Low-resolution FT-IR spectrometer

Several of the early low-resolution FT-IR spectrometers sold by Nicolet incorporated flex-pivot drives, and the interferometer made by Midac has a porch-swing drive by which a resolution of up to 0.5 cm can be achieved [7]. The major... 

For low-resolution spectrometers (Av > 2cm ), amax is usually so great that all the light allowed by Eq. 2.40 cannot be focused onto the detector. In this case, the detector element itself acts as the limiting aperture and there is no need to install a J-stop in the optics. As a result, the optics of low-resolution FT-IR spectrometers can be as simple as those of the instrument shown in Figure 6.5. 

P R. Griffiths, R. L. Richardson, D. Qin, and C. Zhu, Open-path atmospheric monitoring with a low-resolution FT-IR spectrometer, in Optical Sensing for Envimnmental and Process Monitoring, O. A. Simpson, Ed., Society of Photo-Optical Instrumentation Engineers, Bellingham, WA, 1995, Vol. 2365, p. 274. 

The optical density (OD) of the sample was varied by repeatedly diluting the mixture with fresh gas until a value of roughly 0.8-1.2 was obtained in some cases, scans were acquired with ODs as low as 0.1. Absorbance measurements were made directly in the cell using a Mattson Research Series FT-IR spectrometer (0.25 cm1 resolution) configured for external beam operation. The optical layout makes it possible to easily switch from making ps pump-probe measurements to recording an IR spectrum. 

Water vapor in the beam of FT-IR spectrometers is a major source of interference. Thus, the optical bench should ideally either be purged with dry air or evacuated completely. Some low-resolution spectrometers are hermetically sealed and desiccated, so that the only humid air is in the sample compartment. These instruments are adequate if the spectra of conventional films, KBr disks, and solutions... 

One of us (P.R.G.) has incorporated neural network software into a low-resolution OP/FT-IR spectrometer. Once the components had been identified, an online PLS calibration and prediction was carried out Fheliminary results are promising. As an example, a mixture of acetone, methyl ethyl ketone (MEK), and diisobutyl ketone was allowed to evaporate close to the beam of an OP/FT-IR instrument. The software recognized the presence of these three compounds and allowed their quantification. The data obtained in this way are shown in Figure 22.8. Acetone and MEK are seen at relatively high concentrations at the start... 

On the other hand, for spectra measured in equal times and with equal optical throughput, the noise on the baseline is inversely proportional to Av (see Section 7.1). Because the separation of lines in the spectra of molecules with a molecular weight greater than 100g mol is usually less than the collision-broadened width of these lines, they can rarely be resolved at any resolution setting of the spectrometer. As a result, the peak absorbance of bands in the spectra of many volatile organic compounds (VOCs) is only minimally affected by resolution until Av exceeds the FWHH of the band (see Figure 22.7). Thus, if OP/FT-IR spectra of VOCs were to be measured at relatively low resolution (e.g., Av = 4 or 8cm ), the noise level of the spectrum is decreased and the minimum detectable quantity of analytes with broad bands is decreased concomitantly. 

The manner in which Fourier Transform Infra-Red (FT-IR) spectra were obtained depended on the physical nature of the substance. The techniques used were formulation into K r pellets and deposition onto KBr, KRS-5 or NaCl windows. Thus, ca. 1 mg of substance was mixed with KBr (50 mg) in a mortar. When transmission of the major peaks was low a larger amount of substance was used until all peaks had a satisfactory intensity. Spectra were acquired at 4 cm resolution using a MID AC spectrometer. Tlie sample and reference cell were scanned 60 times at 1.4 s/scan and the mean difference spectrum was plotted. FT-IR spectra were checked in a second laboratory using different instrumentation and conditions. 

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