Tetracyclines are broad-spectrum antibiotics that are active against both gram-positive and gram-negative bacteria. These compounds have been widely used in confined animal feeding operations (CAFOs) poultry products for meat, milk from cows and fish production, and analytical methods developed to date have primarily focused on the determination of tetracycline residues in food samples . Recently, concerns have been raised regarding public health issues over the occurrence of antibiotics in the environment  as well as by indications of increased bacterial resistance in waste effluent from hospitals and pharmaceutical plants . The occurrence and fate of tetracycline antibiotics in CAFO waste water and the possible contamination of ground water is largely unknown Thus, there is a need for a sensitive and rapid analytical method to measure concentrations of tetracyclines in animal waste lagoons and in ground and surface water that may be impacted by CAFO waste water. 

Liquid chromatography–mass spectrometry (LC–MS) is becoming more commonly used in the analysis of antibiotics because of its high sensitivity and ability to provide compound confirmation as compared to conventional liquid chromatography–UV detection (LC–UV) or liquid chromatography–fluorescence detection (LC–FD)  Capillary electrophoresis–mass spectrometry (CE–MS)  has also been used for the analysis of tetracycline residues in milk, serum and urine samples. Radioimmunoassay has also been reported as a screening method for antibiotics, however this method is only semi-quantitative, not selective, and needs further confirmation using LC–MS. Reported LC–MS methods for the analysis of tetracyclines also include particle beam (PB). fast atom bombardment (FAB) thermospray (TSP) , atmospheric pressure chemical ionization (APCI) and mass spectrometry. APCI and ESI ionization methods combined with tandem mass spectrometry are preferred due to their higher sensitivity, better reproducibility, and commercial availability.

Cheng et al. present a method for extracting tetracyclines from porcine serum using a recently developed cartridge containing a macroporous poly(divinylbenzene-co-N-vinylpyrrolidone) sorbent (Oasis HLB, Waters, Milford, MA, USA). Serum samples fortified with 0.5–2.5 mg l−1 tetracycline and minocycline were extracted with quantitative (88–101%) and highly reproducible (±2%) results. Major advantages for the Oasis HLB cartridge include a more rugged extraction, improved recovery for both polar and non-polar compounds in complex matrices, and greater capacity than reverse-phase silica based sorbents 

Check out item #4 in the linked report (link provided below).  It says the microbial activity was checked by the European commission to be similar for all 3 of them.

http://www.ema.europa.eu/docs/en_GB/document_library/Maximum_Residue_Limits_-_Report/2009/11/WC500015378.pdf

There are no differences between the three (tetracycline, clortetracycline, and oxytetracycline)

Thanks to all, but still if i used to prefer it, which one is better choice?