Dynamic Light Scattering (DLS) is an established and precise measurement technique for the characterization of particle sizes in suspensions and emulsions. Microtrac is a pioneer of particle analysis technology and has been developing optical systems based on Dynamic Light Scattering for over 30 years.
Dynamic Light Scattering (DLS) is an established and precise measurement technique for characterizing particle sizes in suspensions and emulsions. It is based on the Brownian motion of particles - this states that smaller particles move faster, while larger ones move slower in a liquid. The light scattered by particles contains information on the diffusion speed and thus on the size distribution.
Dynamic light scattering enables the analysis of particles in a size range from 0.3 nm to 10000 nm. While Laser Diffraction (LD) often reaches its limits for particles smaller than 100 nm due to the weak signal and the low angular variance in scattering signal, this is where the strength of dynamic light scattering lies.
It is a technique suited to the analysis and characterization of nanoparticles. Other advantages include measurements of both highly concentrated and highly dilute samples, and the ability to determine Zeta Potential, molecular weight and concentration, which is built into many analyzers.
The diffusion coefficients (D) of the particles are inversely proportional to the size (dp, hydrodynamic diameter) of the particles according to the Stokes-Einstein relationship.
(k=Boltzmann constant, T=temperature, η= viscosity )
To determine the particle size accurately, the precise value of parameter T (temperature) and η (viscosity) of the liquid must be known.
The Dynamic Light Scattering (DLS) technique measures motion optically by recording the scattered light signal at a fixed angle. The particles are illuminated with a monochromatic, coherent light source (laser) and the light scattered by the particles is recorded.
The temporal fluctuation of the scattered light signal is important here, because it contains information about the movement of the particles. The fluctuations are caused by the fact that the particles scattering the light move relative to each other, resulting in constantly changing interferences within the total scattered light. The light scattered by the particles thus contains slight frequency shifts caused by the time-dependent position or velocity of the particles. Measured over time, motion causes a distribution of frequency shifts.
These shift frequencies can be determined by comparison with a coherent optical reference. In Dynamic Light Scattering, the shift frequencies are on the scale of 1 Hz to 100KHz, which can be easily measured.
Two approaches exist for optical reference: homodyne detection (also called "self-beating" or "self-reference") and heterodyne detection ("reference beating" or "controlled reference").
In the homodyne approach, the scattered light itself provides the reference for determining the frequency shift. In contrast to this, controlled reference, or heterodyne detection, superimposes the scattered light on a portion of the incident light, which provides the reference for determining the frequency shifts. The resulting detector signal in both methods contains a distribution of frequencies that is representative of the size of the particles in suspension.
Of the two approaches, the heterodyne mode with "controlled reference" offers many advantages over the homodyne setup in a dynamic light scattering analyzer. The most important of these is signal intensity. This is proportional to is2, the mean scattered light intensity squared, in the homodyne measurement. In contrast, the signal intensity in the heterodyne measurement is proportional to is x i0, the product of the scattered intensity and intensity of the reference.
This results in a much stronger measurement signal and allows the use of laser diodes as light source and silicon photodiodes as detector. The improved signal strength also facilitates the measurement of very small, low-scattering particles down to the lower nanometer range.
λ= wavelength in suspending medium, ω = frequency,
ωo = frequency from particle at the half-height,
η = viscosity, θ = scatter angle, is = scattered optical intensity, io= reference optical intensity, r = particle radius, k = Boltzmann constant, T = temperature
The Dynamic Light Scattering signal can be evaluated in different ways: via a time-dependent autocorrelation function or a frequency power spectrum (FPS), one being the Fourier transformation of the other. Homodyne measurement with autocorrelation is the basis of the widely used "photon correlation spectroscopy" (PCS). This requires an autocorrelator and determines only an average intensity-based mean size (z-average) and a "polydispersity index", which is a rough indication of the width of the distribution. To calculate the distribution, instrument-specific curve fitting algorithms are required.
However, the frequency power spectrum (FPS) method is more reliable and clearly superior to PCS in terms of sensitivity, accuracy, and resolution. The DLS signal from the detector is mathematically transformed into a frequency power spectrum by the Fast Fourier Transform and, after iterative error minimization, provides a direct indication of the size distribution.
The frequency power spectrum takes the form of a Lorentzian function. The characteristic frequency, ω0, is inversely proportional to the particle size. The figure represents the frequency-power spectrum for different particle sizes. The inverse relationship of the characteristic frequency to the particle size is obvious.
1. Detector | 2. Reflected laser beam & scattered light | 3. Sapphire window | 4. Y-beam splitter | 5. GRIN lens | 6. Sample | 7. Laser beam in optical fiber | 8. Laser
Microtrac has taken an innovative approach to dynamic light scattering (DLS) by using a proprietary probe design to deliver and collect light. By focusing the laser probe at the material interface, Microtrac combines the benefits of a short path length with reference beating and 180° backscatter, delivering the best accuracy, resolution and sensitivity.
Strongest optical signal and accuracy at lowest concentrations: All dynamic light scattering measurements use a form of ‘beating’ to strip away the high optical frequency from the scattered light, leaving the particle motion-induced lower frequencies required for size analysis. Microtrac’s heterodyne detection principle uses the probe to collect 180° backscattered light mixed with incident light.
The geometry of the components enables light to reflect from the interface and combines it with collected scattered light. The reflected light enables reference beating. The total optical signal is amplified by the high intensity of the reflected component. The result is the highest possible optical signal providing accurate measurements in the lowest possible concentrations.
The heterodyne measurement principle with reference beating also allows for sizing fluorescent particles.
Microtrac’s probe used in dynamic light scattering analyzers focuses the laser at the interface between probe and particle suspension. Light penetrates the suspension and scattering takes place with the encountered particles and 180° backscattered light. Mixed with the incident light it returns to the photodetector. The total path length is minimized, while the collected scattered light is maximized. This results in accurate measurements at the highest particle concentrations.
Dynamic Light Scattering (DLS) is used in Microtrac's NANOTRAC particle analyzers.
Our team of experts will be happy to advise you on your application and on our product range.
Dynamic Light Scattering is a widely used method for particle size measurement. It is particularly suitable for the characterization of nanomaterials. The Brownian motion (diffusion coefficient) of the particles in a liquid is determined and a hydrodynamic particle diameter is obtained via the Stokes-Einstein equation. Temperature and viscosity must be known for the evaluation.
In particle analysis with Dynamic Light Scattering the sample is illuminated by a laser beam and the scattered light is recorded at one detection angle (in most cases in backscatter direction) over a period of usually 30-120 seconds. The movement of the particles causes intensity fluctuations in the scattered light. From these fluctuations, the diffusion coefficient can be determined, and thus also the particle size.
The measurement range for Dynamic Light Scattering is from 0.3 nm to 10 µm. This largely overlaps with laser diffraction, which has a measuring range starting from 10 nm up to the millimeter range. With decreasing particle size, the method of dynamic light scattering becomes better and better compared to laser diffraction. For larger particles, laser diffraction on the other hand has advantages over dynamic light scattering.
In addition to the possibility of analyzing extremely small particles, Dynamic Light Scattering also offers the advantage of measuring in a wide concentration range from a few ppm to 40 vol % (sample dependent). The measurements can be carried out in various vessels or a probe can even be immersed directly in the sample to be examined. Furthermore, many Dynamic Light Scattering instruments offer the possibility to additionally measure zeta potential.
Dynamic Light Scattering is used in many industries for different applications. Typical samples for dynamic light scattering are particles smaller than 1 micrometer. These include pigments, inks, microemulsions, ceramics, pharmaceuticals, beverages and foodstuff, cosmetics, metals, glues, polymers, colloids, organic macromolecules, and many more.
The method of Dynamic Light Scattering for particle size analysis and the measurement of particle size distribution is described in ISO 22412. Additionally, zeta potential analysis which may often be carried out with a dynamic light scattering analyzer is described in ISO 13099.
There are different methods to acquire and evaluate a Dynamic Light Scattering signal. The heterodyne (or reference-beating) technology, which uses a part of the incident beam as reference for the scattered light, has proven superior in terms of signal to noise ratio. The time-dependent signal is converted to a frequency power spectrum via a Fourier transform. Particle size can be obtained from this power spectrum.
