FT-NIR Spectrometers

An alternative approach for NIR hyperspectral imaging to that described above is to use a Fourier transform NIR (FT-NIR) spectrometer. As the design of FT-NIR microspectrometers is more similar to that of instruments for mid-lR hyperspectral imaging than the dispersive instruments described above, they will be described later (see Section 1.6). 

Figure 9.39 Schematic representation of the diffuse-reflection measurements with a single-element detector FT-NIR spectrometer. Reproduced with permission from Ref [68] 2008, Society for Applied Spectroscopy.

Early NIR work was performed using either a UV-Vis instrument with extension units for low wavenumbers or IR spectrometers with accessories for high wave-numbers. With these instruments, the quality of the collected spectrum was low. However, in modern times, good quality dispersive- and FT-NIR spectrometers exist that provide high quality spectra. 

Nimaiyar (2004) used a Perkin-Elmer Spectrum One Fourier-Transform NIR (FT-NIR) spectrometer to scan ground soybean samples in reflectance mode. She had a calibration set of 74 samples and a validation set of 24 samples. Of 15 amino acids, she was able to best predict (% of dry weight) aspartic acid, serine, glutamic acid, and alanine. Threonine and tyrosine could not be predicted. 

Fourier transform near-infrared (FT-NIR) spectrometers produce reflection spectra by moving mirrors. Once plagued by noise, modern FT-NIR spectrometers boast noise levels equivalent to grating-based instruments. FT-NIR spectrometers are full-spectrum instruments. 

Fourier Transform-Near infrared (FT-NiR). Only within the last 20 years has FT-NIR instrumentation (Fig. 4.1.14) become available. Even then, the first commercial instmments had a distinct disadvantage compared to grating-based scanning instruments. FT-NIR spectrometers employ an entirely different method for producing spectra. There is no dispersion involved. Energy patterns set up by an interaction with a sample and a reference and moving mirrors (or other optical components) produce sample and reference interferograms that are used to calculate the absorbance spectrum of the sample. 

Figure 4.1.14. The Bruker Matrix-E, a Fourier transform near-infrared (FT-NIR) spectrometer (Bruker Optics Inc., 19 Fortune Drive, Manning Park, BiUerica, MA 01821-3991) (A) illustrating the noncontact measuring concept and (B) mounted for analyzing sugarcane pulp passing underneath.

Bruker Optics Inc 19 Fortune Drive, Manning Park Billerica, MA 01821-3991 TEL (978) 439-9899 FAX (978) 663-9177 URL optics brukeroptics.com FT-NIR spectrometer Fiber optics... 

The NIR transmission measurements were made in the region of 12,000-4000 em by using a Bruker Vector 22/N FT-NIR spectrometer equipped with a Ge diode detector. A total of 32 scans were collected on each sample with a spectral resolution of 4 cm . The quartz cell was thermostatted at 25°C during the measurements. A total of 120 NIR spectra were acquired for each adulterant. 

Cannes advantage. The intrinsic wavelength scale in an FT-NIR spectrometer provides wavelength repeatability better than one part in a million. The wave number scale of an FT-NIR spectrometer is derived from a HeNe laser that acts as an internal reference for each scan. The wave number of this laser is known very accurately and is very stable. As a result, the wave number calibration of interferometers is much more precise. If a calibration standard such as the NIST standard reference material SRM1920 is used, the wave number calibration is more accurate and has much better long-term stability than the calibration of dispersive instruments. 

Negligible stray light. Because of the way in which the FT-NIR spectrometer modulates each source wavelength, there is no direct equivalent of the stray light effects found in dispersive spectrometers. 

NIR reflectance spectra were collected using a Laser Precision PCM 4000 Fourier transform near-infrared (FT-NIR) spectrometer, equipped with CaF beam splitters and a thermoelectrically cooled PbSe detector. An Axiom difftise/specular reflectance attachment, set at 15" C, was used to collect the reflectance spectrum from each sample coupon. Each sample spectrum was the result of a 5 scan... 

A noncontact FT-NIR spectrometer with several light sources has also been developed. The system has light collection capabilities for the sample placed between 17 and 50 cm from the optical head. Moving samples that may contain large particles, are measured by an infrared beam projected at approximately 25 mm [65]. The instrument window of the analyzer has a built-in wiper to keep the window clean in demanding process environments. 

With an FT-NIR spectrometer and fibre optics the process operator virtually looks into a process stream reactor, vessel or extruder/pelletiser and can determine variations in composition of the liquid, gas or solid with real-time feedback control at various stages in the process. As no sample preparation or dilution is required results are generated in 30 seconds compared to several hours or more required for off-line laboratory analysis. For example, the Perkin-Elmer PIONIR 1024 process NIR analyser determines up to 20 quality parameters (of petroleum refinery products) within 15 sec, with remote signal acquisition possible via fibre optics [175]. 

Atmospheric Vapor Compensation on the Spectrum 100 FT-IR and lOON FT-NIR Spectrometers Technical note from PerkinElmer http //las. perkinelmer.com/content/TechnicalInfo/TCH SpectrumlOOFTIRAtmos-VaporConc.pdf. 

Similar arguments can be made for measurements made in the near-infrared region e.g., from 9000 to 4000 cm . In this case, the best performance is usually in the windows between the atmospheric water bands at about 5300 and 7200 cm . Good 200-cm -wide windows for measuring the optimum performance of FT-NIR spectrometers are 4600 to 4400 and 6200 to 6000 cm , while interferometer drift is best examined in the region above 8000 cm ... 

Two types of FT-Raman spectrometer are commercially available. The first is an instrument that can be operated as a conventional FT-NIR spectrometer and modified to measure FT-Raman spectra the second is a dedicated FT-Raman spectrometer. Examples of the two types of instruments are shown in Figures 18.8 and 18.9. Provided that all the considerations discussed above are accounted for properly, acceptable Raman spectra can be measured on both types of instruments. However, since the dedicated instrument has been designed explicitly for Raman spectrometry, the optical efficiency is usually superior. If an instrument is going to be used primarily for Raman spectrometry, we recommend that a dedicated instrument be purchased. If only a few Raman measurements are expected to be needed each week and NIR transmission and refiection measurements are also needed, it may well be financially advisable to purchase the more versatile instrument. 

Secondly, NIR spectrometers are often used for practical or routine purposes. Spectrometers developed for such purposes should, therefore, ideally be (i) portable, small in size, and light in weight, (ii) capable of use under severe conditions such as at high or low temperatures, in dusty places, or with mechanical disturbances, and (iii) inexpensive. Since FT-NIR spectrometers cannot fiilly satisfy these conditions, various other types of spectrometers have been developed and used. A dispersive spectrometer with a multichannel detector can be used under the conditions with mechanical disturbances because it has no moving mechanism such a spectrometer can be installed, for example, on a rotary blender of powdered dmgs. On the other hand, spectrometers with optical filters and AOTF spectrometers, which are light in weight and inexpensive, are suitable for outdoor in-the-field type measurements. 

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