Bubble trouble: the control of bubbles in Zetasizer samples

Introduction

While bubbles are generally associated with fun and games, in the world of scientific measurements they are often more toil and trouble. Ordinarily, bubbles are rarely an issue when samples are prepared and immediately measured for zeta potential. However, when preparing samples in advance – such as for use with Sample Assistant¹, the automated sample loader accessory for Zetasizer Advance – these problematic bubbles can form as part of the natural (and decidedly un-fun) process of degassing. This is the release of dissolved gas molecules in a medium, posing increasing risks of bubble formation over time. The issue arises when these gases become trapped as bubbles within samples in the cell capillaries prior to zeta potential measurements. 

The presence of trapped bubbles can affect zeta potential measurements in several different ways, depending on where they occur. One major effect is on the flow within the capillary of a DTS1070 cell. The impacted flow capabilities of the sample will change the observed electrokinetics used in the calculation of zeta potential, subsequently returning inaccurate results. Additionally, a bubble present in the measurement window of the cell will affect the detected scattering light from the sample, which is essentially used to determine zeta potential. This will result in reduced data quality and inaccurate zeta potential results. Furthermore, bubbles coming into contact with the cell electrodes may cause a change in conductivity leading to increased measurement errors. 

In this application note, we demonstrate how you can preserve the quality and consistency of zeta potential results for samples prepared ahead of time using ultrasonic degassing in conjunction with Malvern Panalytical’s patented Diffusion Barrier method². This easy, effective and efficient process removes gas during sample preparation, minimizing bubble-associated risks, when used with the Sample Assistant accessory for the Zetasizer Advance range.

Experimental 

This experiment was set to simulate the conditions of samples when using the Sample Assistant. ZTS1240³ (zeta transfer standard consisting of latex in a borate buffer) was used as its properties are well-known and documented. The sample types used are shown in Table 1 below. The Diffusion Barrier method was utilized so sonication of the buffer could be carried out separately to prevent any damage or disruption to the sample.

Table 1: Different sample types monitored for bubble formation

Bubble Measurement Method

DTS1070 cells were flushed with ethanol and then water, the standard cleaning procedure. The cells were filled with each sample type shown in Table 1. For the sonicated samples, the borate buffer was placed in an ultrasonic bath for 10 minutes using the degas function. 

The samples were left at room temperature for 24 hours, with the number of bubbles in each sample recorded every hour between 0-10 hours, and measured a final time at 24 hours. 

Zeta Potential Measurement Method

The samples looked at for zeta potential measurements were:

• full cells with ZT1240
• diffusion barrier cells with sonicated buffer and ZTS1240

The precleaned DTS1070 cells were filled, and zeta potential measurements were recorded at 0 hours. Further cells were prepared in the same way and left for 24 hours before the zeta potentials and number of bubbles were recorded.

Results And Discussion

Figure 1 demonstrates the reduced accuracy of zeta potential results when bubbles are present in samples. The distribution of zeta potential results from samples left for 24 hours (with an average of 21.5 bubbles) shows a much larger distribution than at 0 hours (with an average of 0 bubbles). 

The larger spread results in samples falling much closer to the edge of the acceptable measurement range for the ZTS1240 standard, proving a need to control bubble formation.

Figure 1: Individual value plot showing the zeta potential results of ZTS1240 at 0 hours and 24 hours

In Figure 2, we demonstrate a significant reduction in the number of bubbles appearing in samples over 24 hours as a result of a diffusion barrier. Ultrasonic degassing of the buffer provided an even further reduction in bubbles, verifying it as an effective method to reduce bubble formation.

Figure 2: Average number of bubbles present in different sample types over 24 hours in Zeta cells

The zeta potential results for ZTS1240, obtained using a degassed buffer with a diffusion barrier, are presented in Figure 3. The findings indicate no reduction in accuracy after 24 hours as bubble formation was effectively prevented in the samples. In contrast to the samples with a large number of bubbles, the spread of data remained consistent, and all measurements stayed within the expected value range for the zeta transfer standard.

Figure 3: Individual value plot showing the zeta potential results of ZTS1240 with a diffusion barrier in a sonicated buffer at 0 hours and 24 hours in comparison to ZTS1240 after 24 hours

Conclusion 

Here we have demonstrated the issues bubbles pose to zeta potential measurements and how you can significantly control their formation with ultra-sonication of buffers and a diffusion barrier. 

This technique should be applied when preparing samples for use with the Sample Assistant for the Zetasizer Advance Range to prevent diminished accuracy of zeta potential results. 

In this experiment, we have used ultrasonication to reduce dissolved gasses in the buffer, but other degassing techniques, such as vacuum degassing or gas purging with inert gas, would be at least as effective - if not better.

References

1. Zetasizer sample assistant details 
2. Diffusion Barrier method – the practical details | Knowledge Center
3. Zeta Potential Transfer Standard | Malvern Panalytical MyStore