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Beginners Guide to Spectroscopy - Transmission


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This is the first part of our introduction to Spectroscopy, more can be found on our blog (http://specac.blogspot.co.uk).


Transmission spectroscopy is the oldest and simplest technique for analysing samples in infrared.

This method of analysis is based upon the absorption of the infrared beam by a sample at specific wavelengths. Different compounds each display a unique infrared spectrum allowing them to be identified.

The extent of absorption ‘A’ is defined by the Beer-Lambert Law: A=abc where ‘a’ is the absorptivity coefficient, ‘b’ is the pathlength, and ‘c’ is the concentration. This law enables the use of Infrared data to determine quantitative information.


In transmission spectroscopy, liquids are analyzed as a thin film sandwiched between two windows in a liquid cell. The type of cell, choice of window material, and pathlength is determined by the sample. Samples can be analyzed neat, or diluted with an appropriate solvent. In order to perform quantitative analysis, the sample should be analyzed in a cell with a known pathlength. A guide to pathlength selection for different concentrations in the mid infrared is shown below.

Analytical Concentration Typical Pathlength
> 10 % 0.05 mm
10 % - 1 % 0.1 mm
1 % - 0.1 % 0.2 mm
< 0.1 % > 0.5 mm

A variety of methods exist for analyzing solid samples by transmission spectroscopy. Thin polymer films can be analyzed directly by using a film holder.

Transmission spectra of solids can also be obtained by grinding the sample together with an infrared transparent matrix, such as KBr, and pressing the resulting homogeneous powder mixture into a thin disk. Another method of analyzing solids is to make a mull by combining and grinding the sample with a liquid paraffin, such as Nujol, and placing the mull paste/mixture between two infrared transparent windows.


Gases have densities several orders of magnitude lower than liquids and solids at standard temperature and pressure.
Therefore, transmission spectroscopy of gases, in the 10’s of ppm range, requires cells with a longer pathlength than those used for liquid or solid analyses, usually 10 cm or longer.

Low concentrations of gases require a pathlength of several meters. This is achieved in a short space by using a multi-pass cell, where the infrared beam is bounced through the sample several times in order to obtain the desired pathlength.

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