Ma$netic separations in chemistry and biochemistrY

W" begin with three observations: . Methods for separating chemicals, materials, and objects are a crit ical part of chemical technology' . Magnetic interactions can achieve separations that are impossible or impractical to achieve l ' ,) ' other techniques. . Magnet ic separat ions are inf reqtrent ly used in chemistry. How is it that chemistry, which has cheerfully exploited separations based on almost every type of physical interaction, has managed to neglect magnetic interactions almost entirel.v? This question has tu'o answers: First, magnetic separatiotts i lre applicable to a smaller set of chemical problems than the more comlnon methocls. In general, magnetic separations are restricted t0 separations of particlut fto(or i1) suspension, and are 'ot applica6le to separations of moleatles from (in) solution. They are not particularly useful in svstems containing only organic ipecies. Second, chemists are too unfamiliar with magnetic separation to recognize promising areas of application. In this article we direct ottrselves to the problem of unfamiliarity. We summarize the physical principles underlying the tu'o major techniques for separations based on magnetic interactiotls, outl ine representative problems in which these techniques have been sttccessftrlly used, and suggest .vpes of problems to u'hic'h thev might be applied in t[e fui,rr". After reacling this article, vou should be able to jldge whether magnetic separations might contribute to the solution of separation problems vou face. I-ike other seltarations, magnetic separations reqttire partit ioning the rnaterials of interest betu'een different iegions of space (Table I ). Separation mal'be [ased on rates of equil ibria and may involve one or manv stages. Regions may be in the same or different phases. Magnetic separations are lsually effective onlv for particles (i.e., collections of molectrles) b""r,rt" the strengt| of the interaction of nragnetic fields rvith single molecules is orclinarily much less than thermal energies in sol t t t iot t (RT 0.6 koal mol-r) ' Macromolecules rnav prove tl l l exception in special c i rcumstances (2). Since feu, chemists have an intuit ive ttttclerstanding of magnetic slscepti| i l i tv (X), let 1s give a brief , nonrigororts explanation of a ferv terms (3. 4), Molectrles and materials may be clivirlecl into trvo grotrps: those rvith unpaired electrons alcl those rvhose electrons are spin-paired. Members of the first group are attractecl to regiols of high magnetic f iel<l (1 ltosit ive). Nlembers of the seconcl group are repelle,cl from high fielt l regiorts (1 negative) (Figrrre l) ' An electron fias a magnetic nr6ment. We interpret this observation in terms of a model in rvhich the electron