Determination of Metalaxyl Technical Material by Wayeal Gas Chromatography

Metalaxyl is a highly effective, low-toxicity systemic fungicide of the phenylamide class. It provides excellent curative activity against diseases such as downy mildew, phytophthora blight, and blackleg caused by Plasmopara, Phytophthora, and Pythium species. It is widely applied in the agricultural production of cucumber, tomato, potato, grape, tobacco, and other crops. The mass fraction of the metalaxyl active ingredient is a core indicator of the technical material's efficacy and directly affects the performance of the final formulation. As a key intermediate impurity generated during the synthesis process, the residual level of 2,6-dimethylaniline reflects the product purity and the degree of process control. To ensure the quality and safety of metalaxyl technical material, it is essential to accurately determine the mass fractions of the active ingredient and 2,6-dimethylaniline.

This article references the Chemical Industry Standard of the People's Republic of China — Metalaxyl Technical Material (HG/T 2206-2015) and employs Wayeal's GC6100 gas chromatograph equipped with an FID detector for the analysis of metalaxyl technical material.

Keywords: metalaxyl; 2,6-dimethylaniline; gas chromatography; FID detector.

1. Instrument Configuration

Table 1 Configuration List of Gas Chromatograph

2. Determination of Metalaxyl Mass Fraction

2.1 Experiment Material and Auxiliary Equipment

Acetone

Metalaxyl reference standard (mass fraction: 99.4%)

Internal standard: diamyl phthalate

Carrier gas: high-purity nitrogen

Hydrogen generator

Air generator

2.2 Test Conditions

GC Reference Conditions

Column: DB-1701 capillary column (30 m*0.32 mm*0.25μm)

Oven Temperature: 200°C, held for 20min

Column Flow: 2.0mL/min

Inlet Temperature: 240°C

Detector Temperature: 300°C

Air Flow: 300mL/min

Hydrogen Flow: 30mL/min

Makeup Flow (N₂): 25mL/min

Injection: Split mode, split ratio 30:1

Injection Volume: 1μL

2.3 Solution Preparation

2.3.1 Internal Standard Solution

Accurately weigh 4.0g of diamyl phthalate and transfer it into a 500mL volumetric flask. Dissolve in an appropriate volume of acetone, dilute to the volume with acetone, and mix thoroughly.

2.3.2 Standard Solution

Weigh 0.1019g of metalaxyl reference standard into a glass stoppered bottle, add 10mL of internal standard solution using a pipette, and mix well.

2.3.3 Test Solution

Accurately weigh 0.1015g of metalaxyl technical material and transfer it into a glass stoppered bottle. Using a pipette, add 10mL of internal standard solution, and mix thoroughly.

2.4 Result and Discussion

2.4.1 Reference Standard Qualitative Test

Fig. 1 Chromatogram of Diamyl Phthalate

Fig. 2 Chromatogram of Metalaxyl Reference Standard Solution

Table 2 Chromatographic Parameters of Metalaxyl Reference Standard Solution

Note: As shown in the chromatogram above, the chromatographic peaks of metalaxyl and diamyl phthalate are well separated, meeting the requirements of experimental analysis.

2.4.2 Sample Test

Fig. 3 Chromatograms of Repeated Injections of Metalaxyl Reference Standard Solution (n=2)

Table 3 Repeatability Test Data for Metalaxyl Reference Standard Solution

The metalaxyl reference standard solution was continuously injected twice in succession. The RSD of the peak areas of metalaxyl between the two consecutive injections was 0.174%, and the RSD of the peak areas of the internal standard was 0.186%, both meeting the standard requirement of RSD less than 1.2%. The analysis was carried out in the order of reference standard solution, test solution, test solution, reference standard solution.

Fig. 4 Chromatogram of Metalaxyl Reference Standard Solution

Fig. 5 Chromatogram of Metalaxyl Technical Material Test Solution

The peak area ratios of metalaxyl to the internal standard measured in the two injections of the test solution, as well as in the two injections of the reference standard solution before and after the test solution, were averaged respectively. The mass fraction of metalaxyl in the test sample was calculated using the following formula.

w1=（r2×m1×w）/（r1×m2）

w1: mass fraction of metalaxyl in the test sample, expressed as a percentage (%);

r2: average peak area ratio of metalaxyl to the internal standard in the test solution;

m1: mass of the reference standard, in grams (g);

w: mass fraction of metalaxyl in the reference standard, expressed as a percentage (%);

r1: average peak area ratio of metalaxyl to the internal standard in the reference standard solution;

m2: mass of the test sample, in grams (g)

Table 4 Test Data for Metalaxyl

Note: The calculated mass fraction of metalaxyl in the test sample is 97.9%, which meets the standard requirement (≥95.0%).

3. Determination of 2,6-Dimethylaniline Mass Fraction

3.1 Experiment Material and Auxiliary Equipment

Acetone;

2,6-Dimethylaniline reference standard, mass fraction: 99.5%;

Internal standard: n-Dodecane;

Carrier gas: High-purity nitrogen;

Hydrogen generator;

Air generator.

3.2 Test Conditions

Gas Chromatograph Reference Conditions

Column: DB-1701 capillary column, 30 m × 0.32 mm × 0.25μm;

Temperature Program: Oven temperature held at 80°C, then ramped to 120°C at 5°C/min and held for 2 minutes, then ramped to 250°C at 30°C/min and held for 5 minutes;

Column Flow Rate: 2.0mL/min;

Inlet Temperature: 260°C;

Detector Temperature: 300°C;

Air Flow Rate: 300mL/min;

Hydrogen Flow Rate: 30mL/min;

Makeup Flow Rate: 25mL/min;

Split Injection: Split ratio 5:1;

Injection Volume: 2μL;

3.3 Solution Preparation

3.3.1 Internal Standard Solution

Weigh 0.025g of n-dodecane into a 500mL volumetric flask, add an appropriate amount of acetone to dissolve, dilute to the mark, and mix well.

3.3.2 Standard Solution

Weigh 0.0529g of 2,6-dimethylaniline reference standard into a 100mL volumetric flask, add acetone and shake to dissolve, then dilute to the mark with acetone and mix well. Transfer 1mL of the above solution using a pipette into a glass stoppered bottle, add 10mL of internal standard solution, and mix well.

3.3.3 Test Solution

Weigh 0.2535g of metalaxyl technical material into a glass stoppered bottle, add 10mL of internal standard solution using a pipette, and mix well.

3.4 Result and Discussion

3.4.1 Reference Standard Qualitative Test

Fig. 6 Chromatogram of n-Dodecane

Fig. 7 Chromatogram of 2,6-Dimethylaniline Reference Standard Solution

Table 5 Chromatography Parameters of 2,6-Dimethylaniline Reference Standard Solution

Note: As shown in the chromatogram above, the chromatographic peaks of n-dodecane and 2,6-dimethylaniline are well separated, meeting the requirements of experimental analysis.

3.4.2 Sample Test

Fig. 8 Chromatograms of Repeated Injections of 2,6-Dimethylaniline Reference Standard Solution (n=2)

Table 6 Repeatability Test Data for 2,6-Dimethylaniline Reference Standard Solution

Two consecutive injections of the 2,6-dimethylaniline reference standard solution were performed. The RSD of the peak areas of the internal standard between the two consecutive injections was 0.023%, and the RSD of the peak areas of 2,6-dimethylaniline was 0.076%, meeting the requirement of RSD less than 1.2% specified in the standard. The determination was performed in the following sequence: reference standard solution, test solution, test solution, reference standard solution.

Fig. 9 Chromatogram of 2,6-Dimethylaniline Reference Standard Solution

Fig. 10 Chromatogram of Metalaxyl Technical Material Test Solution

The peak area ratios of 2,6-dimethylaniline to the internal standard obtained from the two injections of the test solution and from the two injections of the reference standard solution (one before and one after the test solution) were averaged separately. The mass fraction of 2,6-dimethylaniline in the test sample was calculated using the following formula.

w2=（r2×m1×w）/（r1×m2×n）

W2 = mass fraction of 2,6-dimethylaniline in the test sample (%)

r2 = average peak area ratio of 2,6-dimethylaniline to internal standard in the test solution

m1 = mass of the reference standard (g)

w = mass fraction of 2,6-dimethylaniline in the reference standard (%)

r1 = average peak area ratio of 2,6-dimethylaniline to internal standard in the reference standard solution

m2 = mass of the test sample (g)

n = dilution factor, n = 100

Table 7 Test Data for Metalaxyl

Note: The calculated mass fraction of 2,6-dimethylaniline in the test sample is 0.006%, which meets the standard requirement (≤0.1%).

4. Conclusion

In this test, Wayeal's GC6100 gas chromatograph equipped with an FID detector was used to determine the mass fractions of metalaxyl and 2,6-dimethylaniline in metalaxyl technical material samples. The experimental results showed that the chromatographic peaks of metalaxyl and diamyl phthalate, as well as those of 2,6-dimethylaniline and n-dodecane, were well separated, meeting the requirements of experimental analysis. Calculated using the formulas, the mass fraction of metalaxyl in the test sample was 97.9%, and the mass fraction of 2,6-dimethylaniline was 0.006%, both complying with the standard specifications. This demonstrates that the method, utilizing Wayeal's GC6100 instrument, meets the analytical requirements for metalaxyl technical material.