Heavy Metals Testing of Essential Oils

Heavy metals are present as impurities in essential oils. Oils need to be free from such impurities especially if they are to be used for foodstuffs or medicinal purposes. Moreover, the presence of heavy metals in scent oils can cause discolouration in products such as soaps and cosmetic ointments. 
This article is a subtle guide to better comprehend the way to test them, the procedure to remove them and to analyze the quality of essential oils on the bases of these metals.

The United States Pharmacopoeia

This scientific non-profit organization with a focus on quality assurance has long been known for the heavy metal sensitive test to ensure the absence of copper and lead-like heavy metals.

  • How this test is deduced?

The test is based upon the fact that H2S will react with the chlorides of these heavy metals to give dark-colored sulfides, which are mostly black or brownish-black in appearance. The following are a few exceptions:

  • The sulfides of Cd, As, and Sn are yellow
  • Sb, orange
  • Zn, white

This test is significant to detect the small amount of Cu and Pb.

Procedure to remove heavy metals from essential oils

Citric/tartaric acid is typically employed for the elimination of metallic impurities from essential oils. This gives rise to complex tartrates and citrates that are unsolvable and that may be filtered off.

The following procedure is used for the removal of heavy metals from essential oils:

  1. A small amount of dry tartaric acid is added to the oil (usually 1 % is sufficient) and mixed thoroughly.
  2. Acid is let to settle and the supernatant liquid is filtered.
  3. If this method fails to remove entire metallic impurities, the oil is agitated with half to 1 % of a saturated liquid solution of tartaric acid; the oil is separated, shaken thoroughly with slat, and filtered.

However, their removal from clove, pimento, bay, and geranium oils commonly requires several treatments.

Determinants of heavy metals

  • The formation of scum at the surface between the oil and water is no confirmation of heavy metals unless the scum is dark in color.
  • Oils synthesized in primitive stills or not thoroughly dried (stored improperly in metal containers) or oils with a large number of free acids will often contain heavy metals. For instance, bay, sweet birch, cajeput, geranium, clove, anise, and sassafras usually hold heavy metals when commercially distilled.  Therefore, it’s better to test them to assure whether they have been treated properly or not to remove such impurities.
  • Iron – metallic impurity excessively seen in essential oils. Oils distilled using such condensers (iron) or stored in improperly lined drums mostly show this impurity.

The tests mentioned above show the sensitivity of a high degree as 10 parts/Million of metallic lead in oil of cloves give a + test. To this, the threshold value is approx. 5 parts/million.

Lastly, these tests have gained much acceptance for conducting heavy metal analysis due to their exceptionally sensitive detection capabilities. So, the quantity and type of elemental constituents in essential oils are possible to determine now due to such feasible methods.