Fish serves as an important food source for both humans and wildlife, but the accumulation of heavy metals and trace elements in fish tissues can potentially pose health risks.[1] Measuring trace elements in fish tissue provides valuable data for research and regulatory purposes. Sample digestion is a critical step in the analytical process for determination of metals in food samples, including fish. This study presents a method for the determination of twenty elements (Be, Al, Ag, Cd, Sb, Th, V, Ni, Co, Cu, Pb, U, Cr, Mo, Ba, Tl, Mn, Zn, As, Se) in fish tissue samples using inductively coupled plasma-mass spectrometry (ICP-MS). Two acid digestion methods – hot block and closed vessel microwave system – were utilized to prepare the samples for ICP-MS analysis. The effectiveness of digestion methods was verified using standard reference materials (SRMs: Oyster Tissue NIST 1566b and Fish Protein DORM-5).

SRMs (~0.25 g) or fish tissue samples (~0.5-1 g) were accurately weighted into the digestion vessels. Thulium was added to each sample as internal standard (final concentration of 20 µg/L). 7 mL of concentrated nitric acid was added to each vessel for hot block digestion, and 10 mL acid for the microwave digestion procedure. Samples were pre-digested overnight at room temperature. 1 ml of hydrogen peroxide was added to each sample. Digestion was performed using a hot block (SCP Science DigiPREP Jr, 45 min to 95 C, 4 hours at 95 C) and a microwave digestion system (Milestone Ethos Easy, 25 min to 180 C, 15 minutes at 180 C). After digestion, samples were diluted and analyzed using an Agilent ICPMS 8800. The results from the hot block and microwave methods were compared. Additional validation was conducted by analyzing ten fish tissue samples using these methods and comparing the results with data obtained from other laboratories employing similar methods.

Using microwave digestion, the element concentrations in SRMs were within the ±10% of the certified values for all elements, except for Al in NIST 1566b and Ni in DORM-5, which had an average of recovery of 70%. Low recoveries of Al and Ni in matrices similar to these SRMs have been reported in the literature.[1,2] With the hot block digestion procedure, the metal concentrations in SRMs were within ±15% of the certified values for all elements, except for Al (20-50%), V (68-75%), Cr (75%), Ni (68-90%), As (115-120%), Se (135%). In fish tissue samples, the concentrations of Be, Ag, Cd, Th (0.003), Al (1.0), and Sb (0.005) were below the limit of detections (mg/kg) using either microwave or hot block digestion procedures. V, U, Cr, Mo, Tl (0.003), Ni, Co (0.01), Pb (0.03), Ba (0.01), and Cu (1.0) were close or below the limit of detection (mg/kg), resulting in inconsistent results between the two digestion protocols. The concentrations of Mn, As (0.003), Se (0.01), and Zn (1.0) in fish samples were above limit of detection (mg/kg) and comparable when using hot block and microwave digestion methods.

Overall, the closed vessel microwave digestion procedure was a more efficient method for digesting and measuring total elements in fish samples while minimizing contamination, which is consistent with the findings of other studies.[2] Due to its cost-effectiveness, the conventional heating system (e.g., hot block) is still widely used by many laboratories. Therefore, comparison studies like this are valuable for selecting the most appropriate SRM and digestion protocol, including the combination of acids, reagents and the heating system, for a specific suite of elements and sample matrix.

References

[1] C. Donadt, C.A. Cooke, J.A. Graydon, M.S. Poesch Environmental Toxicology and Chemistry, 2021, 40, 422–434.

[2] S. Ashokaa, B.M. Peakea, G. Bremnerb, K.J. Hagemana, M.R. Reid Analytica Chimica Acta, 2009, 653, 191–199.

[3] K.J. Lamble, S.J. Hill, Analyst, 1998, 123, 103R–133R.