FT-NIR instrument

In the late 1980s and early 1990s, interferometer-based instruments were introduced by companies already producing IR equipment Nicolet, Bomem, Perkin-Elmer, and others. Often, these FT-NIR instruments were merely adapted FT-IRs that were already in existence and were not suited for pharmaceutical samples pure raw materials, blends and granulations, tablets and capsules, and larger plastic containers. 

The popularity of interferometers in mid-range infrared has carried over to NIR. The FT-NIR instruments are becoming quite common in the field. Speed and high resolution spectra are the strengths of FT instrumentation. Since most spec-troscopists are familiar with FT-IR instrumentation, it is logical that they would lean toward an instrument that seems familiar. 

A NIR-laced article by Heise et al. discusses the technologies used in noninvasive glucose monitoring [167]. In this article, he describes his own work with a FT-NIR instrument, but also provides a nice overview of other NIR applications plus luminescence, optical activity, and Raman spectroscopy. He gives 38 references on these topics. 

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. 

The potential of interferograms in analytical chemistry [17] will increase in popularity in the years to come. No-moving-parts FT-NIR instruments will make this a much needed exploitation. 

The reader should bear in mind that, since this study was done, several vendors have introduced FT/NIR instruments. If the initial effort had been done with one of these instruments, results would have been comparable, albeit likely very faster. 

Several parameters affect the NIR results, for example, are the seeds whole or ground Grinding samples make a nonhomogenous sample more homogenous, which often improves the repeatability and reproducibility of the results. Particle size can affect NIR results, so the shape and distribution of the ground samples are important. This is particularly important for filter instruments in monochromator and FT-NIR instruments, scatter corrections and derivatives can be used to correct for particle size differences. 

We have seen that one of the key aspects of Fourier transform NIR analyzers is their control of frequency accuracy and long-term reproducibility of instrument line shape through the use of an internal optical reference, normally provided as a HeNe gas laser. Such lasers are reasonably compact, and have acceptable lifetimes of around 3 to 4 years before requiring replacement. However, we also saw how the reduction in overall interferometer dimensions can be one of the main drivers towards achieving the improved mechanical and thermal stability which allows FT-NIR devices to be deployed routinely and reliably in process applications. 

Several new methods and instruments based on infrared spectroscopy are being developed for food applications. Advances in spectroscopic instruments and data analysis have enabled the rapid and nondestructive analysis of cheese parameters in just a few seconds (e.g., Nicolet Antaris FT-NIR by Thermo Electron Corp.). Another recent development is the miniaturization of FTIR instrumentation, which would enable onsite analysis, while the cheese is being produced. The TruDefender FT handheld FTIR by Ahura Scientific, Inc. (Fig. 5.7) is a portable handheld spectrometer that could be applied to food analysis. With numerous developments in FTIR spectroscopy and several potential food analysis applications still unexplored, there is great research potential in this technique that could benefit the industry and research institutions. 

In general, AOTF-based instruments are rugged and fast, capable of numerous readings per second. As long as there is no radio frequency interference, the wavelength accuracy and reproducibility is excellent. While individual instruments vary, the nominal spectral resolution is around 10 nm. This is quite sufficient for process purposes, but if greater resolution is needed, a FT-NIR is required. 

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). 

In 1986, a Raman instrument based on NIR excitation (1064 nm) and a Michaelson interferometer became available [16]. This development revolutionized Raman spectroscopy. In addition to the advantages of throughput and multiplex inherent to Fourier Transform (FT) techniques, this instrument overcame the obstacle of fluorescence. Fluorescence was eliminated by excitation at a NIR wavelength where electronic transitions in most samples are absent. Availability of such NIR FT-Raman instruments was particularly useful in the studies of lignin. 

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. 

Each change - even an improvement - in the system does have some up-front costs. Since the economics are unfavorable in the short term, changes are avoided at almost any price, even if they show the potential to maximize the return on investment in the future. A method, once it has been established, has a long life - independently of its analytical quality. Routine HPLC will face intense competition from spectroscopic methods such as quantitative AAS, UV and especially (FT-)NIRS as well as titrimetry and specific instant test procedures. The importance of HPLC as a routine analysis method will be maintained if applications of specific, simple, almost maintenance-free, self-controlled and inexpensive instruments are available and the probability of user errors decreases to a minimum. 

Particularly, food sector showed interest towards NIR and Vis/NIR instruments, both mobile and on-line. Devices based on diode array spectrophotometers and FT-NIR desk systems proved to be the best for this sector. 

With the introduction of Fourier-Transform (FT) Raman instruments (19,20), near-infirared (NIR) Raman spectroscopy has become an excellent technique for eliminating sample fluorescence and photochemistry in Raman measurements. Recently, Ae range of NIR Raman techniques was extended to include NIR SERS (6,21). Most SERS studies to date have been performed using visible excitation sources such as Ar-ion lasers the demonstration of NIR SERS offers the possibility of using solid-state Nd YAG and diode lasers. 

Raman spectroscopy may be performed on either a dispersive instrument or a FT instrument. All of the spectra provided in this chapter were obtained from a FT-Raman instrument (see Fig. 62), featuring a NdiYAG solid-state laser, and a InGaAs (indium-gallium arsenide) detector, combined with a silicon on quartz beam splitter. Note that in the FT-Raman experiment, the sample effectively becomes the source to the FT spectrometer. Dispersive Raman instruments are also popular, and these usually feature a silicon-based array detector (CCD array) in combination with either a visible laser (doubled YAG or HeNe) or a short-wavelength solid-state NIR laser. 

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. 

Analect Instruments Applied Instrument Technologies 2771 North Garey Avenne Pomona, CA 91767 TEL 909-593-3581 Toll Free 800-326-2328 FAX 909-392-3207 Email AIT hs.utc.com URL www.orbital-ait.com FT-NIR... 

Thermo Nicolet Process Instruments 2555 N. Interstate 35 Round Rock, TX United States Toll Free 877-843-7668 TEL 561-688-8700 URL http //www.thermo.com/ FT-NIR... 

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