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 equal to or greater than 50%, it indicates that the levels of organophosphorus or carbamate pesticides in the vegetables exceed the national standards. Any sample with an inhibition rate of 50% or higher must be tested two or more times to confirm the results.
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 perform a blank detection countdown.
Step 3: After the blank control test is completed, the instrument automatically displays the result (Ao), opens the cover, and allows you to remove the control cuvette.
For the sample test:
Step 1: Press the "Calibrate" button again.
Step 2: Open the instrument door, place the cuvette with the sample solution into the designated channel, close the cover, and press the "Test" button. The instrument will begin the test countdown.
Step 3: Once the test is complete, the instrument automatically displays the test result (Ai) for the pesticide sample. It also saves both the control value and the test result. Then, open the cover and remove the sample.
Note: If the inhibition rate is between 40% and 50%, it may indicate a suspected presence of pesticide residue. If repeated tests show an inhibition rate above 50%, it suggests that the pesticide residue exceeds the standard. In such cases, further testing using gas chromatography or other advanced methods is recommended for accurate identification and quantification.
To print the test results, simply press the "Print" button on the interface displaying the pesticide residue data. The instrument will start printing the results.
Result Interpretation:
The results are expressed as the degree of enzyme inhibition, known as the inhibition rate. If the vegetable extract inhibits the enzyme by 50% or more, it indicates the presence of organophosphorus or carbamate pesticides. These samples should be retested multiple times to ensure accuracy. For further analysis, additional methods can be used to determine the exact type and concentration of the pesticide.
Technical Specifications of the Enzyme Inhibition Method:
(1) Sensitivity: The minimum detectable level of the enzyme inhibition method should meet the following standards: Methylamine 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 of 50% or higher, the matching rate with gas chromatography should be over 80%.
Important Notes:
1. After turning the instrument on, you must re-establish the blank value.
2. Reagent Storage and Handling:
- The applicable national standard for the reagent is GB/T 5009.199-2003.
- Store the reagents in a freeze-dried state and refrigerate when necessary.
- The shelf life varies depending on the storage conditions. For example, the enzyme solution can last up to one year when stored at -20°C, while the substrate can remain stable for up to 7–10 days when refrigerated.
Some vegetables, such as amaranth, ginger, onion, garlic, pepper, and carrots, contain substances that may interfere with enzyme activity. When handling these samples, avoid cutting them too finely or extracting for too long. In some cases, it’s better to use the whole plant for extraction.
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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