ICP-MS Analysis of Heavy Metals in Hemp Containing Personal Care Products

Introduction

Hemp (or: industrial hemp) and cannabis are strains of the Cannabis sativa plant differentiated on the basis of total delta-9-tetrahydrocannabinol (delta-9-THC) content (delta-9-THC and tetrahydrocannabinolic acid (THCA)). Cannabis sativa plant material that exceeds 0.3% wt/wt THC on a dry weight basis is considered to be cannabis (marijuana) under the US Controlled Substances Act. Industrial hemp is Cannabis sativa with a THC threshold below 0.3% (= % THCA x 0.877 + % Delta-9-THC). Definitions of hemp and cannabis are regulated in a specific manner in many parts of the world. In this work various food products containing hemp have been analyzed, but all descriptions are valid for cannabis containing food as well.

Hemp is known to accumulate heavy metals such as lead, cadmium, arsenic, mercury, chromium or nickel in its roots, shoots, buds and seeds, and has been used for the remediation of contaminated soil (phytoremediation and phytoextraction).^1,2,3,4 Due to potentially hazardous effects of these metals, this property may hinder the use of hemp or hemp extracts in cosmetics or medical industries. In addition, product contamination can also occur during the manufacturing process. As a consequence, personal care or pharma products containing hemp or hemp extracts must be tested for their heavy metal content.

This report describes the analysis of arsenic, cadmium, mercury and lead (elements typically referred to as the “big four”) and chromium, barium, silver, selenium, antimony, copper, nickel and zinc in hemp containing personal care product samples by inductively coupled plasma coupled to mass spectrometry (ICP-MS). A premixed metal standard solution containing all twelve element standards was utilized – an approach covering a broad range of analytes. In total, seven different personal care products containing hemp seed oil or hemp seed extract were analyzed:

• Liquid soap
• Soap bar
• Cannabidiol oil
• Caring oil
• Face cream
• Skin cream organic
• Gel cream organic

General Procedures

Sample preparation and preparation of internal standard, blank and food sample solutions

Sample preparation

All analyzed samples but the soap bar were liquid and did not need to undergo any type of homogenization or grinding procedure. In case of the soap bar, small pieces of soap were cut from the bar at room temperature utilizing a plastic blade.

Preparation of Internal Standard, Blank and Food Sample Solutions

1. General: Weigh samples on a calibrated analytical balance with a readability of 0.1 mg. Use single-channel pipettes with variable volume and gravimetrically tested polypropylene pipette tips for reagent, standard and sample transfers.
2. Indium standard solution (internal standard): Pipette 3 mL nitric acid 60% and 1000 µL of indium ICP standard (1000 mg/L) into a 100 mL volumetric quartz flask. Fill flask to mark with ultrapure water to obtain a final concentration of 10 µg/mL. Note: Rhodium ICP standards may also be used as alternative internal standard.
3. Blank solution: Pipette 3 mL nitric acid 60% and 1 mL hydrogen peroxide 30% into a 15 mL quartz microwave digestion vial and digest using a microwave digestion system (for conditions see section below). After completed digestion quantitatively transfer solution into a 50 mL polypropylene tube, combine with 50 µL of indium standard solution and fill to 50 mL with ultrapure water.
4. Sample solution: Weigh 50 ±1 mg of personal care product sample into a 15 mL microwave quartz vial. Add 3 mL of nitric acid 60% and 1 mL hydrogen peroxide 30% and digest sample utilizing a microwave digestion system. After completed digestion quantitatively transfer obtained solution into a 50 mL polypropylene tube, combine with 50 µL of indium standard solution and fill to 50 mL with ultrapure water.

Calibration: Preparation of Heavy Metal Standard Addition Solutions

In order to compensate for sample matrix effects, a standard addition approach utilizing a Certified Reference Material (CRM) Metal mix I for cannabis testing standard solution was applied for the preparation of all calibration curves. The final calibration curve was comprised of four data points (three standard addition solutions plus sample solution).

General Process for the Preparation of Calibration Solutions for ICP

1. Weigh 50 ±1 mg of ground sample into a 15 mL microwave quartz vial.
2. Add 3 mL nitric acid 60%, 1 mL hydrogen peroxide 30% and different volumes of CRM Metal mix I for cannabis testing standard solution and digest sample using a microwave digestion system.
3. After completed digestion, quantitatively transfer obtained solution into a 50 mL polypropylene tube, combine with 50 µL of indium standard solution and fill up to 50 mL with ultrapure water.
4. Subject resulting sample to ICP-MS analysis.

Preparation of Standard Addition Solutions

Preparation of standard addition solutions using Metal mix I for cannabis testing standard solution (As, Cd, Hg, Pb, Ag, Cu, Co, Ni, Sb, Se, Ba and Zn).

The chromium content of one sample made the preparation of an additional set of higher concentrated standard solutions necessary. The compositions are listed below.

Preparation of standard addition solutions using Metal mix I for cannabis testing standard solution (Cr).

Instrument Parameters

Food Sample Digestion

After digestion the obtained solution should be clear and particle free. Conditions must be adapted to the specific microwave system used.

ICP-MS Conditions

The analysis was performed in the sequence: Blank, samples 1 – x, addition solutions.

Results

All hemp containing personal care products were analyzed by ICP-MS and as duplicates. The recovery rate (RR) for the samples was determined using Metal mix I for cannabis testing standard solution. RRs for all metals were in the range of ±6% or better, with the exemption of mercury (up to -24%) and one barium test result (+17%, data not shown).

Liquid Soap

Sample contains Cannabis sativa hemp seed extract.

Soap Bar

Sample contains Cannabis sativa hemp seed oil.

Cannabidiol Oil

Sample contains Cannabis sativa hemp seed oil and Cannabis sativa hemp seed extract (5% CBD).

Caring Oil

Sample contains Cannabis sativa hemp seed oil.

Face Cream

Sample contains Cannabis sativa hemp seed oil.

Skin Cream Organic

Sample contains Cannabis sativa hemp seed extract.

Gel Cream Organic

Sample contains Cannabis sativa hemp seed oil.

The antimony, arsenic, cadmium, mercury, lead, selenium and silver content of all samples was below the limit of detection (LOD; Se: 1.0 µg/g, Hg: 0.5 µg/g, all others: 0.1 µg/g). Both barium and copper displayed values below or slightly above the LOD, zinc with a maximum of 2.3 µg/g, chromium up to 1.1 µg/g (with the exemption of the gel cream at up to 44.0 µg/g) and nickel below or slightly above the LOD (plus the soap bar sample at 4.5 µg/g).

Conclusion

This work demonstrates a comprehensive ICP-MS workflow, using the standard addition calibration method, for determination of metal content in hemp containing personal care products. A crucial elements in the process is the use of an accurate traceable Certified Reference Material metal mix standard solution. Samples were digested utilizing a specific digestion protocol optimized to provide clear digestion solutions. The resulting solutions were subjected to ICP-MS analysis. Calibration data was obtained by the preparation and analysis of standard addition solutions obtained by utilizing a CRM metal mix containing a set of twelve element standards (Sb, As, Pb, Hg Ag, Ba, Cr, Cu, Co, Ni, Sb, Se, Zn).

Calibration data revealed an excellent linearity over the entire calibration range for all metals and samples. This finding prove the suitability of the internal calibration approach in the analysis of matrix rich personal care products. Observed recovery rates were very high and in the range of ±6% or better for all metals, with the exemption of mercury (up to -24%) and one barium test result (+17%).

The final results of duplicate analyses of all samples were consistent and revealed an antimony, arsenic, cadmium, mercury, lead, selenium and silver content below the limit of detection (LOD; Se: 1.0 µg/g, Hg: 0.5 µg/g, all others: 0.1 µg/g). Both barium and copper displayed values below or slightly above the LOD, zinc with a maximum of 2.3 µg/g, nickel below or slightly above the LOD and chromium up to 1.1 µg/g. A slight deviation was observed for nickel in the soap bar sample (at 4.5 µg/g) and a substantial chromium concentration in the organic gel cream sample (up to 44.0 µg/g).