Development of greener ultrasound and microwave-based sample preparation methods for spectroscopic determination of heavy metals in pet food samples

Abstract

Contamination of pet food with heavy metals is a crucial problem worldwide and it results in adverse health effects on pets, massive recall of the product, damaging of the brand name and financial loss. Therefore, researchers have been constantly monitoring heavy metals in pet food samples using spectroscopic techniques especially inductively coupled plasma- optical emission (ICP-OES). However, food matrices cannot be directly introduced into the standard ICP-OES there is always a need for a sample preparation step. Mostly reported wet chemistry sample preparation method have had limitations due to large use of concentrated acids. Thus, development of new methods, that is; minimizing use of concentrated acids, reduction of energy consumption and utilizing reusable materials are the future trends for principles of green chemistry. Therefore, the current study aimed at developing and validating greener microwave and ultrasound-based sample preparation methods for determination of cadmium (Cd), arsenic (As), lead (Pb), tin (Sn), and chromium (Cr) prior to ICP-OES analysis. The optimum conditions (200 °C, 40 mins, 0.5g, and 5 mol/L for temperature, digestion time, sample mass and H2O2 concentration, respectively) for microwave- assisted hydrogen peroxide digestion (MW-AHPD) were investigated using multivariate tools, certified reference material wheat flour (FAP80467) and fish (ERMBB422). The MW-AHPD demonstrated excellent accuracy (96-98%), reproducibility (≤ 2.1%) and method detection limits of 0.0675 to 0.3765 μg/g. Application in real pet food samples made from maize, rice, vegetables, fish, and wheat reported concentration levels of Cd, Pb, As, Cr and Sn ranged between 11.2-22.6, 6.4-11.9, 3.44-13.4, 0.44-2.98 and 0.18-0.98 μg/g, respectively. Furthermore, the optimum conditions for ultrasound assisted hydrogen peroxide extraction (UA-HPE) were: 80 °C, 60 mins, 0.5g, 5 mol/L, for sonication temperature, extraction time, sample mass, and H2O2 concentration, respectively. The UA-HPE method was accuracy (>95%), precise (≤ 1.9 %) with acceptable method detection limits (0.3498 and 0.49 μg/g). The reported concentration levels by UA-HPE were 0.86-11.34, 4.50-11.45, 2.61-12.5, <DL-7.94, and <DL-1.04 μg/g for Cd, Pb, As, Cr and Sn, respectively. Both methods (UA-HPE and MW-AHPD) were assessed for greenness using the AGREEPrep metric tool and had similar scores of 0.74 (UA-HPE) and 0.76 (MW-AHPD), confirming the greenness. It is worthy to indicate that maximum concentration limits for Cd, Pb, As, Cr and Sn in pet food samples are: (10, 10, 10, 12.5 and 200 μg/g respectively) as reported by South African regulators. However, the current study showed that Cd (11.2-22.6 μg/g), Pb (11.45-11.9 μg/g) and As (12.5-13.4 μg/g) were above the maximum tolerable limits, for the fish and wheat-based pet food samples. Therefore, these results imply that South African pets are at risk from consuming wheat and fish-based pet foods. More studies on health risk assessment are required to further confirm the findings.