Alexis St-Gelais, chimiste & Patrice Rondeau, technicien – Popularization
Do you manufacture cosmetic products to be sold in Canada? If so, you should comply with Canadian guidelines for heavy metal impurities in such products, for which PhytoChemia can assist you efficiently. Even if you are in other businesses that use plant-sourced ingredients, our heavy metals testing is a great asset in your toolbox. Let’s look at this topic into greater detail.
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Quick highlights
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In this post, we will have a closer look at heavy metals testing in cosmetics, how they are monitored, and how you can implement heavy metal limits as part of your batch-to-batch quality control. This will be supported by an example certificat of analysis for an anonymized cosmetic product.
[Skip the theoretical explanation, let’s see how a certificate of analysis looks!]
Heavy metals in our environment
Heavy metals, like arsenic, lead and mercury, are elements of the periodic table that are part of our planet’s composition. They can be found in certain minerals and spontaneously in some types of soils and natural water sources. Human activities can also contaminate the environment. For example, addition of tetraethyl lead to fuels has been a widespread practice for decades, which resulted in wide dissemination of lead particles around roads.
Our bodies can handle some trace amounts of e.g., arsenic on their own, and some heavily exposed human populations have even developed some degree of resistance to it [1]. However, significant intake of elements like lead, mercury, cadmium, arsenic and antimony can result in severe health disorders, because these substances can interfere with regular metabolic functions. Hence, it is logical to limit our exposure.
Avoiding heavy metals completely is virtually impossible, because they are part of our environment. When they are found in e.g., plant-sourced products, it is not because they have been added on purpose. This is rather more likely the result of contamination from soil or water that was used during cultivation. As plants need mineral elements to grow, they can absorb those heavy metals and accumulate them alongside. Extracts and products obtained thereof can advantageously be controlled for absence of heavy metals to make sure that they are safe to use.
How to test for heavy metals
The following section has been prepared with guidance from the book Practical Inductively Coupled Plasma Spectroscopy by John R. Dean [2].
Extracting the metals
The first step to testing for heavy metals is to get rid of the tremendous amount of interefering material that you can find in plants, extracts, or cosmetics. In order to do that, we use an acidic digestion method, which briefly involves immersing the organic material in strong acids and heating to munch through the organic matter. More on this topic will be convered in a separate blogpost.
Measuring metals with inductively coupled plasma – mass spectrometry
Inductively coupled plasma – mass spectrometry is more generally noted ICP-MS for short (figure 1). Let us first have a look at the plasma part, which is the core of this instrument, and then examine how data is extracted from it by mass spectrometry.
Figure 1. An ICP-MS instrument to the right, with the autosampler unit to the left.
ICP – Inductively coupled plasma
Matter can be found as three familiar states: solids, liquids, and gases. The level of energy lying in matter increases from one state to another. In solids, atoms clump together, and bear low energy so they do not move around much. Heating them up to obtain liquids gives the atoms more energy, allowing them to slide next to each other, hence the resulting fluidity. More heat, past the boiling point, increases the amount of energy of the molecules, which can then freely move around with low overall cohesion. If a gas is heated more and more, at some point, the energy reaches a point where it is high enough to further dissociate some of the electrons from their atoms. In this state, atoms cannot remain bonded to each other, because electrons are needed for such bonds to exist: instead, matter then exists as a ionized gaseous mixture of neutral undissociated atoms, of electrons, and of positively charged atom nuclei. This fourth state of matter is called plasma. In inductively coupled plasma (ICP), argon, an inert gas, is generally used to generate the plasma. It is preferred for various reasons, including the fact that it cannot form stable compounds with other atoms, and is cheaper than other noble gases like helium or xenon, because it makes up 1% of our atmosphere.
Plasma generally exists at high temperatures, because high energy is required to maintain electrons apart from atoms. In ICP, this temperature is generated within a copper wire coil which converts electrical energy into a powerful magnetic field around the torch. Argon introduced in that torch is first lit by a brief electric spark, which provides some free electrons. Given their high energy, they can collide with argon atoms and rip them off an additional electron. The high magnetic field keeps transmitting energy to those electrons, which keeps this process active even after the initial spark ceases and despite the fact that some electrons recombined with the ionized argon to regenerate neutral atoms. A continuous tradeoff between neutral argon atoms and dissociated argon ions and electrons is established and constitutes the plasma, which remains active as long as the magnetic field keeps inductively inputting energy into the system. Physically, plasma is a luminous flux of gas that looks like a flame (figure 2), which can be as hot as 7000 to 10 000 Kelvins – which is equivalent or hotter than the sun’s photosphere, its visible “surface” (other parts of the sun can be much hotter though).
Figure 2. A lit plasma torch.
When the sample is to be analyzed, liquid is pumped (figure 3) and dispersed as an aerosol with argon by a nebulizer, which then passes into a spray chamber. The latter captures the largest droplets and leaves only the ideal aerosol particles to enter the plasma, ensuring it does not cool down and extinguish. Atoms of heavy metals that enter the plasma are also subjected to the intense electrons bombardment and can in turn ionize too. This is when the detector comes into play.
Figure 3. Pump (in black) and nebulizer (above) of the ICP-MS instrument, which continuously introduce the sample into the plasma torch.
Mass spectrometry
Mass spectrometry involves sorting out ionized molecules or atoms on the basis of their mass, and counting them. The elements of the periodic table feature various atomic masses, which depend on the number of protons and neutrons found in an atom’s nucleus. For example, arsenic atoms contain 33 protons (hence its atomic number 33) and, in nature, 42 neutrons, for a total of 75 atomic mass units.
Since ICP delivers a flux of ionized atoms of heavy metals when those are present, ions are already available for analysis. The plasma flux is therefore interfaced to a mass spectrometer through a pair of cones, which allows a smooth transition between the ambient pressure of the plasma torch and the vacuum inside the mass spectrometer. Therefore, arsenic within the plasma can be ripped off an electron and move on to the mass spectrometer with a single positive charge associated to 75 atomic mass units.
Within the mass spectrometer, a variable magnetic field is generated thanks to four electrically charged metallic rods, called a quadrupole. Ions entering the mass spectrometer, since they carry an electric charge, respond to this magnetic field and will have a trajectory that is defined by it. By varying the magnetic field, the mass spectrometer allows to sort ions according to their masses: those are lighter or heavier than wanted will follow a trajectory that ends up colliding with one of the metal rods, which neutralizes them. On the other hand, ions of the right charge-to-mass ratio will have a stable trajectory and pass through the quadrupole without colliding. If we apply a magnetic filter to only allow ions of mass-to-charge value of 75 to cross the quadrupole, then only arsenic ions will reach the detector.
When an ion reaches the detector, it collides with it and generates electrons which are amplified and generate an electric current. The more ions coming into the detector per second, the more electric current generated: this gives a metric for the number of ions. Measuring the amount of current generated by ions of a mass-to-charge ratios of 75 therefore gives an indication of the concentration of arsenic in the sample introduced in the instrument. This can be compared to a series of calibration solutions containing known amounts of arsenic, to finally convert the signal into readable units of concentration in the cosmetic product.
Canadian guidance on heavy metals in cosmetics
Limits of heavy metals in cosmetics
In 2012, Health Canada has issued a guidance document intended for manufacturers of cosmetics across the country. This document contains a Policy statement that defines maximum tolerated contents for five heavy metals:
- Lead (atomic symbol: Pb) cannot exceed a content of 10 ppm
- Arsenic (atomic symbol: As) cannot exceed a content of 3 ppm
- Cadmium (atomic symbol: Cd) cannot exceed a content of 3 ppm
- Mercury (atomic symbol: Hg) cannot exceed a content of 1 ppm
- Antimony (atomic symbol: Sb) cannot exceed a content of 5 ppm
The document also states that a manufacturer of cosmetics is responsible for ensuring that his finished products do not exceed those limits. Health Canada retains the right to ask, at any point in time, testing results supporting compliance with those limits. Therefore, the guidance document concludes that it is recommended for manufacturers to be proactive and have those results ready on a preventive basis.
Which products or ingredients should be tested?
Under the Canadian definition, a cosmetic product is:
[…] any substance used to clean, improve or change the complexion, skin, hair, nails or teeth. Cosmetics include beauty preparations (make-up, perfume, skin cream, nail polish) and grooming aids (soap, shampoo, shaving cream, deodorant). [Source: Health Canada]
Therefore, if you manufacture any of those products (for example from natural ingredients), you should make arrangements to regularly test your production batches to make sure that they are compliant with heavy metals limits (figure 4). This is especially true if you use plant-sourced ingredients, because plants can leech up heavy metals from contaminated soil or water, even unbeknownst to the producer.
Figure 4. Various cosmetic products that could be tested: hand cream, deodorant recharge cartridge, lipstick and face powder.
Incidentally, if you want to prevent any unpleasant surprise to see a final product batch coming back as unacceptably contaminated with heavy metals, it can be a good practice to adopt upstream testing of plant-sourced ingredients. This could be done systematically, or instead to qualify a new supplier at least once at the beginning of your business relationship.
Verifying method efficacy
Since there is a plethora of potential cosmetic products, it is useful to check whether the assay is effective. Since most cosmetic products are expected not to contain heavy metals, we have to make sure that if there had been heavy metals, we would have detected them properly and at the expected concentration.
A good way to control that in routine is to use spikes. We take a subsample of the cosmetic product and voluntarily add a known amount of heavy metals to it. We then submit it to the same analysis procedure and check whether we obtain the expected reading. This helps making sure that the method appropriately allows to recover and quantitate heavy metals of interest, even if the matrix is different than the previous time.
Example of heavy metals testing in cosmetics: hand cream
A sample of commercial moisturizing cream (figure 5) containing a plant extract was subjected to heavy metals analysis. Portions of the cream were accurately weighed and diluted with predefined amounts of hydrochloric and nitric acids. (Do not worry, the metallic cup was used only for the picture – actual tests were run in single-use volumetric plastic tubes to avoid any external contamination).
Figure 5. A hand cream sample, with hydrochloric and nitric acids in the background. Actual tests take place in single-use plastic tubes, not metal plates.
At this point, two aliquots were supplemented with known amounts of mercury, lead, arsenic, cadmium and antimony, while the third one was kept in its initial state. Then the tubes were capped.
The samples were left overnight in the acids to perform a cold digestion. The following day, the caps were loosened and the temperature was gradually increased to 110 °C for several hours until the cream was completely digested. After cooldown, the tubes were completed to volume with demineralized water and agitated. A portion of the liquid was then filtered through a fine 0.45 µm pore size filter, and further diluted with demineralized water, for each sample.
The ICP-MS instrument was calibrated with a series of 6 solutions containing known dilutions of heavy metals, plus a blank of demineralized water to indicate zero value. This provided with a calibration matrix for each element* to monitor (figure 6).
Figure 6. Calibration curves for the heavy metals as measured by ICP-MS at known concentrations.
The hand cream did not contain any detectable heavy metal. To make sure that this result was not due to some problem with the method, the spiked samples could be used to verify that the method was able to recover the expected metal content within 75-125%:
| Parameter | Content, in ppm | ||||
| As | Cd | Hg | Sb | Pb | |
| Expected spike | 0.400 | 0.400 | 0.400 | 2.00 | 0.400 |
| Spike 1 result | 0.398 | 0.426 | 0.302 | 1.85 | 0.424 |
| Spike 2 result | 0.410 | 0.396 | 0.380 | 1.91 | 0.414 |
| Recovery (%) | 101% | 103% | 85% | 94% | 105% |
Since the results were verified, the sample could be considered compliant with the Canadian Guidance of heavy metals impurities in cosmetics. Here is a glimpse of what the certificate of analysis would look like for this test:
Such a report, duly signed by a chemist from PhytoChemia, would appropriately document a manufacturer’s efforts to track heavy metals and could be presented to Health Canada in case of examination. And a similar approach could be applied to plant extracts, raw ingredients, or vegetable oils too!
*Some elements can be monitored for several atomic masses, called isotopes. This is why the image shows more than 5 analytes, as several isotopes can be checked for a single heavy metal.
