature to 400 , and heat-clean for 4 8h. It can be used after cooling down.

Domestic or Nissan TCD detectors can be contaminated by the following methods. After the instrument is stopped, remove the gas inlet of the TCD, and use a 50ml syringe to sequentially inject acetone (or toluene, according to the chemical nature of the sample) anhydrous ethanol and distilled water from the air inlet 5 to 10 times. Blow slowly from the air inlet with a suction ball to blow out impurities and residual liquid, and then reinstall the air inlet connector. After starting up, raise the column temperature to 200 ° C and the detector temperature to 250 ° C. Pass the ratio analysis operation The carrier gas is 1 to 2 times larger until the baseline is stable.

For severe pollution, the air outlet can be blocked with a dead plug. Fill the air inlet with acetone (or toluene, you can choose a different solvent according to the chemical properties of the sample), keep it for about 8 hours, drain the waste liquid, and then treat it as described above. .

FID detectors have good stability in use, relatively low requirements for use, and widespread use. However, during long-term use, problems such as detector nozzles and collecting polar carbon are prone to occur, or organic matter is deposited on the nozzles or collecting electrodes. And so on. For FID carbon deposits or organic matter deposition, you can first clean the detector nozzle and collector with organic solvents such as acetone, toluene, and methanol. When the carbon deposit is thick and cannot be cleaned, you can carefully grind the thick part of the detector with fine sandpaper. Be careful not to damage the detector during the sanding process. After the preliminary grinding is completed, the contaminated part is further wiped with a soft cloth, and finally washed with an organic solvent, which can generally be eliminated.

Common Gas Chromatograph Failures

1. Failure of chromatographic peaks after injection

The gas chromatograph has no change in the detection signal after the injection, the instrument does not show a peak, and the output is still straight. In this case, the inspection should be performed one by one in the order from the sample needle, the injection port to the detector ^[3] .

(1) First check if the syringe is blocked. If there is no problem,

(2) Check the graphite gasket of the inlet and detector for tightness and airtightness.

(3) Then check the column for any leaks.

(4) Finally, observe whether the detector exit is unblocked.

The unblocking of the detector exit is very important. Some people will encounter such problems in the work: the instrument was working normally the previous day, but there was no response to the peak signal after starting the next day. Check that the inlet, syringe, gasket, and column are all normal, but the peak does not appear. I accidentally find that the inlet pressure of the inlet does not reach the set value, which is always high. Then I suspect that the ECD tester outlet is not working properly. Unblocked. Because ECD emissions are radioactive, ECD outlets are directed outdoors. It was at the turn of autumn and winter, and the rainwater was frozen after entering the ECD outlet, which caused the ECD outlet of the instrument to be blocked, the column head pressure remained high, and the gas could not flow in the gas path, so it could not carry samples to the detector, so Out of the peak.

Baseline question

Gas chromatographic baseline fluctuations and drift are both baseline problems. Baseline problems can increase measurement errors and sometimes cause the instrument to malfunction.

(1) When encountering a baseline problem, you should first check whether the instrument conditions have changed and whether there are new gas cylinders and equipment accessories in the near future.

(2) If there are changes or conditions change, first check whether the baseline problems are caused by these changes. Generally, such changes are often the cause of baseline problems. Some people have encountered this situation at work: the new carrier gas is not pure enough, and the baseline gradually rises after the carrier gas is changed (due to the carrier gas purification tube, the baseline does not change immediately). After turning on the next day, the baseline was very high, accompanied by strong baseline jitter, and all peaks were lost in noise and could not be detected. After inspection, the problem appeared on the newly changed carrier gas. After the carrier gas was replaced again, it returned to normal immediately.

(3) When the above reasons that may cause baseline problems are eliminated, the sampling pad should be checked for aging (it is a good habit to replace the sampling pad regularly);

(4) Should the quartz wool be replaced?

(5) Whether the liner is clean. It is worth mentioning that when cleaning the liner, it can be fully soaked with the solvent at the final volume of the test, then cleaned with ultrasonic waves for a few minutes, and then placed in a high-temperature furnace to a temperature slightly higher than the working temperature, and then reinstalled.

(6) In addition, detector contamination may also cause baseline problems, which can be solved by cleaning or thermal cleaning.

3. Failures that cause peak loss

There are two reasons for the peak loss: one is pollution in the gas path, and the other may be that the peaks are not separated.

The first case can be solved by multiple dry runs and cleaning the gas path (inlet, detector, etc.).

(1) In order to reduce the pollution to the gas path, the following measures can be adopted: the final stage of the program temperature rise should have a high temperature cleaning process;

(2) The sample injected into the inlet should be clean;

(3) Reduce the use of oils with high boiling points;

(4) Use the highest possible inlet temperature, column temperature, and detector temperature.

The second case of peak loss is that the peaks are not separated. In addition to the above reasons, it may also be caused by the reduction of column efficiency caused by system pollution, or due to column aging, but the peak loss caused by column aging is gradual ,slowly. False peaks are generally caused by system pollution and gas leaks, and the solution is also checked by checking for gas leaks and removing pollution. In normal work, the baseline situation in normal time should be recorded for reference during maintenance.

Here are just three methods for troubleshooting common problems in the work. There are many points of failure of the gas chromatograph and the time to recover from the failure is long. Therefore, the key to the maintenance of the equipment lies in the correct analysis of the cause. Every time you check a part, you need to compare the results of the analysis before and after, so as not to enlarge the problem. I believe that through repeated attempts, the problem will be successfully solved.