Insights on extracting mechanisms and interference factors of solid-phase extraction for trace sulfamethoxazole...

Sulfamethoxazole (SMX) has garnered global attention due to its presence in trace amounts (ng L^-1 to μg L^-1) in water bodies. Solid-phase extraction (SPE) is a key pretreatment step for analyzing trace SMX, and its efficiency significantly impacts the accuracy of final detection results. However, optimizing the SPE conditions is empirical and time-consuming, and systematic understanding of this process remains inadequate. The underlying mechanism of SPE for SMX extraction was elucidated in this study, alongside a systematic evaluation of the specific factors affecting extraction efficiency. The HLB cartridge achieved the highest SMX extraction efficiency (75.7∼99.3%) among three options, despite a general decline in efficiency was observed with lower SMX initial concentrations (0.5∼50 μg L^-1). This was primarily driven by hydrogen bonding and π-π interactions. HCO₃^- exhibited the strongest interference at 0.5 μg L^-1 SMX, reducing extraction efficiency to 3.0 ± 0.3%, compared to approximately 77.0% with Cl^-, SO₄^2-, or H₂PO₄^-. Under the interference of Humic acid (HA), the SMX extraction efficiency decreased from 75.5 ± 2.6% (1000 μg L^-1 HA) to 36.8 ± 5.5% (20000 μg L^-1 HA). Additionally, low-molecular-weight HA (<30 kDa) caused stronger interference with only 52.8 ± 2.0% extraction efficiency than high molecular weight HA (>30 kDa, >73.9%). These indicate that HA concentration and molecular weight significantly influenced HLB performance. Different from HA (49.7 ± 5.1%), other natural organic matters (NOMs) showed no interference on extracting process (81.7∼85.6%). SPE exhibits relatively low pre-treatment efficiency for SMX at trace concentrations, with its efficacy readily influenced by environmental factors. Consequently, when applying this technique to extract SMX from complex aquatic environments, measured concentration values should undergo numerical correction to approximate true concentrations. In summary, this study contributes to improving the detection accuracy of trace SMX and establishes a detection foundation for researching its removal.