Gas chromatography is a chromatographic separation and analysis method using gas as mobile phase.The vaporized sample is carried into the chromatographic column by the carrier gas (mobile phase). The molecular force of the stationary phase in the column is different from that of each component in the sample. The outflow time of each component from the chromatographic column is different, and the components are separated from each other.Using an appropriate identification and recording system, make a chromatogram that marks the time and concentration of each component flowing out of the chromatographic column.According to the peak time and sequence shown in the figure, the compounds can be qualitatively analyzed;According to the height and area of the peak, the compound can be quantitatively analyzed.It is characterized by high efficiency, high sensitivity, strong selectivity, fast analysis speed, wide application and simple operation.It is suitable for qualitative and quantitative analysis of volatile organic compounds.Non volatile liquid and solid substances can be analyzed after pyrolysis and gasification at high temperature.It can be used together with infrared absorption spectrometry or mass spectrometry, and chromatography can be used as a means to separate complex samples to achieve high accuracy.[3]It is an important analytical means to detect organic compounds in judicial expertise.
Gas chromatographyGas chromatography (GC) is a kind of chromatography.There are two phases in chromatography, one is mobile phase and the other isstationary phase。If liquid is used as mobile phase, it is called liquid phasechromatographicUsing gas as mobile phase is called gas chromatography.
Gas chromatography can be divided into two types due to the different stationary phases used. The one with solid adsorbent as the stationary phase is called gas-solid chromatography, and the one with monomer coated with stationary liquid as the stationary phase is called gas-liquid chromatography.
According to the chromatographic separation principle, gas chromatography can also be divided intoAdsorption chromatographyAnd distributive chromatography. In gas-solid chromatography, the stationary phase is adsorbent, gas-solid chromatography belongs to adsorption chromatography, and gas-liquid chromatography belongs to distributive chromatography.
According to chromatographic operation mode, gas chromatography belongs toColumn chromatography, depending on thechromatographic columnDifferent in thickness, it can be divided into general packed column and capillary column.Generally, the stationary phase is installed in a glass or metal tube with an inner diameter of 2-6mm.Capillary column can be divided into hollow capillary column andPacked capillary columnTwo.The hollow capillary column is to directly coat the fixed liquid on the inner wall of glass or metal capillary tubes with an inner diameter of only 0.1-0.5mm. The packed capillary column was developed in recent years. It is to put some porous solid particles into thick wall glass tubes, and then heat and draw them into capillaries with an inner diameter of 0.25-0.5mm.
In practice, gas chromatography is mainly gas-liquid chromatography.
detector
There are many kinds of detectors that can be used in gas chromatography, the most commonly used ones are flame ionization detector (FID) andThermal conductivity detector（TCD）。Both detectors are sensitive to many analytical components and can measure concentrations in a wide range.TCD is universal in nature and can be used to detect any substance except carrier gas (as long as their thermal conductivity is different from that of carrier gas at the temperature detected by the detector), while FID is mainly sensitive to hydrocarbons.FID is more sensitive to hydrocarbon than TCD, but it cannot be used to detect water.Both detectors are powerful.Since the detection of TCD is non-destructive, it can be used in series with destructive FID (connected before FID), thus giving two complementary analytical information to the same analyte.
somewhatGas chromatographIt is connected to the mass spectrometer and uses the mass spectrometer as its detector. This combined instrument is called gas chromatography-mass spectrometry (GC-MS)GC-MSAlso withNuclear magnetic resonance spectrometerThe latter is used as an auxiliary detector. This instrument is called gas chromatography mass spectrometry nuclear magnetic resonance (GC-MS-NMR).Some GC-MS-NMR instruments are also associated withInfrared spectrometerThe latter is used as an auxiliary detector. This combination is called gas chromatography mass spectrometry nuclear magnetic resonance infrared (GC-MS-NMR-IR).However, it must be pointed out that this situation is very rare, and most analytes can be solved by simple GC-MS.
principle
Gas chromatography
The gas chromatography system is composed ofPipe stringThe adsorbent (Table 1) inside or the stationary phase coated with liquid on the inert solid and the mobile phase of the gas continuously passing through the string are composed.After adding the sample to be separated and analyzed from one end of the stringComponentsDifferent adsorption or dissolution capacity, that is, the amount ofpartition coefficientThere are differences. When the components are distributed repeatedly in two phases and move forward with the moving phase, the speed of each component moving along the column will be different. The components with small distribution coefficient will stay in the stationary phase for a short time and can flow out from the end of the chromatographic column quickly.Plot the concentration c of each component flowing from the end of the column against the time t after injection, and the graph obtained is called chromatogram.When the chromatographic process is the flushing method, the chromatogram is shown in Figure 1.It can be seen from the chromatogram that the retention time tR required for the component after injection to its maximum concentration flowing out of the chromatographic column, the time tM for the component to pass through the chromatographic column space, and the retention time of the component in the columnAdjust retention timeThe relationship between t'R is: the ratio of t'R to tM in the formula indicates how many times the component stays in the stationary phase compared to the mobile phase, which is called the capacity factor k.
Gas chromatography
It can also be seen from the chromatogram that the chromatographic peak flowing out from behind the column is not a rectangle, but a curve approximating Gaussian distribution. This is because there are eddy diffusion, longitudinal diffusion, mass transfer resistance and other factors when components move in the chromatographic column, resulting in regional expansion.There are two ways to store the stationary phase in the chromatographic column. One is to put granular adsorbent in the column, or put inert solid particles coated with stationary liquid [carrier or support (Table 2)];The other is to apply the fixative orChemical crosslinkingOn the inner wall of the capillary column.The chromatographic column prepared by the former method is called packed chromatographic column, and the chromatographic column prepared by the latter method is calledCapillary column(or called open string).
chromatographic analysis
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overview
It can be seen from the chromatogram that the chromatographic peak is the result of the operation of the component in the chromatographic column, which is the basis for judging what the component is and its content. The chromatographic method is based on the moving speed and size of the chromatographic peak to obtain the qualitative and quantitative analysis results of the component.
qualitative analysis
Equation 4
Under given conditions, the adjusted retention time representing the moving speed of the component in the chromatographic column is an indicator to judge what the component is, that is, the t annoyance value of a component under given conditions must be a certain value (Figure 1).In order to avoid the inconvenience caused by changes in operating conditions such as carrier gas flow rate, column length, fixed liquid dosage, etc. when using t annoyance value as a qualitative analysis index, the relative retention value α of components orRetention indexFor qualitative analysis.The formula for calculating the retention index Ii of component i at a given column temperature and stationary phase is (Formula 4). In the formula, n and n+1 are the number of carbon atoms of normal alkanes flowing immediately before and after component i. Gas chromatography is the adjusted retention time of these two normal alkanes.
After chromatographic analysis of the sample, the pure substance is used for experiments under the same experimental conditions, or the literature is consulted to compare the qualitative indicators (α value, t annoyance value or I value) obtained from the two. If the sample and the pure substance have chromatographic peaks with the same qualitative indicator value, then there is this substance in this sample.
Since it can only be said that the same substance has chromatographic peaks with the same retention value, rather than that the chromatographic peaks with the same retention value are all one substance, in order to better conduct qualitative analysis of chromatographic peaks, other means are often used for direct qualitative analysis, such as gas chromatography andmass spectrumOr spectrometry, using a selective chromatographic detector, usingChemical ReagentsDetection and utilization of chemical reaction, etc[1]。
quantitative analysis
The size of chromatographic peak is determined by the height or area of the peak.The peak height can be measured manually, and the peak area can be expressed as the product of the peak height h and the peak width ω full at half the peak height.A=hω┩。New chromatographs all have integrators or microprocessors to give more accurate peak heights or areas.It should be noted that the corresponding chromatographic signal size (peak height or peak area) generated by components entering the detector varies with the type of detector used and the carrier gas, sometimes even affected by the substance concentration and instrument structure.Therefore, the chromatographic signal obtained must be corrected to be consistent with the amount of components, that is, the weight of components needs to be corrected with the following formula:
Equation 5
W=f ′ A where f ′ is theQuantitative correction factor。The weight of the corresponding component can be obtained from the chromatographic peak area (or peak height) according to the above formula, and the content Wi of component i in the sample can be further calculated using one of the following methods: ① The normalization method multiplies the chromatographic peak area of the component by their respective quantitative correction factors, and then calculates it according to the following formula (Formula 5). The advantage of this method is that the method is simple, and the impact of the sample amount and the carrier gas flow rate is small;The disadvantage is that the components in the sample must have their respective peak areas in the chromatogram, and the correction factors of each component must be known.
Equation 6
② Internal standard method: add a substance called internal standard substance to the sample for chromatographic analysis, and then use it for quantitative analysis of components.For example, take Wm grams of sample, add W φ grams of internal standard into it, and after chromatographic analysis, the chromatographic peak areas of the component to be determined and the internal standard are Ai and A φ, respectively. It can be deduced that (Formula 6) this method has no disadvantage of normalization method, but the disadvantage is that it requires accurately weighing the sample and internal standard, and selecting appropriate internal standard.
③ With the external standard method, under the condition that the analysis conditions such as the injection volume, chromatographic instrument and operation are strictly fixed, first use pure samples with different component contents to inject the same amount of samples for chromatographic analysis, and then calculate the relationship between the content and the chromatographic peak area with the following formula. This method is applicable to factory control analysis, especially gas analysis;The disadvantage is that it is difficult to achieve fixed injection volume and stable operating conditions[1]。
Analytical method
The analysis method is actually based on a specificGas chromatographic analysisA series of conditions used in.The establishment of analysis method is actually the process of determining the best conditions for a certain analysis.
In order to meet the requirements of a specific analysis, the conditions that can be changed include the temperature of the injection port, the temperature of the detector, the temperature of the chromatographic column and its temperature control program, the type of carrier gas and its flow rate, the stationary phase, the diameter of the column, the length of the column, the type of the injection port and its flow rate, the amount of sample, and the injection method.The detector may also have other adjustable parameters, depending on thedetectorType.Some gas chromatographs also have valves that can control the flow direction of samples and carrier gas. The opening and closing time of these valves may also have an important impact on the analysis effect.The instrument has twovalve, which is used to control the carrier gas to enter the quantitative tube.When the quantitative tube is filled with sample gas, switch the valve, and the carrier gas will pass through the quantitative tube.The pressure of the carrier gas will bring the sample into the chromatographic column for separation.
Carrier gas selection and carrier gas flow rate for gas chromatography
Typical carrier gases include helium, nitrogen, argon, hydrogen and air.Generally, the carrier gas selected depends on the type of detector.For example, the discharge ionization detector (DID) needs helium as the carrier gas.However, when analyzing a gas sample, the carrier gas is sometimes selected according to the parent of the sample. For example, when analyzing the mixture in argon, it is better to use argon as the carrier gas, because this can avoid the argon peak in the chromatogram.Safety and availability also affect the selection of carrier gas. For example, hydrogen is combustible, while high-purity helium is difficult to obtain in some areas.(See: helium distribution and production) In many cases, the detector not only determines the type of carrier gas, but also determines the purity of carrier gas (although the requirements for sensitivity also greatly affect the requirements for carrier gas purity).Generally speaking, the purity of carrier gas used in gas chromatography should be above 99.995%.Typical trade names used to identify purity include "zero gas level", "high purity (UHP) level", "4.5 level" and "5.0 level".The effect of carrier gas flow rate on the analysis is similar to that of temperature (see below).The higher the carrier gas flow rate, the faster the analysis speed, but the worse the resolution.Therefore, the selection of the optimal carrier gas flow rate, like the selection of the column temperature, requires a balance between the analytical speed and the resolution.The carrier gas flow rate of gas chromatographs produced before the 1990s is often controlled by the pressure at the carrier gas inlet (pressure before the column), and the actual carrier gas flow rate passes through the outlet of the columnElectronic flowmeterorSoap film flowmeterCarry out the measurement.Such a process is often complex, time-consuming and frustrating.During the whole operation process, the pressure in front of the column cannot be changed, and the air flow must be stable.The relationship between gas flow rate and pre column pressure can be determined byCompressible fluidPoiseuille equation.However, many modern gas chromatographs have been able to automatically measure the gas flow rate with circuits and control the flow rate by automatically controlling the pre column pressure.Therefore, the pressure and flow rate of carrier gas can be adjusted during operation.The pre column pressure/air flow control program (similar to the temperature control program) appears.
Type and flow rate of injection port for gas chromatography
The type of injection port and injection technology are usually related to the form of the sample (liquid, gaseous, adsorbed, solid) and whether there is solvent to be gasified.If the sample is well dispersed and its properties are known, it can be directly injected through the cold column head injection port;If it is necessary to evaporate some solvent, use split/non split injection port (usually use syringe for injection);Gas samples (such as those from cylinders) are usually injected with gas valve samplers.The adsorbed sample (such as on the adsorption tube) can be desorbed through an external (online or offline) desorption device (such as a capture purge system) or in a split flow/non split flow injector (using solid phase microextraction technology).
Sample size and injection technique of gas chromatography
Injection technologyOne tenth principle in gas chromatography The real gas chromatographic analysis process starts from the sample entering the chromatographic column.With the development of capillary gas chromatography, the injection technology is facing many practical problems.The on column injection technique is often used in packed columns but not in capillary columns.stayCapillary gas chromatographThe sample injection technology in should meet the following two conditions: the injection volume should not exceed the capacity of the column;Compared with the sample broadening caused by the unfolding process, the plug flow width after injection should be very small.If this requirement cannot be met, the separation capacity of the chromatographic column will decline.A general rule is that the injected volume, Vinj, and detector volume, Vdet, should be only one tenth of the volume of the part of the sample containing the analyte when leaving the column.The following are some general requirements that should be met by good injection techniques:The chromatographic column should reach its optimal separation efficiency;For a small amount of representative (typical) samples, the injection should be accurate and reproducible;The sample composition cannot be changed (for different boiling point, polarity, concentration andThermodynamic stabilityThere should be no difference in the injection process);It should be suitable for both trace analysis and samples with relatively high concentrations.
Selection of Column for Gas Chromatography
Column temperature and temperature control programA gas chromatograph thermostat that has been disassembled to display the internal capillary column The chromatographic column in the gas chromatograph is placed in the thermostat whose temperature is precisely controlled by the electronic circuit.(When the analyst says "column temperature", he actually refers to the temperature of the incubator. However, this difference is not important, so the two are not distinguished in the following.) The rate of sample passing through the chromatographic column is positively related to the temperature.The higher the column temperature, the faster the sample passes through the chromatographic column.However, the faster the sample passes through the column, the less interaction it has with the stationary phase, so the separation effect is worse.Generally, the selection of column temperature is the result of comprehensive consideration of separation time and separation degree.The method of constant column temperature during the whole analysis process is called constant temperature method.However, in most analysis methods, the column temperature increases gradually with the analysis process.The combination of initial temperature, heating rate (temperature "slope") and final temperature is called temperature control program.The temperature control program enables the analyte eluted earlier to be fully separated, and at the same time, it shortens the time for the analyte eluted later to pass through the chromatographic column.
Application of gas chromatography
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application
The substances that can be gasified without decomposition under the conditions allowed by the gas chromatograph can be determined by gas chromatography.Some thermally unstable substances or substances difficult to gasify can still be analyzed by gas chromatography through chemical derivatization.
It has been widely used in petrochemical industry, medicine and health, environmental monitoring, biochemistry, food detection and other fields
Analysis of biological materials such as body fluids and tissues: such as fatty acidstriglyceride, vitamins, sugars, etc.
3. Application in drug analysis
antiepilepticVolatile components in Chinese patent medicinesalkaloidDetermination of Class A drugs, etc.
advantage
① The separation efficiency is high and the analysis speed is fast. For example, more than 200 chromatographic peaks can be separated from the gasoline sample in two hours, and the general sample analysis can be completed in 20 minutes.
② The amount of sample used is small and the detection sensitivity is high. For example, the amount of gas sample used is 1ml, the amount of liquid sample used is 0.1 μ L, and the amount of solid sample used is several μ g.An appropriate detector can detect impurities in the range of 10 parts per million to several parts per billion.
③ Good selectivity, can separate and analyze azeotropic mixtures, substances with similar boiling points, someisotope, cis and trans isomers, ortho, meta, para isomers, optical isomers, etc.
④ It has a wide range of applications. Although it is mainly used to analyze various gases and volatile organic substances, it can also analyze high boiling point substances and solid samples under certain conditions.The main application fields include petroleum industry, environmental protectionClinical chemistry、Pharmacology, food industry, etc.
shortcoming
In the direct qualitative analysis of components, the known substances or data must be compared with the corresponding chromatographic peaks, or combined with other methods (such as mass spectrometry, spectrum) to obtain directly positive results.In quantitative analysis, it is often necessary to use known pure samples to correct the output signal after detection.
