Fractionating Polymer Microspheres as Highly Accurate
Density Standards
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Abstract
This paper describes a method of
isolating small, highly accurate
density-standard beads and characterizing their densities using accurate
and experimentally traceable techniques. Density standards have a
variety of applications, including the characterization of density
gradients, which are used to separate objects in a variety of fields.
Glass density-standard beads can be very accurate (±0.0001 g
cm<sup>–3</sup>) but are too large (3–7 mm in diameter)
for many applications. When smaller density standards are needed,
commercial polymer microspheres are often used. These microspheres
have standard deviations in density ranging from 0.006 to 0.021 g
cm<sup>–3</sup>; these distributions in density make these
microspheres impractical for applications demanding small steps in
density. In this paper, commercial microspheres are fractionated using
aqueous multiphase systems (AMPS), aqueous mixture of polymers and
salts that spontaneously separate into phases having molecularly sharp
steps in density, to isolate microspheres having much narrower distributions
in density (standard deviations from 0.0003 to 0.0008 g cm<sup>–3</sup>) than the original microspheres. By reducing the heterogeneity in
densities, this method reduces the uncertainty in the density of any
specific bead and, therefore, improves the accuracy within the limits
of the calibration standards used to characterize the distributions
in density