## 1 Introduction  
Deep eutectic solvent (DES) has received attention in the recent years. DES is an emerging class of safe materials related to ionic liquid (IL), which has abnormally deep melting point depression at the eutectic mixture of specific hydrogen bond donors (HBDs) and acceptors (HBAs). DESs are divided into five categories depending on the method of preparation [1; 2]. Type I is composed of a quaternary ammonium salt and a metal chloride, Type II is a mixture of a quaternary ammonium salt and a metal chloride hydrate, Type III is a mixture of a quaternary ammonium salt and an HBD, Type IV consists of a metal chloride hydrate and an HBD, and Type V consists of only nonionic, molecular HBAs and HBDs. All types of DESs have high thermal stability, low volatility, low vapor pressure, and tunable polarity. Their preparation does not need any extra solvent so no purification step is needed. Therefore, they are applicable substitutions for volatile organic compounds (VOCs), used widely throughout research and industry with the aim of environmental remediation, especially magnetic solid-phase extraction (MSPE) [3; 4; 5]. MSPE is a developed generation of solid-phase extraction, which uses magnetic or magnetically modified adsorbents to extract analytes with high efficiency. This tactic overcomes some weak points in solid-phase extraction such as decreasing the time of preconcentration, sample loading, and filtration or centrifugation steps [6; 7; 8]. Magnetic graphene oxide is an example of an MSPE adsorbent. Graphene oxide (GO) is a fascinating new class of two-dimensional carbon nanostructures. It has high surface area, good chemical stability, and strong thermal stability. GO contains hydroxyl, epoxide, carboxyl, and carbonyl functional groups; additionally, its hydrophilic property increases its negative charge and dispersibility in aqueous solution to form a stable suspension. GO has strong interaction with magnetic nanoparticles; accordingly, mGO can be a good candidate to be applied MSPE as an adsorbent. GO can be functionalized with organic components via \(\pi\)-\(\pi\) interaction to control selective adsorption of analyte(s) [9; 10; 11; 12]. DES is an example, which can be a modifier, disperser, and fictionalizer for mGO. Many scientists have used DES to improve the extraction or preconcentration methods. For example, a DES of ethylene glycol and ammonium-based salt functionalized carbon nanotube to adsorb methyl orange from aqueous solution [13]. Additionally, a novel DES was applied as a modifier for graphene and graphene oxide to remove chlorophenols [14]. In additional to organic materials, DES can be a qualified candidate to adsorb cations such as heavy metals, because it has active negative sites and can grab cations via electrostatic force [15]. In this study, magnetic graphene oxide modified with DES (EgLiCl-mGO) was applied as an efficient nanoadsorbent for preconcentrating Pb(II) and Cd(II) simultaneously in legume samples. Pb(II) and Cd(II) are toxic elements and their accumulative characters cause serious problems for humans, plants, and animals [16]. According to specific properties of EgLiCl-mGO, it could be a highly applicable adsorbent. mGO facilities the preconcentration procedure because graphene oxide has a high surface area and magnetic properties enable easy and fast separation of EgLiCl-mGO [7; 17; 18; 19; 20]. The presence of DES provided a wide surface area with functional groups, which has a synergistic effect on adsorption efficiency. More importantly, EgLiCl is inexpensive, biodegradable, nontoxic, and easy to prepare. For the evaluation of the effective variables on the preconcentration efficiency, the one-at-a-time method was employed and pH, adsorption time, adsorbent amount, desorption time, and kind of eluent were optimized. Analytical figures of merit, interference effect of various ions, reusability, swelling behavior, adsorption isotherm, adsorption kinetics, and adsorption thermodynamics were reported.