BeNano 90 Zeta

1) Optical Layout of the BeNano 90 Zeta 

2) Dynamic Light Scattering

Dynamic light scattering (DLS), also known as photon correlation spectroscopy (PCS) or quasi-elastic light scattering (QELS), is a technology used to detect the fluctuations of the scattering intensities caused by the Brownian motion of particles.

In the dispersant, smaller particles move faster, while larger particles move slower. An avalanche photodiode (APD) detector aligned at 90°collects the scattering intensities of the particles and records them with time. The time-dependent fluctuation is converted into a correlation function using the correlator. By applying a mathematic algorithm, the diffusion coefficient D is thereby obtained. The hydrodynamic diameter D_H  and its distribution are calculated through the Stokes-Einstein equation.

3) Electrophoretic Light Scattering

Particles usually carry charges on the surface in aqueous systems, surrounded by counter-ions that form a firmly inner Stern layer and an outer shear layer. Zeta potential is the electrical potential at the interface of the shear layer. Suspension systems with higher zeta potentials tend to be more stable and less likely to form aggregates.

Electrophoretic light scattering (ELS) is a technology for measuring electrophoretic mobility via Doppler shifts of the scattered light. When an incident light illuminates dispersed particles that are subjected to an applied electric field, the frequency of the particles' scattered light will be different from the incident light due to the Doppler effect. The frequency shift is measured and converted to provide the electrophoretic mobility and hence the zeta potential of a sample by Henry's equation.

4) Static Light Scattering

Static light scattering (SLS) is a technology that measures the scattering intensities of the sample, weight-average molecular weight (Mw), and second virial coefficient A₂ through the Rayleigh equation: 

where c is the sample concentration, θ is the detection angle, R_θ is the Rayleigh ratio used to characterize the intensity ratio between the scattered light and the incident light at angle θ, Mw is the sample’s weight-average molecular weight, A₂ is the second virial coefficient, and K is a constant related to (dn/dc)².

During molecular weight measurements, scattering intensities of the sample at different concentrations are detected. By using the scattering intensity and Rayleigh ratio of a known standard (such as toluene), the Rayleigh ratios of samples at different concentrations are computed and plotted into a Debye plot. The molecular weight and the second virial coefficient are then obtained through the intercept and slope from the linear regression of the Debye plot.

5）Phase Analysis Light Scattering (PALS)

The traditional ELS converts the correlated scattering signals into frequency distribution and then calculates the frequency shift Δf of the scattered light, compared with the reference light. Phase analysis light scattering (PALS), an advanced technology based on the traditional ELS technology, has been further developed by Bettersize Instruments Ltd. to measure zeta potential and its distribution of a sample. By analyzing the phase information Φ of the original scattered signal, PALS obtains the frequency information of that light. The phase shift with time dΦ/dt is proportional to the frequency shift Δf. PALS technology can suppress the influence of the Brownian motion of particles on the results, thereby providing higher statistical accuracy. In various applications, PALS can effectively measure the zeta potential of particles whose charge approaches the isoelectric point, for instance, particles with very slow electrophoretic mobility at a high salt concentration.

6) Temperature Trend Measurement 

Temperature trend measurement includes:

• Size vs. Temperature 
• Zeta Potential vs. Temperature

Investigating the particle size and zeta potential of the samples under different temperatures is significant in many applications. The function of Programmed Temperature Change, ranging from -10°C to 110°C, makes Temperature Trend Measurement available in the BeNano Series.

Benefits

• This feature benefits users who need to study the stability of protein formulations. Generally, the higher the denaturation temperature of the protein, the more stable the formulation. 
• Besides, it is useful for users who need to simulate real-time aging using elevated temperatures to manually speed up the aging process.

7) Viscosity Measurement 
For the sample of unknown viscosity, the viscosity measurement could be implemented using tracer particles with known sizes (e.g., standard samples with nominal sizes). When the measurement ends, input the accurate size of the tracer particles, and the viscosity of the sample could be determined.

After the measurement, choose and right-click on the corresponding result. Click "Viscosity Calculator" on the pop-up menu. By inputting the nominal values of the tracer particles and clicking on "Calculation", the viscosity of the sample could be finally ascertained.

1)Dynamic Light Scattering

For small particles The BeNano 90 Zeta is equipped with a 50mW Solid-state laser, a high-sensitivity APD detector, single-mode fibers and a high-speed multi-mode correlator, which provides unprecedented sensitivity and accurate measurement for extremely small particles with fast diffusion speeds. Even for molecules smaller than 1 nm such as vitamin B1 (as shown on the right), under very diluted conditions of 5% wt, the BeNano 90 Zeta can effectively detect its scattering intensity and fast decay signals to obtain the particle size and size distribution.
For large particles  Large particles diffuse slowly and are likely to sediment. Applying DLS technology for large particles requires the intelligent adjustment of the scattering intensity and ensures enough correlation time for the slow decay. The highly effective detection system of the BeNano 90 Zeta can offer enough correlation time to provide an accurate calculation of slow decay signals. The figure on the right is the measurement result of a 5 μm polystyrene latex.
Resolution  The resolution of the DLS technology depends on the algorithm. Usually, for two narrowly sized-distributed components with a size difference of over 3:1, the algorithm discerns two individual peaks by adjusting the resolution to a higher level. The BeNano 90 Zeta provides several algorithms with different resolutions to meet the high-resolution requirements of many applications. The figure on the right is the result of a 60 nm and a 200 nm latex mixture.
Repeatability The BeNano 90 Zeta optical system is robust and stable. It has an automatic intensity adjustment and intelligent signal judgment system to ensure high stability and repeatability of the measurements. The figure on the right shows the measurement repeatability of the 60 nm polystyrene latex. As shown, the system provides excellent repeatability with a relative standard deviation less than 1%.

2)Electrophoretic Light Scattering

Stability of Particles Zeta potential is a key indicator of the stability of the particle system. With a high zeta potential, the repulsive force between particles is strong and the system tends to be stable. Alternatively, with a low zeta potential, the repulsive force between particles is weak, the particles are easy to agglomerate or flocculate, and the system stability is poor. The main factors affecting zeta potential include the dispersant pH, ionic strength (salt concentration), and the concentration of small molecule additives.
The pH of the dispersant is one of the important factors affecting the zeta potential of the particles. Usually, at a lower pH, the particles tend to be more positively charged, and at a higher pH, they tend to be more negatively charged. It should be noted that even particles with the same chemical composition may have different zeta potentials under the same dispersant environment if the sample source is different.
The ionic strength of the dispersant is also one of the important factors affecting the zeta potential of the particles. In general, a higher ionic strength will result in a stronger shielding effect, meaning the absolute value of the particle’s zeta potential is closer to zero, and a smaller electrophoretic mobility of the particle in the electric field. It should be noted that some ions can be specifically adsorbed on the surface of the particles, which will additionally increase the amount of charge distributed on the surface of the particles.

3)Particle Interactions

Light scattering technology can provide information on the intermolecular forces and stability of a given suspension system. Specifically, the second virial coefficient A₂ (B₂₂) and interaction parameter kD can be determined by a concentration-dependent SLS measurement and DLS measurement, respectively. Besides, the zeta potential of the particles can be obtained by an ELS measurement. 
Using quantifiable parameters such as A₂, zeta potential, and kD, users can access accurate and comparable information regarding the intermolecular forces of the particles.