5 Reasons Why Zetasizer Is Still The Most Widely Used Dynamic Light Scattering (DLS) System

Dynamic light scattering (DLS) is now a ubiquitous tool in many laboratories, and offers an accessible and accurate way to determine hydrodynamic size distribution in minutes. The non-invasive technique requires very little sample and is quite easy to use for a range of user abilities. 

The newly released Zetasizer Lab, Pro and Zetasizer Ultra are the latest editions to the Zetasizer range. Adding ease and performance to the popular Zetasizer Nano range, they offer unique updated measurement features, hardware capabilities and software intelligence that are unmatched. In this article we deep dive into the top 5 features and updates of the Zetasizer Pro and Ultra systems and share some insights into how you can get the most out of your DLS instrument – and your analysis.

An overview: how DLS technology works

In DLS, the speed at which particles diffuse due to Brownian motion is measured. This is done by shining light directed to a sample contained in a cell. For dilute samples most of the laser light passes through the sample but some light will be scattered by particles in all angles. A detector is used to measure the intensity of the scattered light. In the Zetasizer advanced series, the detector position will be either at 173° (non-invasive backscatter) or 90° (side scattering) or 13° (forward scattering). 

The intensity of scattered light must be within a specific range for the detector to successfully measure it. If too much light is detected, then the detector will become saturated. To overcome this, an attenuator is used to reduce the intensity of the laser source and hence reduce the intensity of scattering. For samples that do not scatter much light, such as very small particles or samples of low concentration, the amount of scattered light must be increased. In this situation, the attenuator will allow more laser light through to the sample. 

The scattering intensity signal from the detector is passed to a correlator which compares the scattering intensity at successive time intervals to derive the rate at which the intensity is varying. This correlator information is then passed to the Zetasizer software to analyse the data and derive size information.

Zetasizer is the standard in light scattering for over 40 years

What sets the Zetasizer apart for other similar systems is the performance, reliability and ease of use. Over 40 years ago, the Malvern correlator opened doors to a field of research and development investigating ever smaller particles, and continuously advancing since then. Since first launched over 2 decades ago, the Zetasizer Nano series has been the standard for performing dynamic light scattering (DLS) measurements on a wide range of particles and materials. The Zetasizer Nano was the first system that combined dynamic, electrophoretic and static light scattering in one instrument. Zetasizer quickly gained well-deserved attention with core features including fast, simple-to-use, yet sophisticated software with built in guidance. Most significantly, Zetasizer is known not only for its ability to provide the highest sensitivity but also the widest concentration range from the then novel Non-Invasive Back Scattering (NIBS). NIBS reduces the effect known as multiple scattering where light from one particle is itself scattered by other particles by moving the focusing lens and changing the measurement position. In this way light passes through a shorter path length of the sample, allowing for higher concentrations and turbid/opaque samples to be measured.

The need to clarify, NIBS and Backscatter are not the same

NIBS is one of the key features with unique functionality that separates the Zetasizer from other DLS systems. The patented NIBS technology enables the highest sensitivity for both small and large particles even for the most concentrated samples. This unique ability to perform at the highest level no matter the application has resulted in the Zetasizer Nano being the most popular instrument for sizing by DLS, with over 100,000 peer-reviewed publications.

DLS has traditionally used 90 ° detection angles. Adding a backscatter angle provides several benefits allowing for higher sensitivity and a higher size range with increasing volumes. Backscatter measurements are also less sensitive to large particulates such as dust, removing the need for time consuming sample preparation traditional 90° measurements require. However, the benefits of backscatter come with compromises; increased volume reduces the high concentration range, increased flare creates more noise and a reduced sensitivity may be unable to detect the presence of important aggregates. 

These are overcome in the Zetasizer using NIBS at a detection angle of 173°. For aggregate detection, a forward angle of 13° is employed to detect the presence of aggregates at much lower concentrations (with higher sensitivity) than backscatter or 90°.

NIBS and automatically determines the optimum measurement position within the cuvette and correct attenuation of the laser for the sample being measured. When analysing very low concentrations or weakly scattering particles, NIBS automatically positions the detector optics at the center of the cell to maximise the scattering volume. As the concentration or scattering intensity increases, it avoids multiple scattering by moving the optics across the cell in small increments. At high concentrations the optics will be positioned at the cell wall, reducing the path length and therefore minimising multiple scattering. This together with the attenuator, which automatically adjusts to ensure the optimum amount of light is used, ensures that no matter what the concentration, size and scattering efficiency, the optimal results are reached covering the broadest range of applications. These features make NIBS unique, providing extremely useful functionality unavailable on other instruments (even those using back scatter detection).

New Zetasizer Advance series – top 5 features

The Zetasizer Ultra has multiple features that help to reduce the time taken for measurements while providing much more detail on sample properties.

1. Faster size with Adaptive Correlation (AC), and better size data. It takes less time to make a measurement. You can also get data from samples that were too noisy before.

Adaptive correlation is a new approach for capturing and processing DLS data. It uses statistical models to highlight any captured data that is not representative of the sample such as rare dust particles. Multiple short sub runs are performed and the resultant correlation functions are averaged to reduce the effects of noise. AC allows the characterisation of consistent and steady size components without data being skewed by intermittent or transient scatters. In this way, the measurements can exclude effects of dust while also increasing measurement speed and repeatability. There is also less need for filtering of samples and dispersants, simplifying sample preparation procedures. 

2. AI guided, neural network help with size data quality advice: Even a novice without any prior light scattering knowledge can make sense of sizing data.

The new ZS Xplorer software offers intuitive, guided workflows that make setting up a method and performing a measurement easy and straight-forward. Using an artificial intelligence (AI) led approach to data quality assessment, it brings attention to any potential measurement issues and provides guidance on how to improve them.

3. Fluorescence filter wheel allows for measurement of fluorescent samples which can cause large background noise in the data. The fluorescence filter eliminates that noise and makes a measurement possible despite the presence of fluorescence. 

For fluorescent samples like quantum dots, light emitted by the sample other than laser scattering will decrease signal to noise. The Zetasizer has an option that can minimise the effect of (incoherent and thus undesirable for dynamic light scattering) fluorescent light: the fluorescence filter eliminates most light that is not very close to the laser wavelength. The fluorescence filter is an optical component consisting of glass with a special coating to reflect light outside the designated wavelength range. Therefore it only allows a select wave length range for transmission.

4. Polarisation filter, both vertical and horizontal polarisation components can be detected, potentially gaining insights into particle rotational diffusion.

Adding a polarising filter can clean up the optical signal by removing any depolarised light which can be a source of noise in measurements caused by multiple scattering. This feature provides versatility to measure over a wide concentration range, improving signal-to-noise without impairing overall system sensitivity. In addition to a vertical polariser, the Zetasizer Pro and Ultra have a horizontal polariser which measures depolarised light. A depolarised DLS signal can be used to detect rotational diffusion which indicates differences in particle shape and whether particles are spherical or have surface differences.

5. Novel 3uL low volume size cell, lowers sample volume and extends concentration range for 90 degrees: You can measure even turbid samples now at 90 degrees, which was previously not possible and required NIBS backscattering. 

As particle size increases, thermal Brownian motion is no longer sufficient to keep particles in suspension, and samples may sediment over time, meaning that the motion of the particles is no longer random. In addition measurements of particles over 1 micron in size may show some difference in variability as a function of temperature, suggesting that thermal effects may influence the artefacts seen in the measured correlation functions. The geometry of the 1mm capillary used in the low volume disposable sizing cell helps to prevent the formation of convection currents and thus allows accurate measurements without modification of the sample dispersant over the entire measurable size range for DLS. Repeatability for polydisperse samples is improved over comparable measurements in a standard cuvette.  The cell also eliminates the errors associated with multiple scattering, allowing samples to be measured over a wider dynamic concentration range than would normally be possible at side scatter (90°).

Contact us for more information or to book a demonstration with one of our specialists.

Additional resources

• Ask An Expert: Advanced Dynamic Light Scattering: When can a polarizer give you extra insight? | Malvern Panalytical
• Zetasizer Advance in bullet points
• Measuring the size of gold nanoparticles using multi-angle dynamic light scattering (MADLS)
• When is a particle not a particle webinar
• Multiple scattering effects on intercept, size, polydispersity index, and intensity for parallel (VV) and perpendicular (VH) polarization detection in photon correlation spectroscopy
• An Introduction to Dynamic Light Scattering (DLS) | Malvern Panalytical
• https://www.materials-talks.com/polarizers-huh-what-are-they-there-for/
• https://www.malvernpanalytical.com//en/learn/knowledge-center/technical-notes/an151119nibsvsbackscatter?
• Pushing the limits of DLS: Measuring large particles on the Zetasizer Ultra with the Low-Volume Sizing Cell | Malvern Panalytical