Spectra are characteristics of the molecules. Frequency of the fundamental vibrations varies with the atomic weight of the
constituents. Further spectra exist due to overtones. These are in general much weaker, but there are still possibilities for these to be used for measurement purposes. The absorption strengths also vary with different molecules and therefore, different path lengths should be provided in order to obtain adequate absorption in the required sensitivity range. Small measuring cells can be advantageous, notably when a rapid response is needed. Optical cells that enable multiple pass or long pass of the emission through the analyte can be used for detection of very small concentrations.
There are strong absorption bands of many chemical agents in the mid-infrared spectral range that allows their detection with sensor devices based on LED-PD optopairs or using LED Microsensor NT photodiodes in combination with other sources of infrared radiation. Some of these chemical agents and their absorption bands are presented here: