Short answer. Two bulk quotes for the same ingredient are almost never comparable as written. Before you divide price by kilogram, four things have to be put on the same footing: what the assay percentage is a percentage of, whether you are buying a salt or the free form, whether the assay is stated as-is or on the dried basis, and what the commercial terms actually include. Normalise those four, then compare cost per kilogram of the moiety you are formulating with. On real quotes the ranking changes often enough that the arithmetic is worth doing every time.
Why 98% versus 97% is not the comparison it looks like
An assay figure is a measurement of a named analyte, by a named method, on a named basis. Change any one of the three and the number means something else. A quote that says only 98% has told you a digit, not a quantity. The most common way a sourcing decision goes wrong is not a supplier misrepresenting a number; it is two honest numbers being subtracted from each other when they were never on the same scale.
This page is the arithmetic our comparison guides keep referring to. It is about how to read and normalise a quotation. It says nothing about what any ingredient does.
The five things that must match before you compare price
1. The analyte: what the percentage is a percentage of
Ask which molecule the assay counts and against which reference standard. A magnesium salt can be quoted as a percentage of the salt or as a percentage of elemental magnesium, and the two differ by a factor of six or more. A botanical can be quoted on a marker compound, on a group of related compounds, or on a total that includes isomers a formulator may not want counted. The analyte belongs on the quote, not only on the certificate.
2. The form: salt, hydrate, ester or free acid
Almost every commercially supplied active is a salt, a hydrate or an ester of the thing being counted. The conversion is fixed and knowable: divide the molecular weight of the moiety by the molecular weight of the supplied form. Two suppliers quoting the same nominal purity of the same ingredient in different salt or hydrate forms are selling measurably different amounts of the same molecule.
3. The basis: as-is or on the dried basis
An assay stated on the dried basis is calculated after the water is mathematically removed. It describes the dry solids, not the powder in the drum. A 98% material on the dried basis with a 5.0% loss-on-drying limit can arrive containing about 93.1% of the analyte; a 98% material stated as-is arrives containing 98%. The two specification lines are visually identical and about five percent apart in what you receive and pay freight on.
4. Carriers, excipients and extract ratios
Standardised extracts and spray-dried powders frequently carry maltodextrin, silica or a similar flow aid, and the carrier fraction is part of the kilogram you buy. A drug-to-extract ratio such as 10:1 is a process description rather than a content specification: it states how much raw botanical entered the process, and it cannot be verified by testing the powder that leaves it. Where there is no marker assay by a named method, there is no quantity to normalise, and that quote should be set aside rather than converted.
5. The commercial terms
Two prices are only comparable when they cover the same scope. Name the Incoterms rule and its named place, because an ex-works figure and a delivered figure differ by freight, duty and customs handling. Fix the packaging, the minimum order quantity, the lead time and the payment terms as well; a lower unit price against a much larger minimum order is a working-capital decision, not a price advantage.
Salt-to-moiety conversion factors
Each factor below is the molecular weight of the moiety being counted divided by the molecular weight of the supplied form. Molecular weights are taken from the PubChem compound records cited by CID, so every row can be checked independently. Multiply a stated assay by the factor to get the moiety content of the supplied material.
| Supplied form (PubChem CID) | Moiety counted | MW, supplied form | MW, moiety | Factor |
|---|---|---|---|---|
| Berberine hydrochloride, anhydrous (CID 12456) | Berberine cation (CID 2353) | 371.8 | 336.4 | 0.905 |
| Berberine hydrochloride dihydrate (CID 160800) | Berberine cation (CID 2353) | 407.8 | 336.4 | 0.825 |
| Creatine monohydrate (CID 80116) | Creatine (CID 586) | 149.15 | 131.13 | 0.879 |
| Calcium L-5-methyltetrahydrofolate (CID 135564391) | Levomefolic acid (CID 135398561) | 497.5 | 459.5 | 0.924 |
| L-carnitine L-tartrate, 2:1 (CID 57349413) | L-carnitine (CID 10917) | 472.5 | 2 × 161.20 | 0.682 |
| Quercetin dihydrate (CID 5284452) | Quercetin (CID 5280343) | 338.27 | 302.23 | 0.893 |
| Sodium ascorbate (CID 23667548) | Ascorbic acid (CID 54670067) | 198.11 | 176.12 | 0.889 |
| Magnesium bisglycinate (CID 84645) | Elemental magnesium | 172.42 | 24.305 | 0.141 |
| Trimagnesium dicitrate (CID 6099959) | Elemental magnesium | 451.12 | 3 × 24.305 | 0.162 |
| Magnesium taurate (CID 13343447) | Elemental magnesium | 272.6 | 24.305 | 0.089 |
| Magnesium L-threonate (CID 45489777) | Elemental magnesium | 294.50 | 24.305 | 0.083 |
| Magnesium oxide (CID 14792) | Elemental magnesium | 40.305 | 24.305 | 0.603 |
Elemental magnesium is taken at an atomic weight of 24.305. Note that the magnesium rows are not a ranking. The forms are chosen for solubility, taste, tolerability and format fit, and the conversion is only there so the magnesium contribution of each quote is known before that decision is made.
The arithmetic: cost per kilogram of active
Effective content = assay × salt-to-moiety factor × (1 − water allowance, where the assay is stated on the dried basis).
Cost per kilogram of active = quoted price per kilogram ÷ effective content.
Freight, duty and any inbound testing are added afterwards, once both quotes have been restated to the same Incoterms rule.
Worked example: the cheaper invoice is the more expensive material
Quote A offers berberine hydrochloride, anhydrous, assay not less than 97% by HPLC, at $48.00 per kilogram. Quote B offers berberine hydrochloride dihydrate, assay not less than 98% by HPLC, at $46.00 per kilogram. Quote B is 4.2% cheaper on the invoice and carries the higher purity figure.
Quote A: 0.97 × 0.905 = 0.878 kg of berberine per kilogram, so $48.00 ÷ 0.878 = $54.68 per kilogram of berberine.
Quote B: 0.98 × 0.825 = 0.809 kg of berberine per kilogram, so $46.00 ÷ 0.809 = $56.90 per kilogram of berberine.
Quote B is therefore about 4.1% more expensive for the same amount of berberine. Nothing here is a judgement about either supplier: both figures are correct as stated, and the difference is entirely the two waters of hydration in the second form. The prices are illustrative and are used only to show the calculation.
Worked example: the same assay on two different bases
Two suppliers quote the same material at 98% and at $60.00 per kilogram. Supplier C states the assay on the dried basis with loss on drying not more than 5.0%. Supplier D states the assay as-is.
Supplier C delivers 0.98 × 0.95 = 0.931 kg of analyte per kilogram, so $60.00 ÷ 0.931 = $64.45 per kilogram of analyte. Supplier D delivers 0.98, so $60.00 ÷ 0.98 = $61.22 per kilogram of analyte. Identical headline numbers, identical prices, 5.3% apart.
Two practical notes. A dried-basis assay is not a defect and is the normal convention in several pharmacopoeial monographs; it simply has to be converted before comparison. And the water limit is a limit, not a measurement, so the worst case is what the specification permits while the certificate of analysis reports what the batch actually held.
How to request quotes so they arrive comparable
Most of this work disappears if the request for quotation asks for the right fields in the first place. A single line added to an RFQ does it:
Please quote the assay as a percentage of a named moiety, state the analytical method, state whether the figure is as-is or on the dried basis, and give the water or loss-on-drying limit, any carrier or excipient content, packaging, minimum order quantity, lead time, and the Incoterms rule with its named place. Please confirm that the certificate of analysis reports the same analyte and the same basis as the quotation.
Then check the returned certificate against the quotation rather than against the specification sheet. Our guide to verifying a supplier certificate of analysis covers what that document should contain, and the stability report guide covers the document that usually arrives next.
Documentation we supply with a quotation
- Specification with the assay stated as a percentage of a named moiety, the method, and whether the figure is as-is or on the dried basis.
- Batch certificate of analysis reporting the same analyte and the same basis as the quotation.
- Water or loss-on-drying limit, and carrier or excipient content where a carrier is used.
- Residual solvent and heavy metal limits with the methods named.
- Packaging, minimum order quantity, lead time, and the Incoterms rule with its named place.
- Certificates under buyer review, as described on our quality and certifications page.
Where a figure is not something we hold for a given material, we say so rather than supply a number with no basis behind it.
What we do not claim
This page is a commercial and analytical method. It makes no statement about what any ingredient listed on it does in a person, and nothing here should be read as an efficacy, structure or function claim. The conversion factors are arithmetic performed on published molecular weights; they describe composition, not performance. The prices in the worked examples are illustrative figures chosen to demonstrate the calculation and are not quotations. Specification setting, label declaration and regulatory review remain with the buyer and the buyer’s own quality and regulatory reviewers.
References
- PubChem, National Center for Biotechnology Information, US National Library of Medicine — compound records for molecular weight, cited by CID in the conversion table: berberine, CID 2353; berberine chloride, CID 12456; berberine chloride dihydrate, CID 160800; creatine monohydrate, CID 80116; calcium levomefolate, CID 135564391; magnesium bisglycinate, CID 84645.
- Incoterms® 2020 rules, International Chamber of Commerce — the reference set from which the delivery term and its named place should be quoted.
Related guides
- How to verify a supplier certificate of analysis
- How to read a supplier stability report
- How to qualify a nutraceutical ingredient supplier
- Berberine Pro-Micelle versus berberine HCl
- L-5-MTHF versus folic acid
- All ingredient selection guides
Frequently asked questions
Why are two ingredient quotes at the same stated purity not comparable?
Because an assay percentage is a measurement of a named analyte, by a named method, on a named basis. Two suppliers can both write 98% and mean different molecules, different reference standards and different moisture conventions. Until the analyte, the method and the basis match, the two numbers are not on the same scale and the prices attached to them cannot be divided against each other.
What does assay on the dried basis mean on a quote?
It means the percentage was calculated after the water was mathematically removed, so it describes the dry solids rather than the powder in the drum. A material quoted at 98% on the dried basis with a 5.0% water limit can contain about 93.1% of the analyte as delivered. A material quoted at 98% as-is contains 98% as delivered. The two lines look identical on a quote and differ by roughly five percent of the material you pay freight on.
How do I convert a salt-form assay to the active moiety?
Multiply the assay by the ratio of the molecular weight of the moiety you are counting to the molecular weight of the form you are buying. Berberine hydrochloride anhydrous carries a berberine cation of 336.4 against a salt of 371.8, so the factor is 0.905; as the dihydrate the salt is 407.8 and the factor falls to 0.825. The same arithmetic gives the elemental magnesium fraction of any magnesium salt.
Does a 10:1 extract ratio tell me how much active I am buying?
No. A drug-to-extract ratio describes how much starting botanical was processed, not what the finished powder contains, and it cannot be verified by testing the powder you receive. If the specification carries no marker assay by a named method, there is no quantity to normalise and the quote cannot be placed on the same basis as a standardised material. Ask for a marker compound, a method and a limit.
What is cost per kilogram of active, and how do I calculate it?
It is the quoted price divided by the effective content of the moiety you are formulating with. Effective content equals the assay, multiplied by the salt-to-moiety factor, multiplied by one minus the water allowance where the assay is stated on the dried basis. Divide the price per kilogram by that figure and the two quotes finally sit on one axis. Add freight, duty and the Incoterms rule afterwards, because an EXW price and a DDP price are not comparable either.
What should I ask a supplier for so that quotes arrive comparable?
Ask for the assay expressed as a percentage of a named moiety, the analytical method, whether the figure is as-is or on the dried basis, the water or loss-on-drying limit, any carrier or excipient content, packaging, minimum order quantity, lead time, and the Incoterms rule with its named place. Then ask for confirmation that the certificate of analysis reports the same analyte and the same basis as the quote.
Request a sample, specification or quotation
Send the material, the target assay basis and the finished format, and we will return a specification and a quotation written on the basis described above, with a batch certificate of analysis reporting the same analyte. Request a sample, COA or quotation, or browse the ingredient catalogue.
Nutrition BioTech
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