Trace detection of heavy metals in tobacco

Simple, rapid, sensitive and accurate method based on ICP-MS

  • Tab. 1: Analytical elements, masses and internal standardsTab. 1: Analytical elements, masses and internal standards
  • Tab. 1: Analytical elements, masses and internal standards
  • Tab. 2: Results, in µg/kg of the hemp tea, and two types of tobaccos
  • Fig. 1: Calibration curves for As, Cd, Hg, and Pb sum are shown in Figure 1a-d. All correlation coefficients r achieve a 0.9998 level and above.
  • Fig. 2: Recovery rate of the calibrated elements for digested reference material.
  • Tab. 3: LOD in tobacco and hemp samples

Plants, and in particular tobacco plants, hemp tea leaves, and flowers (a non-psychoactive variant of cannabis) are well-known to absorb trace elements from the soil and to accumulate them in their leaves. Some of these elements are toxic to humans even in minute quantities. As they grow, plants can bioaccumulate metals in their tissues that originate from the soil and water in their surroundings. These metals can naturally occur in soils and water as part of their mineral content. They may also be artificially introduced in the form of fertilizers or crop protection products to increase the yields.

The emerging leader in metal analysis is ICP-MS because of its high sensitivity and high sample throughput resulting from quasi simultaneous multi-element data acquisition. An argon plasma as ion source coupled with a rapid quadrupole for mass filtering are the main components of the system. Here, the applicability is explored and discussed of the Shimadzu ICPMS-2030 to the detection of the “Big Four” heavy metals arsenic, cadmium, mercury, and lead (As, Cd, Hg, and Pb) and additionally beryllium , cobalt, chromium, nickel, and selenium (Be, Co, Cr, Ni, and Se) in a digested hemp flower (Sativa flowers) and two tobacco reference samples for compliance.

Total Concentration

Since speciation information is not available using ICP-OES (inductively coupled plasma optical emission spectrometry) or ICP-MS (inductively coupled plasma mass spectrometry), the analyst operates under the assumption that, to be within the acceptable limits, the total concentration of that element must be below the PDE (Permitted Daily Exposure) for the more toxic inorganic forms of these elements. It is worth noting, however, that speciation concentrations can be quantitated by coupling an inert HPLC system to the ICPMS. Using HPLC allows the analyst to separate metals based on their oxidation state and/or associated complexes and elute them into the ICPMS for concentration analysis. Shimadzu offers both the hardware and software to enable such analyses.

Equipment

The ICPMS was used for analysis in combination with an autosampler.

In-line addition of internal standards to calibration and unknown samples was accomplished using the Internal Standard Addition Kit. Based on the internal diameter of peristaltic tubing used for sample and internal standard injection, the approximate dilution of the internal standard solution was 90 %. High-purity reagents (Carl Roth GmbH, Germany) were used during sample preparation and dilution to ensure minimal contamination. Ultra-pure water (≥ 18.1 MΩ∙cm; Sartorius, Germany) along with trace metal grade nitric acid was used for all dilutions and acidifications. All standards and unknown samples were prepared in cleaned single-use containers to minimize any cross-contamination between analytical runs.

Sample Preparation

The method uses liquid samples of the hemp flower and the tobacco (taken from cigarettes) which had to be digested in a closed microwave vessel to make the metal atoms available for ICPMS. All samples were prepared in digestion vessels that contained nitric acid. The matrix samples also contained about 0.5 g of hemp flower or tobacco. The digestion vessels were tightly sealed and placed into a rotor within the microwave. The digestion temperature and vessel pressure were stepwise increased over a period of 35 minutes and afterward cooled down to room temperature. The resulting digested solution was poured from the digestion vessels and were brought to a final volume of 100 mL by diluting with ultra-pure water. No further dilution of the samples is required prior to analysis using the ICPMS system.

Analytical Methodology

Thanks to the discrete dynode electro multiplier detector, the system associates high sensitivity (trace detection) with a wide dynamic range (109) which is the key for simultaneous determination of major and trace elements. Due to the unique environmentally-friendly Eco-Mode associated with the energy-saving Minitorch plasma unit, the system is able to reduce running costs by half. The flat-octupole collision cell assures a high accuracy for all elements measured. Using Helium gas for Kinetic Energy Discrimination principle (KED), this cell suppresses most of the spectroscopic interferences (polyatomic interferences). The efficiency of interferences suppression and enhancement of sensitivity are improved by a cooled cyclonic chamber and well controlled torch positioning.

Moreover, the system can detect the whole spectrum from samples measurements and allowing results treatment without need of reanalysis. Thanks to its development assistant, Labsolutions ICPMS software is able to propose the optimum parameters for each element in the sample. Method development has never been so easy and fast.

Operating conditions for the system and analyte elements, masses, and calibration concentrations are provided in Table 1. Calibration curves were generated immediately prior to analysis to ensure the most accurate quantitation. The calibration curves (fig.1) show here on x-axis concentration and on y-axis internal standard corrected intensity (kcps).

Results

The recovery rate for the calibrated elements are between 89 % (202Hg) and 108 % (52Cr). Even if these values already show the accuracy of the method, one more thing must be considered: The used standards Polish certified reference material, Oriental basma tobacco leaves (INCT-OBTL-5) and Polish virginia tobacco leaves (INCT-PVTL-6) for multielement trace analysis were also part of a study of 19 laboratories using ICPMS technology.

Aim of the study (Proficiency test report, pub. Sep 2015) was to determine As, Be, Cd, Cr, Co, Ni, Pb and Se [3] in the above mentioned reference material. Some of the determined elemental concentrations (52Cr, 78Se, 202Hg) differ much from the certified values or were not re-analyzed (Hg). Basically, the same deviation was found in this experiment. Therefore, for a better comparison with measured laboratory values, it was decided to use only for 202Hg and for 78Se the certified values.

Hg was not part of the comparable study [3] and Se was differing too much in the study between 36+/-7 µg/L to 799 +/-216 µg/L. Therefore, the certified Se value 88 ng/g was taken. All the other elements were compared using the average vales of the study. For more detail please see STS-CTR-Metals Proficiency Study – September 2015 [3].

The quantitation results in figure 2 and table 2, demonstrate that the system is able to quantify simultaneously the various elements present in the leave samples. Result of sample measurement are presented in table 2.

Discussion

Three samples were determined by inductively coupled plasma mass spectrometer with microwave digestion: Hemp flower tea (Hemp A) and, two types of cigarette tobaccos (Tobacco B, and C). The results shown in figure 2 and table 3 demonstrate the method accuracy and the ability of the system to handle acid-digested leaves (hemp, tobacco) matrixes to provide recovery rates within the commonly accepted deviation range for such kind of samples.

The detection limits (LOD) in ppb of As, Be, Cd, Co, Cr, Hg, Ni, Pb and Se were automatically calculated by Labsolution ICPMS software using 3 sigma criteria.

Conclusion

The experiment shows the applicability of the system to the detection of various heavy metals (As, Cd, Hg, Pb, Be, Co, Cr, Ni, and Se) in a digested hemp flower and two tobacco samples for compliance. The applied method is simple, rapid, sensitive and accurate; it meets the demand for trace heavy metal elements analysis in tobacco or hemp samples due to the system’s high sensitivity and its wide dynamic range.

 

Authors
Johannes Hesper and Jan Knoop

 

Contact
Dr. Johannes Hesper

Product Manager Spectroscopy
Shimadzu Europa GmbH
Duisburg, Germany
info@shimadzu.de
 

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References:

[1] Polish Certified Reference Material for Multielement Trace Analysis, oriental Basma Tobacco Leaves (INCT-OBTL-5), Institut Chemi I Techniki Jadrowej. Warszawa Poland.

[2] Polish Certified Reference Material for Multielement Trace Analysis, oriental Basma Tobacco Leaves (INCT-PVTL-6), Institut Chemi I Techniki Jadrowej. Warszawa Poland.

[3]K. Wagner., Smokeless Tobacco Sub-Group, Proficiency Test Report, The Determination of As, Be, Cd, Cr, Co, Ni, Pb and Se in Reference Material, Sep. 2018.

Contact

Shimadzu Europa GmbH


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