Purge and trap is the use of inert gas into the liquid sample (or solid surface), through the adsorption container analysis, and then heating the adsorbent, so that the adsorption component desorption, the gas into the gas chromatography column for analysis. It is widely used in environmental analysis such as the analysis of organic pollutants in drinking water or wastewater. It is also used for the analysis of volatiles (such as odor components) in food.

Purge and trap method belongs to the category of gas phase extraction. It uses nitrogen, helium or other inert gases to extract the analyte from the sample, so that the gas is continuously extracted in an adsorbent or cold trap, and then analyzed and measured. Therefore, the purge and trap method is also known as the dynamic headspace concentration method.

Factors Affecting Purge and Trap Results.

(1) Purge and trap and purge time are equal to the product of purge flow and purge time. Control of the gas volume is usually used to select the appropriate blowing efficiency. The greater the total volume of gas, the higher the blowing efficiency. However, the total volume is too large, which is not conducive to the subsequent capture efficiency, and will blow away the analysis of the adsorbent or the capture in the cold trap. Therefore, it is usually controlled between 400 and 500 ml.

(2) The higher the solubility of the sample, the lower the purging efficiency. For highly water-soluble components, only by increasing the purge temperature can the purge efficiency be improved. The salt effect can change the solubility of the sample. In general, the salt content can be increased to about 15% to 30%. Different salts have different effects on purge efficiency.

(3) Timely, rapid and repeated desorption temperature is the key to purge gas chromatography analysis, which affects the accuracy and repeatability of the whole analysis method. The higher desorption temperature can better feed the volatiles into the gas chromatography column and obtain narrow chromatographic peaks. Therefore, a higher desorption temperature is generally selected. For organics in water (mainly aromatics and halides), the desorption temperature is generally 200°C. After determining the desorption temperature, the shorter the desorption time, the better to obtain a good symmetrical chromatographic peak.

(4) Capture efficiency The blowout is captured in an adsorbent or a cold trap. Capture efficiency also has a great influence on purge efficiency. The higher the capture efficiency. The higher the purge efficiency. The cold trap temperature directly affects the capture efficiency. The capture efficiency can be obtained by selecting the appropriate capture temperature.

(5) Increasing the purge temperature is equivalent to increasing the steam pressure. Therefore, the purge efficiency is also improved. Vapor pressure is the pressure exerted on a solid or liquid during purging. This depends on the ratio of the purge temperature to the vapor and liquid phases. When the purge contains highly water-soluble ingredients. The purge temperature has a great influence on the purge efficiency. However, the increase in the amount of water vapor caused by high temperature is not conducive to the next step of adsorption, which also brings difficulties to the separation of the non-polar gas chromatography separation column. Water can also quench the flame detector, so generally choose 50 ℃ as the common temperature. For high boiling and strongly polar components, higher purge temperatures can be used.