A fully automated method has been developed for analysis of eighteen antibacterial compounds, including penicillins, cephalosporins
and sulfonamides, in animal feed with limits of quantification in the range 0.25–5.79 μg kg
−1. The method is based on pressurized liquid extraction of 3 g homogenized feed with water and online clean-up of 500 μL of
the extract with C
18HD cartridges. The purified sample was directly analysed by liquid chromatography–electrospray tandem mass spectrometry (SPE–LC–ESI-MS–MS).
Chromatographic separation was achieved within 10 min by use of a C
12 Phenomenex Hydro-RP reversed-phase analytical column and a mobile phase gradient (water + 0.1% formic acid–methanol + 0.1%
formic acid). The method was validated, revealing capability for detection of concentrations as low as 0.09 μg kg
−1, decision limits (CCα) and detection capabilities (CCβ) in the range 10–174 μg kg
−1 and 22–182 μg kg
−1, respectively, and inter-day precision ranging from 0.7 to 8.3%. Recovery, with internal standard correction, was in the
range 93–134% for all analytes. The method was then applied to analysis of fifteen feed samples, nine of which contained at
least one antimicrobial at concentrations between 0.006 and 1.526 mg kg
−1. The performance data and results from the method were compared with those from a previous method developed by our group,
using offline SPE, by analyzing the same set of samples by both methods. The online SPE approach resulted in slightly improved
sensitivity, with LODs of 0.09–2.12 μg kg
−1 compared with 0.12–3.94 μg kg
−1 by the offline approach. In general, better recovery was achieved by use of online purification (for 72% of the analytes)
and the correlation between the two methods was good. The main advantages of the new online method are rapid and automated
sample pre-treatment, and reduction of sample manipulation, enabling high-throughput analysis and highly accurate results.
Because of all these characteristics, the proposed method is applicable and could be deemed necessary within the field of
food control and safety.
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