Commercially available DLS instruments generally incorporate an avalanche photodiode (APD) as a single photon counting module (SPCM). APD-based SPCMs are excellent and robust for DLS analysis thanks to their rapid, high-gain response; the NanoStar utilizes a temperature-stabilized APD coupled to an advanced double density, multi-tau (DDMT) correlator for analysis of diffusion coefficients, size and size distributions. The DDMT correlator incorporates 512 bins covering time scales of 100 nsec to 1 hour in order to address a huge span of particle sizes - ranging over hydrodynamic radii Rh of 0.18 nm - 2500 nm.
While size is the primary application of DLS, the measurement of molar mass is often of interest as well. The molar mass of macromolecules having a specific conformation (e.g., globular proteins) may be often estimated from the measured Rh. However, this model-based estimation of mass is often incorrect since many molecules of interest do not correspond to these models. In fact, molar mass may be measured directly from first principles by means of static light scattering (SLS).
Other DLS instruments use the APD count rate for static light scattering, an unsatisfactory solution since the APD response is nonlinear and subject to relatively high noise levels. In addition to the SPCM-based DLS detection channel, the NanoStar incorporates a separate, optimized SLS detection channel featuring a highly linear, low noise photodiode - in fact, the same photodiode used in our on-line multi-angle light scattering (MALS) detectors. This SLS channel guarantees sensitive and accurate molar masses from 300 Da to ~10 MDa (the upper limit depends on the molecular conformation, and the maximum accurate size may be estimated from the measured Rh).
And More Unique Technology
In order to insure that the NanoStar never goes out of range, it comes equipped with automatic attenuation over five orders of magnitude. Its air-launched laser, coupled with proprietary optics for maximum sensitivity, make single-mode optical fiber technology obsolete. The temperature control system in the NanoStar is unequalled in comparable dynamic light scattering detectors, providing rapid temperature ramps with an overall temperature range of -15°C to +150°C.