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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!

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    $\begingroup$ You might say why you suspect that the filters are in the wrong position. You might ask for the help of the near lab if the filter can be easily transported. It will be very quick to check what is what. But I agree with Farooq's answer. IR isn't something fitting your requirement of characterising aqueous solutions. At least not in standard analysis. One can rarely say never. But it would always be the wrong technique. $\endgroup$
    – Alchimista
    Commented Feb 13, 2021 at 8:42
  • $\begingroup$ Thanks for your reply, Alchimista. This is the thermopile sensor: dexterresearch.com/product-finder/… Those windows are just under 1 mm across and the optical filters are sealed inside the can. I need to test the device intact. $\endgroup$
    – John Ladasky
    Commented Feb 14, 2021 at 9:53

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Disclaimer: I assume you refer to compounds which may be «soluble in water», not necesarily to constrained to «recording the spectra in aqueous solution» for limitations of IR transmission spectroscopy which are a bit lesser present with IR ATR spectroscopy, and basically not present for Raman (M. Farooq's answer). I assume you mention water soluble because you want to clean your setup after recording the data simply with water. Note: often, the recordings mentioned below are literally about thin films of theses substances, i.e., an optical pathlength across the sample much less than $\pu{1 mm}$.

Complementary to NIST webbook chemistry offering a search by the vibrational energy here is the Japanese Spectral Database for Organic Compounds, SDBS. (Querries may be either in Japanese, or English.) It equally offers a «reverse search» starting with spectral information you already have in hand:

enter image description here

(credit)

I'm not aware about the spectral width of the sensors you mention, if these record e.g., at $\pu{(1200 \pm 10) cm^{-1}}$, or say $\pu{(1200 \pm 50) cm^{-1}}$. Obviously, the larger the allowance, the list of compounds indexed in the database matching this criterion will increase. In a query with the former threshold, browsing across the list of hits however were entries like formic acid (attention, potential danger because it is corrosive), or acetone (attention, potential danger because it is flammable).

By back-and-forth between the listing (to rule out compounds not water soluble, requiring some intuition / knowledge) and the spectra, you may identify if there are other bands of (dis)interest to you, too, e.g.

enter image description here

(example acetone, loc. cit.)

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We happen to be measuring aqueous solutions instead, so I am trying to identify useful reference solutions we can prepare.

This experiment is really problematic. mid-IR is not meant for aqueous solutions. Water is a no-no in infrared spectroscopy. Look at the low transmittance of water in mid IR range. Liquid water is going to swamp everything. What will be your container (cuvet) material? This is why Raman spectroscopy is the way to go with aqueous solutions.

enter image description here

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  • $\begingroup$ Thanks for your reply. We know that water is a pain to work with. Notice that all three of our optical filters are targeted to the 1400 - 1000 cm⁻¹ range where water transmission is relatively high. For this quick experiment, I don't even plan to use a transmission cuvette. I will point the sensor at an open container of the test solution. I don't need quantitative or sensitive results. $\endgroup$
    – John Ladasky
    Commented Feb 13, 2021 at 5:41

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