Marker residue, definition

By definition, the determinative procedure must be able to quantify the concentration of the marker residue. For compounds with a tolerance, it is critical that the analysis be able to determine accurately if the concentration of the marker residue is above or below the tolerance in the target tissue. The CVM guidelines for determinative procedures call for an average recovery >80% with a coefficient of variation (CV) of <10% for marker residue tolerances of lOOpgkg or greater and an average recovery of >60% with a CV of <20% for marker residues with a tolerance below 100 ppb. 

In those cases where the total residue in not represented by a single marker compound, a more complex residue definition is necessary. The hydrolytically unstable ester of bromoxynil octanoate is presented as an example here (Table 1). 

In summary, it has been demonstrated that a number of compounds have specific analytical requirements due either to extensive metabolism or the legal definitions of the marker residues to be used for detection of use. For the quinoxaline-type antibiotics, the question of the most appropriate marker of use remains unanswered at present. For ceftiofur, differences in legislation worldwide mean that analytical requirements differ from region to region. As with the quinoxalines, a question has been raised as to whether the generally accepted marker residue is the optimum. 

Na and Nb are the numbers of atoms in the two groups A and B and S is the switching function. With the group-based switching function, it is necessary to define the distance between the two groups (i.e. the two points Ta and Tb). There is no definitive way to do this. As with cutoffs, a special marker atom can be nominated within each residue, or the centre of mass, centre of geometry or centre of charge may be used. 

---