Infrared Spectroscopy

The “spectroscopy” is a class of investigation methods based on the energy exchange between matter and electromagnetic fields (historically speaking, visible light). During the time the meaning of the term has been extended so that it addresses nowadays also the interaction of the matter with mechanical/acoustical waves, electrical alternate currents and microscopic particle beams (others than photons).

The infrared (IR) spectroscopy makes use of invisible light with the wave number between 4000 cm-1 and 400 cm-1 (wavelength 2.5 µm – 25 µm) whose photons have the same energy range (0.5 eV - 0.05 eV) with the molecular vibrations.

Both adsorption and emission of electromagnetic radiation can be employed to estimate different specific parameters of the investigated molecules such as: excitation energy of different vibrational modes, the probability of transition between vibrational modes, the influences of the external perturbations, etc. On this basis chemical species present in complex macroscopic systems can be recognised and their amount (concentration), bonding nature and strength can be inferred.

The standard instrument dedicated to IR spectroscopy is the IR spectrometer. The apparatus consist of an integral IR light source, a light guiding and conditioning system (either a monochromator, whose typical dispersive element is a prism, or a Michelson interferometer), a fixture/recipient for the sample and an IR detector. When using the classical monochromator, the acquired information is delivered in the frequency domain, as absorbance dependency on the wave number (the absorption spectrum). The direct response of the spectrometer provided by a Michelson interferometer is an interferogram in the position/time domain; the absorbance spectra are computed on or off line through a mathematical procedure – the Fourier transformation. The last version of apparatus, known as Fourier transform infrared spectrometer (currently abbreviated as FTIR) is the most used one because, due to its high operation speed, reduces duration of the experiments and enables the acquisition of time resolved spectra.

In the field of gas sensors with organic materials (cavitands, polymers, and macromolecular structures) the FTIR spectroscopy under operando conditions is a very powerful tool. It makes possible the identification of the gas sensing mechanisms and the evaluation of the device sensing properties. For such application the most relevant results are the absorption spectra acquired during the gas exposure. The appearance, disappearance or shift of bands/peaks gives indications about the processes induced by absorption of the gaseous analyte in the sensing material.

Polyacrylic acid (PAA), for instance, shows good response to ammonia and humidity. The water vapour sorption in this polymer causes several changes in the infrared spectrum, especially in the wave number region around 3500 cm-1 (symmetric stretching vibration of hydrogen bonded water molecules). An additional background of ammonia results in supplementary IR absorption bands between 3100 and 2750 cm-1 (N-H stretching vibrations of the ammonium radical and changes in the polymer conformation resulting in an absorptions increase due to symmetric and asymmetric CH2 stretching vibrations of the polymer backbone).

The absorbance evidently correlates with both concentration and chemical nature of the analyte


[Hö8] Melanie Hörter. Influence of ammonia and water sorption on the chemical and electrochemical properties of polyacrylic acid and its derivates. PhD thesis, Eberhard-Karls Universität Tübingen, 2008.

[HOBW08] M. Hoerter, A. Oprea, N. Bârsan, and U. Weimar. Chemical interaction of gaseous ammonia and water vapour with polyacrylic acid layers. Sensors and Actuators B: Chemical, 134(2):743–749, 2008. [OBW+08] A. Oprea, N. Bârsan, U. Weimar, M. L. Bauersfeld, D. Ebling, and J. Wallenstein. Capacitive humidity sensors on flexible RFID labels. Sensors and Actuators, B: Chemical, 132:404 – 410, 2008.

[SOBW07] M. Sahm, A. Oprea, N. Bârsan, and U. Weimar. Water and ammonia influence on the conduction mechanisms in polyacrylic acid films. Sensors and Actuators B: Chemical, 127(1):204 – 209, 2007. Special Issue: Eurosensors XX The 20th European Conference on Solid-State Transducers, the 20th European conference on Solid-State Transducers.