Simple purge and trap, also an analytical method, is generally a step: dynamic headspace extraction-adsorption-capture-thermal desorption-GC analysis.

The method of sampling collection and analysis is to inject inert gas into the liquid sample (or solid surface), blow away the components to be analyzed, heat the adsorbent after the adsorbent is concentrated, separate the adsorbed components, and introduce the gas phase chromatographic column with carrier gas for analysis. The dynamic headspace analysis method has rich functions and is beneficial to the analysis of trace components. The main problem is that it takes a long time, impurities may be introduced during the cleaning process, and the performance of the adsorbent is selected.

The method is used for sample pretreatment, does not need to use organic solvents, does not cause secondary pollution to the environment, has small sampling volume, high collection efficiency, small matrix interference, and convenient online detection. However, the cleaning method is easy to produce foam, resulting in overload of the instrument. In addition, with the blowing out of water vapor, it is not conducive to the next step of adsorption, which brings difficulties to the separation of the non-polar gas chromatography separation column. Water also has a quenching effect on the flame detector.

Helium was used as the purge air, which was used to soak the sample solution in most purge gassing. The volatiles in the sample escape with the helium during the continuous gas flow purge and are then concentrated by the sorbent collection device. After a certain purging time, the component to be measured will enter the trap in whole or in a certain amount. At this time, the purge air is turned off and connected to the carrier gas line of the GC through an on-off valve. At the same time, the rapid heating of the trap tube, absorption of the captured sample components, with the carrier gas into the GC separation analysis.

Instructions for use of purge and trap units:

1. Purge capture should consider purge efficiency and capture efficiency. It is difficult to extract the purge component, which can increase the total amount of purge gas and improve the purge efficiency. When the purge gas flow is kept constant, the purge time and recovery can be increased. Increasing the amount of purge gas can increase the purge rate of gases with boiling points below 35°C, but due to the increase in the flow of purge gas, these gases will pass through the trap well to reduce their adsorption efficiency. The effect of purge gas volume and purge time shall be considered, and the recovery rate of each purge equipment shall be considered.

2. Adsorption efficiency and the component to be tested and the adsorbent, such as the vapor pressure of the component, the surface area of the adsorbent, the interaction between the component and the adsorbent, etc. In general, adsorbents improve the capture efficiency of various components. To avoid absorption tube penetration, the capture temperature should be 25°C ± 2°C, less than 30°C. Sometimes a cooling device is needed to capture some of the compounds at room temperature.

3. The main water removal methods of purge and trap are permeation and condensation. The osmosis method is very effective for removing moisture and polar substances, but when measuring polar substances in the sample, such as ketones, they cannot be removed by the osmosis method. Condensation is a commonly used dehydration method, which does not affect the recycling of polar compounds.

4. During the purge capture process, sample foaming will contaminate the automatic purge capture device. The application of anti-foaming agent can inhibit the foaming of the sample, but it will also have a certain impact on the characteristics of the sample, leading to the uncertainty of the analysis results. Filling inert glass microspheres in the purge airway can effectively prevent the sample from foaming. The use of a foam filter neither prevents foaming of the sample nor easily introduces errors.