Fish are valuable sources of nutrients and micronutrients and play an important role in human nutrition (FAO 2018; Hasselberg et al. 2020). Besides being a rich source of protein and essential amino acids, it is a dietary source of omega-3 long-chain polyunsaturated fatty acids, essential minerals, such as Ca, P, Zn, Fe, Se, and I, and vitamins, mainly A, B and D (FAO 2018). Otherwise, fish can also be contaminated despite their nutritional importance, depending on the habitat, trophic position, proximity to anthropic activity, and toxic elements, among other factors (Hasselberg et al. 2020; van der Oost et al. 2003). Even in low amounts, elements such as As, Cd, Hg, and Pb are highly harmful to humans and can compromise food quality (Apostoli et al. 2006).
Arsenic is a naturally occurring element; its toxicity depends on its chemical form. Arsenobetaine (AsB) is the most abundant form of As in fish. Together with other organic species, such as methylarsonic acid (MMA) and dimethylarsinic acid (DMA), present slight toxicity concerning the inorganic species As(III) and As(V) (Apostoli et al. 2006; Schmidt et al. 2018). On the other hand, cadmium is found very dispersed in nature. However, there is an increase in its concentration in certain matrices due to industrial activities that use Cd for various purposes (Apostoli et al. 2006). The As and Cd elements have no known biological function and can be toxic, even at low concentrations and/or in chemical forms, presenting potential human health risks. Due to mutagenic and carcinogenic effects, As can affect the neurological, cardiovascular, and respiratory systems. Cadmium, in turn, can lead to kidney failure and also affect the prostate (Mertz 2018; Oosthuizen 2012).
In fish, Cd usually is present in low concentrations. The amount of As that can be found depends on the habitat and trophic position, among other factors (FAO 2018; Hicks et al. 2019). In this context, several studies that aim to develop analytical methods for the determination of As and Cd in fish were proposed. Among the techniques employed to determine trace elements, inductively coupled plasma mass spectrometry (ICP-MS) is one of the most sought-after. This is due to its high sensitivity, good precision, low limits of detection, and multi-element capability. However, the high cost of the analysis derived from high acquisition cost, high argon consumption, and several matrices and isotopic interferences limits its application (Hirata et al. 2006). Microwave-Induced Plasma Coupled Optical Emission Spectrometry (MIP OES) is an attractive technique for nutrient and contaminants determination. The relatively low cost is its most significant advantage, and this is due to nitrogen use instead of argon. A nitrogen generator can be used, avoiding using a gas cylinder. However, matrix effects and low sensitivity are observed in this technique due to the low plasma temperature (5000 K) (Williams et al. 2019). In this context, several strategies have been investigated to minimize these limitations, such as hydride generation, cold vapor (Lima et al. 2015; Matusiewicz and Ślachciński 2010), and preconcentration methods, an excellent alternative for minimizing susceptible matrix effects and increasing sensitivity to MIP OES. Some preconcentration strategies with this objective have been employed, such as solid-phase extraction (SPE) (Sa et al. 2020), cloud point extraction (CPE) (Sa et al. 2021), liquid-liquid dispersive microextraction (DLLME) (Silva et al. 2020), and coprecipitation (Moreira et al., 2020), among others (Fontoura et al. 2022).
Coprecipitation is a remarkable separation/preconcentration method, MIP OES still requires research. It has advantages such as simplicity, low consumption of reagents, and high preconcentration factor, which are essential in the determination of trace elements (Moreira et al. 2020). In addition, multiple analytes can be separated and pre-concentrated in a single matrix step using different organic or inorganic precipitants Tuzen and Soylak 2009; Chen et al. 2016). This procedure involves the addition of analytes from a soluble liquid phase into the precipitate. The ability of the precipitate to capture quantities at low concentrations can be used to concentrate trace elements. Coprecipitation mechanisms include surface adsorption, ion exchange, surface precipitation, and occlusion (Skoog 2014).
The literature presents a small number of researches exploring coprecipitation for the separation/preconcentration of trace elements in fish samples. One of these uses the MIP OES as the analytical technique Moreira et al. 2020). Aydin et al. applied the Cu-2-mercaptobenzothiazole complex as a carrier to separate Cd and Pb by Graphite Furnace Atomic Absorption Spectrometry (GFAAS) (Oymak et al. 2009). A carrier method was studied employing N-Benzol-N,N-diisobutylthiourea in the coprecipitation and speciation of Sn by GFAAS (Uluozlu and Tuzen 2015). A method for Cr(III)/Cr(VI) speciation using Ni2+/2-Nitroso-1-naphthol-4-sulfonic acid was evaluated by Uluozlu et al. (2009). The same research group used Zr(OH)4 for coprecipitating Pb, Co, Cu, Cd, Fe, and Ni determined by Flame Atomic Absorption Spectrometry (FAAS) (Citak et al. 2009). The coprecipitating agent Mn(II)-Br-PADAP was studied for Cd preconcentration determined by FAAS (Chen et al. 2016). The analytical parameters of these papers are described in Table 9.
It is important to emphasize that coprecipitation is a valuable strategy to improve the features of MIP OES. In this context, following the principles of green chemistry regarding the use of low amounts of reagent and low waste generation, we applied Box-Behnken design to establish the best conditions to determine traces of As and Cd in fish. The proposed procedure explores the combination of coprecipitation using silver chromate and MIP OES.