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.
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.
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).
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