## 1 Introduction  
Antibiotics used in human and animal therapy has been one of the leading scientific achievements of the last century that revolutionized the treatment of infectious diseases worldwide [1]. These substances are known to kill or inhibit the growth of microorganisms at very low concentrations. In this context, quantifying the antibiotics in pharmaceutical formulations is extremely important for quality control and quality assurance [2]. In recent years, many studies have reported the existence of antibiotics in the environment, these drugs being released through diverse paths, such as discharges from wastewater treatment plant effluents to surface water, where hospitals and private households have significant contributions [3]. Consequently, their occurrence and persistence in the environment, and harmful and deleterious impact, has attracted significant attention [1, 4]. Therefore, monitoring antibiotics that reach the environment, even at very low concentrations, is necessary. Amoxicillin (Amox) and doxycycline (Box) (Figure 1) are some of the most prevalent antibiotics prescribed in dental practice due to their broad range of activity and low cost [1; 4; 5]. Amox, which belongs to the \(\beta\)-lactam antibiotic class of high pharmacological, clinical and economical status, is one of the most commonly used to treat various conditions and patient groups against Gram-positive and Gram-negative bacteria [5; 6]. Amox has better absorption from the intestinal tract, higher capacity for achieving effective concentrations at the receptors, and an enhanced ability to penetrate Gram-negative microorganisms' cellular wall compared to other \(\beta\)-lactam antibiotics [7]. The literature depicts various analytical approaches for the quantitative determination of Amox in commercial drugs and wastewater samples, namely spectrophotometry [8; 9], voltammetry [10; 11], fluorimetry [12; 13], microbiological methods [14], capillary electrophoresis [15; 16], and high-performance liquid chromatography (HPLC) [2; 6; 17; 18; 19; 20; 21; 22]. Dox is a broad-spectrum antibiotic belonging to the tetracycline class, usually used in the management and treatment of various infections caused by Gram-positive and Gram-negative bacteria, rickettsiae, chlamydiae, spirochetes, mycoplassmas, and protozoans [23; 24]. Compared with other tetracyclines, doxycycline has a better absorption profile, longer half-life, safety profile, and excellent efficacy for susceptible microorganisms, even at lower concentrations [25; 26]. Doxycycline monohydrate and doxycycline hydrate are equally effective, but doxycycline hydrate is more frequently uwatersed in pharmaceutical samples due to its good water solubility [24]. Many efforts have been dedicated to the quantitative determination of Dox in both commercial pharmaceutical forms and wastewater samples, such as spectrophotometry [27; 28; 29], capillary electrophoresis [30], micellar electrokinetic chromatography [31], and HPLC [32; 33; 34; 35; 36; 37]. HPLC is a valuable tool in the quantification of antibiotics in various types of samples due to its simplicity, speed, low cost, and applicability to various analyte types. Moreover, HPLC has been renowned as a robust separation technique with high selectivity [20]. Unfortunately, several HPLC methods validated for the simultaneous determination of different antibiotics in the same sample suffer from several drawbacks, such as the need for extensive runtime or large sample volumes. Moreover, in some of these methods, the mobile phases following the antibiotic need to be analyzed [38]. In recent years, HPLC-DAD (Diode Array Detector) has been demonstrated to be a fast, reliable, and highly reproducible method for a drug screen for clinical research and forensic toxicological applications, where the ruggedness and the fairly simple sample preparation are impetuously required [39].