FTIR Comes of age - Mark Pinkney

Over the last few years, FTIR technology has moved from the laboratory to the emissions monitoring field, becoming a powerful method of measuring gas mixture components continuously with asingle instrument. Today industry is beginning to recognise FTIR as the future of emissions monitoring. This article explains the technology and some of the problems and outlines a typical application.

WHAT IS FTIR?
An ‘FTIR’ (Fourier Transform Infra Red) is an infrared spectrometer able to collect full IR spectra, typically in the mid-IR range used for measuring gases. The unit will detect anything with an IR absorbance – in practice this includes most inorganic and organic compounds except for certain gases such as hydrogen, oxygen, nitrogen and inert gases such as argon.

In use, the vent gas is pumped into the sample cell for measurement and the FTIR produces an IR absorbance spectrum of it. If the cell contains no IR absorbers e.g. nitrogen only, then there will be no absorbance and the sample spectrum is a flat baseline. Generally, compounds in the cell will absorb IR and the result is a series of peaks, shown below. The pattern of peaks is unique to each compound (its spectrum) and can be used for identification (fig 1). The height of the peaks is proportional to the concentration in the cell.

Results are extracted from the sample spectrum by identifying peaks specific to components, and measuring the peak heights or areas. Concentrations are then calculated by comparing the sample peaks with corresponding peaks in calibration spectra of the pure material of known concentration. In practice the analysis may be more difficult, as many peaks will overlap and interfere and it may not be possible to locate unique ones. Today analysis is carried out automatically using software and more complex mathematical routines.