How to Check Supplement Purity

Checking supplement purity takes about five minutes once you know what to look for. Most people never do it. You buy the tub, read the front label, maybe glance at the ingredients, and trust that someone somewhere checked. For most supplements, that trust is probably justified. But for algae, where the organism concentrates whatever is in its growing environment, five minutes of verification is worth the effort.

Step 1: Find the Batch Number on Your Supplement

Turn the tub over. Look on the bottom or back of the packaging for a batch number, lot number, or production code. It is usually small, sometimes stamped rather than printed, and easy to miss. This identifier links your specific product to a specific production run. Without it, you cannot request testing results for the product you are actually holding.

If there is no batch number, that is the first red flag. A product with no batch traceability cannot be independently verified. You are trusting the label without any way to check it. Some brands print batch codes in locations that are difficult to find or read. Check the base of the tub, the crimp of a sachet, or the side of the box.

Step 2: Request the Certificate of Analysis for Your Batch

Email the manufacturer and ask for the CoA for your batch number. A company that takes purity seriously will respond within a few days with a document from an independent, accredited laboratory. In the UK, look for UKAS accreditation. Internationally, ISO 17025 is the standard for testing laboratory competence.

How the company responds tells you as much as the document itself. A prompt reply with a batch-referenced, dated document from a named lab is a trustworthy sign. A delayed or generic response with no batch reference is concerning. No response at all tells you everything you need to know.

We described what a complete CoA should contain in our guide to reading a certificate of analysis.

Step 3: Check What Was Tested and at What Detection Limits

The Standard Heavy Metal Panel for Algae Supplements

For chlorella, spirulina, and phytoplankton supplements, the minimum purity panel should cover four metals: lead, cadmium, arsenic, and mercury. If only one or two are listed, the testing is incomplete for this product category. The EU sets maximum limits for three of these in food supplements: lead at 3.0 mg/kg, cadmium at 1.0 mg/kg (3.0 for seaweed-based products), and mercury at 0.1 mg/kg (Regulation EC 1881/2006).

Why Detection Limits Change Everything

"Not detected" is meaningless without knowing the detection limit. If the laboratory's limit for lead is 10 ppm and the result is "not detected," your product could contain up to 9.9 ppm and still show that result. The EU limit is 3 ppm. A detection limit above the regulatory limit renders the test useless for safety purposes.

ICP-MS, the analytical method used in competent laboratories, routinely achieves detection limits of 0.001 to 0.01 mg/kg for most metals. You need the detection limit to be at least ten times lower than the regulatory threshold for the result to be genuinely reassuring. When we publish our CoA results, we include the detection limit alongside every measurement for exactly this reason.

Microbial Testing for Whole-Food Powders

Also check for microbial testing: total aerobic plate count, yeast, mould, E. coli, and Salmonella. For whole-food algae powders, microbial testing is essential, particularly for products from open-pond cultivation where environmental exposure is higher.

Step 4: Evaluate Arsenic Speciation for Marine-Origin Products

If you are taking an algae supplement with a marine or aquatic origin, total arsenic alone is not enough. Arsenic exists in multiple forms. Inorganic arsenic is a Group 1 carcinogen. Arsenosugars, the dominant form in most algae, have intermediate and poorly understood toxicity. Arsenobetaine, found mainly in fish, is considered non-toxic.

In some algae products, inorganic arsenic was up to 62 per cent of total arsenic (Almela et al., J Agric Food Chem, 2002). In others, it was negligible. A CoA showing only "total arsenic: 5.0 mg/kg" cannot tell you whether you are consuming a safe product or an unsafe one.

Ask the manufacturer whether inorganic arsenic was measured separately. If they do not know, the testing was not thorough enough for this product category.

There is no EU or UK maximum limit for arsenic in food supplements. This regulatory gap means your own scrutiny is the only safeguard for this particular contaminant.

Step 5: Verify the Laboratory Is Independent and Accredited

The CoA should name the laboratory and include an accreditation number. In-house testing by the manufacturer is better than no testing, but it lacks the independence that third-party verification provides. A laboratory with no named accreditation may not follow standardised methods, which makes the results harder to trust or compare.

In the UK, UKAS is the national accreditation body. Internationally, ISO 17025 certification confirms that the laboratory operates to a recognised standard. If the CoA does not name the lab or provide an accreditation reference, treat the document with caution.

How to Check Supplement Purity FAQs

What should I do if the manufacturer cannot provide a certificate of analysis?

Stop taking the product. If a manufacturer cannot provide batch-specific testing documentation from an independent laboratory, there is no way to verify what you are consuming. Switch to a product from a brand that provides this information as standard practice.

How quickly should a supplement company respond to a CoA request?

A well-organised company with proper documentation systems should respond within a few working days. Longer delays may indicate that batch-specific records are not maintained or that the company does not routinely conduct independent testing.

Is in-house testing as reliable as independent third-party testing?

In-house testing can be technically competent, but it lacks the independence that gives results credibility. The manufacturer has a financial interest in the outcome. Third-party laboratories with UKAS or ISO 17025 accreditation follow standardised methods and have no commercial stake in the result.

Which metals should be tested in algae supplements specifically?

At minimum: lead, cadmium, arsenic, and mercury. These are the four metals most relevant to this product category. For marine-origin algae, ask whether arsenic was speciated into inorganic and organic fractions, because the total figure alone does not indicate risk level.

Does Phytality publish its test results?

Yes. We test every batch through independent laboratories using ICP-MS and make results available on request with detection limits stated alongside each measurement. We consider this a baseline requirement for any algae supplement manufacturer, not a premium feature.

This article is for informational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Consult your GP or a qualified healthcare professional before starting any supplement.

Methodology and Disclosure

EU maximum contaminant levels for food supplements cite Commission Regulation (EC) No 1881/2006 as retained in UK law. Arsenic speciation data cites Almela et al. 2002 (J Agric Food Chem). Laboratory accreditation standards cite ISO/IEC 17025:2017. Detection limit guidance reflects standard ICP-MS analytical capabilities as reported across the cited literature.

Vendor disclosure: Phytality is the publisher of this article and the manufacturer of algae-based supplements. We test every batch through independent laboratories and make results available on request. Our interest in purity transparency is both editorial and commercial.