Heavy metals in herbs are tested by dissolving a weighed sample in strong acid, then measuring the metals in the solution with an instrument. The usual instruments are ICP-MS, ICP-OES or atomic absorption spectrometry, with a separate method often used for mercury.
The four metals most buyers ask for are lead, arsenic, cadmium and mercury. Plants take them up from soil and water, and dust adds more. Roots and leaves grown near roads, mines or industry tend to carry higher levels.
What happens in the lab
The method matters less to you than the steps being done properly. A good lab follows a sequence like this.
- Sampling: a representative sample is drawn from several bags in the lot, not one handful from the top bag.
- Grinding: the sample is milled to a fine, even powder so a small portion represents the whole.
- Digestion: a weighed portion is heated with acids, often in a closed microwave system, until the plant material dissolves.
- Measurement: ICP-MS measures several metals at once at very low levels. ICP-OES and atomic absorption are also used where limits allow.
- Mercury: often measured by cold vapour atomic absorption or a direct mercury analyser, because mercury is volatile and easily lost.
- Quality control: blanks, spiked samples and certified reference materials are run alongside to prove the result is reliable.
Points to check on the report
A heavy metals certificate is only useful if you can tell what was tested and how. Our guide to reading a lab certificate covers the general points. For metals, look at these.
- The lab's accreditation scope covers heavy metals in plant or food matrices, not only water.
- The limit of quantification for each metal is well below the limit you must meet.
- Results are reported per kg of product as sold, and the basis (dry or as-is) is stated.
- The sample reference matches the lot number on the bags and the invoice.
- Arsenic is reported as total arsenic, or as inorganic arsenic if your market sets limits on that form.
Why one test is not enough
Metal levels depend on where the plant grew. A supplier who buys from several collection areas or traders can ship a clean lot one month and a high lot the next. A test from last season tells you little about this season's crop.
Powders carry extra risk. Grinding equipment can add metal, and sand or soil left on roots raises the reading. Acid-insoluble ash is a useful cheap companion test, because high ash often points to soil contamination.
For a new supplier, test the approval sample and the pre-shipment sample at an independent lab. Once results are steady over several lots, you can agree a testing frequency in the contract. Many buyers keep testing every lot for roots and powders.
Setting the limit in your spec
The limit you need depends on your market and on how the product is sold: as a food, a herbal tea, a supplement or a medicinal ingredient. Limits differ between these categories. Our article on limits for lead, arsenic, cadmium and mercury explains where to find them. Put the numbers in your product specification, with the test method and lab named.
How we help
We draw samples from the actual lot and send them to NABL-accredited labs for heavy metals, then share the original reports with you. Where a supplier's results swing between lots, we trace the collection area and raise it before you book the next order. Ask for a free sourcing review.