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In Situ FTIR Inspection Is a Convenient New Tool for Analyzing and Viewing Samples

Fourier Transform Infrared (FTIR) spectroscopy is widely used in industry to analyze organic compounds and to determine the chemical structure of many inorganic ones. Applications include material identification and evaluation, quality control screening,and characterization of surface residues.In a glove box environment where air-sensitive materials or nuclear materials are handled, the difficulty of making analytical measurements compared to making them in a routine laboratory environment increases dramatically.

FTIR spectroscopy has been used in the glove box environment in NMT Division for many years, notably in the Molecular Laser Isotope Separation Program, where in-line gas cells were used to monitor the process. However, varied sample matrices to be analyzed, including parts as well as samples from various processes, usually have to be transferred to the glove box where the spectrometer is situated.

Figure 1. Figure 1.Common objects such as the rock in the background (magnification ~10x) and the penny (magnification 7.5x and 32x) demonstrate the view seen directly through the diffuse reflectance accessory of the infrared spectrometer. The direct-viewing capability enables researchers to perform spectroscopy in the precise area of interest.

Recently, the availability of portable FTIR spectrometers allows in situ examination of samples in a glove box environment. This article describes a simple, portable FTIR inspection system that allows sharing of instrumentation and accessories among several glove boxes. Sharing is accomplished by mounting the FTIR on a standard service panel on top of each glove box where FTIR measurements are performed. Thus, the system can be moved to any glove box that has been fitted with the appropriate service panel. The prototype instrumentation and accessories described here include a remote inspection accessory with interchangeable sampling heads, including diffuse reflectance (DR) and specular reflectance, and a novel visualization accessory that allows in situ viewing of the sample with variable magnification (Figure 1). This system is now commercially available.

The heart of the system is the patented, portable FTIR spectrometer manufactured by Surface Optics Corporation (SOC). The system was designed to be able to record infrared (IR) spectra in field situations. For example, G. L. Powell (co-developer with SOC at Martin Marietta Energy Systems, Y-12, Oak Ridge) has used the system to record spectra of aging paints on airplane wings for NASA. This spectrometer incorporates a source, interferometer, and DR sampling optics with a built-in detector in a package that weighs about 20 pounds and occupies about 0.8 cu. ft.

This FITR sampling method makes use of the most efficient DR collection system available today, known as the patented "barrel-ellipse head" (Harrick Scientific and SOC), shown schematically in Figure 2.

Figure 2. Anatomy of the barrel ellipse diffuse reflectance head. The instrument can be used for verification of a cleaning process or for the identification of undesirable organic residues or molecular species adsorbed on the surface of metal oxide powders.

Diffuse reflectance may be used to examine most surfaces that are able to scatter light. Characterization of organic films on machined metal surfaces is thus easily achieved with this technique so it can be used to certify cleaning processes, for example. Another important application among many is the ability of the DR technique to measure the IR spectra of molecules adsorbed on metal oxide powders. These are two applications of interest in NMT Division, and they are highlighted in this article.

Several instrument modifications were necessary for their use in NMT Division. The most important changes were to redesign the barrel-ellipse so it could be operated as a replaceable head on the end of a gold-coated light pipe and to incorporate a novel sample-viewing system that allows, with the flip of a switch, concurrent video imaging of the sample surface to be examined by the IR beam. While sample-viewing attachments are available with other sampling optics in IR spectrometers today, no other systems are able to view directly through the DR accessories. This is a noteworthy enhancement of the instrumentation, brought about by collaboration of NMT personnel and the industrial partner. The viewing accessory is crucial in determining the correct location on a part of the sample that is to be examined by FTIR. Its value can readily be appreciated if one considers the problem of spectroscopic examination of the interior of a storage can-inside the glove box-using an invisible beam.

Figure 3. Scene from the viewing accessory monitor when the bottom of a stainless steel dressing jar is a few millimeters below the focal plane of the barrel ellipse. The figure in the corner of the image shows spectra of a smudge on the can surface taken 2 mm apart.

Figure 3 shows what is seen on the viewing accessory monitor when the bottom of a stainless steel dressing jar is a few millimeters below the focal plane of the barrel ellipse. The figure in the corner of the image shows spectra of a smudge on the can surface taken 2 mm apart. By further rotating and translating the can one beam width at a time, one obtains hundreds or thousands of similar spectra correlated with the examined position on the can surface. The surface of the can is then plotted as a grid, and the peak heights of the chromophore that we wish to map (the CH stretch at 2900 cm-1, for example) are scaled to a color and plotted at each position on the grid. In this way one obtains maps of the location of various films and their thicknesses on the can interior. This method can be used for verification of a cleaning process or to identify undesirable organic residues left behind after a process. In the case shown here, one can see that the smudge is more complex than a simple oil (the region-1800-600 cm-1-would consist of only 2 peaks). Actually, the smudge is an intentionally smeared fingerprint.

Another application of interest to NMT Division is the identification of the molecular species (such as impurities) adsorbed on the surface of metal oxide powders. Diffuse reflectance is a convenient and sensitive sampling technique for such powders. There are several reasons for this: first, clean metal oxides do not absorb IR light above 1000 cm-1; therefore, any molecular fragments adsorbed on the oxide surface that absorb IR light will be seen. Second, the high surface area of these powders provides a large number of sites for adsorbates. Third, the extensive scattering of the IR light in the powder leads to a very long effective path for absorption by the adsorbed molecular fragments. For example, when a clean metal oxide surface is exposed to atmospheric water, the water molecules dissociate on the surface to form hydroxyl groups (-OH). As water continues to adsorb and dissociate, more sites become occupied by -OH; with continued exposure to water from the gas phase, water molecules will eventually adsorb on the surface as molecular water. Figure 4 shows the DR/IR spectrum of lanthanum sesquioxide (La2O3, lanthana) powder that has been aged in air. Both OH (sharp, 3600 cm-1) and molecular H2O (broad, 3200­3500 cm-1) spectral features can be seen in the top panel.

Figure 4. Lanthanum oxide powder before and after mild heating. Even a mild heat treatment causes changes in the amount of adsorbed molecular fragments as shown by the changes in the IR spectra. This same technique will be used with PuO2 powders to determine the changes in composition and desorption of the molecular adsorbates as a function of temperature.

Other gases may also react on the oxide surface. Carbon dioxide is readily adsorbed from air and forms various types of carbonates on the oxide lattice. The carbonate region is shown in the lower panel of Figure 4. In fact, for lanthana, samaria, and other rare earth oxides aged in air, it is known that the oxide lattice is partially transformed into hydroxycarbonate-like phases La2(OH)2(3-x)(CO3)x, where x is ~1.

The strong absorptions seen in Figure 4 are due to these hydroxycarbonate phases within the oxide lattice. The weaker peaks above 3600 cm-1 are due to hydroxyl groups on different sites of the oxide lattice. The upper trace is for the sample prior to mild heating (~300šC), while the lower trace was obtained after heating in air for almost an hour. Temperature-programmed desorption described in the literature shows water desorbing from lanthana at 300šC and at 450šC. Our simple demonstration is consistent with this. These are only two examples of the type of analyses that will be carried out on analogous actinide materials inline using our portable FTIR instrumentation. The system will be installed this quarter in one of the Bldg. PF4 laboratories.

This article was contributed by Joe Baiardo (NMT-11). Others who work on the project include John Ward (Laboratory Fellow, Ret., NMT-11) and Trish Wright (NMT-15). Stephanie Hale (formerly NMT-5) obtained capital equipment funds for development of the prototype instrumentation. Other support for this project was provided by Doug Kautz (NMT-5) and Patrice Stevens (NMT-15). The glove-box-adapted system including the visualization accessory has also been implemented by Y-12.


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