NIR Spectrometers

Many FT-IR spectrometers have external ports for optical coupling to dedicated accessories. The IR radiation is conveniently directed to/from the external ports by computer-controlled flip mirrors. A large variety of accessories, like an IR microscope, interfaces for gas chromatography (GC/FT-IR), liquid chromatography (HPLC/FT-IR), thin layer chromatography FT-IR (TLC/FT-IR), etc., is commercially available. This type of method combination is usually called a hyphenated technique. FTIR spectrometers can even be supplemented by a FT-Raman accessory. The versatile combination of FT-IR spectrometers with other instruments has substantially contributed to their abundance in most analytical laboratories. 

Absorption of electromagnetic radiation in the NIR region is caused by overtone and combination vibrations. Polyatomic molecules exhibit many overtone and combination vibrations, their spectral bands overlap and make typical NIR bands look very broad and featureless. Nevertheless, NIR spectra contain molecular information about the sample, and this information can be extracted by means of chemo-metric methods (cf Chapter 13). A prerequisite for chemometric evaluations is high quahty of the collected spectral data. Therefore, wavelength precision, resolution, photometric precision and signal-to-noise ratio are important criteria for the selection of an NIR spectrometer. 

Diode array spectrometers (fixed-grating spectrometers) 

AOTF (acousto-optical tuneable filter) spectrometers 

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In Chapter 41, based on reference [2] we derived the following expression for the noise of a transmission measurement, for the case of constant detector noise, as is commonly found in IR and NIR spectrometers ... 

As a general statement, the one thing that all NIR spectrometers, built in the past 25 years, have in common is that they are all single beam. This means that periodic wavelength, noise, and linearity checks must be made. In typical chemical or instrumental analyses (i.e. titrations or HPLC), a standard is run in parallel with an unknown and the result calculated from the response of the two. This is defined as an analysis or assay. 

In a series of controlled experiments, Dosi et al. used the conversion of glucose to lactic acid as a model for the potential of controlling (automatically by computer) the concentrations of the constituents.45 Nice schematics for the reactor setup and connections to the computer/NIR spectrometer/microhltration unit, etc., are shown. Six cases are described, each using a conventional batch process. Transition from batch mode to automatic was triggered by predefined criteria such as degree of substrate conversion or biomass concentration. Control charts and comparisons of NIR data with conventional assays are given for all six cases. 

UV-VIS-NIR diffuse reflectance (DR) spectra were measured using a Perkin-Elmer UV-VIS-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by BaS04. Spectra of sample in 5 mm thick silica cell were recorded in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see Ref. [5], The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(R ,) = (l-R< )2/2Roo, where R is the diffuse reflectance from a semi-infinite layer and F(R00) is proportional to the absorption coefficient. 

Wang, X. Soos, J. Li, Q. 8t Crystal, J., An Acousto-Optical Tunable Filter (AOTF) NIR Spectrometer for On-line Process Control Process Contr. Qual. 1993, 5, 9-16. 

There are many examples of second-order analyzers that are used in analytical chemistry including many hyphenated spectroscopic tools such as FTIR-TGA, IR-microscopy, as well as GC-MS, or even two-dimensional spectroscopic techniques. Another hyphenated technique that is being developed for the study of solid-state transitions in crystalline materials is dynamic vapor sorption coupled with NIR spectroscopy (DVS-NIR).26 DVS is a water sorption balance by which the weight of a sample is carefully monitored during exposure to defined temperature and humidity. It can be used to study the stability of materials, and in this case has been used to induce solid-state transitions in anhydrous theophylline. By interfacing an NIR spectrometer with a fiber-optic probe to the DVS, the transitions of the theophylline can be monitored spectroscopically. The DVS-NIR has proven to be a useful tool in the study of the solid-state transitions of theophylline. It has been used to identify a transition that exists in the conversion of the anhydrous form to the hydrate during the course of water sorption. 

Figure 2.1 shows representative second-derivative normalized spectra from the sample compared with pure component spectra of acetaminophen, aspirin and caffeine. The single-pixel spectra are from 9.7 x 9.7 jxm areas on the surface of the tablet, each with signal to noise greater than 5000 1, which is comparable to a standard NIR spectrometer. This comparison between pure component spectra and single-pixel spectra ensures that the assignment of the component pixel spectra from the tablet is accurate. The spectra of these three components show spectral... 

R 18] [A 13] This bus concept allows the integration of standardized flow cells and inline sensors as described above. Currently, the integration of a commercial flow cell coupled to an inline NIR spectrometer is under development. 

Greensill, C.V., Wolfs, P.J., Spiegelman, C.H., and Walsh, K.B., Calibration transfer between PDA-based NIR spectrometers in the NIR assessment of melon soluble solids content, Appl. Spectrosc., 55, 647-653, 2001. 

At-line sampling may involve a flow-through cell in the NIR spectrometer in one process a glass-lined steel reaction vessel was used in combination with a fibre optic probe for measurements in a full scale chemical plant reactor [57]. Fibre optic bundles can be used to transmit NIR radiation to the reaction matrix and take signal back to the spectrometer. NIR is notoriously sensitive to changes in temperature and methods for keeping the temperature constant must be incorporated into the instrumentation. 

In UV-vis-NIR spectrometers, the monochromator and detector are switched simultaneously. Step-like artifacts can be generated at this switch, and it is then questionable which part of the spectrum represents the correct absolute intensity. By nature, NIR detectors are susceptible to thermal radiation, and the step at the change-over to or from the NIR range and also the noise in the NIR range increase with temperature (Melsheimer et al., 2003). Sometimes authors present the UV-vis and NIR sections of the spectrum separately, disguising step-like artifacts at the transition. 

The NIR spectrometer used for method development and sample analysis was a Foss NIR Systems Model 6500 Forage Analyzer with a sample transport module and a standard reflectance detector array. The transport module moves the sample compartment up and down during data collection, thereby allowing a more representative spectrum to be obtained from bulky heterogeneous samples. The reflectance array uses two silicon detectors to monitor visible light from 400-850 nm and four lead-sulfide detectors to monitor NIR light from 850-2500 nm. Natural product sample compartment cells in 1/4-cup and 1-cup sizes were used as sample holders in the transport module. This instrument has a maximum resolution of 2 nm. 

A key step in the application of rapid analysis methods is the collection of high quality NIR spectra. To minimize the effect of water in the biomass spectra, each sample was air-dried to less than 10 percent moisture prior to NIR spectroscopic analysis. Spectroscopic techniques were used that enable a high quality, reproducible, and representative NIR reflectance spectrum to be obtained. For each sample, a total of 35 spectra were collected and averaged to compensate for sample heterogeneity. Each calibration sample was subsampled three times. Sample cells were emptied and repacked between subsamples. Instrument reproducibility tests demonstrated that the reproducibility limits of the NIR spectrometer contributed less than 0.1 percent to the absolute prediction errors in the rapid analysis method. 

The first coupling of a LINAC with infrared spectroscopy has been performed by Palmese et al. in order to study in situ kinetics of radiation-induced cationic polymerization of epoxy systems. The aim of the study is to understand the curing behavior of polymers under irradiation. A UV light source and an electron beam (10 MeV pulse width of the beam from 2.5 to 10 pm) are coupled to a portable near infrared (NIR) instrument. Briefly, a portable NIR spectrometer (Control Development Incorporated, South Bend, IN, USA) is used,... 

Gravimetric or manometric techniques have been used to establish adsorption data of gases on zeolites. Both techniques present problems, manometric equipment has an accumulation of the error and data obtained by the gravimetric method are influenced by effects associated with flow patterns, bypassing, and buoyancy. In the mixture s adsorption behaviour, isomers mixtures have the highest degree of difficulty to study. Isomers can not be differentiated in standard commercial adsorption equipment. This problem has been solved in this study by coupling a manometric apparatus with an NIR spectrometer, which allows us to measure the gas phase composition (in time, if necessaiy). In this paper we report this new approach to study the adsorption of mixtures of butane and iso-butane. 

In a combined simulation and experimental project we set out to assess the adsorption properties of a series of zeolites. In the present work the adsorption properties of n-butane and iso-butane on MFI are being studied. The experimental part consists in the validation of the molecular simulation model, by confirming its results. The experiments were performed in a constructed in-house manometric apparatus coupled with a NIR spectrometer (Perkin Elmer, FT-IR system, GX Spectrum). Figure 1 is a scheme of the experimental set-up. 

Figure 1. Scheme of the manometric apparatus coupled with the NIR spectrometer. 

Using the presents used manometric set-up coupled with a NIR spectrometer produced results that are in agreement with literature data, and in agreement with simulations ( simulations details are provided on ref [4]). 

We conclude that with a manometric set-up coupled to a NIR spectrometer it turns out to be possible to measure equilibrium and kinetic data for single components and mixtures. 

Fig. 12.23 (a) Photograph showing light emission of carbon black upon 785-nm laser excitation in an argon atmosphere. The corresponding emission spectrum was recorded using external UV-VIS-NIR spectrometer. The lines represent the calculated blackbody emission curves of 50-nm carbon black particles at different temperatures... 

The spectral composition of the emitted light was recorded using an external UV-NIR spectrometer. The temperature is determined by comparing the corrected experimental data [59, 116] with the calculated emission spectrum [59, 117, 118]. The blackbody spectrum of the nanocrystalline graphite sample was calculated for three different temperatures. Based on the maxima of the emission curve, the local sample temperature was 2,600°C (Fig. 12.23b). 

Absorption spectra were measured on spin-cast films using a Perkin-Elmer Lambda 19 UV/VIS/NIR spectrometer. Photoluminescence (PL) and EL were... 

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