The normal way of doing this is to prepare a standard curve: take your standard and prepare a series of standard dilutions, running each dilution on the HPLC and graphing the (known) amount of nicotine in the sample and the (measured) area under the curve on the HPLC trace. When you run your unknown sample, you should be able to measure the area under the curve, find that point on your standard curve, and it will tell you the amount of nicotine in the sample.