title: Fractionating Polymer Microspheres as Highly Accurate Density Standards.
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(-3)) 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(-3); 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(-3)) 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.
authors.affiliation: †Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States.|‡Department of Physics, Harvard University, 17 Oxford St., Cambridge, Massachusetts 02138, United States.
authors.forename: Blox
authors.initials: B
authors.lastname: Bloxham
authors.affiliation: †Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States.
authors.forename: Jonathan W
authors.initials: JW
authors.lastname: Hennek
authors.affiliation: †Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States.
authors.forename: Ashok A
authors.initials: AA
authors.lastname: Kumar
authors.affiliation: †Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford St., Cambridge, Massachusetts 02138, United States.|§Wyss Institute for Biologically Inspired Engineering, Harvard University, 60 Oxford St., Cambridge, Massachusetts 02138, United States.|∥Kavli Institute for Bionano Science and Technology, Harvard University, 29 Oxford St., Cambridge, Massachusetts 02138, United States.
authors.forename: George M
authors.initials: GM
authors.lastname: Whitesides
mesh_terms: D002138:Calibration / Q000592:standards; D005591:Chemical Fractionation; D008863:Microspheres; D010316:Particle Size; D011108:Polymers / Q000737:chemistry* / Q000302:isolation & purification*; D012015:Reference Standards; D014867:Water / Q000737:chemistry
chemical_list: D011108:Polymers; D014867:Water