|DYNAMIC LIGHT SCATTERING
measurement solutions from MICROTECH
A revolutionary high resolution optical
diffractometer, with outstanding specifications (patent
static and dynamic light scattering
5° to 150° angular range,
5 ms angle settling time
static and dynamic light scattering measurements
5° to 150° angular range, 1°resolution
5 ms angle settling time
Tutorial Three basic elements constitute diffractometers (also called scatterometers): thermostatic cell, light source, photodetector. The thermostatic cell contains the sample to be analysed, typically in a liquid or gaseous status, and allows to regulate the sample temperature according to a programmed law, under computer control. A red or green laser beam is commonly used as a light source, especially for dynamic light scattering measurements, where narrow light bandwidth and high coherence are mandatory. Due to the low scattering levels of many samples, a photomultiplier or an integrating CCD-camera is often used as a photodetector.
The above mentioned elements can be combined in a variety of configurations, depending on the mechanical and optical characteristics of the whole system. The most diffused configuration consists of a thermostatic cell with a HeNe laser beam pointing at the sample. In this case the photodetector is mounted on a moving arm, which can rotate around the axis of the sample. The scattering angle is determined as the angle between the laser and the photodetector. On most sophisticated systems the rotation is performed under computer control. A similar configuration consists of stationary photodetector, with a laser mounted on the rotating arm. In this case, only small and low power lasers can be employed. Both systems guarantee a good angular precision, but their use can be particularly cumbersome - a stiff arm must rotate around the cell. Moreover, changing the observation angle usually takes several minutes, while measurements must be suspended. This limitation prevents the simultaneous acquisition of scattering intensity from several angles, which is of special interest in the study of the evolution of fast kinetic phenomena (sol-gel transitions, etc.). In a different approach, the simultaneous acquisition at different angles is obtained using many photodetectors. For space reasons, fibers can be employed to convey the light from the sample, at the right observation angle, to the photodetector. The main drawbacks of this configuration are:
In the new DLS instrument from MICROTECH, the thermostatic cell, the laser and the photodetector are all mounted on a steady frame. Thanks to the original optical setup in the instrument, there is no need for heavy moving components, and measurement from different angles can be achieved almost simultaneously.