Abstract: The widespread occurrence of trace estrogens (ESHs) in aquatic environments poses significant ecological and human health risks. Conventional laboratory-based sample preparation methods, such as liquid-liquid extraction (LLE) and solid-phase extraction (SPE), require time-consuming and labor-intensive sample collection, storage, and transport. Furthermore, adsorption and degradation of the target analytes may occur during transport. Consequently, developing portable, efficient on-site enrichment techniques is imperative for the reliable monitoring of ESHs.
In this context, the present study introduces an innovative analytical approach that addresses these challenges by leveraging abundant and renewable agricultural residue-wheat straw-as the precursor for synthesizing a functionalized carbonaceous sorbent (WSC). Through controlled pyrolysis and chemical activation, the lignocellulosic structure of wheat straw was transformed into a porous carbon material endowed with a high specific surface area and abundant surface functional moieties, including hydroxyl, carboxyl, and carbonyl groups. This sustainable synthesis route not only minimizes material cost and environmental impact but also aligns with the global imperative of waste valorization and circular economy principles. The as-prepared WSC was subsequently packed into a disposable microextraction cartridge fabricated from a conventional 200 μL pipette tip, thereby constructing an integrated in-tip microextraction (ITME) device. This miniaturized format eliminates the need for complex instrumentation during field sampling and facilitates straightforward operation by non-specialist personnel.
The analytical performance of the WSC/ITME system was systematically evaluated through rigorous optimization of all pertinent extraction variables. Parameters such as sorbent mass, sample volume, loading flow rate, elution solvent composition and volume, sample pH, and ionic strength were carefully investigated to achieve maximal extraction performance. Each parameter was optimized sequentially to balance extraction efficiency, enrichment factor, and analysis time. The extraction mechanism was elucidated through adsorption studies, which revealed that the abundant active sites on the WSC surface facilitate the extraction of target ESHs via a combination of hydrogen bonding, π-π stacking, hydrophobic interactions, and weak electrostatic forces. These multiple interactions work synergistically, allowing the WSC/ITME system to efficiently capture analytes from complex sample matrices. Further investigation of the adsorption isotherm confirmed that the adsorption process followed a multilayer adsorption mechanism, which is characteristic of heterogeneous surface interactions. This behavior indicates that the WSC surface possesses a distribution of active sites with varying affinities, enabling adsorption beyond a simple monolayer and thereby enhancing overall extraction capacity.
Under the optimized conditions, the developed WSC/ITME technique was successfully coupled with high-performance liquid chromatography for quantitative determination of target ESHs. The method exhibited excellent sensitivity, with limits of detection (S/N=3) ranging from 0.010 to 0.028 μg/L, and satisfactory linearity across the studied concentration ranges. The accuracy of the method was validated through recovery tests in diverse real water matrices, including river water, lake water, and tap water, yielding spiked recoveries between 81.6% and 116% with satisfactory relative standard deviations. These analytical figures of merit demonstrated that the proposed method is fully capable of meeting the stringent requirements for trace-level estrogen monitoring in complex environmental samples, while substantially reducing the consumption of organic solvents and generation of laboratory waste compared with conventional protocols. In addition, the established method effectively circumvented the challenges inherent in the conventional lab-based sample preparation procedure.
In summary, this study establishes a novel, cost-effective, and environmentally benign on-site enrichment platform for the determination of trace ESHs in aquatic environments. By circumventing the logistical and technical limitations inherent in traditional laboratory-based sample preparation, the WSC/ITME approach ensures the preservation of sample integrity and the reliability of analytical results. The utilization of agricultural waste as a high-performance sorbent material further underscores the potential of bio-based materials in advancing green analytical chemistry. It is anticipated that the developed methodology can be readily adapted for monitoring other emerging organic contaminants, thereby offering a versatile and scalable solution for field-based environmental surveillance and risk assessment programs.

Key words: in-tip microextraction (ITME), biochar, on-site sample pretreatment, estrogens (ESHs), environmental water