HPLC and SDS-PAGE Purity: Which Number Should You Trust?

HPLC and SDS-PAGE measure different properties. Understanding what each purity figure captures helps developers compare recombinant proteins fairly.

Two Methods, Two Questions

Purity is not a single property, so a single number cannot describe it. HPLC methods such as size-exclusion or reversed-phase chromatography separate species by size or hydrophobicity in solution and report the proportion of the main peak. SDS-PAGE separates denatured proteins by molecular weight and reports the proportion of the main band. The two methods disagree because they interrogate different behaviours: one observes native aggregation and hydrophobicity, the other chain length and fragmentation. A high value from one method does not compensate for an unmeasured property in the other.

What Each Figure Captures

An HPLC purity value is sensitive to aggregates, fragments and isoforms that differ in surface properties, and it measures material that may not enter a gel. An SDS-PAGE value reflects the staining intensity of separated bands and is comparatively insensitive to non-covalent aggregates that dissociate under reducing conditions. Neither method detects the other's blind spots. A low-molecular-weight impurity may dominate a gel while being poorly resolved by a size-exclusion method, and the reverse can also hold.

Reading Them Together

Report both figures with their methods and conditions, and treat the pair as a description of the material rather than a single verdict. Where a specification matters, name the method explicitly, for example size-exclusion HPLC or reducing SDS-PAGE, so comparisons across suppliers rest on the same basis. If only one purity figure is quoted, ask which method produced it and whether the other was measured. The question is not which figure to prefer, but what each was designed to detect.

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