Quality control of pesticides via laser diffraction particle size analysis

From the key active ingredients to the final pesticide product, particle size is known to play a critical role in the residual period, the biological activity, and the stability properties of pesticides.

Measuring the particle size distribution is crucial when establishing the quality of pesticides during QC inspection. This article outlines a study in which the laser diffraction method measured particle size distributions of suspension concentrate samples.

Typical size values and size distribution curves were contrasted to assess the quality of pesticide samples and to help to enhance the milling process and create a pesticide that is fit for a particular application.

Pesticides are extensively used in the agricultural industry: a pesticide is defined as any substance or mixture of substances intended for preventing, destroying, or controlling any pest, according to the Food and Agriculture Organization (FAO).1

The pesticide can be a suspension concentrate (SC), emulsion in water (EW), suspoemulsion (SE), wettable powder (WP), or microemulsion (ME), etc. ² Measuring the particle size distribution (PSD) is regarded as a critical part the quality inspection process throughout the pesticide industry.

The PSD of the pesticide’s active ingredient is affiliated with its photodegradation speed, volatility, and specific surface area. Thus, the biological activity and residual period of the pesticide are contingent on the PSD of the active ingredients.

Regarding the final products of the pesticide, the PSD of particles, which includes powders but specifically droplets, impacts the coverage area. In the case of crop spraying, it is crucial to determine if a spray’s droplets are too large, as they are more likely to fall from the leaves onto the ground.

Pesticides on the ground do little to help control pests living on the crop surface. If the spray is too fine, the droplets can be carried away by the wind, meaning the spray does not treat the targeted area and instead ends up on barren land or in another farmer’s field.

Therefore, a PSD of appropriately sized droplets – not too large, not too fine – is desired. Additionally, the PSD of the dispersed active ingredient particles affects the dispersity and stability of the pesticide product.

As a result,  in the preparation and production of pesticides, both the PSD of the active ingredients and the PSD of the final product are essential factors to control.

The Bettersizer ST laser diffraction particle size analyzer was employed for assessing the PSD of pesticide products and their active ingredients used in this study. The patented dual lens optical system (DLOS) in the Bettersizer ST delivers a measuring range of 0.1-1000 μm, which is appropriate for the majority of quality inspection processes in the pesticide industry.

Besides offering a broad measuring range, the DLOS technique also allows the Bettersizer ST to offer the user highly repeatable and efficient PSD measurement results.

Evaluate Pesticide Products with PSD Results

The suspension concentrate (SC) is produced by dispersing insoluble solid active ingredient particles in the aqueous continuous phase. Generally speaking, the smaller the active ingredient particles, the shorter the residual period, which enhances the biological activity of the SC product.

In the study detailed in this article, the PSD of three SC samples was measured to assess the quality of these samples. These SC samples are made up of various formulations that contain different wetting agents and dispersants but are primarily made up of the same active ingredient particles.

The standard size values of the three SC products are displayed in table 1. The D50 of formula 2 is considerably smaller than formula 1 and 3, meaning that it possesses a better suspension and the stability of the SC product is also better.

Table 1. Typical size values of SC in different formulations. Source: Bettersize Instruments Ltd.

The particle sizes are largest in Formula 1, more than 5 μm. As a result, it is vulnerable to sedimentation, decreasing its storage stability and shelf life.

PSD Results Help the User to Optimize and Control the Milling Time of SC Product

Pre-milling is an extensively used technique in the manufacture of SC products. In this study, the effects of different milling times on the particle size distribution of SC were evaluated. The trend plot and standard size values are displayed in table 2 and figure 1.

Table 2. Typical size values of SC under different milling times. Source: Bettersize Instruments Ltd.

Figure 1. Trend size plot of SC under different milling times. Image Credit: Bettersize Instruments Ltd.

It is apparent from both table 2 and figure 1, that when milling time increases, D50 decreases from 1.015 μm to 0.812 μm, and D90 progressively decreases from 12.39 μm to 4.749 μm. The standard size values did not change much when the milling time was prolonged from 3.5 hours to 4 hours.

As the particles shrink, the surface area becomes larger, and agglomeration is most likely to occur whenever the Iso Electric Point (IEP) is approached for the formulation. In the absence of an active dispersant, a finer particle size distribution cannot be acquired by extending milling time, so the standard size values become stable.

The BeVision S1 image particle size analyzer was employed to examine the milled SC particles. As displayed in Figure 2, it can be determined that the coarse particles above 10 μm were present at 0.5 hours, and the size of particles is much finer once the milling time reaches 4 hours.

Figure 2. Images of SC under the milling time of 0.5 hours(a) and 4 hours(b). Image Credit: Bettersize Instruments Ltd.

It can be concluded that the distribution of SC under different milling times can be monitored effectively by the laser diffraction particle size analyzer, thus safeguarding the stability of the product performance.

Conclusions

Firstly, measuring the PSD of active ingredient particles is crucial when assessing the quality of SC pesticides. With PSD curves, manufacturers could enhance the formula of the components efficiently to guarantee the optimal performance of SC pesticide products.

Subsequently, when processing pesticides, the Bettersizer ST can examine the product particle size distribution and effectively ensure the stability of the product's performance.

To sum up, for those that need to diagnose problems related to the PSD, the Bettersizer ST is a practical tool to provide rapid, consistent and reliable PSD results which can be used to improve the production process.

References

1.Food and Agriculture Organization of the United Nations. (2002) International Code of Conduct on the Distribution and Use of Pesticides. FAO, Rome, Italy.

2.CropLife International. (2017) Catalogue of pesticide formulation types and international coding system (7th edition). CropLife International A.I.S.B.L., Brussels, Belgium.