Heavy Metals on a Peptide COA: ICP-MS and USP 〈232〉/〈233〉

Most research-peptide certificates end with a short block headed something like Heavy Metals (ICP-MS): four or five elements, a limit beside each, a result, and a pass or fail. It is the least discussed part of the page. This article explains what that block measures, the instrument behind it, the two USP chapters it usually points to, and how to read the numbers. It finishes with a worked comparison you can run on our own certificates.

Elemental impurities, not "heavy metals"

"Heavy metals" is the older name, inherited from USP's earlier general chapter 〈231〉 Heavy Metals. USP replaced that approach with two newer chapters, 〈232〉 Elemental Impurities—Limits and 〈233〉 Elemental Impurities—Procedures, which became applicable to drug product monographs on January 1, 2018, according to USP's elemental impurities page. The modern term is elemental impurities: trace elements that end up in a material without being part of it.

The international guideline on the subject, ICH Q3D(R2), lists where they come from. They can be residual catalysts added on purpose during synthesis, or they can arrive by contact with processing equipment and container systems, or from other components. Its scope explicitly includes drug products containing synthetically produced polypeptides, which is why the same thinking shows up on peptide certificates.

The usual elements

ICH Q3D sorts elements into classes. Class 1 is arsenic (As), cadmium (Cd), mercury (Hg) and lead (Pb). The guideline describes them as having limited or no use in pharmaceutical manufacturing and as typically coming from commonly used materials, and it requires them to be evaluated across all potential sources. That is why these four appear on almost every panel. Some laboratories add others. Chromium, which Q3D places in Class 3, is a common fifth.

One distinction is worth keeping clear. An element that is part of the molecule itself is not an impurity. The copper in GHK-Cu is the compound, not a contaminant, and a panel of impurity elements is not designed to measure it.

What ICP-MS does

Inductively coupled plasma mass spectrometry turns a sample in solution into a fine aerosol and passes it into an argon plasma. The plasma breaks the sample down into its constituent elements and converts them into ions, as Thermo Fisher describes it. The ions are then separated by mass-to-charge ratio and counted at the detector. Thermo Fisher puts the working range at milligram-to-nanogram levels per liter for most of the periodic table, which is why ICP-MS is the standard tool for trace work.

Two properties matter when you read the result:

• It reports each element separately. Every element on the panel gets its own result.
• It measures the total element, not its chemical form. USP 〈233〉 states that procedures for speciation, meaning the element's oxidation state or compound, are not included in the chapter. An arsenic result is total arsenic.

USP 〈233〉 vs 〈232〉: procedure vs limit

These two chapters are often cited together, but they do different jobs.

〈233〉 is about method. The current harmonized text describes two analytical procedures: Procedure 1 by ICP-OES and Procedure 2 by ICP-MS. It also sets the validation criteria and permits any procedure that meets them. A laboratory that says its metals test follows 〈233〉 is making a claim about how it measured.

〈232〉 is about acceptance. It sets limits, and USP has revised it to align with ICH Q3D. Those limits are written for finished drug products. They are derived from toxicity data and from the characteristics of a specific product, so the chapter does not give a single concentration that transfers to a material that is not a drug product.

This is why certificates for research-use material usually show a laboratory-set limit instead of a 〈232〉 figure, in the form "NMT 1.0 ppm" (not more than). Read that limit as the laboratory's own acceptance criterion for the panel. It is not a pharmacopeial specification, and a pass against it is not a claim of 〈232〉 compliance. A certificate that implies otherwise is overstating what it shows.

Units and "Not Detected"

• ppm by mass = µg/g = mg/kg. ICH Q3D expresses permitted concentrations in µg/g. A result of 0.1 ppm means 0.1 microgram of the element per gram of sample.
• Check that the result and the limit use the same unit. A result that carries no unit inherits its meaning from the limit beside it, so confirm that it does.
• "Not Detected" is not zero. It means the element was below what the method could detect in that run. A careful certificate states the detection or reporting limit. If it doesn't, that is a fair question to ask the laboratory.

Worked example: compare our PDF with the lab's record

Our certificates, from ILS Laboratories, list five elements under ICP-MS: arsenic, cadmium, lead, mercury and chromium, each with its own laboratory limit and status. ILS's verification page states that peptide testing is research-use-only work outside its accreditation scope, so we do not describe these results as accredited.

There is a discrepancy we want you to know about before you look. On 14 of our lots, the heavy-metal values in the PDFs we host differ from the values in ILS's own portal record. Every value in both versions is within the laboratory's limits. We are not quoting the PDF metals values here, because the laboratory's record is the authoritative one.

To check it yourself:

1. Open a lot's certificate PDF from our COA library. Every lot and its access code are also listed on our lot verification page.
2. Go to ils-lab.com/verify, which we do not host, and enter the access code printed on that certificate. You can also scan the certificate's QR code.
3. Find the heavy-metals section in both documents.
4. Compare them row by row: element, result, unit, limit and status.
5. Where the two differ, use the laboratory's record.

The same comparison will also show that the endotoxin method wording differs between our PDFs and the lab's record. The rule is the same: the lab's record governs. This check works on any supplier's certificate, and the full method is in how to verify a COA with the issuing laboratory. For where the metals block sits among the other tests, see how to read a peptide COA.

For laboratory research use only — not for human or veterinary use.