Sample Preparation Technology for Gas Chromatography
In order to expand the range of gas chromatography and improve the sensitivity of detection, some samples need to be pre-treated before being injected into the column. Commonly used sample pretreatment techniques include headspace method, derivatization method, lysis method, and adsorption tube method. The headspace method and derivatization method with more applications are briefly introduced below.
First, the headspace analysis method
For volatile components in some solid or liquid samples, headspace techniques can be used for pretreatment of the sample. In this method, a solid or liquid sample is quantitatively added to a container of a certain volume, and the mouth of the container is sealed with a plug of a material such as silicone rubber, and kept at a constant temperature for a certain period of time, so that volatile components in the sample are volatilized to the gas phase in the upper portion of the container. In the (headspace), the distribution of the components in the gas phase and the liquid phase (or solid phase) is balanced, and then the gas in the upper portion of the vessel is extracted by a syringe for analysis. Since the concentration of the volatile component in the gas phase is proportional to its content in the liquid phase, quantitative analysis can be performed by comparing the chromatographic peak areas of the sample with known standards under the same experimental conditions.
At present, the headspace analysis device has been made into a compact gas chromatograph-specific accessory for users to purchase. In general, the laboratory uses a container such as a serum bottle as a sample bottle, a water bath as a thermostat, a syringe as a sampler, and a simple headspace analysis, but the accuracy and repeatability are sometimes not good enough.
Since the solid sample and the liquid sample with complex composition cannot be directly injected into the gas chromatograph, the headspace method can be used for the pretreatment of these samples, which can avoid other complicated and complicated sample pretreatment methods and improve The sensitivity is detected. Moreover, since the gas injected into the gas chromatograph is relatively “clean”, the chromatographic separation is easy, and the life of the column can be extended. For example, measuring volatile harmful components in sewage; measuring volatile organic components in biological materials (body fluids, etc.); measuring volatile aroma components in coffee, tea or other beverages.
Second, the sample derivatization method
There are many compounds that are difficult to analyze by gas chromatography due to their poor volatility, too strong polarity, or severe tailing of chromatographic peaks. Derivatization of the sample can usually be used to achieve the following objectives:
1. Increase the volatility of the sample.
2. Reduce the polarity of the compounds (acids, phenols and alcohols).
3. Increase the thermal stability of the sample.
4. A group (eg, CF3) that is particularly sensitive to a selective detector (eg, ECD) is introduced.
5. The sample is effectively extracted from the aqueous phase (e.g., acylation of phenolamine).
Usually, the derivatization treatment of the sample is carried out by chemically reacting the active hydrogen atoms of some polar compounds (for example, compounds containing NH, OH, SH groups). Derivatization methods can be classified into five types depending on the reagents used and the reactions that occur, namely, acylation, alkylation, silanization, condensation, and esterification.