The design and optimization of a novel type alkali flame ionization detector (AFID) for capillary gas chromatography is presented. The design differs in that the alkali salt is continuously introduced, as a solution in water, into the detector by means of a liquid chromatographic syringe pump. This design permits continuous refreshment of the enhancement source and circumvents the need of source replacement, thereby eliminating detector response fatigue over time. Modification of an existing nitrogen-phosphorus detector (NPD) to allow direct introduction of the alkali salt into the detector will be described. Next, important operational parameters such as the location of introduction, type and concentration of the salt and the flow of the various detector gasses are optimized experimentally with regard to detector sensitivity, selectivity, minimum detectability and noise level. Moreover the linearity, reproducibility and stability of the detector are evaluated. It will be shown that the new detector design exhibits excellent response characteristics for phosphorus compounds which are comparable to or better than those observed with commercially available AFIDs or NPDs. Finally, the performance of the detector will be demonstrated by two environmental applications.