Low temperature secondary thermal desorption apparatusIt is the core equipment for analyzing volatile and semi volatile organic compounds (VOCs/SVVOCs), achieving high-sensitivity detection of trace substances through two-stage thermal analysis and low-temperature focusing technology. Its working principle is based on the deep integration of thermal desorption and gas chromatography analysis, mainly divided into two stages: the first stage of thermal analysis, in which the sample tube is rapidly heated at a high temperature of 280 ℃ -300 ℃, and the target compounds on the adsorbent (such as benzene, toluene, bisphenol A, etc.) desorb and enter the secondary system with the carrier gas (helium/nitrogen); In the secondary thermal analysis stage, the compound is enriched in a low-temperature focused cold trap at -30 ℃ to -40 ℃, and then the cold trap is instantly heated to 320 ℃ to achieve narrowband release of the compound. It is synchronized with the gas chromatography (GC) or mass spectrometry (MS) injection port to form sharp chromatographic peaks.
Low temperature secondary thermal desorption apparatusThe main components of:
1. Sample Management and Injection Module
Autosampler:
Usually designed as a turntable or track, it can accommodate multiple (such as 50-100) adsorption tubes.
Responsible for automatically grasping, locating, and replacing adsorption tubes according to preset programs, achieving unmanned batch analysis.
Adsorption tube bracket/clamp:
Fix the adsorption tube to ensure accurate positioning and good sealing during heating and airflow transmission.
Mechanical arm/gripping device:
Under the control of the automatic sampler, move the adsorption tube from the storage position to the primary desorption position, secondary cold trap position, and cooling position.
2. Primary Desorption Module
Primary Oven:
Heat the entire sampling tube that has adsorbed VOCs (usually within the temperature range of 150 ° C-400 ° C).
Under the action of reverse airflow (carrier gas, such as high-purity helium), most VOCs are desorbed from the adsorbent.
Primary Transfer Line:
Inert (such as siliconized) stainless steel tubes connecting the primary heating furnace and the secondary cold trap.
Usually equipped with heating function (100 ° C-200 ° C) to prevent VOCs from condensing during transportation.
3. Secondary Focusing Trap
冷阱 (Cryogenic Trap/Cold Trap)：
This is the "low-temperature" core of "low-temperature secondary".
It is usually composed of a three-stage inert capillary tube or U-shaped tube, placed in a refrigeration system.
When VOCs are first desorbed, they are deeply frozen (ranging from -30 ° C to -150 ° C, depending on the refrigeration method) and then re captured and concentrated.
Common refrigeration methods: mechanical refrigeration (compressor), liquid nitrogen refrigeration, semiconductor refrigeration.
Focus function:
The cold trap "focuses" the broad first-order desorption peak into a narrow initial peak, greatly improving the peak intensity and resolution entering GC.
4. Secondary Desorption Module
Secondary Oven:
After the cold trap is captured, it is rapidly heated at high temperatures (usually 300 ° C-350 ° C).
Instantly desorb concentrated VOCs to form an extremely narrow "plug" airflow, which is injected into the gas chromatograph in a pulsed manner.
Secondary Transfer Line:
Connect the inert heating pipeline between the cold trap and GC injection port to ensure non-destructive transfer of the sample.
5. Pneumatic control system
Gas supply:
High purity carrier gas (He or N ₂), used for blowing, transportation, and desorption.
Additional gas may be required (such as gasket blowing gas).
MFC Mass Flow Controller:
Accurately control the gas flow rate at each stage of primary desorption, purging, and blowback.
Electromagnetic valve and six way valve/ten way valve:
Controlling the direction of airflow and switching between different gas paths (such as desorption, purging, bypass, injection GC, etc.) are key components for achieving automated processes.
Pressure sensor:
Monitor system pressure to ensure airtightness and normal processes.
6. Temperature control system
Multi channel temperature control unit:
Independently control the temperature of the primary furnace, secondary furnace, cold trap, and various transmission pipelines.
Use high-precision platinum resistors (Pt100) or thermocouples for temperature monitoring.
Ensure precise execution of heating, insulation, and cooling procedures.
7. Control and Data Processing System
Control unit/industrial computer:
The "brain" of the instrument runs control software to set and execute desorption programs (temperature, time, flow rate, valve switching, etc.).
Operation interface (HMI):
Touch screen or computer software interface, used for parameter setting, method editing, status monitoring, and fault diagnosis.
Communication interface:
Connect with gas chromatograph (GC/GC-MS) to achieve synchronous triggering and data transmission.
8. Auxiliary and safety systems
Cooling system:
Used for rapid cooling of the primary heating furnace and adsorption tube, preparing for the next injection. It can be air cooling or water circulation cooling.
Waste liquid collection device:
Collect the water or high boiling point condensate generated during the cold trap water removal process.
Leakage detection and alarm system:
Monitor the pressure of the gas path and the status of critical components, and issue an alarm or automatically shut down in case of abnormalities.
Exhaust port:
Discharge the exhaust gas generated by blowing and backblowing from the instrument or connect it to the ventilation system.