Dynamic Light Scattering

The Nicomp 380 Submicron Particle Size Analyzer uses the principle of Dynamic Light Scattering (DLS) to obtain the partcile size distribution of colloidal systems whose sizes range from 0.5 nanomters to 6 microns. DLS works by illuminating a group of particles in suspension with a focused laser beam which gives rise to many scattered light waves. These waves interfere with each other and produce a net scattered intensity that fluctuates as a function of time at some distant detector. Diffusion, or Brownian motion, of the particles causes random variations in the phases of the individual waves, resulting in a fluctuating light intensity. The particle size distribution can be obtained by analyzing the time behavior of these fluctuations using an autocorrelator. The autocorrelation function for a single uniform size distribution is a decaying exponential function where particle diffusivity is easily obtained from the decay time. Finally the particle radius can easily be calculated using the Stokes-Einstein relationship.

In general most samples are not uniform instead they are quite often polydisperse, having a range of particle sizes. The autocorrelation function then consists of a combination of decaying exponential functions, each having a different decay time and the analysis of the autocorrelation function is no longer quite simple. The instrument using varying deconvolution algorithms must invert the raw data in order to arrive at the best estimate of the true particle size distribution. The Nicomp excels at characterizing these difficult particle size distributions by utilizing a group of unique deconvolution algorithms ranging from a simple Gaussian approximation to a proprietary high resolution multi-modal deconvolution analyses called the “Nicomp Distribution".

Some of the unique features found on Gaussian analysis mode is a baseline adjust parameter which provides aggregate compensation that excedes the sensitivity found on most other instruments which employ a dust or dirt factor. The Gaussian analysis mode also allows for the user to specify a solid or vesicle weighting mode for analysing thin walled colloidal systems like liposomes. The Nicomp Analysis Mode is a proprietary high-resolution deconvolution algorithm that was first introduced over 25 years ago. It has historically proven its ability to accurately analyze even the most difficult closely spaced bimodals (e.g. 2:1 apart) and even certain trimodal distributions. This is extremely useful in finding the native peak of the aggregate distribution.

The standard Nicomp 380 is equipped with a 12 mW laser diode and PMT detector with an optical fiber set to 90°. Sample is introduced with drop-in cells.The 380 is the only Dynamic Light Scattering Instrument designed using a modular approach. Its capabilities may be enhanced by adding one or more modules:

Autodilution

This patented module eliminates the need for manual dilution of concentrated sample. Autodilution makes particle size analysis quick and easy, with no training required. Results are highly reproducible.

380/HPLD High Power Laser Diodes

PSS offers an array of high power laser diodes to meet the needs of our most demanding applications. Higher power lasers are needed to extend the lower limit of our instrument by providing adequate scattering from small particles. They are also useful when measuring large particles such as dextrans, which yield insufficient scattering intensity because of index of refraction properties. The result is a more versatile instrument, ideal for sizing microemulsions, surfactant micelles, proteins and other macromolecules. It can even estimate the extent of aggregration of biopolymers after reconstitution, without chromatographic separation.

Avalanche Photo Diode (APD) Detector

The Nicomp 380 can be equipped with various high-powered lasers as well as a high-gain Avalanche Photo Diode Detector (APD which provides approximately seven times the gain of a conventional photomultiplier tube. The APD is used to increase signal-to-noise and sensitivity in systems that do not scatter light well. Proteins, micelles, other macro-molecular-based systems, and nanoparticles are often dilute (1 mg/ml or less) and are made of atoms that do not scatter light well. The Avalanche Photo Diode coupled with a nominally higher powered laser diode module offers a low cost solution for accurately analyzing nanoparticles in a short period of time.

380/MA Multi-angle Goniometer

Particles larger than 100 nm do not scatter light isotropically in all directions. It is possible to make DLS measurements more sensitive to certain sized particles by changing the angle of detection. The Nicomp 380 can be equipped with a mini-goniometer that moves the optical fiber between 12° and 175° by 0.9° increments.

Zeta Potential

The Nicomp 380/ZLS is designed to measure the electrophoretic mobility and zeta potential of charged particles in liquid suspension. The main reason to measure zeta potential is to predict colloidal stability. The interactions between particles play an important role in colloidal stability. The use of zeta potential measurements to predict stability is an attempt to quantify these interactions. The zeta potential is a measure of the repulsive forces between particles. Since most aqueous colloidal systems are stabilized by electrostatic repulsion, the larger the repulsive forces between particles, the less likely they will be to come close together and form aggregates. Thus the more stable a colloid will be. The Nicomp 380/ZLS combines the techniques of Dynamic Light Scattering (DLS) and Electrophoretic Light Scattering (ELS) to measure both sub-micron particle size distributions and zeta potentials in one compact instrument.

The Nicomp 380/ZLS uses the method of Electrophoretic Light Scattering (ELS) to measure Zeta potential. To make a measurement, a small aliquot of sample is typically placed in a disposable plastic cuvette. Then the palladium electrodes are inserted. The entire cell is placed into the Nicomp 380. Because of the unique cell design, there is no need to align the cell to the stationary plane. After the cell is in place, a simple click of the mouse starts the measurement. Since ELS requires the use of heterodyned light, the scattered light must be properly mixed with a reference beam (split off from the incident light beam) prior to entering the detector. The software will begin a measurement by automatically adjusting the incident light intensity to optimize the mixing between the scattered light and the reference beam. Once this is completed, a reference power spectrum is measured while the electric field is off. Then the electric field is applied and another power spectrum is measured. The change in the frequency of the peak in this power spectrum when compared to the reference spectrum is the Doppler shift. The Doppler shift is used to calculate the average mobility. Using the Smoluchowski equation, the zeta potential is determined.

Autotitrator

The Autoitrator module gives the Nicomp 380/ZLS the ability to automatically make multiple measurements on the same sample over a series of different pH's or ionic concentrations. This allows iso-electric points to be determined.

Phase Analysis Light Scattering (PALS)

The use of Phase Analysis (instead of the standard amplitude analysis) allows for the more accurate and precise measurement of small Doppler shifts. This means that zeta potential measurements can be made in high ionic strength or high dielectric environments (like alcohols and organic solvents).