such as the ir source 14 and the uv source 15 and associated fiber optic 17 in fig1 the separate uv and ir sources are combined through optical diffractive and or reflecting optics to pass through a common optical fiber or fiber bundle 19 in this case it may be desirable to employ the uv radiation for determining the concentration of no while the ir radiation is used for the other chemical species of interest and also for the ref channel by example the monolithic embodiment of the sensor assembly 60 shown in fig5 utilizes the single fiber 22 or fiber bundle 56 to illuminate chemically specific filter detector combinations in this embodiment there are three mid wave 3 5 micrometers ir responsive filter detector pairs co hc co 2 and one uv responsive filter detector pair for no x the spectral regions that may be utilized by various embodiments of this invention include but are not limited to the uv below approximately 0 4 micrometers the visible approximately 0 4 0 8 micrometers short wavelength ir swir approximately 0 8 3 micrometers mid wavelength ir mwir approximately 3 8 micrometers long wavelength ir lwir approximately 8 12 micrometers very long wavelength ir vlwir approximately 12 20 micrometers and the far ir fir approximately 20 1000 micrometers further in accordance with this invention the sampling cell 12 is provided with one or both of a mechanism for retarding an accumulation of an emission gas constituent upon the.