Journal of Magnetism and Magnetic Materials 194 (1999) 248—253 Size characterization of magnetic cell sorting microbeads using flow field-flow fractionation and photon correlation spectroscopy S. Kim Ratanathanawongs Williams*, Hookeun Lee, Midori M. Turner Department of Chemistry and Geochemistry, Colorado School of Mines, Golden, CO 80401, USA Abstract Flow field-flow fractionation (FFF) and photon correlation spectroscopy (PCS) were used to determine the size distribution of magnetic cell sorting (MACS) microbeads. The nominal 50 nm microbeads, when coupled to monoclonal Anti-Fluorescein isothiocyanate (FITC), have an average diameter of 125 nm. FFF results showed that two different batches of microbeads had similar average diameters.  1999 Elsevier Science B.V. All rights reserved. Keywords: Field-flow fractionation; Photon correlation spectroscopy Field-flow fractionation (FFF) is a group of elu- tion-based separation techniques capable of simul- taneously separating and characterizing particles and macromolecules ranging in size from 1 nm to 100 m [1—4]. Field-flow fractionation has been used to analyze biological, pharmaceutical, envir- onmental, food, and industrial materials. Examples of biological and pharmaceutical materials include proteins and protein aggregates, antibodies and antibody—antigen complexes, lipoproteins, viruses, bacteria, parasites, cells, blood substitutes, lipo- somes, and parenteral emulsions. The separation aspect of FFF provides a major advantage over non-separation techniques because complex sam- ples can be analyzed and changes in individual sample components can be monitored. * Corresponding author. Fax: #1-303-273-3629; e-mail: krwillia@mines.edu. The applicability of FFF to sample species in the nanometer-to-micrometer size range arises from the open channel structure (no packing material). The separation process is carried out in the thin rectangular channel shown in Fig. 1. The flow pro- file between the two parallel walls is parabolic with the highest flow velocity located near the center of the channel and decreasing flow velocity towards the walls. An external field is applied to drive the sample to the accumulation wall at the same time that diffusion causes migration of the sample away from the region of higher concentration at the accu- mulation wall. At equilibrium, there is no net flux of sample across the channel (despite continuous dif- fusion and interaction with the field) and each sample component forms a layer with a character- istic thickness. Components A and B occupy differ- ent transverse positions because each component has a different diffusion coefficient and degree of interaction with the external field. The faster 0304-8853/99/$ — see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 5 5 1 - 4