Some factors affecting assay variability in the determination of chlortetracycline in premix feeds

There are many sources of error in the microbiological determination of high levels of chlortetracycline in premix. Among the critical factors that can be controlled by the analyst are careful weighing of the reference standard and careful dosing of cylinders. A spectrophotometric approach and automatic filling of cylinders are suggested to ensure proper monitoring of these operations. Among factors that cannot be controlled by the analyst is the assay design. A 3-point design which is a modified version of the (3+3) design is considered, and its advantages are illustrated.