= Bioassay-guided fractionation
\b[<Bioassay-guided fractionation>] separates a mixture repeatedly while using a <bioassay> to decide which separated portions retain the biological response of interest. The physical separation may use <chromatography>; the selection criterion is biological activity, rather than the largest chemical peak. The goal is to connect a reproducible response to particular material while tracking what is recovered and what is lost.
A historically important example is the separation of tumour-derived angiogenic material by <Judah Folkman>, <Ezio Merler>, <Charles Abernathy> and <Gretchen Williams>. Their 1971 study combined <size-exclusion chromatography> with biological testing of separated fractions in a <rat dorsal air sac assay>.<Isolation of a Tumor Factor Responsible for Angiogenesis by Judah Folkman and others>[Printed p.279, chromatography on Sephadex G-100 and Biological Assay of TAF]{ref} Following activity through a separation is evidence about where the activity resides; it is not by itself proof of a chemically pure molecule.
An original bookkeeping example assumes that the assay has been converted to calibrated, additive activity units over the concentration range used. An initial $100\,\mathrm{mg}$ mixture contains 1000 units, or 10 units per milligram. Fraction A contains $10\,\mathrm{mg}$ and 600 units, while fraction B contains $90\,\mathrm{mg}$ and 100 units. Thus A has
$$
\frac{600}{10}=60\ \mathrm{units\,mg^{-1}},
\qquad \text{enrichment}=\frac{60}{10}=6,
\qquad \text{activity recovery in A}=\frac{600}{1000}=60\%.
$$
The combined recovery is 70 percent even though the recovered mass is 100 percent. Loss of activity could reflect degradation or removal of a required interacting component. Conversely, removal of an inhibitor could increase measured activity. Neither mass conservation nor a larger response per milligram settles the molecular explanation.
The additive-unit assumption matters. A visual grade or a percentage response at a single test <concentration> cannot automatically be added across fractions. Fractions should be compared with controlled solvent exposure and an appropriate range of <concentrations>, using matched <negative controls>. Recombining separated portions can test whether the original response required their joint presence. Independent chemical characterization then tests composition; a positive <bioassay> alone does not determine molecular identity or clinical effectiveness.
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