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Particles and Number Density

In particles mode, it is possible to measure the size and number density of a sample using just a DAWN HELEOS II. ASTRA supports several sophisticated models to determine the size of a macromolecule based on the angular variation in the light scattering intensity:

  • Zimm, Debye, and Berry formalisms: Traditional multipole expansions to determine rms radius of sample regardless of geometry.
  • Random coil and sphere: Closed form expressions for particles with known geometries. Applicable in the Rayleigh-Gans-Debye limit where solute and solvent indices of refraction are similar.
  • Mie: Full power Mie calculations to determine geometric radius of spherical samples. Applicable to samples with any refractive index.
  • Coated sphere: Ideal for modeling lipid vesicles, where thickness of spherical shell is known.
  • Rod: If the radius of the rod is known, this model can be applied to determine the length.

number density analysis GUI

Number Density

If the index of refraction of the sample is known, it is possible to use ASTRA's proprietary number density calculations. As shown in the screen shot, the geometric radius and number density of a sample are determined for each eluting slice using just a DAWN. The number density calculations make it possible to measure the total number of particles in a peak.

Moreover, using ASTRA's powerful distribution calculations, cumulative and differential number fractions can be reported. For example, the number fraction of a sample between different size ranges can be determined. Finally, it is possible to plot quantities such as absolute number density vs. volume, as shown below for a 100 nm nanosphere sample.

100 nm nanosphere sample



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Wyatt Technology is the recognized leader in light scattering instrumentation and software for determining the absolute molar mass, size, charge and interactions of macromolecules and nanoparticles in solution.

Wyatt's line of multi-angle static light scattering products couple to size exclusion chromatography (SEC-MALS), field-flow fractionation (FFF-MALS), and stop-flow composition-gradient systems (CG-MALS). Our dynamic light scattering (DLS) products operate in traditional cuvette as well as on-line and automated, high-throughput modes. We also offer unique instruments for electrophoretic light scattering (MP-PALS), differential refractometry, and differential viscosity.

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