Using HR ICP-OES for Quantitative Analysis of Cannabis and Hemp

With a history spanning across several millennia, hemp and cannabis are one of the oldest crop plants to be found. Their first and primary application was as textile fibers, which continues to be their main use in ropes, textiles, waterproofing, and insulation. Cannabis and hemp are additionally used in the pharmaceutical and food industries for their recreational effects and nutritional value.

Since cannabis was legalized in 2014 for recreational and medical use, the analysis of toxic trace elements gained more attention in order to validate cannabis product safety in the same way as other drug and food products.

Individual guidelines were produced by the areas that legalized hemp and cannabis products because a global recommendation for the analysis of these products intended for human consumption does not yet exist.

For example, the California Bureau of Cannabis Control dictates that laboratories should analyze processing chemicals and residual solvents, microbiological impurities, pesticides, mycotoxins, moisture content and water activity, foreign material and filth, and heavy metals in cannabis and its associated products.

Local authorities often specify elements to be analyzed in line with the FDA and the US EPA regulations. In USP Chapter <232>, the elements Cd, Hg, Pb, and As are known to be the most harmful in pharmaceutical products. As such, these ‘toxic four’ are frequently investigated in cannabis samples as well.

Optical emission spectroscopy with inductively coupled plasma (ICP-OES) is commonly used to perform the measurement of trace metals in plant materials. However, a highly sensitive analyzer is required to determine the commonly low trace levels of Cd, Hg, Pb, and As.

In this respect, mainstream ICP-OES instruments frequently lack the resolving power to accurately detect trace levels in the sub mg/kg range. Routine laboratories are frequently forced to use sensitivity optimizing accessories such as an ultrasonic nebulizer (USN) along with ICP-OES or the more sensitive ICP-MS technology.

Compared to the solutions detailed above, the technique outlined in this article uses a high-resolution ARRAY ICP-OES, the PlasmaQuant 9100 Elite, which can attain detection limits in the sub µg/L range in a standard measurement solution of plant materials.

The PlasmaQuant 9100 Elite provides a degree of sensitivity that removes the requirement for expensive and complex sample feed accessories (such as USN) due to the long analytical zone of its plasma, counter gas technology, and a reduced loss of light throughout the optical path.

Materials and Methods

Sample and Reagents

1 g of dried plant material was precisely weighed and moved into a microwave digestion vessel (PM 60) for the preparation of the measurement solutions. 1 mL of H2O2, 1 mL of HCl, and 5 mL of HNO3 were introduced to the vessel.

The pre-installed “vegetables and leaves” digestion program (Table 1) was used to digest the sample utilizing the TOPwave microwave system. After the samples had digested in full and had cooled to room temperature, the clear solution was filled with deionized water to reach 50 mL, along with the introduction of internal standard Tb and Au to stabilize Hg (200 μg/L Au in the final solution).

Yttrium, the common internal standard element, could not be utilized, as several samples included a significant concentration of Y. Terbium was selected as an internal standard instead as it was not identified in any of the samples.

Table 1. Digestion program for hemp sample.

Step T [°C] Pmax [bar] Ramp time [min] Hold time [min]
1 150 40 5 5
2 200 40 5 15
3 50 0 1 20

 

Instrumentation

Instrument Settings

A PlasmaQuant 9100 Elite ICP-OES featuring a standard sample introduction kit, an additional seaspray nebulizer, and a Teledyne Cetac ASX 560 autosampler were used for the analysis. Table 2 provides an overview of the complex system configuration.

Table 2. Configuration of the PlasmaQuant 9100 Elite equipped with standard kit.

Parameter Specification
RF Power 1300 W
Plasma Gas Flow 12 L/min
Nebulizer Gas Flow 0.6 L/min
Auxiliary Gas Flow 0.5 L/min
Nebulizer Seaspray, 0.4 mL/min, Borosilicate
Spray Chamber Cyclonic spray chamber, 50 mL, Borosilicate
Injector Quartz, inner diameter 2 mm
Outer Tube/Inner Tube Quartz/Quartz
Pump tubing PVC (black, black)
Sample Pump Rate 1 mL/min
Rinse/Read delay 45 s
Auto sampler Teledyne Cetac ASX 560

 

Method Parameters

Table 3. Overview of method-specific evaluation parameters.

Element Line [nm] Plasma view Integration mode Read time [s] Evaluation
No. of pixel Baseline fit, pixel no. Polyn. degree Correction
As 188.979 axial peak 5 3 ABC1 auto Tb
Cd 214.441 axial peak 1 3 ABC auto Tb
Cr 267.716 axial peak 1 3 ABC auto Tb
Hg 184.886 axial peak 5 3 ABC auto Tb
Pb 220.353 axial peak 5 3 ABC auto Tb
Tb 350.917 axial peak 1 3 ABC auto -

1 ABC = Automatic Baseline Correction

Calibration

Multi-element calibration standards were assembled with single element standards, Tb internal standard, and Hg stabilizer in 10% (v/v) HNO3 and 2% (v/v) HCl. Table 4 presents data regarding the concentration levels of the calibration standards and Figure 1 presents the related calibration functions.

Table 4. Concentration of calibration standards

Element Unit Cal.0 Cal.1 Cal.2 Cal.3 Cal.4 Correlation coefficient
As µg/L 0 5 10 25 50 0.9997
Cd µg/L 0 5 10 25 50 0.9995
Cr µg/L 0 5 10 25 50 0.9986
Hg µg/L 0 5 10 25 50 0.9997
Pb µg/L 0 5 10 25 50 0.9997

 

Calibration data.

Figure 1. Calibration data.

Results and Discussion

Table 5 outlines the targets for inhalable and all other cannabis products detailed by the California Bureau of Cannabis Control. The target limits in the real measurement solution range from 2 to 60 µg/L with the applied dilution factor 50.

The method detection limits achieved (MDLs) (see Table 5) are well below the regulated limits and therefore provide a dependable measurement of the trace metal contents of Cd, Cr, Hg, Pb, and As. Applying a lower dilution factor (e.g. 20) can produce a further reduction of the MDLs.

Table 5. Limits of heavy metal concentrations in solid samples and measurement solutions, MDLs of PlasmaQuant 9100 Elite.

Element Limit of all inhalable cannabis products Limit for other cannabis products Method detection limit
Sample [µg/g] Measurement solution1 [µg/L] Sample [µg/g] Measurement solution1 [µg/L] Sample [µg/g]
As 0.2 4 1.5 30 0.034
Cd 0.2 4 0.5 10 0.002
Cr n.d. n.d. n.d. n.d. 0.006
Hg 0.1 2 3.0 60 0.009
Pb 0.5 10 0.5 10 0.024

1 dilution factor: 50

Spike recovery tests at the regulation target levels were performed to validate the accuracy and sensitivity of the method.

Table 6 provides the results for one digestion of a spiked sample and a hemp sample. For all elements, the spike levels were 10 µg/L. The methodology is further validated by the fact that all spike recoveries are within ± 7% of the added amounts.

Table 6. Results and spike recoveries for the analysis of hemp samples.

Element Hemp sample Spiked Hemp Sample2 Instrument Detection Limit
Sample [µg/g] Measurement Solution1 [µg/L] Recovery [%] Measurement Solution1 [µg/L]
As 0.17 3.44 100 0.67
Cd 0.02 0.43 93 0.04
Cr 0.09 1.76 98 0.12
Hg < LOD < LOD 103 0.17
Pb 0.11 2.29 93 0.48

1 dilution factor: 50
2 spike level: 10 μg/L

Two different emission lines for each element were analyzed to investigate the consistency of the results. Even lines that are highly prone to spectral interferences, for example, the Cd/As line pair at 228.8 nm, can be utilized for accurate measurement because they are base-line separated by the PlasmaQuant 9100 Elite’s high-resolution optics (Figure 2).

As-recorded spectrum of Cd/As line pair at 228.8 nm with base-line separation for interference-free quantification.

Figure 2. As-recorded spectrum of Cd/As line pair at 228.8 nm with base-line separation for interference-free quantification.

Conclusion

The measurement of trace metals in hemp and cannabis products by ICP methods has become a routine task for laboratories in the pharmaceutical and food industries.

The methodology outlined in this article utilizes a highly sensitive HR-ARRAY ICP-OES, the PlasmaQuant 9100 Elite, with a standard sample introduction system to accurately measure trace levels of Cadmium, Arsenic, Lead, Chromium, and Mercury.

As a result of the high spectral resolution of the PlasmaQuant 9100 Elite (2 pm @ 200 nm), it is possible to perform an interference-free measurement of all investigated elements. This removes the requirement for lengthy data analysis of multi-line evaluation.

The PlasmaQuant 9100 Elite provides results of the highest accuracy by single line evaluation in a routine task like the analysis of cannabis. The method detection limits reached are in the low µg/kg range, which is well below the target range provided by local authorities and can be attained with a standard sample introduction system (such as a cyclonic spray chamber or pneumatic nebulizer).

The use of a hydride system (As, Hg), a highly sensitive ultrasonic nebulizer (USN), or applying less dilution to the measurement solutions can make it possible to reduce the MDLs further.

A significantly higher initial investment cost and higher complexity when operating the instrument are results of these additions, especially with the use of a USN.

Due to this, the aim of the investigation was to assess the possibilities of a standard configuration for ICP-OES as a reliable, easy to use, cost-effective, and robust solution for the measurement of trace metals in hemp and cannabis samples.

The PlasmaQuant 9100 Elite also offers a viable alternative to ICP-MS instrumentation for the investigation of cannabis and hemp.

This information has been sourced, reviewed, and adapted from materials provided by Analytik Jena US.

For more information on this source, please visit Analytik Jena US.

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