Miniaturized spectral analytical systems become ever increasingly important for in situ monitoring of natural waters’ nutrients, such as nitrate, phosphate or ammonium. A miniaturized UV spectral photometer has been developed for online detection using liquid core wave guides (LCW), UV transmitting optical fibers, a low-cost miniature polychromator, and a deuterium light source. The LCWs were manufactured by coating the insides of silica glass capillaries with Teflon AF 1600. Due to this setup our instrument needs only a few microliters of sample for each measurement. Nitrate can be directly detected by UV absorption spectroscopy in a spectral range between 200 and 350 nm. To separate the nitrate absorption from the superposition of other UV absorbing contaminations, a multi component analysis (MCA) software was applied to the measured spectra. With this developed photometer, nitrate levels can be determined online in inland and seawater or, if needed, in situ. It was evaluated twice in the field and the results for the measured amounts of nitrate in reservoir samples and in the North Sea are also presented in this work.