Dear Anuj, 

Usually  FTIR spectra draw the both axis have been better you take  x-axis is wavenumber cm-1 and Y- axis is %Transmission . However, this is general concept.

Thanking you, 

N. Prakash

No problem. It depends in which mode you are more comfortable. 

When you submit a paper. however, just have a look to the  journal reuqests

You can use any of them, but if you want to quantification, overall soft are developed for use in absorbance.

However you can get information transmittance. After transforming to absorbance. This is done in a simple way any soft.

In general people are more comfortable, particularly non-experts, with seeing a spectrum in % transmission.  However if you are going to do any spectral processing ensure your spectrum is in absorbance.  This is a linear scale; %T is not!

Also note it is absorbance units, not % absorbance as mentioned in this discussion.

Usually % transmission is set as default in many spectrometer. However if you need to do some calculation with your data, e.g. concentration of your analyte, then measuring absorbance (not % absorbance) can save you some calculation time later.

Use  %transmission! The transformation of transmission data into absorbance by spectrometer software is always done applying the Beer-Lambert law, which is not generally valid (it is a good approximation only in very exceptional cases!).

I would also suggest you to record transmission. It is very easy to obtain the absorption spectra from that by simple calculus.

In this way, you are also free in the choice of your reference .

Just to repeat, but a little bit more specific: Please consider that measurements are nearly always recorded in transmission (rare exceptions are a photoacoustics or laser-induced deflection, where you really measure the absorbance) and then converted into absorbance: However, the conversion according to A = - log10 T (Beer-Lambert law) can easily cause errors of up to an order of magnitude...

Absorbance is actually logarithmic scale, while transmittance is linear:

A = - log10 T

The transmittance spectra provide better contrast between intensities of strong and weak bands because transmittance ranges from 0 to 100% T whereas absorbance ranges from infinity to zero.

In practice, Absorption ranges from three to zero if you are lucky. And if you want it to be linear in the concentration than you should not go much beyond the range from one to zero...

It is clear that the type of test you choose depends on what you're looking.

So if you want to evaluate for example the association constant in a transparent mixture, you will choose to follow some / s band depending on the composition

Please note.  Absorbance is a linear scale and obeys the Beer Lambert Law.  Most instruments produce a %Transmission (or % Reflectance) value which is a logarithmic scale.  Before doing any spectral subtractions, library searches or quantitative measurements a %T spectrum must be converted to Absorbance.

Yes, and this conversion is actually often the most problematic part... ;-)

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