Typically, lignins are characterized by IR spectroscopy in the solid state. The examination of lignins in solution has long been a domain of UV and NMR spectroscopy. However, with the advent of the FTIR technique, the IR spectra of liquid samples now can be readily obtained. The need to measure IR spectra in solution exists when there is not enough time to isolate lignin from solution as, for example, is the case when spent pulping liquors have to be analyzed rapidly during a pulping process for process control. This section describes briefly the common liquid sampling accessories and applications of solution state FTIR spectroscopy to lignins. For more details on FTIR spectroscopy, the reader is referred to Chapter 4.1.

Fourier Transform Infrared Spectroscopy

A spectral analysis of whole EDTA blood was undertaken by using attenuated total reflection and Fourier-transform infrared spectroscopy. The concentration of blood glucose was measured by an enzymatic method using glucose dehydrogenase and ranged between 40 and 290 mg/dL with an average concentration of 90.4 mg/dL. Multivariate calibration with the partial least-squares (PLS) algorithm was performed on spectral data between 1500 and 750 cm-1 showing a varying background from different unidentified interfering compounds. Cross validation was carried out for optimizing the PLS model. PRESS was 19.8 mg/dL, which was calculated on the basis of 127 standards, whereas the estimated standard deviation for the calibration fit was computed to be 11.9 mg/dL. Infrared spectroscopy can be used for monitoring glucose levels within the normal physiological range in a complex matrix like whole blood as an alternative to electrochemical sensors.

Multivariate determination of glucose in whole blood by attenuated total reflection infrared spectroscopy

Effects of different exposure times and concentrations of sodium hypochlorite/ethylenediaminetetraacetic acid on the structural integrity of mineralized dentin.

A spectral analysis of human blood plasma was undertaken by use of a Fourier-transform infrared spectrometer with a circular attenuated total reflection cell. The concentrations of total protein, glucose, triglycerides, total cholesterol, urea, and uric acid were measured by chemical or enzymatic methods. For these constituents the partial least-squares (PLS) algorithm was used for a multivariate calibration including the infrared fingerprint region of the plasma spectra. Best results were achieved for total protein with an average prediction error (PRESS based on cross validation) of 2.1 g/L; other PRESS results were for glucose 22 mg/dL, triglycerides 33 mg/dL, cholesterol 31 mg/dL, urea 4.4 mg/dL, and uric acid 1.6 mg/dL.

Multivariate calibration for assays in clinical chemistry using attenuated total reflection infrared spectra of human blood plasma.

FTIR spectra of the tropical hardwood Guibourtia sp. (Caesalpinia-ceae) have been recorded using the KBr pellet technique and diffuse reflection (DRIFT) in KBr powder. It was demonstrated by variation of the wood concentration in KBr and the particle size of the milled wood that the spectroscopic data of wood- mainly those of the intense carbohydrate band around 1100 cm−1—are strongly influenced by these parameters. The KBr pellet technique gives reproducible results for concentrations between 0.1 and 0.5% with a particle size below 25 μm. The. DRIFT spectra of wood powders in KBr are very much influenced both by particle size and concentration. Wood spectra are also presented which were obtained by transmission spectroscopy and FTIR microscopy of 25 μm microtome cuts and by reflectance on solid wood surfaces. The influence of heterogeneous dispersion of wood in KBr and the roughness of wood surfaces on reflectance spectroscopy is discussed. A simple baseline correction is recommended to increase interlaboratory comparability of wood spectra.

The influence of particle size and concentration in transmission and diffuse reflectance spectroscopy of wood

The introduction of Fourier Transform techniques and the increasing use of computers in infrared spectroscopy has made new techniques of investigation available to the spectroscopist, such as photoacoustic spectroscopy (PAS) and IR microscopy. These methods now complement the techniques of specular reflectance and attenuated total reflectance. Thin films on metals, sometimes with a thickness of much less than a micron, can be studied by various specular reflectance methods. The physical basis of the attenuated total reflectance technique (ATR) leads to a penetration of the radiation in the order of a few microns. It is, therefore, especially suitable for the investigation of surfaces and of layers close to the surface. By changing the modulation frequency of the IR radiation, i.e. the mirror velocity of the FTIR spectrometer, photoacoustic spectroscopy (PAS) can be employed to study layers at various depths below the surface of a sample. Therefore, this technique allows depth-profile analysis, so that PAS reveals itself as a complementary method to attenuated total reflectance spectroscopy. Samples with inhomogeneous profiles, e.g. laminated polymer films, can often be prepared as microtome slices perpendicular to the layered structure. Using infrared microscopy it is now possible to investigate different regions of the cross-section easily. The size of the regions that can be studied in this way may be as small as a few microns.

Surface and depth-profile analysis using FTIR spectroscopy

The growing capabilities of FTIR spectrometers and computers have opened the use of new sample preparation techniques in infrared spectroscopy. In addition to the established KBr pellet technique and ATR spectroscopy, diffuse reflectance and photoacoustic spectroscopy are increasing in importance. A systematic experimental comparison of these techniques has been made in order to make proper use of their mutual advantages.

A critical comparison of solid sample preparation techniques in infrared spectroscopy

Infrarotspektroskopische Harnsteinanalyse: Automatisierte Spektrenauswertung mit Hilfe der Faktoranalyse

Bestimmung der Hydroxylzahl von Polyolen mit Hilfe der FT-NIR-Spektrometrie

Until recently photoacoustic detectors coupled to infrared spectrophotometers were low signal-to-noise devices. Often long acquisition times and low resolution were used in order to obtain any spectrum at all.

Joachim Oelichmann

Papers

Surface and depth-profile analysis using FTIR spectroscopy

A critical comparison of solid sample preparation techniques in infrared spectroscopy

Infrarotspektroskopische Harnsteinanalyse: Automatisierte Spektrenauswertung mit Hilfe der Faktoranalyse

Bestimmung der Hydroxylzahl von Polyolen mit Hilfe der FT-NIR-Spektrometrie

Industrial problem solving using infrared photoacoustic spectroscopy