Ultrasound titrations with SOP Architect: determining optimal conditions for wet sample dispersion

Introduction

Ensuring your sample is adequately dispersed is a pivotal step in the particle size method development process. For wet dispersions, this often means applying ultrasound as an energetic means of breaking apart agglomerates. But it can be a challenge, especially for less experienced users, to find that ‘sweet spot’ in terms of ultrasound intensity and duration to achieve dispersion whilst avoiding unwanted side effects…or so we thought! With the addition of ultrasound titrations to the Mastersizer 3000+’s SOP Architect, optimizing dispersion conditions for wet samples has never been easier. 

About SOP Architect

SOP Architect is an intelligent tool within the Mastersizer Xplorer software for developing wet dispersion methods. It uses a combination of on-screen prompts, links-out to additional information and automated measurement actions to guide a user, from start to finish, in how to confidently develop an excellent quality method for their sample. It is a comprehensive solution with eight distinct stages within its workflow, including the new ultrasound titrations stage – see Figure 1.

Figure 1: Method development workflow used by SOP Architect

The importance of sample dispersion 

In computer science, there is a saying that goes ‘garbage in, garbage out’ – in other words, flawed or poor-quality data will yield a result of similar low quality. In the particle size domain, the same idea can be applied to sample dispersion and method development – ‘incorrectly dispersed material in, inaccurate data out’. 

Your sample dispersion conditions must be suitable and well-controlled to achieve the state of sample dispersion aligned with your intended aims (e.g. primary particle size determination). Otherwise, your particle size distribution (PSD) results will not be accurate, no matter how thorough the rest of your method development may be.

This underlines the importance of performing ultrasound titrations; the application of ultrasound is one of the key steps in dispersing wet samples, along with ensuring appropriate wetting and stabilization with surfactants. Indeed, ultrasonics is listed as an important method parameter in the fishbone diagrams in Annex G of ISO 13320:2020 ^[1]. 

Given the importance of ultrasound for dispersing wet samples, several of the Mastersizer 3000+’s wet dispersion accessories (Hydro EV, Hydro MV, and Hydro LV) incorporate in-line ultrasonic probes into their design. This allows ultrasound to be applied directly to the sample circulating within the Mastersizer, controlled by the Mastersizer Xplorer software, and for immediate feedback on particle size changes to be recorded. And thanks to Smart Manager functionality, it is even possible to track the health of the ultrasonic probe, so you know when it is time for a replacement before data quality is impacted! 

Ultrasound titrations – what are they? 

The process for determining optimal dispersion conditions of wet samples involves an experiment called an ‘ultrasound titration’. This enables you to determine the power and duration of ultrasound required to disperse the sample to its primary particle size, and it typically consists of the following stages:

1. A set of measurements is conducted to assess the impact of stirring without ultrasound applied – this helps to determine whether ultrasound is necessary or if stirring alone is enough to disperse the sample.
2. At a set intensity, ultrasound is applied in increments of a known duration, and particle size data (usually Dv10, Dv50, and Dv90) is recorded after each applied increment. 
3. Ultrasound intensity is varied if required, to higher or lower intensities depending on the outcome of the previous stage. The use of external ultrasound (using a probe or bath) could also be considered. 

An example of an ultrasound titration is shown in Figure 2. The sample gradually disperses under the action of stirring, but it is only when ultrasound is applied that the sample starts to disperse at a faster rate until the particle size percentiles stabilize. Once the ultrasound is stopped, the results remain stable.

Figure 2: Example of an ultrasound titration

There are several possible outcomes from the ultrasound titration – including:

• Dispersion achieved: As above, the particle size percentiles decrease with the application of ultrasound until stable values are reached as the sample becomes fully dispersed. 
• Stable: If the plots of particle size percentiles (Dv10, Dv50 and Dv90) vs ultrasound duration are flat, and no reduction in particle size is observed, then the sample may be fully dispersed without the use of ultrasound.
• Continual size decrease: If a continual decrease in particle size is observed during ultrasound, then the primary particles may be being broken by the ultrasound. In this case, a lower ultrasound intensity would be recommended. 
• Continual size increase: If a continual increase in particle size is observed during ultrasound, then the sample may be agglomerating, or heat generation is leading to beam steering. 

If possible, you should verify the state of dispersion of your sample using a microscope or a static or dynamic particle imaging instrument (such as the Hydro Insight). Making observations before and after ultrasound should be able to show you if agglomerates have been dispersed or if the particle shape has changed due to breakage.

In this case, we have shown how particle size changes in real-time as ultrasound is applied. However, this is not always possible as other effects can skew the PSD whilst ultrasound is applied, especially if your dispersant is heating up. SOP Architect uses a different approach by applying ultrasound first and then allowing a delay before starting the measurements. 

How does SOP Architect conduct ultrasound titrations? 

The ultrasound titration stage of SOP Architect uses the internal ultrasonics of the Hydro EV, Hydro MV or Hydro LV accessories. 

The ultrasound titration can be selected in addition to other tests (stability check, stirrer speed titration and laser obscuration titration), as part of your full method development process, or individually, depending on your specific requirements. And even if you do not want to conduct an ultrasound titration, SOP Architect allows you to define internal ultrasound conditions or record details of external ultrasound. See Figure 3 for details of the titration selection page.

Figure 3: Titration selection page in SOP Architect – with ultrasound titration not selected

The default ultrasound intensity used in the titration is 100 %. But when it comes time to start the ultrasound titration, SOP Architect provides flexibility in its setup. SOP Architect asks you whether your sample is ‘fragile or prone to milling’, and if you select this option the ultrasound intensity is reduced to 50%. Lower intensities can also be accessed under the ‘Advanced Settings’ – see Figure 4. 

Figure 4: Ultrasound titration setup 

The ultrasound titration starts with the usual pre-measurement steps such as dispersant filling, alignment, and background measurements, before you are prompted to add your sample in its pre-dispersed state. 

Firstly, 6 measurements are performed on the sample without ultrasound having been applied. Ultrasound is then applied in 30 second increments up to a cumulative total of 210 seconds, with 6 measurements performed after each increment. A pre-measurement delay is also applied (30 seconds for water, and 180 seconds for non-aqueous dispersants) to ensure adequate dissipation of heat before data is recorded.

Data is assessed whilst the ultrasound titration is ongoing; average data points generated after each ultrasound increment are compared by algorithms with the measurements before and after to assess stability. If %RSD values fall below relevant ISO 13320:2020 criteria, the sample is determined to be stable and SOP Architect can recommend appropriate dispersion conditions. SOP Architect will also finish the titration early if it identifies sample stability has been reached, saving the user time!

The algorithms used by SOP Architect during the ultrasound titration have been tested with a varied set of sample types, showing a 100% success rate in recommending similar conditions to particle size experts – see Table 1. 

Table 1: Comparison of SOP Architect performance vs an expert user

Following evaluation of the data, SOP Architect will present you with one of the outcomes given below.

If the titration has been successful, SOP Architect will recommend the intensity and duration of ultrasound to use with the sample. This includes the possibility of SOP Architect advising that no ultrasound is required. Either way, these details will be added to the SOP generated once SOP Architect has completed.

Alternatively, SOP Architect might indicate that additional method development is required and provide recommendations on next steps for you to follow. Take the recommendations in Figure 5 for example. 

Figure 5: Example of recommendations provided by SOP Architect after ultrasound titration

In this case, SOP Architect has identified that at the set ultrasound intensity, the sample is not completely dispersed or has not reached stability. SOP Architect provides you with two options:

• Either to increase ultrasound intensity 
• Or disperse the same using external sonication

 You can select the check box for the preferred option and then be provided with additional support. If you choose to increase the ultrasound intensity, you will then be taken back to the ultrasound titration start page and the ultrasound intensity is automatically increased.

Case study: kaolinite 

The ultrasound titration within SOP Architect was performed using a sample of kaolinite clay, a material used in many varied industries. For example, kaolinite is commonly used as a filler material in the paper industry. The particle size of the kaolinite used can have impact on the brightness of the paper, its printability and the cost efficiency of using it as a filler material. As such, it is critical to have an accurate report of its particle size distribution with agglomerates effectively dispersed. 

As shown in Figure 6, the kaolinite sample is bimodal when dispersed in water. Without ultrasound, there is a distinct shoulder at larger sizes, but this is removed by applying 100% ultrasound (within the Hydro MV) as agglomerates are dispersed. Interrogation of the particle size vs ultrasound total duration plot shows that by 120 seconds of total ultrasound, no further particle size reduction is observed – and full dispersion at this stage is confirmed through imaging (Figure 7).

Figure 6: Results from the ultrasound titration in SOP Architect when performed with a kaolinite sample - left: overlay of PSDs; right: particle size trend data versus total ultrasound duration

Figure 7: Particle images recorded by the Hydro Insight. Agglomerates are observed before ultrasound is applied (left), but following sufficient ultrasound only primary particles remain (right)

In this case SOP Architect automatically did the task of assessing the data for us and it came to the same conclusion – 120 seconds at 100% is the correct combination to disperse this kaolinite sample (Figure 8).

Figure 8: Ultrasound titration guidance from SOP Architect for kaolinite sample

Summary

The new ultrasound titration stage of SOP Architect offers significant guidance in performing one of the more advanced stages of method development. And it’s not just inexperienced users of laser diffraction who will benefit from using this as part of a comprehensive method development package. Expert users find it useful to perform the ultrasound titration as a standalone experiment – one example is when changing from the use of external sonication to internal sonication within a Hydro dispersion accessory. 

But no matter how you plan to use the ultrasound titration, be assured that SOP Architect with Mastersizer 3000+ will help you to:

• Standardize or simplify method development
• Embed best-practice in method development, and so achieve consistently high standards
• Reduce reliance on person-to-person knowledge transfer
• Incorporate Malvern Panalytical’s experience into your workflow
• Boost team independence with decision-making
• Move towards automation of your laser diffraction measurements with SOP Player

Do you want to tighten up your method development? Find out more about SOP Architect on the Mastersizer 3000+ today.

References

• [1] ISO13320:2020 – Particle size analysis – Laser diffraction methods