Infrared Spectra. Free Download of FTIR Library
Download free infrared
spectra library – FTIR Library (go to page bottom).
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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:
The same spectrum in
transmittance units looks like this (Picture 2):
http://en.wikipedia.org/wiki/Infrared_spectroscopy
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm
http://wwwchem.csustan.edu/Tutorials/INFRARED.HTM
http://wwwchem.uwimona.edu.jm/spectra/IRIndex.html
http://www.ut.ee/katsekoda/IR_Spectra/links.htm
http://www.forumsci.co.il/HPLC/FTIR_page.html
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.
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:
Glass or quartz cannot be used – their own absorption is too strong in IR region.
Typical ATR
materials: ZnSe, Si, Diamond, Ge…
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.
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.
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.)
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Created
by NICODOM Copuright NICODOM 2009