"Fraction V", "heat shock", "chromatographically purified". The words on an albumin label describe how the protein was pulled out of plasma, and they tell you more about how it will behave in your assay than the purity percentage does.
Cold-ethanol fractionation (Cohn Fraction V)
Edwin Cohn's wartime process separates plasma proteins by adding ethanol in steps at sub-zero temperature while the pH and ionic strength are adjusted. Each step precipitates a group of proteins. Fraction I is mostly fibrinogen, Fractions II and III carry the immunoglobulins, Fraction IV the alpha and beta globulins, and Fraction V is albumin. Because the cuts are made cold, the albumin comes through with little denaturation and its binding sites intact.
Fraction V is the general-purpose grade. Our BSA Cohn Fraction V pH 7 and the pH 5.2 version are used as blocking agents in ELISA and Western blotting, as protein standards, and as enzyme stabilizers and diluents. Residual globulins are low but not zero, which is why IgG-free grades exist for immunoassays that would see them.
Heat-shock albumin
The heat-shock method leans on albumin's unusual thermal stability. Plasma, or a crude albumin fraction, is held near 65 to 70 °C, often with a fatty acid such as caprylate bound to protect the albumin. The other proteins denature and precipitate while the albumin stays in solution. It is simple and scales well, and it gives an inexpensive reagent-grade product. The trade-off is some aggregate and a slightly different fatty-acid profile than fractionated albumin. For blocking, diluent and stabilizer work those differences don't matter, and heat-shock BSA is a perfectly good choice.
Chromatography
Ion-exchange and affinity chromatography separate albumin on charge or on specific binding rather than solubility, so they can remove the trace globulins, lipids and small ligands that survive fractionation. That is what you want when the application is sensitive to those traces: diagnostic calibrators, serum-free cell culture, drug-binding work. It is also the route used to produce albumin for pharmaceutical use (PMC). The extra purity costs more and comes in smaller batches, and in practice chromatography is usually a polishing step after fractionation rather than a replacement for it.
Which one you need
For blocking and general reagent use, Fraction V or heat-shock albumin is fine; the assay won't tell the difference. Move to a low-endotoxin or IgG-free grade when cells or an immunoassay would notice, and to a chromatographically purified product when trace ligands or globulins are the thing you're measuring. For more on the protein itself, see key attributes of albumins.
We supply albumin from bovine, rabbit, mouse and other species as lyophilized powder. If your specification calls for a fraction, pH or species you don't see listed, send it to us and we'll tell you whether we can source it.