Dynamic Light Scattering Systems

Nanoparticle Size, Charge, and Molecular Weight

Dynamic Light Scattering Instrument Nanotrac WaveOur proprietary approach to Dynamic Light Scattering allows us to accurately measure from sub-nanometer to several microns across the widest concentration range on the market.

Probe technology – unique design, superior results

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.

180º backscatter and GRIN lens focusing for accuracy at highest concentrations

Microtrac’s probe 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. Result: accurate measurements at the highest particle concentrations.

Reference beating technology – 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. Reflected light enables reference beating. The total optical signal is amplified by the high intensity of the reflected component. This results in 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.

Reference beating technology

DLS: Frequency Power Spectrum

Dynamic Light Scattering is based on the Brownian motion of particles in suspension. Smaller particles move faster, larger particles move slower. Light scattered by moving particles carries information about the size distribution. The conventional method to analyze scattered light in DLS is called Photon Correlation Spectroscopy (PCS). It requires an autocorrelator and provides just a mean size, or needs vendor-specific curve-fit algorithms to estimate a size distribution. The Frequency Power Spectrum (FPS) method is different – the intensity signal of the photo detector is transformed mathematically by Fast Fourier Transformation into a Frequency Power Spectrum and directly provides a size distribution by iterative error minimization.

Brownian Motion

Zeta Potential

Zeta potential is the electrokinetic potential in colloidal dispersions. Theoretically, the Zeta potential of a particle is the electric potential in the interfacial doublelayer at the location of the slipping plane relative to a point in the bulk fluid away from the interface. In other words, Zeta potential is the potential difference between the dispersion medium and the stationary layer of fluid attached to the dispersed particle.

Zeta Potential Explained

Particle Suspension with Optical Probe
Suspended charged particles are in constant Brownian motion. Applying an AC electric field creates an electrophoretic mobility that combines with Brownian motion, resulting in a modulation of the Brownian motion power spectrum called the Modulated Power Spectrum (MPS) signal. The MPS signal is proportional to the zeta potential of the particles:

The 180° backscattering probe by Microtrac allows for measuring the highest concentration of any optical Zeta potential and enables you to operate at or closer to your suspension’s designed concentration.

High concentration – The Microtrac probe is paired with an electrode. A voltage is applied, establishing an electric field between those two points, enabling accurate Zeta potential analysis across a broad concentration range

High accuracy – The Microtrac zeta probe measures the modulated power spectrum signal (MPS) using high frequency modulation, eliminating the need to correct the electro-osmotic flow

High precision – Achieved by inclusion of Brownian motion frequency spectrum

Low sample volume – Compact probe / sample interface allows for lowest volume available: 150 µL

Full Size Distribution And Peak Analysis

Distribution analysis – For full featured, accurate particle size distributions from nm to µm

Mode analysis – Quick results with specified size and volume concentration; this approach enables you to resolve and report accurate multi-mode distributions

Legacy calculations – Enables measurement of historical specifications for data consistency with legacy instrumentation

Beyond Size & Zeta Potential

Molecular weight
By using the Frequency Power Spectrum method, the molecules’ scattered light intensity – which goes into Debye plot calculation as a prime input – can be accurately measured with the prime input. The molecule index of refraction, a constant required for the Debye plot technique, can be determined with the Microtrac probe through the built-in index calculator.

Volume concentration
Mode analysis determines not only the size, but also the volume concentration (in cc/mL units) of each mode of the sample through the power spectrum magnitude. This
method is ideal for monitoring reactor processes and nano attrition milling.

At-Line/On-LineMicrotrac Nanotrac Flex probe utilized with a reactor

The Nanotrac Flex can be connected to liquid handling devices at customers‘ sites. The FLEX software controlling the Nanotrac Flex has an interface to connect to external process control units, to initiate measurements on command, and export data to other computers or network drives.

The external probe has been integrated into high throughput workstations to measure particle size automatically in formulation vessels and synthesis reactors. An automated dilution and flushing can be added, if necessary.

A flow cell is available for on-line bypass measurements without dilution, under pressure up to 6 bar.

Microtrac’s Dynamic Light Scattering Instruments include:

Nanotrac Wave II – Can measure particle size, zeta potential, and molecular weight in one self contained chassis.

Nanotrac Flex – A nano-particle size analyzer equipped with unique probe technology.

Nanotrac Wave II Q – nanoparticle size analyzer with cuvette sample cell.

All systems are compliant to ISO 22412 and meet safety requirements according to CFR21 Part 11.

See which Dynamic Light Scattering instrument is right for you by viewing our product family brochure.

Did you know you can simultaneously measure size and concentration with Microtrac’s Dynamic Light Scattering systems?

 Watch this presentation to learn more about our unique capability.

To speak to a knowledgeable staff member about Microtrac’s DLS instruments, please click here.


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