H Fourier Transform Infrared Spectrometry

The infrared region of the electromagnetic spectrum is the region on the longer-wavelength side of the visible spectrum. Infrared radiation can not be "seen" by our eyes, but we sense it as heat (all hot objects give off considerable infrared radiation). Infrared reflectography described earlier in this book, uses "near infrared" radiation, that is, infrared radiation just beyond the red part of the visible light spectrum. (The infrared radiation used in IR reflectography goes out to wavelengths of about two micrometers, or two millionths of a meter.) Many pigments are more transparent in near-infrared radiation than they are in visible light, and thus the technique allows underdrawing or underlying paint layers to be seen through overlying thin paint layers. An underdrawing or underpainting that may be totally obscured in visible light can often be readily seen by infrared reflectography.

Radiation in the "mid infrared" region extends from about 2.5 to 25 micrometers (or about 2.5 to 25 millionths of a meter); this is a conventional definition, fairly arbitrary. Just as many materials absorb some of the wavelengths of the visible spectrum and transmit others, many materials also absorb some wavelengths of the mid-infrared region and transmit others. Absorption in the visible range causes electronic transitions, as was discussed in Appendix C. Such electronic transitions also affect the transparency of some pigments in the near infrared region utilized in infrared reflectography. In contrast, mid-infrared radiation is not energetic enough to produce changes in electronic states in atoms or molecules. Instead, mid-infrared radiation can cause chemical bonds to bend or stretch, or cause the atoms in the crystalline lattices of compounds to vibrate relative to one another. Such reactions occur at different wavelengths, depending on the specific compound absorbing the radiation and the types of bonds it contains. Infrared spectrometers are analytical instruments that record the absorption (or transmittance) of infrared radiation by a material as a function of wavelength. The resulting pattern of absorption or transmittance can be used to identify a material, sometimes quite specifically.

In the analysis of painting materials, infrared spectrometry can be used for analysis of many pigments (both organic and inorganic), binders, and varnishes. Many organic compounds with similar chemical compositions and structures have similar patterns of absorption in the IR range. This is true, for example, of the protein-containing binders, such as glue, egg white, casein, and the protein portion of egg yolk (see Figure H.1). Thus, this instrumental technique is useful for the identification of the general class of a binder, but not usually for specific binder identification.

The most modern generation of infrared spectrometers, called "Fourier transform" instruments after the mathematical technique used to compute spectra from the information acquired by the instrument, can be attached to infrared microscopes. With the aid of an infrared microscope, very small specks of paint can be analyzed. Paint chips can also sometimes be sliced very thinly and analyzed, which permits analyses of individual paint layers to be carried out. The technique of infrared microscopy cannot, however, be applied with great success at the mo-

Rabbit Hide Glue Microscopic

Wavenumbers (cm-1)

Fig. H.1. FTIR spectra of several protein-containing binders. Rabbit skin glue, the particular casein shown here, and egg white are almost entirely made up of proteins. Although each binder contains different specific proteins, these proteins have virtually identical spectra, and the three binders cannot be distinguished from one another by infrared spectrometry alone. Egg yolk contains a substantial amount of oil in addition to protein. The oil component makes it possible to readily distinguish egg yolk from rabbit skin glue, casein, and egg white. It should be noted, however, that a mixture of one of the nearly pure protein binders and a drying oil would produce a spectrum similar to that of egg yolk, so in certain instances there could be some ambiguity.

Wavenumbers (cm-1)

Fig. H.1. FTIR spectra of several protein-containing binders. Rabbit skin glue, the particular casein shown here, and egg white are almost entirely made up of proteins. Although each binder contains different specific proteins, these proteins have virtually identical spectra, and the three binders cannot be distinguished from one another by infrared spectrometry alone. Egg yolk contains a substantial amount of oil in addition to protein. The oil component makes it possible to readily distinguish egg yolk from rabbit skin glue, casein, and egg white. It should be noted, however, that a mixture of one of the nearly pure protein binders and a drying oil would produce a spectrum similar to that of egg yolk, so in certain instances there could be some ambiguity.

ment to cross sections. Infrared microscopy is an attractive technique because of the wealth of information obtainable from a single quite small sample. The sample itself is not damaged or destroyed during the analysis, so it can be used for additional analyses after the IR spectrum is acquired. IR spectrometry is virtually the only common instrumental technique that can simultaneously provide information about both pigments and binding media.

One difficulty in the study of paint samples is the presence of pigments, which often absorb radiation in the mid-infrared range and can partially mask information from the binders in the sample, sometimes to the point where the binder cannot even be generally identified.

One example of general binder identification on a paint sample is shown in Figure H.2. The sample is from the early Netherlandish painting shown in Figure 3.8. The sample contains the pigment lead white

Ftir Lead White

Fig. H.2. FTIR spectrum of a white paint sample from Rogier van der Weyden's St. Luke Drawing the Virgin, ca. 1434.

The sample contains lead white pigment (specifically, the compound hydrocerussite) and a drying oil medium (probably linseed oil). The spectrum shows absorption bands for both the pigment and the binder. In addition, interaction between the pigment and medium has produced additional bands, including the large one at about 1550 wavenumbers. This sample spectrum is quite typical for old lead-white-containing oil paint.

Fig. H.2. FTIR spectrum of a white paint sample from Rogier van der Weyden's St. Luke Drawing the Virgin, ca. 1434.

The sample contains lead white pigment (specifically, the compound hydrocerussite) and a drying oil medium (probably linseed oil). The spectrum shows absorption bands for both the pigment and the binder. In addition, interaction between the pigment and medium has produced additional bands, including the large one at about 1550 wavenumbers. This sample spectrum is quite typical for old lead-white-containing oil paint.

and an oil binder. For comparison, reference spectra of lead white and oil are shown. The paint sample probably consists of about 80-90% pigment and 10-20% paint medium, so it is not surprising that the spectrum more closely resembles that of the reference pigment than that of the pure reference binder.

A second example, in Figure H.3, is from a 1938 painting by the American artist Arthur Dove. Analysis of the sample by chromatography techniques (discussed in the next section) showed that the binder contains both wax and drying oil. While the infrared spectrum indicates the presence of oil, the presence of wax cannot be unambiguously established.

IR spectrometry can often produce a general class identification of the binder (for example, oil, protein, or gum). Because of interference from pigments, even this general class identification cannot always be carried out. If a mixture of media is present, identification of the different media may not be, indeed often is not, possible by IR spectrom-etry alone.

Infrared Spectroscopy Bend Stretch Paint

Fig. H.3. FTIR spectrum of a blue-green paint sample from Arthur Dove's Dancing Willows, 1941.

Pigments in the sample include two that can be readily identified in the spectrum: the mineral celadonite, the major component of many green earth pigments; and barium sulfate, a white compound. Other features of the spectrum suggest the presence of oil. Also present are fatty acid salts, perhaps a result of interaction between the medium and some pigment in the paint. Analysis of a sample of this paint by gas chromatog-raphy/mass spectrometry indicated that the medium also contains some beeswax and a little conifer (probably pine) resin, neither of which can be positively identified in the IR spectrum. The top spectrum is of pure beeswax. The distinctive pairs of bands around 1450 and 720 wavenumbers cannot be seen in the paint sample.

Fig. H.3. FTIR spectrum of a blue-green paint sample from Arthur Dove's Dancing Willows, 1941.

Pigments in the sample include two that can be readily identified in the spectrum: the mineral celadonite, the major component of many green earth pigments; and barium sulfate, a white compound. Other features of the spectrum suggest the presence of oil. Also present are fatty acid salts, perhaps a result of interaction between the medium and some pigment in the paint. Analysis of a sample of this paint by gas chromatog-raphy/mass spectrometry indicated that the medium also contains some beeswax and a little conifer (probably pine) resin, neither of which can be positively identified in the IR spectrum. The top spectrum is of pure beeswax. The distinctive pairs of bands around 1450 and 720 wavenumbers cannot be seen in the paint sample.

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