Thermal Desorption

When using solid adsorption materials for enrichment and concentration to collect atmospheric and liquid (water) samples, as well as preparing chromatographic analysis samples using techniques such as solid-phase extraction, purge capture, and membrane separation, the problem of the tested components being adsorbed on the solid adsorbent is involved. How to desorb these tested components from the solid adsorbent and send them to the chromatographic analysis system for analysis will directly affect the operation and results of chromatographic analysis.

1、 Principle of Thermal desorption 

There are two ways to decompose and adsorb the tested component from a solid adsorbent: thermal desorption and liquid desorption. At present, most of them use thermal desorption method. To ensure that all adsorbed samples enter the chromatography. Usually, secondary cold focusing technology is used, and the use of non splitting and injection port programmed heating technology can effectively improve the sensitivity and resolution of GC determination. However, solvent desorption technology is used for activated carbon adsorption, and the activated carbon has strong adsorption capacity, requiring a higher thermal desorption temperature, which will lead to degradation of samples and increase the error of analysis and determination. Liquid desorption mostly uses low boiling point solvent extraction, such as carbon disulfide, dichloromethane, pentane, benzene, etc.

Compared with thermal desorption, solvent extraction allows a longer adsorption bed, higher flow rate and larger sampling volume, so it is possible to select appropriate determination techniques to analyze the concentrated samples and obtain more accurate determination results. However, trace analysis requires that the sample volume extracted by solvent be as small as possible, so it is often necessary to evaporate a portion of the solvent to further concentrate the sample. As a result, the evaporation and concentration process may cause some problems, such as contamination by glassware or other solvents during sample concentration, which may evaporate some volatile components in the sample. In addition, solvents in the sample may mask or interfere with other components in GC analysis. The theory and methods of liquid desorption are detailed in the chapters on elution in liquid-solid extraction and solid-phase extraction in this book.

According to adsorption theory, the lower the temperature, the stronger the adsorption force between the adsorbent and the adsorbed substance; As the temperature increases, the adsorption force between the adsorbent and the adsorbed substance becomes weaker. Therefore, heating can cause the desired component adsorbed on the adsorbent to decompose and absorb. The heating temperature, i.e. the thermal desorption temperature, is related to the boiling point, thermal stability of the desired component, and the thermal stability of the adsorbent. Low thermal desorption temperature may result in incomplete decomposition and desorption of components in the sample, low recovery rate, and large residual amount in the tube; A high thermal desorption temperature may cause thermal instability of certain components, resulting in low recovery rates. In addition, some adsorbents have catalytic activity towards certain substances, resulting in a decrease in their recovery rate. There are reports that Carbotrap (graphitized carbon black) and Tenax GR have catalytic reactions on a-pinene and aldehydes during the thermal desorption process.

The process of thermal desorption is influenced by the heating rate and final temperature, so strict control of the heating rate and final temperature is required during thermal desorption. The faster the heating rate, the higher the final temperature, and the faster the desorption rate. The initial sample band entering the chromatographic column becomes narrower.

The final temperature depends on the thermal stability of the tested component and adsorbent, usually below 300 ℃, as most polymer adsorbents begin to decompose at 300 ℃.

The flow rate of the carrier gas during the thermal desorption process also affects the thermal desorption. Generally, the faster the flow rate of the carrier gas, the more favorable it is for thermal desorption.

2、 Thermal desorption device

Thermal desorption is generally carried out by heating to 200~250 ℃. If the temperature control is not good, or if the temperature of the adsorption tube does not rise sufficiently despite the rise of temperature, the chromatographic peak may be divided into two or cause tailing. When the heating rate of the thermal desorption device is slow, the substances adsorbed by the adsorbent gradually desorb, widening the initial sample spectral band entering the chromatographic column, resulting in a widening of the final chromatographic peak and reducing the resolution of the factory chromatography. Therefore, the thermal desorption device should heat the adsorption tube evenly and rapidly. The adsorption tube is placed in a heating furnace controlled by a heating controller, which controls the heating temperature and rate of the heating furnace. The components that have been thermally resolved are analyzed by entering the separation column with the carrier gas.

3、 Precautions when using Thermal desorption techniques

In order to improve the recovery rate of adsorption-thermal desorption of the adsorption sampling tube, the filled adsorbent should be a substance with high collection efficiency and easy to be recovered by heating. 

In order to improve the recovery rate of adsorption-thermal desorption of the adsorption sampling tube, the filled adsorbent should be a substance with high collection efficiency and easy to be recovered by heating. The dryness and wetness of the air during sampling have an impact on the recovery rate of adsorption-thermal desorption of some organic compounds. A good adsorption trap should have as large an adsorption capacity as possible at a temperature below normal temperature, and can simply expel various compounds when heated at 100 deg C, that is to say, it has a small breaking capacity at a high temperature. However, even when a good adsorption trap tube is used for adsorption at room temperature, the phenomenon of chromatographic peak deformation is often found in the gas chromatographic measurement. Substances that are difficult to thermally desorb at high temperatures can broaden chromatographic peaks. In addition, even for the substance that is easy to desorb, if the sampling amount is too large and close to the breakthrough volume, the distribution of the components of the substance to be measured in the whole adsorption trap tube will produce a time difference when the thermally desorbed components enter the chromatographic column, and the chromatographic peak may be divided into two or become wider. Also, if the amount of the adsorption material filled is too large, the distribution range of the component to be measured becomes wide during the passage through the adsorption trap tube, resulting in a broader chromatographic peak.

In order to prevent these situations, the filling of the adsorbent material in the adsorption tube should be controlled to a minimum amount, and the temperature should be raised to a high temperature as quickly as possible during thermal desorption, and all components should be desorbed instantaneously. Or all the components obtained by the primary thermal desorption are subjected to secondary concentration (secondary cold focusing) at a low temperature, and then heated and introduced into a chromatographic column.

Due to repeated heating and cooling of the adsorption tube, the adsorption material may break and undergo particle size changes, resulting in a change in the speed of the purged gas passing through the adsorption tube and causing the chromatographic peak to deform. In addition, as the surface area increases, the adsorption capacity also changes, which sometimes widens the chromatographic peak width. At this point, small adsorbent particles can be removed through a sample sieve or replaced with new adsorbent materials.

4、 Application of thermal desorption technology

Usually, the thermal desorption technology can be used when there are volatile components that can be thermally desorbed in the following four types of sample matrices:

① The composition of volatile aroma and flavor compounds in food;

② Composed of thermally degradable compounds in the matrix, such as plasticizers, additives, monomers, etc. in polymeric materials;

③ Unwanted components in the sample matrix, such as residual solvents in the product;

④ Targeted collection of volatile components in the sample matrix, such as volatile organic pollutants (VOCs) in the air collected on adsorption tubes.

The first type of sample is food. Analytical chemists have used thermal desorption technology for food analysis for many years, which can not only determine flavor substances in natural foods. And it can detect residues and pollutants in food. For example, at 50 ℃, the aroma components of red apples can be collected. Place the apple in a sealed container with controllable skimming (with a diameter of 95mm and temperature control). Then, a vacuum pump is used to extract the air from the container and pass it through a Tenax trap with a flow rate of 25mL/min, collect it for 10 minutes, and then transport the sample obtained from pyrolysis and desorption (275 ℃ for 2 minutes) from the trap to the separation column [0.53mm (i.d.)] in the chromatography for determination (FID). By using this sampling method, changes in food flavor can be compared, the status of volatile organic compounds related to a food can be monitored, and potential changes in food over time can be identified.

The second type is additives in the samples, such as plasticizers and additives in polymer products. The degradation products of these samples after thermal desorption are helpful for the analysis of residual debris in arson cases, for the qualitative determination of pollutants in soil, and for the performance analysis of polymer materials. For example, after the contaminated 20mg soil sample is directly placed in a quartz tube and quickly heated to 400 ℃ (using platinum wire), it is determined online by GC-MS. After being purged by carrier gas, it passes through a 0.25mm (i.d.) capillary tube and directly enters the MS, polycyclic aromatic hydrocarbons such as pyrene and fluoranthene can be quickly determined without any other sample preparation procedures. Also, the plasticizer in the polymer sample can be viewed using the above device. When 1mg of polyoxyethylene plastic is heated to 300 ℃, a very strong chromatographic peak -2-ethylhexyl phthalate can be determined.

The third type of sample is the determination of residual volatile components in substances, such as residual solvents in pharmaceuticals, residual monomers in polymers, and other oligomers. For example, 10mg of silica gel sample is heated to 275 ℃ and held for 3 minutes, and the components obtained from helium blowing (30mL/min) are collected in a Tenax trap. Then, under the condition of 300 ℃, the desorption product is transported to a large-diameter capillary column for GC-FID determination by reverse blowing the trap of the pyrolysis suction well. The chromatographic determination results indicate that at least 15 oligomers of methylsiloxane have been determined.

The last type of sample is the use of adsorbent tubes to collect volatile organic pollutants in environmental atmospheric samples. After pre concentration of the environmental sample through a sampling tube, the desorption product is blown out through thermal desorption, and then directly transported to GC or cold focused on the column for GC analysis. The results showed that 100mL of gas sample was taken from the sampler and passed through a Tenax trap, followed by thermal desorption into GC-PID. The volatile components measured include 2- and 3-chloroethylene, toluene, ethylbenzene, xylene, etc.

The solvent free pyrolysis sample preparation technology has several advantages:

① Thermal desorption can perform 100% chromatographic analysis of sample components instead of partial analysis, greatly increasing sensitivity. The early thermal desorption technology was mainly applied in the separation of environmental samples, which could concentrate and determine substances at 10-12 levels in the samples.

② In chromatographic analysis, there are no solvent peaks and a wide range of volatile matter analysis can be performed. Sample components with short chromatographic retention values will not be disturbed by solvent peaks.

③ Thermal desorption does not use solvents, reducing and eliminating the impact of solvent vaporization and waste on environmental pollution.

——The above content is excerpted from "Chromatographic Analysis Sample Processing" by Wang Li, Wang Zhengfan, et al