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Introduction and application of headspace gas chromatography
Top air phase chromatography concept
Headspace gas chromatography is an indirect method for the gas chromatographic analysis of volatile constituents in liquids or solids. It is carried out in a closed system with a thermodynamically balanced vapor phase and the sample being analyzed. This method, from the perspective of a gas chromatograph, is a sampling system called a "headspace sampling system." Main features of headspace gas chromatography
In the previous headspace analysis, most of the volatile components in liquid (or solid) samples were determined by infrared spectroscopy, ultraviolet spectroscopy, mass spectrometry, and the like. Due to the limited sensitivity of these test methods and the lack of ability to separate the mixtures, it is difficult to obtain the desired analytical results when several components are present in the gas.
Since the advent of gas chromatography, many chromatographers have applied gas chromatography to headspace analysis. Gas chromatography has the advantages of good separation efficiency, high sensitivity, low sample consumption, fast analysis speed, and wide application range. To make the headspace analysis show a new prospect. The top air phase chromatography analysis has the following characteristics.
1. Widely applicable
Headspace gas chromatography can analyze both volatile components in liquid samples and volatile materials in solid samples. It is suitable for both one-component volatile gas samples and complex volatile groups. The mixture is separated and analyzed; it can be used for both constant head air sample analysis and low content of volatile components.
2. Quick and easy
Headspace gas chromatography analysis directly takes a volatile gaseous sample of a liquid sample (or solid sample) and sends it to an air-phase chromatograph for separation and analysis. In many cases, the sample preparation operation can be omitted, so the method is faster and easier than ordinary chromatographic analysis.
3. Low detection limit
Headspace gas chromatography sometimes yields lower detection limits than conventional GC analysis because it avoids the solvent interference caused by conventional sample preparation and the specificity of the headspace sample. . For liquid or solid samples that are easily decomposed and cannot be directly injected for analysis, they have more practical value.
Basic type of headspace gas chromatography
1. Static headspace gas chromatography
Static headspace gas chromatography is a gas chromatographic analysis of volatile gaseous constituents that are in equilibrium with a liquid (or solid) sample in a closed, thermostated system.
A sampling device for static headspace gas chromatography analysis, using a syringe to take a gas sample, and then sent to a gas chromatography column for separation analysis. The analytical instrument used was a general laboratory gas chromatograph equipped with a gas six-way valve sampling device.
2. Dynamic headspace gas chromatography
Dynamic headspace gas chromatography, also known as purge-trap gas chromatography, uses an inert gas (usually nitrogen) to pass into a liquid (or solid) sample, which is purged of volatile gaseous components for selection. Sex enrichment (adsorbents, cold traps, etc.), followed by heating (or other methods) to carry the separated gaseous components from the carrier gas into the GC column for separation and analysis.
The sampling device for dynamic headspace chromatography is shown in Figure 2-38. In the figure, 1 is the collecting tube, 2 is the cooling water, 3 is the sample tube, 4 is the water bath, and 5 is the gas washing bottle. The analytical instrument used is General laboratory gas chromatograph with gas six-way valve sampling device.
Application of headspace gas chromatography
Because of the unique superiority of headspace gas chromatography, it has been widely used in the fields of food science, environmental science, materials science, and biochemical science.
Headspace gas chromatography (HS-9) is a chromatographic technique that adds a headspace sampler in front of the gas chromatograph inlet, often interpreted as an instrument that combines a headspace sampler with a gas chromatograph. It uses the sample to be tested (gas-liquid and gas-solid) to heat balance, and then takes its volatile gas portion into the gas chromatograph. It is specially designed for the analysis of volatile trace components, such as many volatile organic solvents such as methanol and ethanol; floral aromas and perfumes in different seasons, Chinese herbal medicines with volatile components, vegetables and condiments with special odors, etc. It can be used for quantitative analysis. Headspace gas chromatography coupled with mass spectrometry is used for qualitative analysis of unknown volatile components.
Headspace gas chromatography is often used for qualitative and quantitative confirmation and analysis of alcohol in the blood of drivers or pedestrians after a traffic accident, standard analysis of residual solvents before and after Chinese and Western medicines are put on the market, and toxic gases and volatile poisons in criminal cases. Identification, etc., and many other industries need to control the quality of products or develop new products. This method of analysis avoids the problem of overloading and contamination of the analytical column by moisture, high boilers or non-volatile materials. Moreover, the operation is simple and fast, and the analysis result is as sensitive, reliable and accurate as the gas chromatogram.
Precautions
1. Since the carrier gas entering the headspace enters the GC at the same time, the gas used for the headspace should also be purified.
2. The headspace bottle heating temperature, the metering tube temperature, the transmission line temperature should be small to large, and the transmission line is less than or equal to the inlet port temperature.
3. When applying headspace, the total flow of GC gas should be the headspace airflow plus the GC gas flow. Care should be taken when calculating the split ratio. It can be calculated after measuring with a flow meter.
4. In the time setting, the time for filling the sample tube should be sufficient, the balance time of the quantitative tube should not be too long, and the injection time should be long enough.
5. If the pressure adjustment of the headspace sampler is manual, the sample pressurization and carrier gas pressure values ​​should be recorded after the method is built to avoid pressure changes due to changes in valve status.
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