ICP-MS Method Validation

The instrument was calibrated using a calibration blank and six working standard solutions of each metal to be analyzed: 0.5, 1.0, 2.5, 5.0, 10.0, and 25.0 μg/L of As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Sn, respectively. The response curves were recorded, and linearity and regression results are presented in Table 2.

The method blank consists of all water and acids used for digestion, without any sample. It is prepared with each batch of samples and is digested, diluted, and analyzed the same as unknown samples.

The laboratory control sample (LCS) is a method blank with a known amount of analytes added. In this case, 250 μL of an intermediate stock solution with 1.0 μg/L each of As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Sn was added to the digestion tube containing the same acid and water as all other samples. After digestion and dilution, the final concentration of each metal was 5.0 μg/L. The percent LCS recoveries for each metal of interest were calculated using the following eq 1. LCS results are used to verify that the laboratory can acceptably perform the analysis in a clean matrix. The target recovery range for LCS samples was 80 to 120%.³⁷

where LCS is the laboratory control sample results (μg/L), MB is the result of the method blank (μg/L), and S is the concentration of the element used to spike the LCS (μg/L).

The relative standard deviation for replicate analyses of the different samples was obtained by dividing the standard deviation by the mean value of the analytical data according to eq 2.

where S is the standard deviation of the nine replicate analyses and $\bar{x}$ is the mean of the nine replicate analyses.

A series of “cold air blank” or procedural blanks were prepared on different days by drawing air through the entire system, without any cartridge or heat, and processed the same way as experimental samples. Their ICP-MS concentrations are found in Table S5 of the Supporting Information. The average value is subtracted from experimental samples to account for any background signal from glassware or solvents.

Accuracy and precision of the instrument method were assessed by the analysis of laboratory control samples (LCS) and matrix spike (MS) samples. Accuracy was evaluated through recovery studies of sample spikes. Precision was evaluated regarding repeatability by estimating the relative standard deviation (RSD) of the recovery percentage for each spiked level. In this study, the recovery test was done by creating a matrix spike as follows: 250 μL of an intermediate stock solution with 1.0 μg/L of nine metals standards was added to 0.2 g of cannabis concentrate stock in a digestion tube and digested using the same parameters as all other samples. After digestion and dilution, the final target concentration of the metals was 5.0 μg/L each. The percentage recoveries of the analyte were calculated using eq 3. MS analysis indicates a potential problem due to the sample matrix itself. The target recovery range for spiked samples was considered to be 75 to 125%.³⁷

where C_s is the concentration of the spiked sample, C_u is the concentration of unspiked sample and C_t is the target concentration in μg/L.

The LOD and LOQ were determined experimentally and defined as 3 and 10 times the standard deviation of the low-concentration spiked samples, respectively. For this study, both the LOD and LOQ were calculated using the six replicates of 0.5 μg/L matrix spike samples. LOD and LOQ sample spike recoveries were calculated using eq 3. The limit of detection (LOD) and limit of quantitation (LOQ) were calculated using eqs 4 and 5, respectively. After factoring in the sample weight (0.2 g) and a final volume (50 mL), the LOD and LOQ concentrations are expressed in μg/g. The percent recovery values for limit of detection were determined by eq 6 where C_s is the concentration of the spiked sample, C_u is the concentration of unspiked sample metal of interest in μg/L.

where SDss is the standard deviation of the six replicate spike samples.

where LOD (%) is the limit of detection in percent recovery units, is the limit of detection in μg/g units, and is the initial spiked oil concentration in μg/g units.

The digested samples were analyzed by ICP-MS for As, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, and Sn, and the concentration in the original sample was calculated using eq 7.

where C is the concentration in μg/L from the instrument software’s evaluation of the calibration standards, V is the final volume of the digested solution (50 mL), df is the dilution factor (4), and m is the mass of the sample (∼0.2 g).

Each of the organic solvent trials (Table 1; MeOH + AA, Ac + AA, and Hex + AA) had an air blank subtracted for both the organic and aqueous impinger by using data from a blank trial with the Ac + AA solvent system. The aqueous samples had the metal concentrations of unused impinger liquid subtracted from them as a blank. In all experiments, both the impinger signals were added together for the total metal recovery. All spiked and unspiked materials had a method blank subtracted from them. When subtracting values, the raw instrument values were subtracted first, and then any unit conversions or calculations to the concentration in the matrix were conducted.