Introduction

The SOLATION^® Inductively Coupled Plasma Mass Spectrometer (ICP-MS) puts the power of trace, multi-elemental analysis in your hands by simplifying and optimizing the typical ICP-MS workflow, inside and out. The system offers high performance, multi-elemental analysis, ideal for environmental, clinical, biomedical, food, agriculture, and geological applications.   

The SOLATION^® offers a state-of-the-art quadrupole deflector that ensures the analyzer and detector stay clean and improves S/N by preventing neutrals and particles from entering the analyzer. The system is also designed for lower argon consumption. 

Key System Infrastructure Includes:

• Ion extraction cones: Triple-cone ion extraction, followed by an Einzel lens, which are electrically controlled to maximize transmission of ions into the vacuum system. 
• RF coil: Plasma generation with water cooled RF coil using industry standard 27 MHz variable frequency generator for rapid impedance matching and ultimate performance with challenging matrices. 
• Torch: One-piece, demountable torch with fast, one-step connection of argon and ignitor. Optional shield to prevent secondary discharge. 

• Nebulizer: High efficiency concentric nebulizer available in glass or quartz for compatibility with the widest range of flow rates and sample composition. 
• Spray chamber: The cyclonic spray chamber with optional temperature control further reduces droplet size and solvent load to ensure stable, efficient plasma. 
• Peristaltic pump: Integrated 4-channel, 12-roller pump for maximum flexibility and ultra-low pulsation. Software controlled flow rate from 1 μL/min to >1 mL/min. 
• Gate valve: Allows quick and easy maintenance and replacement of the cones whilst maintaining vacuum integrity. 
• 90° Quadrupole deflector: Ensures that the analyzer and detector are not in line with the plasma beam, preventing neutrals and particles from entering the analyzer, improving S/N and preventing contamination. 

• Octupole collision cell: Acts as an ion guide and a collision cell with He gas to provide Kinetic Energy Discrimination (KED) to remove interferences. 
• Quadrupole Analyzer: High frequency mass filter design with the highest stability to simultaneously maximize transmission, resolution, and abundance sensitivity. 
• Dual function detectors: Measures in both analog and pulse detection modes with seamless transmission between the two, to allow measurement of high and low levels in a single analysis with more than 9 orders of magnitude of linear dynamic range. 

• Pulse Detection: captures ions generating pulses shorter than 20 ns; accurate and linear to minimum dwell time of less than 100 μs 
• Analog Detection: used for higher ion signals while pulse detection is deactivated to extend detector lifetime. 

• Mass Dependent software control: Software designed to optimize specific mass ranges independently to allow for mass specific tune optimization.  

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Instrumentation: SOLATION^® ICP-MS

INTRODUCTION

With the growing acceptance and legalization of hemp and cannabis in the US, Canada, and several other countries, cannabis products are more widely available than ever before. Now approved for medical, recreational and health supplement uses, increased production and consumption have highlighted the need for routine testing and development of testing standards for toxic chemicals, including heavy metals, in cannabis plant material, and all the byproducts made from them, to ensure safe products for the consumer.  With the adoption of chapters <232> and <233>, US Pharmacopeia (USP) specifies a list of elements and maximum exposure limits based on toxicity and routes of administration for pharmaceutical products.  

Many states that have legalized medical and recreational cannabis base their exposure limits on the USP values.  California, Colorado, and Massachusetts are examples with Permissible Daily Exposure (PDE) limits by inhalation for As, Cd, Hg, and Pb. These values are summarized in Table 1. 

Table 1: PDE limits for states that use the USP guidelines for heavy metal exposure by inhalation. USP<233> also defines the accuracy, repeatability, and ruggedness required for the analysis of these toxic elements:

Validation Criteria

Accuracy: The matrix and materials under investigation must be spiked with target elements at concentrations that are 50%, 100%, and 150% of the maximum permitted daily exposure (PDE). Mean spike recoveries for each target element must be within 70%-150% of the actual.

Repeatability: Six independent samples of the material under investigation must be spiked at 100% of the target limits defined and analyzed. The measured percent relative standard deviation (%RSD) must not exceed 20% for each target element.

Ruggedness: Carrying out the repeatability measurement testing procedure by analyzing the six repeatability test solutions either on different days, either with a different instrument or by a different analyst. The %RSD of the 12 replicates must be less than 25% for each target element.

In this study, we used the Advion SOLATION^® ICP-MS and a microwave digestion system to digest and analyze hemp samples using the validation methods described in USP General Chapter <233>.  The sensitivity, ability to handle complex matrices, and the ability to remove interferences with a helium collision cell makes this the ideal system for heavy metal analysis in the cannabis industry.

EXPERIMENT

Sample Preparation

A sample of medicinal hemp flower was purchased locally.  Around 14 grams was finely ground and homogenized, then 0.5g +/- 0.005g was weighed into digestion vessels and the appropriate amount of a spike solution added.  Nine mL of concentrated HNO3 and 1 mL concentrated HCl was added to each vessel and the samples were allowed to react for 15 minutes prior to sealing and placing the vessels on the turntable of the closed vessel microwave digestion system.  The program controls the microwave energy such that the samples ramped up to the optimal digestion temperature of 200°C over 20 minutes, maintained 200°C for 10 minutes, and then cooled back to room temperature.  

This method resulted in the complete digestion of all samples resulting in a clear, particulate-free solution once brought to volume with 18MΩ water in a 50mL volumetric flask.

The calibration standards and spikes were based on the action levels in table 1.  The sample set included a hemp sample, a duplicate, the 50%, 100%, and 150% spikes, and NIST 1575 Pine needles to further validate the results, which were run in duplicate.  For the ‘ruggedness’ specification from USP<233>, there were 6 samples of the 100% spiked hemp.  The spike values are summarized in Table 2.

Table 2: Spike values based on the action limits defined in USP<233>

A second, 1:4 dilution was performed post digestion to bring the final acid concentration to 5% for an overall dilution factor of 400x.  The calibration blank and standards were prepared using the same acid concentration, 5% of 9:1 HNO₃/HCl, for matrix matching.  To stabilize mercury and help with wash-out, Gold was added at 20x the mercury concentration.  The internal standards were added to all the samples, standards and blank for a final concentration of 10 ng/g (ppb).  The standard concentrations and internal standards are summarized in Table 3.

Table 3: Analyte masses, calibration standards, and internal standards.

Instrumentation

The Advion SOLATION^® ICP-MS incorporates a robust solid-state generator, orthogonal ion optics for keeping the most sensitive components of MS clean, and easy-to-use control and data processing software. 

Since HCl was used in the sample digestion, there is a significant amount of chlorine present which creates an isobaric interference on ⁷⁵As from ⁴⁰Ar³⁵Cl⁺.  The collision cell effectively eliminates the contribution that ArCl⁺ makes to the signal at m/z 75 by taking advantage of kinetic energy discrimination (KED) to separate polyatomic interferences from analyte ions resulting in accurate quantitation of low levels of Arsenic.  Arsenic is the only analyte in the suite with this type of interference, so the collision cell isn’t used for Cd, Hg, or Pb.  

A glass concentric nebulizer fitted to a cyclonic spray chamber, connected to the standard torch with an injector ID of 2mm, was used for sample introduction.  Instrument running parameters are summarized in Table 4.

Table 4: ICP-MS Parameters

RESULTS AND VALIDATION

Sample Results