GB/T 24572.6-2026Standard practice for the separation and concentration of ignitable liquid residues from fire debris samples - Part 6: Thermal desorption (English PDF)

5.2 Container Material

5.2.1 Evidence Bag Including polyamide nylon, polyethylene terephthalate (PET), and polyethylene (PE). Evidence bags should be made of plastics such as ethylene (PE) and polyvinyl chloride (PVC). Evidence bags should ideally be airtight. Nylon and PET evidence bags.

5.2.2 Container Various types of containers with good airtightness. Containers should preferably have an air inlet at the bottom and an air outlet at the top, and be sealed with silicone rubber, fluororubber, or similar materials. A special metal container for the ring.

5.3 Adsorbent Materials Adsorbent materials include.

a) Dry powder fire extinguishing agents, diatomaceous earth, and other micron-sized high specific surface area fine powders with a specific surface area of 10m^2/g to 300m^2/g;

b) Soft, porous materials such as plant fibers, hair, oil-absorbing cotton, activated charcoal, or activated charcoal fibers;

c) Polydimethylsiloxane (PDMS), divinylbenzene (DVB), and other polymers Solid-phase microextraction fibers and films with coatings.

5.4 Sample Tubes This includes empty stainless steel tubes and adsorption tubes filled with adsorbent packing material. The adsorbent packing material should preferably be poly(2,6-diphenyl-p-phenylene ether) (Tenax- TA) and poly(2,6-diphenyl-p-phenyl ether) containing 30% graphitized carbon black (Tenax-GR).

5.5 Sampler This includes syringes and sampling pumps with a volume of at least 50 mL. Small-volume sampling pumps with flow control functionality are preferred.

5.6 Heating Equipment For ovens or other heating equipment with temperature control functions, the heating temperature should be between 40°C and 100°C.

5.7 Aging device The technical requirements for aging sample tubes in an aging device shall meet the following criteria.

---The set temperature should not be lower than 400°C;

---The carrier gas flow rate is adjustable, and the maximum carrier gas flow rate should not be less than 100 mL/min.

5.8 Thermal Desorption Instrument Manual loading of the thermal desorption unit or fully automatic thermal desorption unit.

5.9 Gas Piping System Gas lines and related accessories used to connect thermal desorption instruments and gas chromatograph-mass spectrometers.

6.2 Sealing of the sample

6.2.1 Static Adsorption Sealing Sealing steps for static adsorption method.

a) Place the sample in an evidence bag, suspend the absorbent tube or absorbent material inside the bag, and then seal it;

b) Place the sample into the container, suspend the adsorption tube or adsorption material on the container lid, and then close and seal it.

6.2.2 Dynamic Adsorption Sealing Sealing steps for dynamic adsorption method.

a) The evidence was placed in a sealed bag. 1) Place the sample in a bag, tie the bag tightly, preferably by folding the edges of the bag opening in half and then binding it, or by using a special tool strip to lock it securely; 2) Connect the air inlet end of the adsorption tube to the evidence bag. Place the evidence bag horizontally, insert the air inlet end of the adsorption tube into the evidence bag, and cut it open. Insert it into the small opening or the special air outlet of the evidence bag and lock it tightly, keeping the air outlet of the adsorption tube sealed.

b) Sealed container. 1) Place the sample into a special container with an air inlet at the bottom and an air outlet at the top, and seal it. 2) Insert the air inlet end of the adsorption tube into the air outlet of the container and lock it in place, keeping the air outlet end of the adsorption tube sealed.

6.3 Adsorption

6.3.1 General Requirements This includes static adsorption and dynamic adsorption, both of which are optional. Static adsorption should be used when the sample clearly shows a positive result for flammable liquids, except... In addition, all samples should be subjected to dynamic adsorption.

6.3.2 Static Adsorption This includes adsorption under volatilization equilibrium conditions at room temperature or under heating volatilization equilibrium conditions. The ideal time for room temperature equilibrium is approximately [time value missing]. 30 min; the heating equilibrium conditions should be a heating temperature of 40°C~70°C and an equilibrium time of about 20 min.

6.3.3 Dynamic Adsorption

6.3.3.1 Negative pressure adsorption (sampling pump adsorption) Connect the outlet end of the adsorption tube, which has undergone dynamic adsorption sealing (see 6.2.2), to the sampling pump for negative pressure adsorption (sampling pump adsorption). After attachment, remove the adsorption tube and quickly seal both ends of the adsorption tube. The adsorption time depends on the volume of the container material and the concentration of the target substance remaining in the flammable liquid; the carrier gas flow rate should be 20 mL/min. 100 mL/min. When the flammable liquid content of the sealed evidence bag is low, sampling should continue until the evidence bag shows slight wrinkles; for evidence in sealed containers, room temperature sampling is recommended. Under suitable conditions, the sampling pump should be used for adsorption. If the content of flammable liquid in the sample is low, a temperature of 40°C~70°C and an equilibration time of approximately 20 minutes are recommended. After heating mode, the sampling pump adsorption is performed.

6.3.2.2 Positive pressure adsorption Positive pressure adsorption includes the following two adsorption methods.

a) Injection adsorption. Use a syringe to draw gas from the sealed container, remove the sealing plugs at both ends of the adsorption tube, and inject the gas along the gas inlet direction on the adsorption tube. Introduce the gas and quickly seal both ends of the adsorption tube after adsorption is complete.

b) Purge adsorption. Remove the plug from the outlet end of the adsorption tube at the top of the container, open the nitrogen purging inlet at the bottom of the container, and allow nitrogen to flow through the sample. The gas then enters the adsorption tube to complete positive pressure adsorption; the purging conditions should be nitrogen flow rate of 10 mL/min to 100 mL/min and heating temperature of 40°C~70°C, purging time 10min~60min; after adsorption is complete, remove the adsorption tube and quickly seal both ends of the adsorption tube. When the sample contains a high moisture content, rapid backflushing should be used to remove the moisture from the adsorption tube. Connect the outlet end of the adsorption tube to a gas... With the gas source and inlet open, initiate gas purging. The backflushing condition should ideally use nitrogen, with a gas flow rate of 100 mL/min to 500 mL/min and a purging duration of 30s~90s.

6.4.1 Thermal desorption method

6.4.1.1 Direct Desorption Fill both ends of an empty stainless steel tube with a small amount of glass wool, and place a tiny piece of evidence with a diameter of less than 4 mm in the middle. Fine powder samples are suitable for this method. The fabric is wrapped into a strip with a diameter of less than 4 mm. The sample tube containing the sample is then transferred to a thermal desorption apparatus for desorption.

6.4.1.2 Desorption of Adsorbent Material A small amount of glass wool is filled at both ends of a stainless steel tube, and adsorption material that has undergone static or dynamic adsorption treatment is placed in the middle. The tube containing the adsorption material is then placed... The sample tube of the material was transferred to a thermal desorption apparatus for desorption.

6.4.1.3 Desorption of Adsorption Tubes The adsorption tubes, after static headspace or dynamic adsorption, are transferred to a thermal desorption apparatus for desorption.

6.4.2 Thermal desorption conditions

6.4.2.1 Sample tube desorption analysis Desorption and desorption temperature selection.

---Organic samples. biological materials, textiles, plastics, and rubber, etc., preferably at temperatures below 100°C;

---Inorganic samples. soil, tiles, cement blocks, etc., should be kept at a temperature below.200°C;

---Adsorption materials. Activated carbon (temperature should be 150°C~200°C), extraction fibers or films coated with PDMS, DVB polymers, etc. The ideal temperature is.200°C~260°C.

---Adsorption tube. The desorption temperature should be set according to the manufacturer's recommended temperature. The desorption thermal desorption time should be 3 min to 50 min.

6.4.2.2 Cold Trap Focusing The temperature should generally be below 10°C. When there is a clear positive result or a high content of flammable liquid residue, the temperature should be 25°C and the time should be 3 minutes.

6.4.2.3 Flow split ratio Depending on the enrichment concentration, split ratios of 40.1, 20.1, and no splitting are used. When using the splitting mode, a trap can be selected... A staged desorption (TrapDesorb) or tube desorption (TubeDesorb) split can be selected, or a single-stage split can be chosen. A single-stage split can be selected. When using this method, it is advisable to adopt the Trap-Desorb split mode to ensure good peak shape and improved sensitivity.

6.4.2.4 Other Recommended Conditions Other recommended conditions include, but are not limited to.

---The flow rate of the high-purity helium carrier gas is 40 mL/min;

---Transmission line temperature is 150°C~200°C;

---The cold trap heating rate is the maximum rate.

Note. Select the optimal desorption conditions according to the different types of instruments.

6.5 Testing The sample obtained after thermal desorption extraction should be tested for the target analyte. For common flammable liquids, the testing should be performed according to GB/T 18294.5. For other volatile flammable liquids, the requirements of GB/T 19267.7 shall apply.