Methomyl is a widely used carbamate insecticide that has been detected in surface and groundwater due to its high solubility and persistence. Methomyl oxime is an intermediate and degradation product in methomyl production and environmental transformation. Both compounds pose potential risks to aquatic ecosystems and human health. Chinese standard HJ 851-2017 specifies an HPLC method for their determination in water. This solution presents a validated HPLC method for the analysis of methomyl and methomyl oxime using a SinoPak SP C18 column.
Standards
Methomyl and methomyl oxime standards: Purity ≥ 98.5%.
Reagents
Acetonitrile, HPLC grade; Deionized Water, ≥18.2 MΩ·cm).
Other materials
Volumetric flasks, pipettes, membrane filters (0.45 μm), etc.
Standard Solution Preparation
Stock solution (10 μg/mL each): Pipette 1 mL of 100 μg/mL methomyl and methomyl oxime standards into a 10 mL volumetric flask and dilute to volume with water.
Working standard solutions: Dilute the stock solution with water to obtain concentrations of 5, 10, 20, 50, 100, 200, 500, and 1000 μg/L.
Filter water samples through a 0.45 μm membrane filter. The filtrate is directly injected into the HPLC system.
HPLC system
Agress 1100 configured with Two P1100 pumps, D1100 UV detector, O1100 column oven, Rheodyne 7725i injector, TD-1-15 gradient mixer, Chromatography data station
Column
SinoPak SP C18 (5 μm, 4.6 × 150 mm)
Pretreatment equipment
Solvent filtration apparatus, vacuum pump, ultrasonic cleaner, etc.
Mobile phase: A: Acetonitrile; B: Water, in gradient (Table 1)
Table 1. Gradient program
| Time (min) | A% | B% |
|---|---|---|
| 0 | 30 | 70 |
| 6 | 30 | 70 |
| 8 | 100 | 0 |
| 14 | 100 | 0 |
Flow rate: 1.0 mL/min
Detection wavelength: 232 nm
Injection volume: 50 μL
Column temperature: 30°C
Typical Chromatogram
A standard solution containing 5 μg/L each of methomyl oxime and methomyl was analyzed. The chromatogram (Figure 1) shows baseline separation with retention times of approximately 4.5 min (methomyl oxime) and 7.2 min (methomyl).
Figure 1. Typical chromatogram of methomyl oxime (peak 1) and methomyl (peak 2) standards (5 μg/L)
Linearity
Calibration curves were constructed over the range of 5–1000 μg/L. Both compounds showed excellent linearity with correlation coefficients > 0.999. Linear equations were y = 0.0367x + 0.0214, R = 0.9995 for Methomyl oxime, and y = 0.0298x + 0.0156, R = 0.9993 for Methomyl.
Detection and Quantification Limits
Based on S/N = 3 and S/N = 10, the instrument and method detection limits are summarized in Table 2.
Table 2. Detection and quantification limits
| Compound | Instrument LOD (μg/L) | Instrument LOQ (μg/L) | Method LOD (μg/L) | Method LOQ (μg/L) |
|---|---|---|---|---|
| Methomyl oxime | 0.7 | 0.8 | 2.2 | 3.2 |
| Methomyl | 1.0 | 0.9 | 3.4 | 3.6 |
Repeatability and Accuracy
Method repeatability was evaluated by spiking blank water samples at three levels (10, 100, and 800 μg/L) with three replicates each. Results are shown in Table 3 - 5. RSDs were below 2.7%, and recoveries ranged from 81.9% to 103.9%, meeting the requirements of HJ 851-2017.
Table 3. Repeatability at 10 μg/L spike
| Compound | Mean measured (μg/L) | RSD% |
|---|---|---|
| Methomyl oxime | 9.82 | 2.08 |
| Methomyl | 8.19 | 2.66 |
Table 4. Repeatability at 100 μg/L spike
| Compound | Mean measured (μg/L) | RSD% |
|---|---|---|
| Methomyl oxime | 91.35 | 2.25 |
| Methomyl | 84.84 | 2.29 |
Table 5. Repeatability at 800 μg/L spike
| Compound | Mean measured (μg/L) | RSD% |
|---|---|---|
| Methomyl oxime | 831.65 | 0.38 |
| Methomyl | 783.28 | 0.71 |
Sample Analysis
Surface water and domestic wastewater samples were analyzed using the method. No methomyl or methomyl oxime were detected (Figures 2).
Figure 2. Chromatogram of (A) surface water sample and (B) sanitary waste.
The HPLC method using a SinoPak SP C18 column provides reliable determination of methomyl and methomyl oxime in water with excellent linearity, sensitivity, and repeatability. It fully complies with HJ 851-2017 and is suitable for routine monitoring.
