Gas chromatography

A new separation and analysis technology

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Gas chromatography (GC) is a major scientific and technological achievement that emerged in the 1950s. This is a new separation and analysis technology, which has been widely used in industry, agriculture, national defense, construction and scientific research. Gas chromatography can be divided into gas-solid chromatography and gas-liquid chromatography.

Chinese name: Gas chromatography
Foreign name: gas chromatography
Abbreviation: GC

Time of occurrence: 1950s
Type: New separation and analysis technology
Classification: Gas solid chromatography and gas-liquid chromatography

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classification

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Gas chromatograph picture

Gas chromatography can be divided into gas-solid chromatography and gas-liquid chromatography. Gas solid chromatographic finger mobile phase Is gas, stationary phase It's solid chromatographic separation method. For example, activated carbon, silica gel, etc. are used as stationary phases. Gas liquid chromatography refers to the chromatographic separation method in which the mobile phase is gas and the stationary phase is liquid. For example, apply a layer of inert material diatomite Squalane Can separate and determine trace methane acetylene 、 propylene , propane and other impurities.

development

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Headspace gas chromatography

The development of GC chromatography is inseparable from the following two aspects. One is the development of gas chromatography separation technology, and the other is the development of other disciplines and technologies.

In 1952, James and Martin proposed Liquid chromatography And invented the first gas chromatography detector 。 This is a Packed column Export-oriented titration A device for detecting the separation of fatty acids. Plot the time with the volume of titration solution to obtain an integral chromatogram. Later, they invented the gas density balance. Ray proposed in 1954 Thermal conductivity meter , creating a modern era Gas chromatography detector The era of. From then on to 1957, it was the era of packed columns and TCD.

It was first proposed by Gloay in 1958 capillary In the same year, Mcwillian and Harley invented FID and Lovelock invented argon ionization The detector (AID) increases the sensitivity of the detection method by 2-3 orders of magnitude.

In the 1960s and 1970s, due to the development of gas chromatography technology, the column efficiency was greatly improved, and the development of environmental science and other disciplines, it was proposed that Trace analysis With high sensitivity and selectivity detector 。 For example, Lovelock proposed in 1960 Electron capture detector （ECD）； Brody and others invented FPD in 1966; In 1974, Kolb and Bischoff proposed the NPD of electric heating; In 1976, the American HNU company launched a practical window light ionization Detector (PID), etc. At the same time, due to the development of electronic technology, the original detector has made significant improvements in structure and circuit. For example, TCD has a constant current, constant hot wire temperature and constant hot wire temperature detection circuit; ECD has constant frequency to current, constant current pulse modulation detection circuit, etc., which improves the performance.

In the 1980s, due to elasticity Quartz capillary column The rapid and wide application of detector The requirements of small size, fast response, high sensitivity and good selectivity are put forward, especially the development of computer and software, which has greatly improved the sensitivity and stability of TCD, FID, ECD, and NPD, and greatly reduced the pool volume of TCD and ECD.

In the 1990s, due to the rapid development of electronic technology, computer and software, the production cost and complexity of MSD decreased, and its stability and durability increased, thus becoming the most common Gas chromatography detector one of. During this period, non radioactivity Of Pulse discharge electron capture detector （PDECD）、 Pulsed discharge helium ionization detector (PDHID) and Pulsed discharge photoionization detector (PDECD) and integrated Pulse discharge detector (PDD), four years later, Varian Company of the United States launched a commercial instrument, which is 100 times more sensitive than the usual FPD. In addition, the rapid development of fast separation technologies, such as fast GC and full two-dimensional GC, has led to the gradual maturity of fast GC detection methods.

characteristic

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Chromatographic outflow curve

Gas chromatography It refers to using gas as mobile phase Chromatographic method of. Since the sample is transmitted rapidly in the gas phase Components In mobile phase and stationary phase The balance can be reached instantaneously. In addition, there are many substances that can be used as stationary phases, so gas chromatography is a fast and separation efficiency High separation and analysis method. High sensitivity and selectivity have been adopted in recent years detector It has the advantages of high analytical sensitivity and wide application range.

principle

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Fig. 1 Gas phase analysis flow chart

GC mainly uses the boiling point Polarity And the difference of adsorption properties to realize the separation of the mixture. The process is shown in the flow chart of gas phase analysis in Figure 1.

The sample to be analyzed is in Vaporization chamber After vaporization inert gas (i.e carrier gas , also called mobile phase ）Bring in chromatographic column , the column contains liquid or solid stationary phase , because each Components The boiling point, polarity or adsorption performance of are different, and each component tends to form a partition or adsorption equilibrium 。 However, because the carrier gas is flowing, it is difficult to establish such a balance in fact. It is precisely because of the flow of carrier gas that the sample components are repeatedly distributed or adsorbed/desorbed during the movement. As a result, the components with high concentration in the carrier gas flow out of the chromatographic column first, and the components with high concentration in the stationary phase flow out after distribution. When the component flows out of the chromatographic column, it enters immediately detector 。 The detector can convert the sample components into electrical signals, and the size of the electrical signals is proportional to the amount or concentration of the measured components. When these signal amplification And when it is recorded, it is the gas phase Chromatogram Has.

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Schematic diagram of gas chromatography detector

Gas chromatograph It is composed of the following five systems: Air circuit system 、 Injection system 、 Separation system 、 Temperature control system 、 Detection and recording system 。

Components The key to separation is chromatographic column ； Whether the components can be identified after separation depends on detector Therefore, separation system and detection system are the core of the instrument.

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stay Petrochemical industry Most of the raw materials and products can be used Gas chromatography To analyze; It can be used to check the latent faults of transformers in the power sector; It can be used to monitor the quality of urban air and water in environmental protection; It can be used to monitor pesticide residues in crops in agriculture; It can be used to inspect and identify the quality of food in the commercial sector; In medicine, it can be used to study human metabolism and physiological function; It is used clinically to identify drug poisoning or disease types; It can be used to automatically monitor the gas in the sealed cabin of the spacecraft.

chromatographic Actually a Russian botanist Tswett (M.S. Tswett) was first discovered in 1901. In March 1903, Zwitt University of Warsaw The term "chromatography" (i.e. chromatography) was formally proposed in a report delivered at an academic conference held in Beijing, marking the birth of chromatography. He was therefore nominated for 1917 Nobel Prize in Chemistry Candidate for. At that time, Zwitt studied Liquid chromatography (LC) separation technology, gas chromatography appeared in the 1940s, the British A.J.P. Martin and Singh (R.L.M. Synge) is studying Distribution chromatography In the process of theory, the possibility of gas flow as chromatography was confirmed, and the birth of GC was predicted. Coincidentally, these two scientists won the Nobel Prize in Chemistry that year. Although the award-winning achievement is their contribution to the theory of distributive chromatography, some later generations believe that they won the award because of GC. This also explains the importance of GC technology to the development of the whole chemistry from another aspect.

Although GC appeared 50 years later than LC, its development in the next 20 years was beyond the reach of LC. From the launch of the first commercial GC instrument in 1955 to 1958 capillary The advent of GC column; From the research of capillary GC theory to the application of various detection technologies, GC quickly changed from laboratory research technology to conventional analytical means, almost forming a situation where GC is the leader in the field of chromatography. Since 1970, the development of electronic technology, especially computer technology, has made GC technology even more powerful. In 1979, the emergence of elastic quartz capillary column has made GC reach a new level. These are the results of high-tech development and the requirements of modern industrial and agricultural production. In turn, chromatography technology has greatly promoted the development of modern material civilization. Chromatographic technology plays an important role in every aspect of modern society. From heaven space shuttle , swimming in the water Aircraft carrier GC is used to monitor the gas quality in the cabin; From food and cosmetics in daily life to process control and product quality inspection of various chemical production, from substance identification in judicial inspection to geological prospecting In Oil and gas fields From disease diagnosis, medical analysis, archaeological excavation and environmental protection, GC technology is widely used.

In petrochemical analysis

Gas chromatograph (Figure 3)

In oil and Petrochemical analysis GC is very important in. From oilfield exploration and development to Oils The quality control is inseparable from GC, which is a method with low cost, fast speed, high resolution and sensitivity. American Society for Materials and Analysis (ASTM) has developed and continues to develop various GC standard methods for petrochemical analysis. The application of GC in petrochemical analysis mainly involves the following aspects:

1. Oil and gas field exploration Geochemistry analysis;

2． crude oil analysis;

3． Refinery gas analysis;

4． Simulated distillation ；

5． Oils analysis;

6. Analysis of single hydrocarbon;

7． Sulfur And nitrogen-containing compounds analysis;

8． Gasoline additive analysis;

9． Aliphatic hydrocarbon analysis;

10． aromatic hydrocarbon analysis;

11. Process chromatographic analysis 。

In environmental analysis

Gas chromatograph (Figure 4)

With the development of social economy and science and technology, human civilization is making rapid progress. On the other hand ecological environment It has caused more and more serious damage, and environmental pollution has become one of the biggest challenges facing mankind. All countries in the world are trying to control and treat various environmental pollution, for example, the United States Environmental Protection Agency (EPA) and the China Environmental Protection Agency have issued a large number of standard analysis methods. The application of GC in environmental analysis mainly includes the following aspects:

1． air pollution Analysis（ Toxic and harmful gases , gaseous sulfide, nitrogen oxide Etc.);

2. Drinking water analysis（ Polycyclic aromatic hydrocarbons 、 pesticide residue , organic solvents, etc.);

3. Water resources (including organic pollutants in fresh water, sea water and wastewater);

4． Soil analysis （ Organic pollutants ）；

5. Solid waste analysis.

In food analysis

1． Fatty acid methyl ester analysis;

2． pesticide residue analysis;

3． essence Spice analysis;

4． food additives analysis;

5． Food packaging materials in Volatiles Analysis of.

In pharmaceutical analysis

Schematic Diagram of Capillary Gas Chromatography

1． Estriol determination;

2. In urine Pregnanediol And progesterone determination;

3. In urine Cholesterol determination;

4． catecholamine Analysis of metabolites;

5. Ethanol, anesthetics and Amino acid derivatives Analysis of;

6. In the blood testosterone Analysis of;

7. Analysis of some volatile drugs.

Physical chemistry research

Gas Chromatogram (Figure 1)

1. Study on specific surface and adsorption performance;

2. Solution thermodynamics Research;

3． Vapor pressure Determination of;

4． Complexation constant determination;

5． reaction kinetics Research;

6． Virial coefficient determination.

Polymer analysis

1． Monomer analysis;

2． additive analysis;

3． copolymer Composition analysis;

4. Polymer structure characterization;

5. Analysis of impurities in polymer;

6． thermal stability Research.

method

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Gas chromatography packed column

Headspace sampling It is a special type of gas chromatography Sample method 。 Applicable to volatilization Sexual Components analysis. Precise weighing during measurement standard solution Put 3-5 ml and 3-5 ml of the test solution respectively into a head space sampling bottle with a volume of 8 ml. Heat each bottle in a 60 ℃ water bath for 30-40 min to make the residual solvent volatilize to saturation, and then put it in the same water bath Heated in empty tubes in Syringe Extract a proper amount of top air (usually 1 ml). Sample injection, repeat the injection for three times, and calculate and treat according to the direct solvent injection method^[1] 。

The headspace injection method allows the sample to be injected after volatilization of the substance to be tested, which can avoid sample removal extraction 、 Concentration And other steps can also avoid the pairing of non-volatile components of tested varieties Column chromatography But it is required that the substance to be tested has enough volatility 。

Headspace analysis is conducted through sample The gas composition above the matrix Components Content in the original sample. Its basic theoretical basis is that under certain conditions, the gas phase and Condensed phase There is a distribution equilibrium between the liquid and solid phases. Therefore, the composition of the gas phase can reflect the composition of the condensed phase. Headspace analysis can be regarded as a gas phase extraction method, which uses gas as "solvent" to extract volatile components in the sample. Therefore, headspace analysis is an ideal method for sample purification. In traditional liquid-liquid extraction and SPE, the sample is dissolved in liquid, and inevitably there will be interference analysis of some co extracts. Besides, the purity of the solvent itself is also a problem, which is Trace analysis Is particularly important. As a solvent, it can avoid unnecessary interference because high-purity gas is easy to obtain and the cost is low. This is one reason why the headspace gas phase is widely used.

As an analytical method, headspace analysis is simple at first. It only takes the gas part for analysis, which greatly reduces the interference or pollution of the sample itself to the analysis. Headspace is the most convenient sample processing method for GC analysis. Secondly, it can inject samples after gasification. Headspace analysis has different modes, which can be suitable for various samples by optimizing operating parameters. Thirdly, the sensitivity of headspace analysis can meet the requirements of regulations. Fourth, headspace sampling can relatively reduce the amount of solvent with high boiling point used to dissolve the sample and shorten the analysis time, but it requires high purity of the solvent, especially the impurities with low boiling point, otherwise it will seriously interfere with the determination. Finally, with GC's quantitative analysis With the combination of capabilities, headspace GC can completely carry out accurate quantitative analysis.

process

Headspace GC usually includes three processes: sampling, sampling and GC analysis.

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Gas Chromatogram (Figure 2)

1 Gas chromatography

Gas chromatography is a kind of gas chromatography mobile phase Of Column chromatography It can be divided into gas solid chromatography (GSC) and gas liquid chromatography (GLC) according to the different stationary phase states.

2 Principle of gas chromatography

The mobile phase of gas chromatography is inert gas ， Gas solid chromatography The adsorbent with large surface area and certain activity is used as the stationary phase. Dangduo Components Mixed sample of chromatographic column After a certain period of time, the running speed of each component in the chromatographic column will be different due to the different adsorption capacity of the adsorbent for each component. Components with weak adsorption are easy to be desorption Get down and leave the chromatographic column first detector The component with the strongest adsorption is the least likely to be desorbed, so it finally leaves the chromatographic column. In this way, each component can be separated from each other in the chromatographic column, and then enter the detector in order to be detected and recorded^[2] 。

3 Gas chromatography process

carrier gas It flows out of the high-pressure cylinder, depressurizes to the required pressure through the pressure reducing valve, purifies the carrier gas through the purification drying tube, and then passes through the pressure stabilizing valve and Rotameter Then, it flows through the gasification chamber at a stable pressure and a constant speed to mix with the gasified sample, and the sample gas is brought into the chromatographic column for separation. The separated components successively flow into the detector along with the carrier gas, and then the carrier gas is vented. The detector converts the change of substance concentration or mass into a certain electrical signal, which is amplified and recorded on the recorder to obtain the chromatographic outflow curve.

Qualitative analysis can be carried out according to the retention time of each peak obtained on the chromatographic outflow curve. According to the size of peak area or peak height quantitative analysis 。

four Gas chromatograph

It is composed of the following five systems: Air circuit system Sample injection system, separation system, temperature control system, detection and recording system.

Whether components can be separated depends on chromatographic column ； Whether the components can be identified after separation depends on detector Therefore, separation system and detection system are the core of the instrument.

5 Several common detectors of gas chromatograph

At present, there are many kinds of detectors, among which the commonly used ones are: Hydrogen flame ionization detector （FID）、 Thermal conductivity detector （TCD）、 Nitrogen and phosphorus detector （NPD）、 Flame photometric detector （FPD）、 Electron capture detector (ECD), etc.

(1) Hydrogen flame ionization detector (FID): It is designed according to the fact that the conductivity of the gas is proportional to the concentration of charged ions contained in the gas. Generally, component steam is not conductive, but under the action of energy, component steam can be ionized to generate charged ions and conduct electricity.

Working principle: When the carrier gas (sample) flowing out of the chromatographic column flows through the hydrogen flame with a temperature of up to 2100 ℃, the organic component to be measured will ionize in the flame, so that a certain amount of positive and negative ions will appear between the two electrodes. Under the effect of the electric field, the positive and negative ions will be collected by the corresponding electrodes respectively. When the carrier gas does not contain the substance to be measured, there are few ions in the flame, that is, the base current is very small, about 10-14A. When the organic matter to be measured passes through the detector, the ionized ions in the flame increase and the current increases (but very weak 10-8~10-12A). A large voltage signal needs to be obtained after high resistance (108~1011), and then amplified by the amplifier, so that the chromatographic peak with enough size can be displayed on the recorder. The current is proportional to the mass of the component to be measured entering the detector in unit time within a certain range, so the flame ionization detector is Mass detector 。

The ionization potential of flame ionization detector is lower than H two The flame ionization detector can only analyze organic substances (carbon containing compounds), which is not suitable for analysis inert gas , air, water, CO, CO two 、CS two 、NO、SO two And H two S, etc.

（2） Thermal conductivity detector (TCD): also called thermal conductivity pool or hot wire detector, is Gas chromatography The most commonly used detector. A heat conduction detector based on the principle that different components and carrier gases have different thermal conductivity.

Working principle: the working principle of the thermal conductivity detector is based on the different thermal conductivity of different gases. The hot wire has the characteristic that the resistance varies with temperature. When a constant DC current is connected to the overheated conducting cell, the hot wire is heated. Due to the heat conduction of the carrier gas, part of the heat of the hot wire is taken away by the carrier gas, and part of the heat is transferred to the pool body. When the heat produced by the hot wire is balanced with the heat lost, the temperature of the hot wire will be stable at a certain value. At this time, the hot wire resistance is also stable at a certain value. Since the reference cell and the measuring cell are filled with pure carrier gas, and the same carrier gas has the same thermal conductivity, the resistance values of both arms are the same, the bridge is balanced, and there is no signal output. The recording system records a straight line. When a sample enters the detector, the pure carrier gas flows through the reference cell, and the carrier gas carries the component gas through the measuring cell. Because the thermal conductivity of the binary mixture of the carrier gas and the component to be measured is different from that of the pure carrier gas, the heat dissipation in the measuring cell changes, resulting in a difference between the hot wire resistance values in the reference cell and the measuring cell hole, and the bridge loses balance, The detector has a voltage signal output, and the recorder draws the chromatographic peak of the corresponding component. The greater the concentration of the component to be measured in the carrier gas, the more significant the change in the gas thermal conductivity in the measuring cell, the more significant the change in temperature and resistance values, and the stronger the voltage signal. At this time, the output voltage signal is proportional to the sample concentration, which is the quantitative basis of the thermal conductivity detector^[3] 。

The thermal conductivity cell (TCD) detector is a general non-destructive concentration type detector, which has been widely used in practical work Gas chromatography detector one of. TCD is particularly applicable to the analysis of gas mixtures Hydrogen flame ionization detector TCD is more unique in the analysis of inorganic gases that cannot be directly detected. TCD will not damage the monitored components during the detection process, which is conducive to the collection of samples, or combined with other instruments. TCD can meet the requirements of high peak and quantitative analysis in industrial analysis, and is very suitable for control analysis in factories.

（3） Nitrogen and phosphorus detector (NPD): It is a mass detector, which is suitable for the analysis of nitrogen and phosphorus compounds with high sensitivity and selectivity. It has a structure similar to that of FID, but is coated with an alkali metal salt such as Na two SiO three ，Rb two SiO three Compound like ceramic beads are placed between the burning hydrogen flame and the collecting electrode. When the sample steam and hydrogen flow pass through the surface of the alkali metal salt, the compounds containing nitrogen and phosphorus will obtain electrons from the reduced alkali metal vapor, and the alkali metal losing electrons will form salts and then deposit on the surface of the ceramic beads^[3] 。

Working principle: above the nozzle of the NPD detector, there is a rubidium bead heated by a large current. The alkali metal salt (rubidium bead) is heated and escapes a small amount of ions. The rubidium bead is applied with a - 250V polarization voltage to form a DC electric field with the cylindrical collector. The small amount of ions that escape move directionally under the action of the DC electric field, forming a small current that is collected by the collector, which is the basic current. When organic compounds containing nitrogen or phosphorus flow out of the chromatographic column, thermal ionization reaction is generated around the rubidium bead, which greatly improves the ionization degree of alkali metal salt (rubidium bead). The generated ions move directionally under the action of DC electric field, and the small current formed is collected

Collector collection, signal amplification by micro current amplifier, and processing by integrator to achieve qualitative and quantitative analysis.

Nitrogen and phosphorus detector With long service life and high sensitivity, it can detect 5 × 10 -13 G/s azobenzene compounds, 2.5 × 10 -13 G/s, such as Marathon pesticide. It has a high response to nitrogen and phosphorus compounds. However, some response values to other compounds are 10000~100000 times lower. Nitrogen and phosphorus detectors are widely used in pesticide, petroleum, food, medicine, spice, clinical medicine and other fields.

（4） Flame photometric detector (FPD): It is a device that uses certain external conditions (that is, burning under hydrogen rich conditions) to promote some substances to produce chemiluminescence. Through wavelength selection, optical signal reception, and amplification, it links substances, their content and characteristic signals. Mainly composed of combustion chamber, monochromator Photomultiplier tube , quartz chip (protective filter), power supply, amplifier, etc.

Working principle: When compounds containing S and P enter the hydrogen flame ion chamber, they burn in a hydrogen rich flame. Organic sulfur compounds are first oxidized to SO2, reduced to S atoms by hydrogen, and then generate excited S2 * molecules. When they return to the ground state, they emit 350~430nm characteristic molecular spectrum, with the maximum absorption wavelength of 394nm. Through the corresponding filter Photomultiplier tube Receive, and record the chromatographic peak with a recorder after amplification. This detector has a logarithmic relationship (proportional to the square root of the concentration of S containing compounds) instead of a linear relationship with compounds containing S.

When phosphorus containing compounds are oxidized to phosphorus oxides, they are reduced to HPO lobes by H in a hydrogen rich flame. This lobe is excited and emits a characteristic molecular spectrum of 480~600nm, with the maximum absorption wavelength of 526nm. Because the intensity of the emitted light (response signal) is proportional to the HPO concentration.

（5） Electron capture detector (ECD): The early electron capture detector is made of two parallel electrodes. Currently, radioactive coaxial electrodes are commonly used. In the detector tank, a The stainless steel rod As the positive pole, a cylindrical radioactive source (3H, 63Ni) is used as the negative pole, and current or pulse voltage is applied between the two poles^[4] 。

Working principle: when pure carrier gas (usually high purity N two ）When entering the detection room, the positive ion (N two + ）And electron e - The generated positive ions and electrons move to the two poles respectively under the action of electric field, forming a current of about 10-8A - basic current. After the sample is added, if the sample contains an element with strong electronegativity, that is, a molecule that is easy to combine electrons, these low-energy electrons will be captured, and negatively charged anions will be generated (electron capture). These anions will combine with the positive ions generated by the ionization of the carrier gas to generate neutral compounds, which will be taken out of the detection room by the carrier gas, thus reducing the base current, generating negative signals, and forming inverted peaks. The inverted peak size (high and low) is proportional to the component concentration, so the electron capture detector is a concentration type detector. The minimum detection concentration can reach 10-14 g/ml, and the linear range is about 103^[5] 。

Electron capture detector It is a highly selective detector. High selectivity means that it only responds to substances containing elements with strong electronegativity, such as compounds containing halogen, S, P, N, etc. The stronger the electronegativity of the material, the higher the detection sensitivity.

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