The phenylthiohydantoin (PTH) derivative can then be identified by thin layer chromatography, gas chromatography, or high pressure liquid chromatography (HPLC) on reversed-phase supports. PTH arginine and PTH histidine could be placed almost anywhere in the chromatogram by small changes of the conductivity; it was therefore necessary to standardize the ionic strength of the buffer by conductivity titrations. One method of obtaining high sensitivity analysis of PTH amino acids is the use of microbore HPLC columns to achieve increased solute detectability by reducing peak dilution. Utilizing relatively simple statistical methods, retention data for PTH amino acids on the columns were used to predict optimum separation conditions for combinations of stationary phase/mobile phase gradients. Considerable differences in chromatographic behavior have been observed between often supposedly similar columns used for the separation of PTH amino acids.