As the core equipment for trace volatile organic compound analysis, the calibration quality of the secondary thermal analyzer directly affects the accuracy and comparability of the detection results. The following are the key calibration steps and technical points covering the entire process:

1、 Preparation before calibration

1. Confirmation of Environment and Equipment Status

Environmental conditions: The laboratory temperature is controlled at 25 ± 2 ℃, and the humidity is ≤ 60% to avoid temperature and humidity fluctuations that interfere with thermal desorption efficiency;

Gas system: Check that the pressure of the carrier gas (high-purity helium gas) cylinder is ≥ 0.4MPa, and there is no leakage in the pipeline (leak detection with soap solution);

Instrument self-test: Preheat the device for 30 minutes, run the self-test program, and confirm that the communication between each module is normal.

2. Preparation of standard materials and consumables

Calibration standard: Select nationally recognized liquid mixed standard samples (such as benzene derivatives and n-alkanes), with concentration gradients covering the detection range;

Adsorption tube: Activate the blank Tenax TA adsorption tube in advance and place it in the dryer for backup;

Micro injector: cleaned with acetone and dried with nitrogen blowing, used for precise labeling.

2、 Core calibration steps

1. First level thermal desorption calibration (initial desorption)

Parameter setting: Set the primary desorption temperature (), holding time, and diversion ratio according to the instructions;

Dynamic adsorption verification: Inject 1 μ L of standard solution into a blank adsorption tube, introduce a carrier gas to drive solvent evaporation, and record the breakthrough curve;

Recovery rate calculation: Collect all desorbed gases through a secondary capture trap, perform GC-MS quantitative analysis, and require a target recovery rate of ≥ 95%.

2. Secondary flash evaporation calibration (focused injection)

Transient temperature rise verification: Set the temperature of the secondary furnace (), with a heating rate>200 ℃/s, and optimize the residence time to transfer;

Peak symmetry test: After injection, observe the tailing factor of the chromatographic peak, which should be<1.5. Otherwise, adjust the temperature difference between the hot and cold zones;

Cross contamination inspection: Inject blank adsorption tubes three times in a row to confirm the absence of residual peaks.

3. Quantitative ring volume calibration

Geometric volume measurement: Use a measuring cylinder to measure the actual volume of the quantitative ring, with an error of<± 5%;

Linear regression analysis: Prepare standard gases with 5 concentration gradients and plot peak area concentration curves with a correlation coefficient R ² ≥ 0.995.

3、 Performance validation and adjustment

1. Determination of detection limit

Dilute the highest concentration standard sample to a signal-to-noise ratio of S/N=3 to determine the detection limit of the method, which should be lower than the hygiene standard value of the target substance.

2. Precision testing

Measure the same concentration standard sample in parallel 6 times, calculate the relative standard deviation RSD, and require the RSD of the main component to be less than 5% and the RSD of the secondary component to be less than 10%.

3. Abnormal diagnosis and handling

Baseline drift: Check the blank value of the adsorption tube and extend the baking time to 300 ℃ if necessary;

Low response value: Check the focusing efficiency of the cold trap and clean the quartz cotton filling material;

Ghost peak interference: Replace the O-ring seal and prohibit plastic products containing plasticizers.

4、 Calibration report and traceability

Original record archiving: Save chromatograms, mass spectra, calibration curves, and calculation processes;

Certificate issuance: issue a calibration report containing calibration date, operator, and reviewer information;

Periodic review: quarterly periodic verification and annual comprehensive calibration.

This calibration process strictly follows the specification of "HJ 644-2013 Determination of Volatile Organic Compounds in Environmental Air", and ensures quality control throughout the entire process from sampling to detection through graded verification. In practical operation, attention should be paid to high temperature protection, avoiding direct contact with hot components, and regularly replacing aging gaskets to ensure system airtightness.