What is thermal desorption unit?
Thermal desorption unit is a laboratory instrument. To use the device, a solid sample or adsorption tube of the substance to be measured is placed in a thermal desorption device (which can be directly connected to the chromatograph or installed independently), and the carrier gas enters the chromatograph inlet through the thermal desorption device. When the instrument is elevated in temperature, the volatile components are released from the solid sample or adsorbent and enter the gas chromatography system with the carrier gas for analysis and determination. So, the thermal desorption unit can be considered as a part of the purge-trap feed.
The unique design of the TDU thermal desorption unit eliminates the interference of water in high humidity sampling sites and removes water from the sample during standard dry purging without causing the loss of analytes. The instrument has an inlet port for liquid specimens for calibration, which is convenient and feasible. It is a fully automatic design, touches a large screen display, has more convenient operation, and can continuously run multiple samples of the new generation. It is also possible to add a secondary desorption component at room temperature or a secondary desorption component at a low temperature according to user requirements. Since the thermal desorption unit can be used both independently and desorbed with the trap tube heating device of the purge-trap sampler, the thermal desorption feed is a special case of the purge-trap feed in this case. It can be mounted directly on the inlet of the chromatograph, at which point it becomes a special case of a lysis feeder. Now it has been widely used in life science, industry, agriculture, environmental protection, pharmaceutical, tobacco, education, petrochemical, toxicology, food, electronics and many other fields.
Working principle of thermal desorption unit
is a process in which a substance is heated to a certain temperature to release the adsorbed compounds. The principle of thermal desorption is to enrich the sample to be measured (volatile and semi-volatile components) by adsorption in a sample tube with the adsorbent material, and then desorb the volatile and semi-volatile components from the adsorbent material by heating the sample tube, and to carry the desorbed components to the gas chromatography for analysis by means of carrier gas. The thermal desorption unit
is the device used to achieve the thermal desorption of the sample.
The thermal desorption units
are divided into primary thermal desorption devices and secondary thermal desorption devices according to whether the volatile and semi-volatile components are desorbed from the sampling tube and then concentrated by cold focusing. In the secondary thermal desorption device, the sample components desorbed from the sampling tube are concentrated and enriched, and the concentrated sample components are introduced into the gas chromatography with a smaller gas load, which improves the injection efficiency and results in a better peak shape.
As an analytical method, thermal desorption
is often used as a pre-concentration tool for GC to enable the analysis of low-concentration compounds. Otherwise, low concentration analytes would not be detected by GC. By adding a valve to the gas path, high concentrations of analytes can be shunted to prevent overloading of the GC column and extend the detection limit.
Working process of thermal desorption
a. Volatile organic compounds (VOCs) are collected onto the adsorbent.
b. The sorbent is heated under a certain airflow to release the compounds in it to concentrate them into a smaller volume.
c. Early thermal desorption instruments used only first-order desorption, where the sorbent tube was heated under the gas stream to release the adsorbed VOCs directly into the GC. However, the latest commercial thermal desorption instruments include a second-order desorption process where the gas stream from the adsorbent tube is collected in a thin tube in the thermal desorption unit, which is often referred to as a focused trap or cold trap.
d. By heating the focusing trap/cold trap, the analyte is released again and injected into the GC. This time, however, the analyte concentration is higher, resulting in higher sensitivity and better GC peak shape.
Thermal desorption units, as sample introduction devices commonly used in gas chromatography, are characterized by high sensitivity, fast and simple operation, and wide range of use. On the one hand, the large volume sampling process allows for the selective enrichment of trace components dispersed in liquids, solids or gases in sampling tubes with different adsorbent materials, thus concentrating the sample and improving the sensitivity of the detection. On the other hand, the sample can be placed on a thermal desorption device after sampling without the use of solvents and without sample pre-treatment. It can be used directly in combination with gas chromatography for sample analysis, making it widely available.
Thermal desorption unit advantages and disadvantages
Advantages of thermal desorption unit
offers many advantages for the analysis of trace volatile and semi-volatile organic compounds (VOCs and SVOCs). The advantages are as follows.
a. High sensitivity
The second-order desorption of the adsorption tube enhances the concentration of the substance by a factor of several, greatly enhancing the detection limit of the GC.
b. High quality analysis
Highly concentrated gas bands are injected from the focusing trap into the GC, which keeps the peaks narrow and thus improves the quality of analysis.
c. Save time and effort
By selecting the appropriate sampling device, sample preparation effort can be greatly reduced or removed entirely, which in turn can be more easily automated.
d. No analytical interference
No sample addition and no sample dilution prior to analysis, thus eliminating analytical interferences (such as solvent interferences).
e. Autonomous selection of analysis
By optimizing the sorbent and thermal desorption settings, only the target compound is injected into the GC.
f. Extended dynamic range
Second-order thermal desorption and sample splitting allow thermal desorption units to analyze analyte concentrations from parts per trillion to parts per hundred.
g. Broader range of analytes
Modern sorbent and thermal desorption instruments are designed to quantify a broader range of substances, from ultra-high VOCs to semi-volatile organics.
h. Sample compatibility
Thermal desorption unit can be combined with different GC sampling methods to sample in a wider range of sample types (whether gas, liquid or solid).
Disadvantages of thermal desorption unit
a. Complete desorption of the sample may take a longer time.
b. Need to examine and calculate the sampling volume.
c. Higher cost of sample handling.
d. Large and heavily contaminated samples or samples containing less volatile components often require long purging times to collect them completely.
Use of thermal desorption unit
a. Due to the high frequency of hot and cold changes during the use of the thermal desorption unit, the gas circuit may be loose, so it is necessary to pay attention to whether the gas circuit is in a sealed state without air leakage.
b. After many times of thermal desorption operation, there is a possibility that tiny carbon particles (or other tiny solid adsorbents) will stay in the bend of the outlet. Therefore, after working for a period of time, it is necessary to use the ear-wash ball to back-blow at the outlet to avoid the accumulation of tiny particles, which will affect the work.
c. A variety of different organic compounds have different stability, as well as different adsorbent properties and thus different thermal desorption efficiencies. In the case of ≥ 75% thermal desorption and < 10% stability of the coefficient of variation, the results are allowed to be corrected by thermal desorption efficiency. In order to keep the compound from decomposition, when the adsorption tube enters the thermal desorption furnace, a little nitrogen should be passed immediately. To obtain a good desorption efficiency, the directionality of the solid adsorption tube to the inlet is very important, i.e., the inlet end is the outlet end when sampling is desorbed.
d. The sampled solid not only adsorbed the measured sample but also adsorbed water vapor, thus the water vapor was desorbed at the same time during pyrolysis. Sometimes small droplets of water can be seen at the outlet of the gas. Before the second sample operation, small droplets of water should be wiped away (using cotton fiber or small filter paper strips to absorb water can be).
e. In order to maintain the beauty of the instrument, it is not allowed to use acid or alkali and other corrosive substances to rub its surface.
f. When the thermal desorption unit is not in use, the power switch should be set to the "off" position. Then cut off the power supply and place the instrument in a clean and dry place. Start once every six months to maintain the performance of the machine.
g. The operator should go through relevant training and read the relevant technical information in detail before operation.
h. Wearable parts should be replaced regularly.
i. Electrical equipment should be regularly maintained.
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