Infrared Spectra. Free Download of FTIR Library


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What are infrared spectra?


Infrared spectra are „fingerprints“ of molecules. The same molecules have the same infrared spectra, different molecules have different infrared spectra.

Infrared spectra are used in analytical chemistry.

Typical infrared spectrum looks like this (Picture 1):





Infrared spectra are curves. The final infrared spectrum is a ratio of sample spectrum and air spectrum (background).

The X-axis (peak position)  gives you information about the wavelenght and is usually presented in units called „cm-1“ = wavenumbers (number of waves in 1 centimeter).

Typical wavenumber range by infrared spectrum is 4000-400 cm-1.

The Y-axis (peak intensity) gives you information about how much the sample absorbs the energy, the units are Transmittance (%T - …peaks go down)  or Absorbance (logarithmic scale, peaks go up) units. Transmittance ranges from 0-100%. Absorbance ranges from 0 to infinity. We recommend to take in account absorbance values 0-1 or maximally 0-2. Although your software will show higher absorbance values, you should not trust to values higher than 2 absorbance units.


About the spectrum on picture 1 you can tell following:

  1. The peak at 1774.2 wavenumbers has the intesity of 0.988 absorbance units
  2. The peak 2968.0 wavenumbers has the intesity of 0.229 absorbance units
  3. There are no peaks in range 2000-2800 wavenumbers
  4. etc.



The same spectrum in transmittance units looks like this (Picture 2):




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Measurement Techniques


You can collect your spectra by transmission techniques (KBr pellets, Nujol mulls, disposable cards, liquid cells, gas cells…) or reflection techniques (ATR, diffuse reflectance, specular reflectance…).

Transmission techniques are classical and will deliver the best quality spectrum, but they are time consuming. 

Today 70-90% of new instruments are delivered together with an ATR (= attenuated total reflection) sampling unit.  Collection by ATR technique does not require any sample preparation. Typical time of the spectrum collection by ATR together with sample adjustment and crystal cleaning is 2-5 minutes.  The quality of the ATR spectrum is fully sufficient for sample identification - you should compare it with spectral libraries collected by the same technique – ATR spectral libraries. The ATR spectrum may have limitations if you look for trace impurities in your sample and in a few other very special cases.


Sampling Materials


All chemical compounds absorb in the infrared region. This means that all sampling materials have their own infrared spectrum. In most cases you need the sampling material, because you cannot put the solid, liquid or gas sample to the spectrometer without sampling material (except polymer foils). If you saw the sampling material peaks in your sample spectrum, that would cause difficulties by the interpretation.

To avoid/remove the unwanted peaks in your spectrum you have following possibilities:

  1. Use KBr as a sampling material (cells, windows, pellets etc.). KBr is the only material which has no absorbtion bands in region 400-4000 cm-1. An important disadvantage of KBr is its extreme sensitivity to moisture. It is time consuming to work with, but the spectra have the best quality.
  2. Use other sampling material other than KBr and use only the part of the spectral region, where the sampling material has no absorption peaks
  3. Use other sampling material other than KBr and compensate the sampling material peaks – collect the sampling material as background or subtract its spectrum
  4. Some of the reflection techniques (diffuse reflectance, specular reflectance, reflection mode in microscopy) do not need sampling materials. The widely used ATR technnique compensates the sampling material peaks automatically (you collect the empty ATR as background – see point 3)


Typical transmission materials: KBr, NaCl, BaF2, ZnSe…

Glass or quartz cannot be used – their own absorption is too strong in IR region.


Typical ATR materials:  ZnSe, Si, Diamond, Ge…



Peaks in infrared spectra


The reason why you collect infrared spectra is usually unknown compound identification, identification/quantification of components in mixture (impurities, additives…), verification of sample identity and purity etc. An expert with help of supporting computer programs evaluates peak positions and/or their intensities.

The peaks in infrared spectra are caused by absorption of characteristic frequencies by molecules. Those absorptions are caused by different vibration modes of a molecule.

Different functional groups of a molecule (like –CH3, COOH, NH2) exhibit peaks at different  ranges of wavenumbers. The peak position gives you information about sample quality (organic x inorganic, alifatic x aromatic, which functional groups are present…) and depends on the symetry of the molecule, atom weight and bond stenght.  Peak intensity gives you quantitative information and depends on concentration.

The qualitative evaluation of infrared spectra can be by following two methods.



Spectral Interpretation


If an expert looks at the infrared spectrum, he is able to interpret it. This means, he can estimate what functional groups are present and in simple cases even establish the chemical composition of the compound.

In the above example, it is well known that the peak at 2968 cm-1 is the -CH3 peak and peak at 1774 cm-1 is the C=O peak. Further we can see peaks of aromates etc.  If you show this spectrum to an expert on the field of infrared spectra of polymers who works with polycarbonates,  he can identify a polycarbonate on the first look, because he remembers the spectrum shape. But he cannot easily identify spectra which he has never seen before.

Generally spectral interpretation is easily possible for simple compounds,  but it is time consuming, expensive and generaly limited for more complicated compounds. Although computer programs have been developed for infrared spectra interpretation, this method is used relatively rarely.


Spectral SEARCH


Another widely used approach is spectral search – comparison of unknown spectra with many other spectra and calculation of the best „hit“ = the most similar spectrum.

The main advantage is that spectral SEARCH is fast and can be used by less qualified personal.

To provide spectral search, you need a special software („Search Software“) which is usually a part of your instrument. The search software has a capability of comparing thousands of curves within seconds and suggest the most similar spectrum. The principle is similar as finger print search in forensic science. The identification is successfull only if the same spectrum is part of the library you compare with.

The absolute values of the hit quality are not so important. 100% would mean identical spectrum and 0% totally different spectrum (some software packages have different scaling of the hit value).  In reality you never reach a hit with value 100%. Sometimes a hit with quality of 70% or even less can be correct. You must always do a visual comparison of the unknown spectrum with the library spectrum. It is you who must decide if the spectrum is identical, not the software.

Further you need collections of infrared spectra – „Infrared Spectral Library“ or „Infrared Spectral Database“.

Some spectroscopists create their own spectral libraries. This is usefull if you work with limited number of compounds, for example in QC laboratory of company manufacturing 100 products or if you want to study newly synthetized or very unique compounds which are not present in any commercial library.

If your application is unknown sample identification, if you study composition of competitive products and if you simply analyze many different sample kinds, it is usefull to buy a commercial infrared spectra library.

Commercial infrared spectra library is typically a collection of thousands infrared spectra (curves) in digital format. Different software and instrument manufacturers use different digital library formats (you cannot use Bruker library with Thermo software etc.).

One from the infrared spectra libraries provider is NICODOM Ltd.  Nicodom offers over 140000 infrared spectra in over 80 libraries in 10 different digital formats.

Their webpage gives you detailed description of the libraries and their prices.

You can choose between general big libraries (e.g. Aldrich – Ichem Library of over 30000 spectra) or smaller special libraries.

The choice of suitable spectral library is important and depends on your application. If your laboratory focuses on one compound type (e.g. pesticides, dyes, polymers…), you better choose a special library than the general one. Universities or forensic laboratories usually choose general libraries.


Nicodom Ltd. offers a free IR spectra library, click on the logo to go to their download section:



Download free infrared spectra library compatible with your format (10 different formats, information about over 140000 IR spectra in over 80 databases.)




Created by NICODOM Copuright NICODOM 2009