These wavy substructures can have many origins and might be hard to find out in detail.

Some possible origins: If your sample (or the substrate/carriers in the spectrometer) is metallic or has otherwise free carriers, you might observe a broad plasmonic, flourescence kind of background. It could be flourescense in general in your sample. Further more, the wavy-ness could indicate some reflection/interferrence resonances, if you are using a thin substrate layer or from interference of reflected signal within your sample (I don't know, if your sample or the sample carrier has several reflecting interfaces, for example, causing an interference pattern). Lastly it could be some kind of interference going on within your experimental setup, e.g. there could be a beamsplitter or optics with multiple interfaces, which causes an interference fringe pattern (the wavy structure). And, of course, it likely will be a combination of any of the above (e.g. a broad flourescent background, which gets wavy by some interference fringes).

The detailed reason may be hard to find out. But some suggestions:

If it is fluorescence, you could try a longer excitation wavelength. The flourescence might be surpressed, if you are exciting with a wavelength below your absorption bandgap.

To check, if it might be interference in the setup or the wavyness comes from the substrate/carrier of the sample you should try to measure a background spectrum of the substrate without the sample. This might allow you to observe the same fringes/wavy-ness minus your signal. This might provide you atleast an explanation, where it comes from (not necessarily what it physically exactly is). With some luck and normalization, you might even be able to substract this background from your sample spectrum.

EDIT: If you have access to the Spectrometer, it might also be helpful to have a calibration sample (something with know spectrum) avaiable. This might also help to clarify that the wavy-ness is a real feature of your sample or sample substrate and not the setup.

Regards,

Michael

Michael Rüsing thank you a lot for your detailed answer. I did not observe the waviness when I analysed the same specimen in the air nor in the demineralised water. That's why I decided that it has something to do with the tested NaCl+BSA solution

If it's an effect of that solution, the interference that Michael Rüsing mentioned might be the reason, maybe caused by an NaCl layer on the sample. The frequency of the pattern depends on the layer thickness. You can determine the average peak distance (in cm-1) and simply apply Bragg's equation:

d="lambda"/2sin(Theta)=1/[2*peak distance*sin(Theta)]

where Theta is the irradiation angle relative to the surface normal. Please check whether that gives a reasonable value for your system.

Jürgen Weippert thank you for your answer. If i understood your suggestion correctly, the values I got do not make sense for my system. The actual solution layer is significantly thicker.

The regular pattern, especially in the black one, looks like an interference fringe. These are seen in IR when a beam passes through a cell or film with reflective parallel sides. The "thickness" can be worked out as mentioned above. The question is where in your set up will you have at situation which generates such a pattern.

You can also use the equation thickness (pathlength) equals the number of fringes divided by 2 times the spectrum distance you have counted the fringes over. In your black one that is roughly 6/2* 1000 cm-1 which is about 30 microns. The refractive index can also be included in the divisor if it is significantly different than 1

Geoffrey Dent thank you for your answer. Is it possible that this effect is related to a specific liquid environment? I don't see such pattern in water or air using the same setup.