The developer was cultured at 30°C for 15 minutes. Then, 100 μl of the substrate was shaken into a cuvette and placed into the instrument for testing. When the inhibition rate is ≥50%, it indicates that the sample contains organophosphorus or carbamate pesticides in excess of the national standard. If the inhibition rate reaches this threshold, the sample must be retested at least twice to confirm the result.
For the blank control test:
Step 1: Press the "Calibrate" button.
Step 2: Open the instrument door, insert the cuvette containing the control solution into the designated channel, close the cover, and press the "Blank" button. The instrument will then begin the blank detection countdown.
Step 3: Once the blank test is complete, the instrument will display the result (Ao), and the cover will open automatically for you to remove the control cuvette.
For the sample test:
Step 1: Press the "Calibrate" button again.
Step 2: Place the cuvette with the sample solution into the designated channel, close the cover, and press the "Test" button. The instrument will start the test countdown.
Step 3: After the test finishes, the instrument will display the test result (Ai) for the sample. It will also save both the blank value and the test result. The cover will open automatically so you can retrieve the sample.
Note: If the inhibition rate is between 40% and 50%, it may indicate a possible pesticide residue above the limit. If repeated tests still show an inhibition rate above 50%, the sample likely exceeds the safety standard. For confirmation, it is recommended to use gas chromatography or other advanced methods.
To print the test results, simply press the "Print" button on the interface displaying the pesticide residue information. The instrument will then proceed to print the data.
Result Interpretation:
The results are presented as the percentage of enzyme inhibition. If the inhibition rate is ≥50%, it suggests the presence of organophosphorus or carbamate pesticides in the vegetable sample. These samples should be tested multiple times to ensure accuracy. Further analysis using alternative methods is advised to identify the specific type and concentration of the pesticide.
Technical Specifications of the Enzyme Inhibition Method:
(1) Sensitivity: The minimum detection limits for the method should be as follows: Methylammonium phosphate at 0.8 mg/L, Trichlorfon at 0.25 mg/L, and Carbofuran at 0.002 mg/L.
(2) Matching Rate: Among 10 or more samples showing an inhibition rate ≥50%, the match rate with gas chromatography should be over 80%.
Important Notes:
1. After turning the instrument back on, the blank value must be recalculated.
2. Reagent Storage: According to GB/T 5009.199-2003, the reagents should be stored in a freeze-dried state. The storage conditions and shelf life vary depending on the reagent type:
| Reagent | Room Temperature | Freezer (-20°C) | Refrigerated (4°C) |
|---------|------------------|------------------|--------------------|
| Enzyme Solution | 1–2 weeks | 1 year | 2–4 days | 7–10 days |
| Substrate | 1–2 weeks | 1 year | 2–4 days | 7–10 days |
| Reagent | 1 year (protected from light) | 6 months (away from light) | N/A | N/A |
| Extract | No limit | No limit | N/A | N/A |
Some vegetables like amaranth, ginger, onion, garlic, pepper, and carrot contain substances that may interfere with enzyme activity. When handling these, avoid cutting too much, keep extraction time short, and if necessary, use the whole plant for leaching.
Comparison of Pesticide Concentration and Inhibition Rate (Experimental Conditions: 37°C):
| Pesticide | Concentration (μg/ml) | Inhibition Rate (%) |
|-----------|------------------------|----------------------|
| Methamidophos | 0.4 | 35.0 |
| | 0.8 | 52.0 |
| | 1.6 | 69.0 |
| Omethoate | 0.4 | 26.4 |
| | 0.6 | 29.4 |
| | 0.8 | 36.7 |
| | 1.2 | 52.7 |
| | 1.6 | 59.6 |
| | 3.75 | 76.3 |
| Trichlorfon | 0.1 | 13.6 |
| | 0.15 | 22.9 |
| | 0.2 | 37.2 |
| | 0.25 | 50.6 |
| | 0.4 | 63.9 |
| | 0.5 | 73.9 |
| Carbofuran | 0.001 | 25.2 |
| | 0.002 | 50.3 |
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