Two labs test the same drum of milk thistle extract. One reports “80% silymarin,” the other “31% silibinin.” Neither is wrong — they measured different things, by different methods, and reported on different bases. Understanding why is the difference between buying the grade you meant to buy and being talked out of good material by a spec you misread.
Silymarin Is a Complex, Not a Molecule
“Silymarin” is the collective name for a group of flavonolignans extracted from Silybum marianum seed: silybin (silibinin) A and B, isosilybin A and B, silychristin and silydianin, accompanied by the flavonoid taxifolin. Silybin A and B together make up the largest and most studied fraction, which is why regulators anchor the assay on it. Because silymarin is a mixture, the number you get depends entirely on whether you measure the whole complex or one component of it.
UV: The Total-Complex Number
UV spectrophotometry measures light absorbed by the whole flavonolignan complex (typically after a colour reaction) and expresses it as total silymarin. It is fast, inexpensive and captures every component at once, so it returns the high figure — the familiar 80% silymarin by UV grade. Its weakness is specificity: because it sums everything that absorbs, it cannot by itself confirm the flavonolignan pattern or rule out interfering material.
HPLC: The Component Number
HPLC physically separates the flavonolignans and quantifies each one, usually reported as silibinin (silybin A + B) against a reference standard. Because it counts only part of the complex, it returns a lower figure — frequently around 30% silibinin for material that is 80% total silymarin by UV. Its strength is exactly UV's weakness: the separated peaks are a fingerprint that confirms identity and exposes adulteration.
The key insight: “80% by UV” and “~30% silibinin by HPLC” can describe the same powder. They are not two grades — they are two rulers. Comparing a UV number from one supplier against an HPLC number from another is the most common milk thistle buying error.
| Basis | What it measures | Typical figure | Best for |
|---|---|---|---|
| Silymarin by UV | Total flavonolignan complex | NLT 80% (also 70%, 65%) | Headline grade, batch potency |
| Silibinin by HPLC | Silybin A + B only | ~30% (varies by material) | Identity, pharmacopoeia, actives |
| Full HPLC fingerprint | All six flavonolignans + taxifolin | Pattern, not a single % | Authenticity, adulteration screen |
What the Pharmacopoeias Say
The European Pharmacopoeia and USP monographs for milk thistle standardise on a silibinin / HPLC basis, not on the UV total. That is why a pharmacopoeia-grade or drug-adjacent buyer will ask for an HPLC silibinin figure even when the commercial label says “80% silymarin.” For a food-supplement buyer, the 80% UV grade is the trade standard; for a pharmaceutical or clinical-format buyer, the HPLC silibinin number is the one that governs.
How to Write It Into a Purchase Order
- State the method next to the number — “silymarin NLT 80% by UV” or “silibinin NLT X% by HPLC.” A bare percentage is ambiguous.
- Ask for identity by HPLC regardless of the assay grade — the fingerprint is your adulteration defence.
- Decide the governing basis up front — UV for supplement trade, HPLC silibinin for pharmacopoeia / clinical formats.
- Request the reference standard used — HPLC silibinin figures move with the standard, so consistency matters batch to batch.
Why This Protects Your Margin
Getting the basis right stops two expensive mistakes: rejecting perfectly good 80%-UV material because a rival quoted an HPLC number that looks higher-value, and overpaying for a “stronger” grade that is simply reported on a different ruler. It also closes the door on adulteration, because a supplier who will put an HPLC fingerprint on the CoA has little room to cut the seed with cheaper thistles — a risk we cover in Silybum marianum adulteration.
SV Botanica supplies 80% silymarin by UV with identity by HPLC as standard, and silibinin-by-HPLC grades on request, each with a batch-specific CoA referencing spec SVB-SPEC-MT-01. To specify the whole material end to end, start with the milk thistle buyer's guide or view the Milk Thistle Extract.
Need Silymarin Specified the Right Way?
80% by UV · identity by HPLC · silibinin-by-HPLC grades on request · batch-specific CoA with method stated · samples for qualified buyers