A complete workflow has been developed for the analysis of a total of twelve heavy metals in hemp-containing beverage products. This workflow includes the following:
Hemp (or: industrial hemp) and cannabis are strains of the Cannabis sativa plant differentiated based on 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 marijuana (cannabis) under the US Controlled Substances Act. Industrial hemp is Cannabis sativa L. 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 beverage products containing hemp have been analyzed, but all descriptions are valid for cannabis-containing beverages 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. It is also known to be used for the remediation of contaminated soil (phytoremediation and phytoextraction).1,2,3,4 Due to the potentially hazardous effects of these metals, this property may of course hinder the use of hemp in, e.g., beverage or cosmetics industries. In addition, product contamination can also occur during the manufacturing process. Therefore, beverage or personal care products containing hemp 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”) as well as of chromium, barium, silver, selenium, antimony, copper, nickel, and zinc by inductively coupled plasma coupled to mass spectrometry (ICP-MS). A premixed metal standard solution containing these twelve heavy metals was utilized – an approach that is simple and that covers a broad range of analytes. Depending on specific lab needs, it can be supplemented by additional element standards – or replaced by a heavy metal mix only containing the “big four”. In total, six different beverage samples containing hemp were analyzed:
All six beverages were liquid and did not contain any solid components such as fruit zest or deposited particles. Therefore, no sample homogenization or filtration was conducted. All the samples containing carbon dioxide were degassed for 30 minutes in an ultrasonic bath prior preparation of sample solutions.
To compensate for sample matrix effects, a standard addition method 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 is comprised of four data points (three standard addition solutions plus a sample solution).
Preparation of standard addition solutions using Metal mix I for cannabis testing Standard Solution (As, Cd, Hg, Pb, Ag, Cr, Cu, Co, Ni, Sb and Se)
The metal mix I for cannabis testing was diluted by a factor of 1:10 using ultrapure water.
After digestion, the obtained solution should be clear. Conditions must be adapted to the specific microwave system used.
The analysis was performed in the sequence: Blank, samples 1 – x, addition solutions.
Standard addition solutions were prepared utilizing Metal mix I for cannabis testing standard solution (As, Cd, Hg, Pb, Ag, Ba, Cr, Cu, Co, Ni, Sb, Se, Zn). Shown are the results obtained for the CBD lemon-lime drink sample. These revealed excellent linearity over the entire calibration range, with R2 values of > 0.9997 for all metals but lead (0.9963). The latter can be considered uncritical, as all analysis results are below the LOD of 0.5 µg/mL and corresponding recovery rates are excellent (see results section). Comparable results were achieved with all other hemp-containing beverage samples.
Figure 1. Calibration curves for As, Cd, Hg, Pb, Ag, Ba, Cr, Cu, Co, Ni, Sb, Se, and Zn obtained on the analysis of four standard addition solutions.
All hemp-containing beverage samples 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 heavy metals were in the range of ±7% or better.
Three out of six beverage samples did not contain any of the twelve heavy metals analyzed. In the hemp energy drink, only zinc was detected (0.1 µg/mL). The antimony, arsenic, chromium, lead, nickel, selenium, and silver content of all the samples was below the limit of detection (Ag: 1.0 µg/mL, Pb: 0.5 µg/mL, all others: 0.1 µg/mL). The CBD drink and the hemp mead samples contained detectable amounts of either one of the metals barium, cadmium, copper, mercury, and zinc, with concentrations of 1.5 µg/mL (Ba), 0.3 µg/mL (Cd), 0.2 – 0.4 µg/mL (Cu), 0.2 µg/mL (Hg) and 1.4 µg/mL (Zn), respectively.
This work demonstrates a comprehensive ICP-MS workflow, using the standard addition calibration method, for the determination of heavy metals in hemp-containing beverage products. Crucial elements in the process include the use of an accurately traceable Certified Reference Material metal mix. 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 twelve metals and heavy metals (arsenic, cadmium, lead and mercury, silver, barium, chromium, copper, cobalt, nickel, antimony, selenium, and zinc).
The final results were consistent and revealed the content of Sb, As, Pb, Hg, Se, and Ag in all the samples below the limit of detection. But the barium, copper, nickel, and zinc concentrations ranged from 0.1 – 38 µg/g, chromium content between 0.1 – 0.3 µg/g (not detected in cereals), and the cadmium concentration of 0.07 – 1.0 µg/g (chocolate and chocolate covered hempseed).
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