Wet Analysis Method Using Laser Diffraction

Wet Analysis Method Using Laser Diffraction 

 

 

Overview

 

The particle analysis method using wet analysis is currently the most widely used method to obtain reproducible results in laser diffraction methods. The wet analysis method allows particle size analysis over a wide range, from pigments of the sub-micron size to sand or sediments. When large particles or a mixture of various sizes are distributed, measurement can be performed using the viscosity of the dispersing medium, much like addressing the problem of particle sedimentation.  

 

In laser diffraction measurement methods, various types of dispersing media can be used, allowing easy dispersion of various types of particles. Additionally, optimization of dispersion stability, which is impossible in dry methods, is possible. This is because the dispersion energy of particles within the dispersing medium in wet methods is higher than the dispersion energy in dry methods.  

 

Therefore, wet analysis is sometimes said to be the only method for reproducible dispersion of sub-micron materials and is particularly suitable for large particles such as viscous or strongly aggregated particles. Compared to dry methods, particle breakage in the wet analysis method is not a significant issue. 

 


Obtaining good results using the wet analysis method is possible, but only when the analyst has developed and applied a precise analysis method. This application note focuses on providing information on important factors to consider when setting up the SOP for wet analysis using the laser diffraction method to device users. Setting up for laser diffraction measurement is easy and simple, but for accurate analysis, considering matters related to the sample, as indicated below, is crucial. 

 


• The representative sample amount for analysis must reflect the entire sample’s characteristics.
• Selecting and stabilizing the dispersing medium for powder dispersion requires various additives. 

 


The user should consider appropriate dispersing devices related to the inherent characteristics of the powder itself for consistent results in both the state of aggregation and well-dispersed states. This is possible by adjusting factors such as those listed below. 

 


• Other analysis option settings such as measurement settings and obscuration settings
• Dispersal energy for sample dispersion using ultrasound. 

 

This document assumes that the user is familiar with the operation of the basic laser diffraction system. In particular, it is relevant to the wet dispersion equipment of the Mastersizer 2000 model using Hydro 2000S, Hydro 2000G, and Hydro 2000MU, all of which allow ultrasonic treatment of samples and ongoing real-time monitoring with the dispersion process. However, the development process of analysis methods introduced here can also be applied to other wet analysis devices using the Malvern laser diffraction system. The part concerning the selection of optical properties was provided elsewhere.

 


Sample Representativeness

 

Sampling the analysis sample from the bulk material is crucial in particle analysis. If large particles (over 70 microns) exist in the sample, it is especially important. Since laser diffraction is a measurement technology based on volume, it is sensitive to small changes in large particles within the sample. This is because the large volume taken up by coarse particles is similar to that of many small particles (the volume of one 100-micron particle is equal to that of one million 1-micron particles, showing the same dispersion response). Sample-related matters are covered in Reference 1. 
 

If sampling is controlled, it is easy to achieve each measurement reproducibility specified in ISO13320-1, and the ISO standard for laser diffraction measurement is within 3% at D(v,0.5), and within 5% at D(v,0.1) and D(v,0.9). If sampling is not controlled, measurement errors can exceed 20%. 



Selection of Dispersing Medium


Choosing the appropriate dispersing medium for wet analysis is the next step. The following aspects are important for dispersing medium selection. 

• Is it transparent enough for laser beam penetration for laser diffraction measurement?
• Does it have a different refractive index from the particles to be measured (the refractive index difference must be sufficient to distinguish the measured scattering intensity)?
• Does not dissolve the substance during measurement.
• Is it possible to measure in a non-agglomerated wet state?
• Is it stable enough to prevent re-agglomeration?
• It should not be too viscous, and no bubbles should form.

 

 




Water, due to its transparency and low cost, is generally the first choice. For large particles, even if the quality of regular tap water might change compared to when the SOP was set, it can be used. Measurements should be carried out with deionized water if possible, and especially for sizes below 20, as salts in tap water can cause particle agglomeration.
The addition of surfactants or other stabilizers may also be necessary to prevent the agglomeration of fine particles.  


 

Many substances, especially pharmaceuticals, dissolve well in water. For such substances, a dispersing medium that reduces polarity is necessary (Table 1). In such cases, the choice of dispersing medium should rely on testing for the substance’s exhibited polarity. The cost of non-aqueous dispersing media is also a factor to consider. 

 

Surfactants 

 


The first step in the wet analysis process is to wet the particle surface with the selected dispersing medium. In some cases, a surfactant may be required to facilitate wetting.

 

Surfactants serve to lower the surface tension of the dispersing medium. This reduces the contact angle between the dispersing medium and particle surface, aiding dispersion.
 

The choice of surfactant depends on the chemical properties of the particle surface and the dispersing medium. Non-ionic surfactants (e.g., Tween 20/80; Span 20/80) can generally be safely used in pharmaceutical or food fields. In some cases, anionic surfactants like SDS or cationic surfactants like CTAB that not only improve wetting by adsorbing on the particle surface but also stabilize particles, preventing agglomeration, can be used. 

 


Guidance on surfactants according to the particle and dispersing medium is specified in ISO 14887. In all cases, the concentration of surfactants must be carefully controlled. Surfactants can be active even at very low concentrations (>0.001g/l). Generally, one to two drops of a surfactant dilution solution (10%) can significantly improve particle wetting. Higher concentrations of surfactants can lead to foam formation in the dispersing device, which can cause detection of non-existent large particles in the sample.
 

Stabilizers 

 

Once the particles are dispersed in the selected dispersing medium, preventing re-agglomeration is crucial. An aqueous dispersing medium can be stabilized by controlling the conductivity of the suspension. This is a simple reason for using deionized water instead of tap water when water is used as a dispersing medium. Additionally, substances like Sodium Hexametaphosphate, Ammonium Citrate, or Sodium Pyrophosphate can also be used as charge stabilizers. Each material increases the zeta potential, thereby increasing the suspension’s stability (this is related to particle charge). However, the excessive addition of stabilizers can also cause problems in dispersion. Typically, stabilizers are added at less than 1 w/v%, otherwise, the solution’s conductivity becomes too high, leading to agglomeration. A good example of optimized dispersion using stabilizers is referenced in [4].  

 

Appropriate Dispersing Medium Selection 

 

It is possible to choose proper dispersion conditions without measuring particle size. Samples during experimentation can be dispersed in bottles with different dispersing media.  

If particles float on the surface of the dispersing medium, poor wetting occurs. This can be resolved by adding a surfactant or switching to a dispersing medium with a lower surface tension. 

Total Dispersing Medium

 

If particles have been wetted by the dispersing medium, perform ultrasonic treatment on the sample and observe sedimentation. For example, in Figure 1, sodium hexametaphosphate is used as a stabilizer in the cerium oxide suspension.

As shown, at low suspension concentrations, it is unstable with definite sedimentation. The suspensions at 0.01 wt% and 0.5 wt% are stable – in this case, the approximate average of 0.25 wt% provides the strongest dispersion stability. 

Finally, the suspension becomes unstable if the concentration of sodium hexametaphosphate exceeds 0.5 wt%. This is due to the increase in ion strength of the solution. 

 

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