BI-APD

High Sensitivity Detector and Small Volume Cells

Two New Options for Measuring Small Particles

Measurements of weakly scattering small-volume samples is now much easier, more convenient, and less expensive with the 90Plus or the BI-MAS option on the ZetaPlus or the ZetaPALS . Using the optional BI-APD , avalanche photodiode detector, and the BI-SM50 , small volume cell, particles with hydrodynamic radii as small as 1.5 nm are now measurable.

The BI-APD has more than ten times the sensitivity of the standard PMT detector at the wavelength of the standard 30 mW laser. Thus, the count rate with the BI-APD is equivalent to a laser with more than 300 mW power. With such sensitivity, measurements on very weakly scattering particles are possible without the need for a more costly laser.

That is only half the story, especially with precious samples like proteins. Often only a milligram or two of sample is available for light scattering studies. If diluted into the normal 2 mL of liquid, the concentration is then too low for good measurements. The answer is the 50 L BI-SM50 disposable cell . This cell fits directly into the 90Plus on top of a custom adapter, the BI-SM50A . Using the insertion/removal tool supplied with each one, the adaptor is easily removed to allow use of the standard cells.

A couple of drops constitute 50 L. The open-well design allows easy removal of the samples after the measurement for sample reconstitution, if desired. The sample is not diluted as it would be with a flow cell.

Measurements with Lysozyme*

Lysozyme is a small, globular protein, well characterized and widely available. Measurements were done at 13.6 kDa, pH 6, in 10 mg/mL NaCl, at 4.6, 9.2 & 15 mg/mL of Lysozyme. The mean hydrodynamic radius, R h , 1.77 0.12 nm. Using the crystal structure, assuming a prolate ellipsoid, this result agrees well with one layer of water attached to the lysozyme surface.

Without the BI-APD , the photon count rate is quite low from such a molecule. The resulting noisy autocorrelation function does not yield reproducible answers with a standard laser. With the BI-APD , excellent repeatability in the size is obtained.

The measurement shown below was made with 9.2 mg/mL in the BI-SM50 cell. The amount of protein used is therefore less than 0.5 mg. And, it is recoverable .

Summary Report of Measurement of Lysozyme in Small Volume Cell
with BI-APD High Sensitivity Detector

Typical Applications

Measurements on small globular proteins, micelles, microemulsions, superfine inorganic oxides or metal particles and even hydrated salt ions are possible without the need for a high-powered, more expensive laser.

Specifications

Detector: High sensitivity, avalanche photodiode, requires return of instrument for retrofitting

For instruments that already have the APD or high power lasers, order cells and the adapter only. See options below.

The BI-APD is also available for use with the BI-200SM . Please contact factory for details.

A policy of continual improvement may lead to specification changes.

Options and Accessories

BI-SM50: Box of 80 plastic cells/tops, 12.5 x 12.5 x 36 mm
BI-SM50A: Custom insert with insertion/removal tool, for use with 90Plus or BI-MAS options on ZetaPlus or ZetaPALS*Berman et al., The Protein Data Bank, Nucleic Acids Research, 28 , 235 (2000). Entry 1LYZ.
Picture from Rasmol V2.7.1.1, Copyright H.J. Bernstein.
Eppendorf UVette