High Resolution as Solution - TA-HR-FTIR for Highly Complex Samples
D. Merz, O. Dregert, J. Vehlow Forschungszentrum Karlsruhe GmbH, Institute for Technical Chemistry, Division of Thermal Waste Treatment, D-76021 Karlsruhe, Germany
Thermoanalysis - FTIR-Spectroscopy has proved to be an efficient tool in investigating the thermal behaviour of different materials and fuels. However, the volatile products evolved during thermal treatment of municipal solid waste result in highly complex gaseous mixtures. Using TA - low resolution FTIR, the vibrational bands of interesting analytes and matrix substances are oftentimes overlapping. Since some matrix substances are still unknown or unidentified, chemometric methods for quantification are difficult to apply. The problem is not a mathematical one, but lies in insufficient analytical data of the complex chemistry.
The advantage of analysis in the gas phase is, that rotational states of the molecule are detectable. High resolution FTIR spectroscopy uses this fact by solving the vibrational bands of gases into rotational vibrational bands. The result is the splitting of broad vibrational bands into lots of sharp rotational vibrational bands, each of them characteristic for one substance. Thus, the most important condition for accurate quantification is offered: distinct and separated bands can be selected for each interesting gaseous analyte.
The challenge of HR-FTIR is the resulting complex spectrum. Due to hundreds of bands, it sometimes becomes demanding, to see the "forest for the trees" - qualitative identification has to be done very precisely. Even rotational vibrational bands may overlap with that of matrix substances. Additionally, time needed for recording the spectrum increases with higher resolution.
We applied TA-HR-FTIR to a recently launched research project, which uses nitrogen oxide reduction as an example of process optimisation in municipal solid waste incineration. In that case, the simultane and online identification of primarily released N species is essential. Since gas cleaning procedures or prior condensation of water are not suitable, the analytes are quantified in a complex matrix.
To learn about the accuracy of the TA-HR-FTIR method, various N?containing waste fractions were used for the thermal treatment in the thermobalance. The combustion gases were transferred into an in-house designed FTIR gas cell. By tracing characteristic rotational vibrational bands of NO, its precursors and potential reaction partners, temperature related release profiles were obtained. Molar concentration ratios at given temperatures are calculated directly from these traces. Our study shows, that High Resolution FTIR Spectroscopy allows the simultaneous and highly accurate online quantification of released N-containing analytes.
It should be stated, that using the right tool for the right task is essential in this kind of analysis. Due to the lower time frequency for spectra recording and the need of a specialist to interpret the spectra, the application of TA-HR-FTIR should be well considered. The tool should fit to the problem to be solved. However, for highly complex gaseous mixtures, the advantage of accurate quantitative data outweighs the additional effort. In these cases, TA-HR-FTIR constitutes an efficient tool for investigations of the chemical reaction mechanisms, allowing fast online screening for differences and changes in gas composition as a result of variations in process parameters.
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