Almost every quercetin specification in circulation says "NLT 95%". Very few state what the percentage is calculated on, and fewer still could reconstruct the number from the chromatogram and the water content. This article unpacks the source chemistry, the assay method and the basis arithmetic — the three things that separate a controlled purified active from a number on a page.
Where Commercial Quercetin Comes From
The economics of quercetin begin with a botanical fact: the flower buds of Sophora japonica accumulate rutin — quercetin bound to the disaccharide rutinose — at 15–20% or more of dry weight. No food crop comes close: onion skin, the richest dietary source, carries free and glycosidic quercetin at a fraction of that level. Sophora buds are therefore the raw material for essentially all commercial rutin, and rutin is the raw material for essentially all commercial quercetin.
The conversion is a controlled hydrolysis. Acid or enzymatic treatment cleaves the glycosidic bond, releasing rutinose and the free aglycone, which is then crystallised, washed and dried to purity. Two facts about this route matter to a buyer:
- The impurity profile is diagnostic. Genuine rutin-derived quercetin shows a characteristic pattern — traces of residual rutin, sometimes its mono-glucoside intermediate isoquercitrin, and related flavonols. Material with a clean single peak and none of this history, or with peaks that do not belong to the Sophora pathway, warrants questions.
- Residual rutin is a process readout. An incomplete hydrolysis leaves glycoside in the product. Rutin is not harmful — it is a supplement ingredient in its own right — but it is not free quercetin, and above a couple of percent it means the buyer is paying quercetin prices for unconverted feedstock. A NMT 2% limit keeps the process honest.
Crystalline Forms and the Water Arithmetic
Quercetin crystallises readily as a dihydrate — the molecule plus two waters of crystallisation. The molecular weights do the rest: anhydrous quercetin is 302.24 g/mol, the dihydrate 338.27, so the water accounts for 10.65% of the dihydrate's weight. That single number generates the industry's most persistent specification confusion.
| Material | Assay, as-is basis | Assay, anhydrous basis |
|---|---|---|
| Pure anhydrous quercetin | ~99% | ~99% |
| Pure quercetin dihydrate | ~89% | ~99% |
| 95% dihydrate (anhydrous basis) | ~85% | 95% |
Read the last row twice: a dihydrate honestly certified at "95%, anhydrous basis" delivers about 85 g of actual quercetin per 100 g of powder. Neither figure is wrong — they answer different questions. The as-is figure tells you what is in the drum; the anhydrous figure tells you the purity of the solid once water is excluded. A specification must pick one, name it, and pair it with a water limit so the two can be reconciled.
The reconciliation habit: as-is assay ≈ anhydrous assay × (100 − water%) / 100. If a CoA states 95% anhydrous and 10.5% water by Karl Fischer, the drum contains about 85% quercetin as-is. If the numbers on a CoA cannot be reconciled by this arithmetic, something on the document is wrong — and it is worth finding out what before the material ships.
How the HPLC Assay Works
Quercetin is assayed by reversed-phase HPLC — typically a C18 column with an acidified water–methanol or acetonitrile gradient — with UV detection around 370 nm, near the flavonol's absorption maximum. The sample is dissolved (methanol, with sonication — quercetin's poor water solubility applies to the lab as well as the factory), and the quercetin peak is quantified against a certified reference standard. Because the method resolves quercetin from rutin, isoquercitrin and related flavonols, one run yields the assay and the impurity profile.
The pharmacopoeial framing is worth knowing: the USP dietary-supplement monograph for quercetin specifies not less than 95.0% calculated on the anhydrous basis, with water determined separately. That is the model a commercial specification should follow — assay and water as two lines that together define the material.
Why UV spectrophotometry is not equivalent
A bulk UV reading at 370 nm cannot distinguish quercetin from anything else absorbing there — rutin included, along with other yellow flavonoids and worse. On a material whose main economic adulterants are themselves UV-active at the same wavelength, a spectrophotometric assay is close to meaningless. If a cheap quotation is supported by a "UV" assay line, those two facts are related.
Writing a Specification That Cannot Be Gamed
- Name the basis. "Quercetin NLT 95.0% w/w by HPLC, calculated on the anhydrous basis." Without the basis clause, an as-is reading of dihydrate material fails and an anhydrous reading passes — on the same drum.
- State the form and require water. "Form (anhydrous/dihydrate) to be declared; water by Karl Fischer NMT 12.0% (dihydrate) or LOD NMT 5.0% (anhydrous)."
- Limit residual rutin. NMT 2.0% by HPLC. Closes the under-conversion route.
- Require the method. HPLC against a reference standard, with the chromatogram available on request.
- Declare carriers. "Carriers and excipients, if any, to be declared by name and percentage." A purified active should not need one — so a declaration line makes any dilution visible.
SV Botanica assays quercetin by HPLC on the anhydrous basis with the form stated and water by Karl Fischer on the dihydrate grade, and reports residual rutin on every batch. See the quercetin product specification for the complete parameter list, or the buyer's guide for how to approach the purchase.
Source High-Purity Quercetin from India
Quercetin NLT 95% by HPLC on a declared basis · 98%, dihydrate & enhanced-bioavailability grades · CoA-backed · samples for qualified buyers