## 1 Introduction  
Graphene oxide (GO) is considered as a chemical compound with a structure similar to graphene, where oxygen is introduced through chemical oxidation. It is described as a heavily oxygenated nanomaterial with the presence of many functional groups that contain oxygen such as epoxide and hydroxyl groups on its basal plane and carbonyl and carboxyl groups at the edges [1; 2]. Due to the presence of these functional groups, GO is able to bind with metal ions, especially with multivalent metal ions, through electrostatic and coordinate approaches [3]. The adsorbed analytes could be eluted by the addition of acids, according to the hydrogen bonding competition [4]. Graphene oxide has been previously used for analytical chemistry purposes, however, in most cases it was anchored onto different surfaces such as silica, monolithic column and Fe\({}_{3}\)O\({}_{4}\)[5; 6; 7]. Moreover, graphene oxide can be functionalized with various polymeric and non-polymeric materials such as polyaniline [4], chitosan [8], ethylenediamine and 1,6-hexanediamine [9], polypyrrole [10], ethylene diamine tetra-acetic acid (EDTA) [11], manganese oxide [12], triethylenetramine [13] and polyimide [14]. It has been proved several times that functionalization of graphene oxide could increase the adsorption capacity for metal ions, however, the investigation of different forms of non-functionalized modified graphene oxide is still a challenge. Reduced graphene oxide (RGO) [15] and oxidized graphene oxide (OGO) [16] are two forms of graphene oxide that have been used for the adsorption of toxic and nutrients metals and rare earth elements, respectively. However, to the best of our knowledge, NaOH-modified graphene oxide (NaGO) has not been used before for the determination of toxic and nutrient elements. Metals are a group of important pollutants because of their potential toxic and carcinogenic effects. In general, they are refractory and non-degradable, and they tend to accumulate in the organisms. Some metals can be toxic at low concentration and, as a result, their presence in the aquatic environment represents water pollution [17]. Meat and meat products are important for human diet in many parts of the world since they provide the well-known trace element contents. Therefore, there is a great concern about the effects of anthropogenic pollution on the ecosystems [18]. Chromium, iron, copper and zinc are essential elements for human health, and they are playing a vital role in human metabolism [19]. On the other hand, lead and cadmium are potential harmful metals and environmental pollution is the main cause of their presence in the food chain. Nickel is a moderately toxic element as compared with other transition metals [20]. Moreover, aluminum and barium are non-essential elements and their biological function for living organisms is unknown [21; 22]. A wide variety of inductively coupled plasma atomic emission spectroscopy (ICP-AES) methods have been reported in the literature regarding the determination of toxic and nutrient metals in meat and meat products after acidic digestion [18; 23; 24; 25; 26; 27]. In this work, different graphene-oxide-derived materials were examined for the preconcentration of metals from meat samples. Following the selection of the optimum material for dispersive solid-phase extraction (d-SPE), experimental design methodology was used for the optimization of the extraction process. Compared to the traditional \"one variable at a time\" approach, which is more time consuming, experimental design or design of experiment (DoE) is a simpler, quicker and more reliable methodology, since it provides the advantage of multivariate optimization [28; 29]. Factorial design is a popular statistical process for multivariate optimization that is used in analytical chemistry because of its usefulness in the identification of the significant variables and the optimum experimental conditions.