Fourier Transform Infrared Spectroscopy in the Pulp and Paper Industry

Abstract

AbstractFourier transform infrared spectroscopy (FTIRS) is the combined use of infrared (IR) light, for illuminating a light‐absorbing sample, with an interferometer, for producing an interference pattern from which an absorption spectrum characteristic of the sample is recovered through Fourier transformation. The majority of current IR pulp and paper applications involve the routine characterization of samples that contain variable amounts of water, a very strong IR absorber. Consequently, qualitative IR analysis of pulp and paper samples is best performed with a Fourier transform infrared (FTIR) spectrometer because of the larger optical throughput offered by an interferometer over a dispersive spectrometer. Also, an interferometer simultaneously records all IR frequencies present in the spectrum. These two advantages enable one to acquire high‐quality spectra in a few minutes or less. Additionally, since an interferometer accurately determines IR frequencies with the output of a visible‐light laser, quantitative determinations which use multivariate calibration methods such as partial least squares (PLS) are much less prone to instrument drift, and hence more accurate. Liquid and solid samples can be analyzed either neat or suitably diluted in a nonabsorbing matrix, through a variety of sampling techniques such as transmission, diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), attenuated total reflectance (ATR), IR microscopy, and photo‐acoustic spectrometry (PAS). Detection limits for quantitative methods vary from 0.01 to 1 percent, depending on the sampling method and the complexity of a sample matrix. However, FTIR spectrometry cannot resolve more than a few components at a time in liquid and solid samples because of the presence of strongly overlapping bands. Therefore, FTIR spectroscopy must often be used in combination with separation techniques such as gas chromatography (GC) or solid‐phase extraction (SPE), especially when a complete analysis of the sample is required.