Interpretation of UV-Vis spectra
(take a look at the attached figure)
For each type of isolated precipitate lignin from black liquor (hemp and flax; modified alkaline anthraquinone cooking) precipitate of sulphuric acid (5, 25,50 and 72 % w/w), a stock solution was prepared by dissolving lignin powder in NaOH (0.1 M). UV-VIS absorption spectra were registered using a solution of lignin on a CECIL spectrophotometer, absorption region 200 -450 nm, scan speed 5 nm/s and resolution 1 nm. Measured values of absorbance were normalized at 222nm (maxim bands). The free and etherified hydroxyl groups contributing to the characteristic absorption maximum of lignin near 280 nm. The lignin fractions showed a characteristic absorption band at 284 nm, which could be assigned to the unconjugated phenolic groups of lignin.
Adsorption maxima of unconjugated quaiacyl and 3,4-dimethoxy-phenyl model compounds fall within the narrow wavelength range from 277 to 282 nm. In alkaline solution, ionization of the hydroxyl groups shifts the absorption bands of compounds with free phenolic towards longer wavelength.
The pronounced shoulders at 242, 257, 280 and 312 nm on the lignin ionization curves in alkaline solution indicate the presence of biphenyl derivatives. These structures may be aromatic, containing non-etherified hydroxyl groups, conjugated carbonyl groups and aromatic carboxyl groups in lignin.
I also need help in the interpretation of the absorption band at 222 nm.
Maybe you should try to do some computational modeling on lignin. It's big system, but maybe some semi-empirical methods will work on it.
How did you get the wavenumbers in areas where there are no distinct peaks? By curve fitting? UV spectra are notoriously difficult to interpret beacause of the broad peaks. Sometimes cyrogenic spectra are sharper due to the less explicit vibrational sturcture. I agree with Olga - semiempirical or nowadays even ab initio quantum chemical calculations are available for predicting electronic transtions of typical low molecular analogs of of lignin. (Probaly the aromatic parts will be most important - in this case even a simple PPP calculation would do). I alos suggest to look for papers on the interpretation of low molecular analogs - they do help in deciphering your own data. Anyway, lignin is a mixture of several components, so you will get an apporximate answer only. I believe more could be extracted from the changes of spectra under different conditions - but even for that you need the interpretation of low molecular analogs.
I looked presented spectrum as an expert in the field of spectroscopy. I am not familiar with the features of the investigated material. I think that before interpreting any optical (or other) spectrum it is necessary clearly and unambiguously establish the presence of its structure. As far as I can see, there is a pronounced peak at 222 nm, strongly broadening band at 284 nm. There are some peculiarities in the form of shoulders at 242 nm (in greater degree) and 367 nm (in less degree). As regard to mentioned by you the bands at 257 and 312 nm, in my opinion, there are no grounds to assert their presence in the spectrum. In my opinion, special interest is the structure of the narrow lines in 200 - 215 nm. So, first of all, It is necessary to understand the nature of these lines.
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