Use of high-performance liquid chromatography to monitor stability of tetracycline and chlortetracycline in susceptibility determinations

Simple continuous and simultaneous determination of tetracycline residues

A new continuous separation method was developed for the determination of five different tetracyclines (oxytetracycline, tetracycline, chlortetracycline, methacycline and doxycycline). A bioassay using minimum medium (MM) seeded with Bacillus subtilis ATCC 6633 was carried out for the detection, with simple extraction from the agar block of the clear inhibition zone on the MM produced by the mixed tetracyclines. The extract was subjected to continuous identification by high performance liquid chromatography using a mu-Bondapack C18 column. The tetracyclines were separated at ambient temperature using a mobile phase of 0.01 M oxalic acid: acetonitrile: N,N-dimethylformamide (74:18:8, v/v/v) at a flow rate of 1.0 ml min-1. A variable-wavelength detector set at 355 nm and recorder set at 4 mm min-1 were used for the detection. The entire mixture was resolved as five peaks with retention times ranging from 2.75 to 9.65 minutes. This continuous, simple and rapid method of detection, extraction and identification may be useful for routine laboratory testing of residual antimicrobial agents in food.

Analysis of chlortetracycline by high performance liquid chromatography with postcolumn alkaline-induced fluorescence detection

A high performance liquid chromatographic method for the analysis of chlortetracycline (CTC) using postcolumn fluorescence detection has been developed. After chromatographic separation of CTC on a polystyrene-divinylbenzene copolymer column, a highly fluorescent derivative isochlortetracycline (iso-CTC) was formed postcolumn in an on-line reaction coil with the addition of 25% NaOH (w/v). Chromatographic separation was achieved on a PRP-1 column, 15 cm x 4.6 mm, with 27:73 acetonitrile:0.2% perchloric acid (v/v), at 1.0 mL/min. Fluorescence derivatization was achieved by the on-line addition of 25% NaOH (w/v), at a flow rate of 0.2 mL/min, into the column eluant in a post-column reaction coil. The reaction coil was 9 m of teflon (1/16 in o.d., 0.3 mm i.d.) knitted into a six-sided coil. The fluorescent derivative was detected at lambda ex 355 nm and lambda em > 389 nm. Using this method after a simple sample cleanup, CTC can be detected in milk at 0.04 micrograms/mL, which is comparable to that obtained by microbiological assays. The detection method was linear between 0.02 micrograms/mL and 4 micrograms/mL. Because of the chromatographic separation, the method is more selective than microbiological assays and more sensitive than ultraviolet detection. With the chromatographic system described, the keto tautomeric forms of CTC and 4-epi-CTC are separated in a system which minimizes their formation on-column. In acidic aqueous organic solutions, the keto tautomer of CTC is the only product formed to any significant amount.