Two onion peel extracts can carry very different headline numbers and both be honest — because they were measured by different methods. "Total flavonoids by UV" and "quercetin by HPLC" answer different questions, and conflating them is the most common way buyers overpay or under-specify. This article explains what each method measures, why the numbers diverge, and how to read an onion peel Certificate of Analysis without falling into the assay trap.
Two Methods, Two Questions
The flavonol content of onion peel extract can be reported in fundamentally different ways:
- Total flavonoids by UV (spectrophotometry). A colorimetric reaction (typically aluminium chloride) measures the whole flavonoid group and reports it as a single percentage. It is fast and inexpensive, but it does not isolate quercetin — kaempferol, isorhamnetin and other flavonols all contribute to the number.
- Quercetin by HPLC. High-performance liquid chromatography separates the sample and quantifies the specific quercetin peak against a reference standard. It is the method that tells you how much quercetin is actually present.
A "10:1 native extract" assayed by herb-to-extract ratio is a third, separate claim — it describes how concentrated the extract is, not what percentage of any single molecule it contains. The grade distinction is covered in the onion peel buyer's guide.
Why the Numbers Diverge
Because UV captures the whole flavonoid family and HPLC captures one molecule, the UV figure is almost always higher than the HPLC quercetin figure on the same powder. Reported side by side:
| Method | What it measures | Typical reading |
|---|---|---|
| UV / AlCl₃ colorimetry | Total flavonoids (group) | Higher — whole flavonol pool |
| HPLC vs standard | Quercetin only (specific) | Lower — one molecule |
| Herb : extract ratio | Degree of concentration | e.g. 10:1 (no % implied) |
The trap: a supplier quotes a high "flavonoid" percentage by UV and lets the buyer assume it is the quercetin content. It is not. Always check which molecule was measured and by which method before comparing two quotes on price.
How to Read an Onion Peel CoA
When you receive a Certificate of Analysis, separate the assay line into its three parts:
- What was measured — total flavonoids, or quercetin specifically? If the line just says "flavonoids," it is almost certainly a group figure, not quercetin.
- By what method — UV/spectrophotometry for groups, HPLC for a specific molecule. A specific quercetin percentage should always be by HPLC against a reference standard.
- Against what basis — a native ratio (e.g. 10:1) is not interchangeable with a standardised percentage. A native CoA will state the ratio and identification (commonly by TLC), not a guaranteed quercetin number.
SV Botanica's stock grade is a 10:1 native extract assayed by UV ratio, identified by TLC — quercetin-rich by nature but not certified to a specific quercetin percentage. When a label needs a guaranteed active, we supply quercetin-standardised grades (5%–40% by HPLC) on request.
Which Should You Specify?
- If you need a label claim — specify a quercetin percentage by HPLC. Do not accept a UV total-flavonoid number as a substitute for a quercetin claim.
- If you want a cost-effective, quercetin-rich botanical without a specific percentage claim — a native 10:1 extract is appropriate, and the UV ratio basis is fine for that purpose.
- If you are comparing suppliers — normalise the quotes: same molecule, same method, same basis. Otherwise you are comparing two different things.
The chemistry behind why quercetin sits at the centre of all this is covered in quercetin & onion flavonols, and the contaminant side of the CoA in regulatory & compliance.
Need a Defined Quercetin Assay?
10:1 native by UV · quercetin 5–40% by HPLC on request · batch-specific CoA · Samples for qualified buyers