Low-Cost Infrared Microspectroscopy
addition, the SNR for the 160-microme- ter aperture (sample size) approaches 60, which is more than acceptable for quali- tative identification (see below). Te performance of the micro-
scope from a qualitative perspective can be observed in Figure 9, which illus- trates spectra of a reference polystyrene film collected using the 60-, 160-, and 250-micrometer-diameter apertures, top to bottom, respectively. In all cases, the SNR is more than sufficient for unknown identification analyses, even when the DTGS detector is used. Te quantitative capability of the
Figure 9: Transmission spectra of a reference polystyrene film collected using 60 (top), 160 (middle), and 250 (bottom) micrometer aperture sizes, 4 co-adds, 4 cm−1
resolution.
micrometers in diameter. Te SNRs associated with particular sample sizes were determined by generating a 100% transmis- sion spectrum and calculating the peak-to-peak noise over the range of 2800 to 2400 cm−1
. Te peak-to-peak noise was then
divided into 100 to give the SNR. Figure 8 illustrates a graph of SNR plotted against aperture size. Te black data points relate to the experimental results, and the blue data points represent the- oretical calculations. Te theoretical values were normalized to the SNR experimentally obtained for the 60-micrometer-diam- eter aperture. Tis normalization was done for subsequent aper- ture sizes by multiplying the SNR value of the 60-micrometer aperture to the ratio of the cross-sectional area of a larger aper- ture to that of the 60-micrometer-diameter aperture. Te results show a reasonable correlation between the experimental results and what would be expected for the increased throughput. In
microscope was evaluated by study- ing colored fibers in which the colorant concentration ranged from 1% by weight to 10% by weight. Bouffard et al. previ- ously used transmission infrared micro- spectrosopy to study yellow, blue, and red pigments added into polypropylene fibers [12]. In this work, the authors used
a Perkin-Elmer Model 1600 FTIR spectrometer equipped with a Perkin-Elmer FTIR microscope and an MCT detector. Te authors were able to identify all three pigments and quantify the concentration of the yellow and red pigments down to 1%w/w and the blue pigment down to 2%w/w. Tis work was success- fully reproduced using the same polypropylene fibers using the ATR feature of the SurveyIR 5X microscope as a proof of con- cept for the accessory. ATR spectra of fibers loaded with 1, 2, 5, and 10% (w/w)
of red, blue, and yellow pigments were collected, and an absor- bance band ratio method was used for quantification with a polypropylene band acting as an internal standard. Quantifica- tion of the colorant was performed by using the peak area of an absorbance characteristic of the colorant and that from the polypropylene fiber itself [13]. Te control band (internal stan- dard) was the polypropylene absorption at 1377 cm−1
corresponding to the sym-
metric methyl bending motion. Strong bands resulting solely from each of the three pigments in the fibers were then selected for quantification (Figure 10). Table 1 lists the results for each of the
colored fibers along with the relative stan- dard deviation for each concentration and the sensitivity and correlation coefficient for the associated calibration curve. Te uncertainty in the band ratios
Figure 10: Reference spectra of the fibers at 10% colorant loading. Arrows indicate the spectral range used for determination of the peak areas.
2020 March •
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reflects the error in the method and the variability of the dye along and between fibers. Five measurements were made along 3 individual fibers for each fiber color and concentration. As expected the error (percent relative standard deviation [RSD]) decreases as the concentration of the colorant increases. Te sensitivity of the method for each fiber is similar, and the correlation is very good.
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