I am working with a custom-built device which is intended to perform some mid-infrared chemical measurements. It consists of four thermopile sensors with optical windows. In front of each window is a band-pass filter. Each filter admits a distinct range of wavelengths. We performed an initial experiment which suggests that the manufacturer may have installed the filters in the wrong positions. I am trying to design a more definitive follow-up experiment. I can't tell the filters apart by looking at them.
These kinds of sensors are typically used in the automobile industry to characterize engine exhaust gases. We happen to be measuring aqueous solutions instead, so I am trying to identify useful reference solutions we can prepare. Ideally, the solutions would absorb well at one of the three wavelengths, and transmit well at the other two. I don't care whether the reagents are organic or inorganic. Safe, water soluble, inexpensive, and already present in my lab are all desirable qualities though.
The center wavelengths of the three optical filters are 6.7 μm, 8.3 μm, and 9.6 μm. In wavenumbers, these are around 1490, 1200, and 1040 cm⁻¹. The bandpass range on each of the filters is about 0.2 μm wide. I also have a fourth, broad-band filter which I will identify by process of elimination.
Using my limited knowledge of IR fingerprints and the NIST Webbook database of infrared spectra, I have identified two candidate compounds. Glycine absorbs preferentially in the 6.7 μm band, and trimethyl phosphate absorbs preferentially in the 9.6 μm band.
I still need a good candidate compound for the middle band: 8.3 μm / 1200 cm⁻¹. The best I have found so far is n-decanoic acid, but it's not really that selective in absorbing at 8.3 μm. It also isn't particularly water-soluble.
I would greatly appreciate it if anyone has any suggestions. Thanks!