A facile in-loop dilution strategy for large volume injection in ultra-sensitive UPLC-MS/MS determination of 37 veterinary drug residues in beef.

Large-volume injections were firstly achieved by a simple stainless steel loop for highly sensitive UPLC-MS/MS analysis of veterinary drug multi-residues in beef. Sample elution strength was effectively attenuated via in-line mixing with initial mobile phase within the precolumn-embedded helical loop assembly. Helical configuration, inner diameter, loop length, and flow rate critically modulated the mixing-dilution process. The developed assembly exhibited seamless integration with commercial LC-MS instruments, enabling injection volumes of acetonitrile solutions up to 20 μL with enhanced sensitivity. Relative to conventional 2-μL injection volume, a 2-14-fold signal enhancement was observed under identical analytical conditions. Validation data for all 37 targets demonstrated satisfactory performance, featuring excellent linearity (R2 ≥ 0.993) in the range of 10-150 μg/kg, acceptable recoveries (60.5%-119.8%), good intra- and inter-day reproducibility (RSD≤14.4%), as well as low LODs (0.001-0.36 μg/kg) and LOQs (0.01-1.2 μg/kg). This in-loop dilution strategy holds strong potential for cost-effective and highly sensitive application in LC-MS analysis of multi-residues at trace levels.