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Low-Cost Infrared Microspectroscopy


Results and Discussion One of the most important char-


Figure 6: Three different colored fibers viewed in transmission, reflection, and oblique illumination modes (left to right, respectively).


acteristics of any microscope is the ability to view and image the sam- ple at high magnification for proper photo-documentation and color rep- resentation. Te microscope has three different optical illumination modes that include transmission, reflection, and oblique illumination. Figure 6 illustrates three different colored fibers (approximately 50 microm-


eters in diameter) with colorant loadings of 5% by weight using transmission, reflection, and oblique illumination. In order to obtain infrared transmission spectra that are


Figure 7: Images obtained in oblique illumination through the IRE (left) and in transmission after contact with the IRE (right).


photograph of the instrument is shown in Figure 5. Te fiber samples, previously described by Bouffard et al., were mounted on a low-E slide for analysis [12]. Spectra of the fibers collected with the MCT detector represent the average of 60 individual scans using a spectral resolution of 4 wavenumbers.


photo-metrically correct (that is, a spectrum in which the absorp- tions are not totally absorbing), one usually needs to flatten the sample. Tis requirement arises from the fact that the path length of light through the sample is approximately the same as the sample thickness. In general, a sample thickness of less than 6 micrometers yields spectra in which the strongest feature does not extend below 20% transmittance. A means to overcome this sample preparation requirement is to collect the spectrum using an ATR accessory where the path length through the sam- ple is limited to the penetration depth. Tis depth, which can be calculated from known parameters, is on the order of a few micrometers across the infrared spectrum. Avoidance of sample preparation is one of the major reasons why ATR spectroscopy dominates the way infrared spectra are collected today. In addition to transmission and reflection infrared modes,


the microscope also has an ATR mode in which optical viewing takes place through a diamond IRE. Figure 7 (leſt) illustrates the same three fibers shown in Figure 6 but now viewed through the diamond IRE using oblique opti- cal illumination. Figure 7 (right) also illustrates the fibers aſter contact has been made with the IRE. Tis later image was obtained in transmission optical illumination mode and illus- trates the sample flattening and area increase on contact with the diamond IRE, as well as the high image quality that SurveyIR 5X delivers in multiple illumination modes. Te performance of the SurveyIR


Figure 8: SNR versus aperture size for (blue).


30 the microscope, experimental transmission (black), theoretical


5X microscope was assessed both qualitatively and quantitatively by collecting 100% transmission spectra, which represent the noise uncertainty in a measurement, and transmission spectra of a reference polystyrene film. Spectra were collected by co- adding 4 individual scans at 4 cm−1 spectral resolution using the internal DTGS detector of the interferom- eter. Te microscope has an internal remote aperture wheel with selectable apertures that correlate to sample sizes of 60, 100, 160, 200, 250, and 2000


www.microscopy-today.com • 2020 March


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