Risk Assessment and Determination of Heavy Metals in Home Meal Replacement Products by Using Inductively Coupled Plasma Mass Spectrometry and Direct Mercury Analyzer

Distilled water was purified through an Fpwps501 Ultrapure Distilled Water Purification System from Human Science (Hanam, Gyeonggi-do, Korea). Hydrogen peroxide (H₂O₂, 30%) was purchased from Merck (Darmstadt, Germany). Ultrapure nitric acid (HNO₃, 70%) was purchased from Duksan (Ansan, Gyeonggi-do, Korea).

Polytetrafluoroethylene membrane filters (0.45 µm) were obtained from Advantec Co., Ltd. (Chiyoda City, Japan). Sodium chloride (NaCl, 25%) was purchased from Samchun (Pyeongtaek, Gyeonggi-do, Korea). Hydrochloric acid (HCl, 98%) was purchased from Duksan (Ansan, Gyeonggi-do, Korea). Toluene (C₆H₅CH₃, 99.5%) was purchased from SK Chemicals (Nam-gu, Ulsan, Korea). L-cysteine (99.0%) was purchased from Samchun (Pyeongtaek, Gyeonggi-do, Korea). The mixed standard solutions for making calibration curves for Pb, Cd, and As were prepared by serial dilution using 7% HNO₃ in concentrations ranging from 0.25 µg/L to 10.00 µg/L. The mixed standard solutions for making the Sn calibration curve were prepared by serial dilution using 7% HNO₃ in concentrations ranging from 0.06 mg/L to 1.00 mg/L. Certified Reference Material (CRM) Standard concentrations (1.12 mg/kg of Pb, 0.80 mg/kg of Cd, 13.66 mg/kg of As, 55.43 µg/kg of Hg, and 23.81 µg/kg of Me-Hg) were purchased from the National Institute of Standards and Technology (Gaithersburg, MD, USA). Sn (224.19 mg/kg) was purchased from Korea Research Institute of Standards and Science (Daejeon, Korea).

From January to September 2020, samples of HMR products were purchased from various convenience and grocery stores located in the Republic of Korea to evaluate the heavy metal content. After removing the non-edible part of each product, the food samples were homogenized with a blender and approximately 10 g stored in zipper bags at −18 °C before digestion and analysis. The samples were sorted into four groups. The first group was Ready to Eat (RTE) products consisting of 100 samples, including ready-to-eat food, fresh convenience food, other processed seafood products, and infants and toddler’s food. The second group was Ready to Heat (RTH) products consisting of 200 samples, including instant cooked foods, sauces, breads, rice cakes, infants and toddler’s food, and other processed seafood products. The third group was Ready to Cook (RTC) products consisting of 140 samples, including meat extract processed products, seasoned meat, infants and toddler’s food, ground processed meat products, and other processed seafood products. The fourth group was Ready to Prepare (RTP) products consisting of 40 samples, including meal kit products.

Fast and convenient sample digestion was conducted by a Terminal 640 microwave-assisted acid digestion system (Milestone, Bergamo, Lombardia, Italy) equipped with a rotor for ten MKM023 HPV-100 TFM vessels. Samples (0.5 g) were placed in a Teflon vessel with 7 mL of ultrapure HNO₃ (70%) and 0.5 mL of H₂O₂ (30%). The vessels were pre-disassembled using a heating block device and distilled water (2 mL) was then added to the pre-disassembled sample and microwave-assisted acid digestion system. Ten vessels were set and operated in the microwave-assisted acid digestion system. The temperature of the microwave was raised to 180 °C for 15 min and maintained for a further 15 min to complete digestion. The vessels were ventilated for 50 min. After cooling at 40 °C for 30 min, ultrapure water was added to the preprocessed solution and volumes diluted to 25 mL. The resulting solutions were filtered through 0.45 µm polytetrafluoroethylene membrane filters for inductively coupled plasma mass spectrometry (ICP-MS) analysis.

Heavy metals ICP-MS analysis was performed with a Thermo Fisher Scientific iCAP-RQ equipped with nebulizer, a Teflon spray chamber and nickel (Ni) sampling cone, and platinum skimmer cone (Thermo Fisher Scientific, Waltham, MA, USA). The solutions were pumped by a peristaltic pump from tubes accompanied by an autosampler ASX-560 (ThermoFisher Scientific, Waltham, MA, USA). After the ICP-MS was stabilized for 20–30 min, the working ability was optimized daily with the tuning solution based on torch horizontal and vertical position, extraction lens, CCT (collision cell technology) focus lens and radio frequency power at 1550 W to minimize interference effects and to maximize signal. Highest purity argon and helium gas (99.99%) were used as the carrier gas in auxiliary flow at 0.8 mL/min, and nebulizer flow of 1.0 mL/min and 5.3 mL/min. For quantification, the analyte isotopes ⁸²Pb, ⁴⁸Cd, ³³As and ⁵⁰Sn were used.