3.3. Application of the method to contaminated pig tissue samples
In addition to the examination of the method performance for spiked samples, the assay was also applied to real world samples for two of the analytes, AOZ and AMOZ.
In the course of an animal feeding study (see also Experimental section), furazolidone and furaltadone were administered in therapeutic doses to pigs over a period of 3 days. After the pigs had been slaughtered, muscle samples were collected, stored at −20°C and analysed by the developed LC–MS–MS method. Due to the scope of the feeding study, tissue samples of nitrofurazone- and nitrofurantoin-treated pigs (leading to free and bound SC and AH metabolites) were not available to us; however, a comparison with literature data could be made with the samples we obtained.
Chromatograms of respective meat samples of the pigs are shown in Fig. 5. Both AOZ and AMOZ could be identified unambiguously and quantified at levels of 100 and 30 ng g−1, respectively. The determined content of the AOZ metabolite is of the same magnitude as the concentration values obtained by McCracken et al. [15] and [16] when similar amounts of furazolidone were applied, considering that no discrimination between free and bound metabolites was made in our work.
The half-life of AOZ residues in vivo has been determined to be between 4 and 9 days [10], depending on the nature of the tissue, with the highest stability in muscle tissues. Judging from these data and from the detection limits of the present method, the illegal use of this group of antibacterial drugs should be detectable at least for several weeks after the last administration depending on the tissue and the individual analyte.
Furthermore, it has been reported that AOZ concentrations in liver tissue are several times higher than in muscle tissue [10], [11], [15] and [16]. This indicates that detection of nitrofuran type antibiotics would be possible over an even longer period of time. Since the nature of the liver matrix is considered to be different and/or more difficult for sufficient isolation of the metabolites and removal of possibly interfering matrix constituents, the developed LC–MS–MS method needs to be tested in more detail for its applicability also to this matrix. This is a matter for further investigations in our laboratory.
However, the sensitivity of the present method and its straightforward clean-up procedure, which may have to be modified for liver tissue samples, may qualify this assay as a suitable tool for the determination of nitrofuran antibiotic residues in muscle tissue, especially considering that there are cases where liver samples might not be readily available for analysis.