Infrared Examination of Micro Samples Using Reflectance Techniques
The past few years have seen a significant increase in the use of infrared reflectance techniques for obtaining information of types not previously associated with reflectivity measurements. The purpose of this paper is to describe several reflectance attachments used by our laboratory and to illustrate a few of their applications in general chemical analyses. In addition to the normal types of reflectivity study, we have found these attachments to be extremely valuable for micro-analyses and thin coating or surface chemistry studies. One attachment is particularly useful at extended wavelengths, i.e., to 300 microns.
Because of its self-contained beam condensing system, the use of a micro-specular reflectance attachment, originally designed to measure the thickness of semiconductor epitaxial films, has been used for the examination of small samples, such as those separated from GC effluents. Samples are mounted on a small reflecting surface so as to reflect the light beam through the sample twice, producing a transmission-like spectrum. Due to the double-pass through the sample and the resulting increase in sensitivity, only about one-half the amount of sample normally required for a micro KBr pellet of equivalent area is necessary. In conjunction with gas chromatographic separations, this technique has been used for the identification of DDT in milk and phenobarbital in blood. In favorable cases using a lxl mm area of sample, good spectra have been obtained on as little as 15 micrograms of sample without resorting to scale expansion techniques. For more weakly absorbing materials, 25 to 50 micrograms is typical. While the system has been applied mainly to gas chromatographic fractions, it has also been found applicable to the identification of residues from solvent wash systems and LSD from milk sugar.
In those cases where very thin coating must be examined, a multiple specular reflectance attachment has been found to be extremely useful. Basically this system consists of two flat highly reflective surfaces, such as aluminum or steel, held 4 mm apart and oriented at 45° to the infrared spectrophotometer beam. Light entering one end of the attachment is reflected back and forth nine times between the two mirrors before being deflected back to the monochromator via two additional mirrors. Like the micro-specular reflectance technique, the multiple reflection system produces transmission-type spectra, but achieves greater sensitivity for a given sample thickness through a multiple reflection process. Whereas a single specular reflection produces spectra about twice as intense as conventional transmission techniques, multiple reflection can produce a spectrum 8, 10 or 18 times more intense, depending on the number of reflections used. Using the multiple specular reflectance and scale expansion techniques, the presence of a typical organic material such as polyvinyl acetate on a highly reflective surface can be detected at the 0.001 micron level.
The growing interest in the far-infrared has prompted the design of several micro beam condensers useful for transmission and reflectance studies in the extended wavelength regions, i.e., beyond 50 microns. Due to the relatively poor efficiency and wavelength limitations imposed by normal beam condensing lenses, front surface mirrors have been used throughout. These systems are typically 80–90% efficient throughout the 2.5–300 micron region and provide a beam size reduction of 4 times in an F-10 system. Use of these systems in an IR-11 far-infrared spectrophotometer allows samples as small as 3×5 mm to be examined at near full instrument energy. Using the condensers in combination with beam aperturing, samples as small as 2 mm2 in area can be readily examined in the region beyond 50 microns. The results of transmission and reflectance studies in the 33–800 cm−1 region are illustrated.