if by "Pre-column HPLC" you mean pre-column derivatisation, then this paper might help. I-norvaline is internal standard and OPA is the derivatising agent. The analytical procedure is given in a reasonable amount of detail.
Hi Rajabdeen, you can also FMOC as derivatsation agent. Excess of FMOC can be captured with aminoadamantane. Derivatisation can be done very comfortable with an autosampler. Very sensitive in conjunction with a fluorescence detector. I've used it in my former company for the determination of proline in casein.