A method and system for determination of contaminants, such as black powder, in a flowing fluid, such as natural gas, is disclosed. The method comprises transmitting a plurality of light beams over a spectrum of wavelengths through the flowing fluid and receiving a plurality of measurements relating to transmitted and scattered light beams over the spectrum of wavelengths. The received plurality of measurements are compared with a plurality of stored patterns and a result indicative of the determination of the contaminants is output.