Why is preheating necessary before using an online chromatograph?

1. Temperature control: The temperature of the injection chamber should be determined based on the injection method and the sample. When sampling by vaporization, the vaporization temperature must be such that the components can be fully vaporized without decomposing (except in the case of pyrolytic sampling). It is advisable to keep the temperature of the detection chamber slightly higher than that of the column, to prevent component condensation or other issues. The determination of the column temperature requires comprehensive consideration, taking into account factors such as the operating temperature range of the stationary phase, the analysis time duration, and the ease of qualitative and quantitative analysis. It is best to operate at a constant temperature; program-controlled temperature rise should only be used for samples with a wide boiling range. The optimal operating temperature must be determined experimentally. 2. Sample pretreatment: The compound to be analyzed is often converted into another compound using chemical reactions; this is known as the preparation of derivatives. Then, chromatographic analysis is performed on the derivatives. The advantages of pretreatment are: ① Many compounds have too low or too high volatility, very low polarity, or poor thermal stability, making it impossible or unsuitable to directly sample them and inject them into a chromatograph for analysis; their derivatives, on the other hand, can be easily introduced into the chromatograph ； ②Some components that are difficult to separate become easier to separate and analyze qualitatively once converted into derivatives ； ③High-sensitivity derivatives can be obtained using a selective detector ； ④Some impurities in the sample were removed because they could not be converted into derivatives.

The chromatography column of a chromatograph generally needs to operate stably at a constant temperature; therefore, before carrying out normal analysis, the column chamber must be heated to a stable temperature, thereby ensuring that it remains within its optimal operating temperature range. Therefore, ordinary chromatography analyzers need to be preheated to the appropriate temperature before they can function properly.

This is related to the principle of measurement. Chromatographs require analysis under constant temperature conditions, and many analyzers also have such features during preprocessing, such as constant-temperature oil baths. This technology is also a key factor in competition among different analyzers, as maintaining a constant temperature is extremely important; even a temperature difference of 0.1°C can make a difference. The error then fails to meet the relevant standard requirements