NIR instrumentation

Prior to World War II, the near-infrared (NIR) region was not considered to be particularly useful for organic compositional spectroscopy. The explanation for this line of reasoning seemed obvious. The NIR region consisted only of overtone and combination bands of fundamental molecular vibrations. It was also obvious that all these combination and overtone bands occur in a relatively narrow region (750-3,000 nm) as compared to the fundamental bands occurring at 2,800-50,000 nm. Thus it was observed that NIR bands are severely overlapped, difficult to resolve, and, once resolved, difficult to interpret. If the same information is obtained with better resolution in the infrared (IR) region, then why would any chemist be interested in the difficulties inherent with NIR spectroscopy  

In the late 1960s, work led by Karl Norris, considered by many to be the father of NIR, demonstrated the potential value of this region for quantitative work by making measurements of agricultural products. The U.S. Department of Agriculture, the then-employer of Mr. Norris, made 

G Fixed grating (Hadamard mask exit sht with fixed grating) 

MEMS (micro-electro-mechanical systems) timable optical filter (MEMS-NIR) 

Ultra-fast 0.01—0.1 s for full range (single scan) 

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The first NIR instruments were, in reality, developed for the UV and Vis regions of the spectrum. They were made by seven companies Beckman, Cary (now owned by Varian), Coleman, Perkin-Elmer, Shimadzu, Unicam, and Zeiss. Based on the work of Karl Norris and coworkers in the USDA, the Illinois Department of Agriculture solicited bids from companies to produce a pure NIR instrument, capable of measuring protein, oil, and moisture in soybeans. 

Why would you guess that NIR instruments are so varied when compared with other spectroscopic methods ... 

J.J. Workman and D.A. Burns, Commercial NIR Instrumentation, in Handbook of Near-Infrared Analysis, 2nd edn, D.A. Burns and E.W. Ciurczak (eds), Marcel Dekker, New York, 2001. 

Filter-based instruments are often limited to applications where there is simple chemistry, and where the analytes can be differentiated clearly from other species or components that are present. Today, we may consider snch analyzers more as sensors or even meters, and the analytical instrument community does typically not view them as trne instraments. Since the late 1980s a new focns on instrumentation has emerged based on the use of advanced measnrement technologies, and as such is considered to be more of the con-seqnence of an evolution from laboratory instruments. Some of the first work on full-spectrum analyzers started with an initial interest in NIR instruments. The natnre of the spectral information obtained in the NIR spectral region is snch that an analyzer capable of measnring multiple wavelengths or preferably a fnll spectrnm is normally reqnired. 

The use of hber optics and hber-optic multiplexing can increase the number of analysis points, and hence can reduce the overall costs related to a single analyzer. This approach has been used successfully with NIR instrumentation, where typically up to eight points can be handled. As noted earlier, the use of hber optics with IR Fourier transform instruments has in the past been limited. New hber materials with improved optical throughput are available, and also with the considered use of IR lasers, the role of hbers for IR applications is expected to increase. Although in the past commercial multiplexers have been available for mid-lR hber systems, their use has not been widespread. 

Acquisition times commonly vary from seconds to minutes, often with negligible time between acquisitions, even when measuring multiple locations simultaneously (multiplexing). The dedication of different areas on the charge coupled device (CCD) detector to each measurement point makes this possible. The detectors used for MIR and NIR instruments cannot be multiplexed in the same fashion and must measure multiple samples sequentially. 

Ensuring that the instrument to be used performs correctly is the first step in developing an instrumental methodology. The European Pharmacopoeia and the guidelines of the Pharmaceutical Analytical Sciences Group (PASG) recommend that NIR instruments be qualified as per the manufacturer s instructions, which should include at least the following ... 

There are three types of data error random error in the reference laboratory values, random error in the optical data, and systematic error in the relationship between the two. The proper approach to data error depends on whether the affected variables are reference values or spectroscopic data. Calibrations are usually performed empirically and are problem specific. In this situation, the question of data error becomes an important issue. However, it is difficult to decide if the spectroscopic error is greater than the reference laboratory method error, or vice versa. The noise of current NIR instrumentation is usually lower than almost anything else in the calibration. The total error of spectroscopic data includes... 

As already mentioned, any multivariate analysis should include some validation, that is, formal testing, to extrapolate the model to new but similar data. This requires two separate steps in the computation of each model component calibration, which consists of finding the new components, and validation, which checks how well the computed components describe the new data. Each of these two steps needs its own set of samples calibration samples or training samples, and validation samples or test samples. Computation of spectroscopic data PCs is based solely on optic data. There is no explicit or formal relationship between PCs and the composition of the samples in the sets from which the spectra were measured. In addition, PCs are considered superior to the original spectral data produced directly by the NIR instrument. Since the first few PCs are stripped of noise, they represent the real variation of the spectra, presumably caused by physical or chemical phenomena. For these reasons PCs are considered as latent variables as opposed to the direct variables actually measured. 

Absorbance of peptide bonds at 2170 nm is usually used as a key band for the calibration of protein content measurement. However, the intensity of the absorption depends on the structure and conformation of the protein. Yamashita et al [8] investigated the change in absorption at 2170 nm caused by the conformational change of protein using bovine serum albumin (BSA) as the model protein. A mixture of dithiothreitol (a reducing agent) and BSA (5%) was taken in a quartz cell attached to the NIR instrument, and then spectra were recorded at 10-minute intervals. Dithiothreitol reduced the disulfide bond in BSA. The... 

The rice taste analyzer, developed first by Satake Engineering Co., Ltd., consists of NIR instrument provided by Bran Luebbe Company. This analyzer is based on the experimental result that rice taste is fixed by the balance of moisture, protein, amylose, and fatty acid. From a practical stand point, milled rice is ground, and the ground sample is kept at a constant temperature oven for more than one hour, after which the NIR measurement is performed to determine the amount of different constituents. From these constituents, taste scores can be calculated using taste-related equation which relates the constituents to taste score. A taste score can be generated in only a few minutes by the NIR instrument. The rice taste analyzer includes the software that calculates the blending ratio to perform lowest price at the same taste, or to perform best taste at the same price. 

The largest application segment for filter photometers is in the area of combustion gases analysis, primarily for CO, CO2, hydrocarbons, SO2, etc. Other major areas of application include the petrochemical industry, with natural gas and other hydrocarbon process gas streams being important applications. As measurements become more complex, there is the need for more advanced instrumentation. Variable or tunable filter solutions (as described above) or full-spectrum FTIR or NIR instruments are normally considered for these applications, primarily in terms of overall versatility. Now that array-based systems are becoming available, there is a potential for an intermediate, less expensive, and more compact solution. Note that compact instrumentation tends to be environmentally more stable, and is well suited for industrial applications. 

In the interest of patient safety, there exists a need for final identification testing of products as they are packaged for delivery to the patient. Herkert etal.lls evaluated a reflectance NIR instrument and found it to be capable of on-line identity check of tablets at-line speeds of 12 000 tablets a minute. 

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,... 

With these differences from the mid-range IR, standard IR and UV/Vis/NIR instruments could not... 

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. 

In a second chapter, Monfre and DeThomas wrote about the calibration of a NIR instrument for the analysis of a prescription vasodilator. Individual tablets were used, but still needed to be crushed for analysis. The important part of the calibration was that no synthetic tablets were produced to extend the range. Using only production tablets ranging from 96%i to 102 /o of label claim, the NIR results were within 0.5 mg of the HPLC values. 

NIRS instruments are typically designed as either transmittance or reflectance, with some allowing the... 

The validation process determines the amount of error owing to variation among the values in the population. It is used to check for the existence of a relationship between the calibration set and the validation set. Manufacturers of NIRS instrumentation include software packages that allow the operator to predict analytical results on data files that have been stored, thus allowing for validation of the calibration equation and testing for errors in the developed calibration. This enables calibration equation performance testing in terms of precision. The validity of these models depends on the ability of the calibration set to accurately represent the samples in the prediction set. 

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