NIR on-line

NIR on-line samplers were first developed to measure the protein content of flour. This is still the most popular application and provides an excellent example of an NIR feedback control system. Dried gluten is commonly used, particularly in Europe, to replace wholly or partly the protein in flour that would otherwise be derived from high-protein wheat in the grist. The success of on-line NIR for monitoring flour protein content therefore led to its incorporation into a closed-loop control system... 

F. Gradenecker. NIR on-line testing in grain milling. Cereal Foods World 48(1) 18-19, 2003. 

FIGURE 27.4 Schematic of a typical powder mix line. NIR on-line analyzers can be used to monitca- addition... 

Sohd-state multi-element detector arrays in the focal planes of simple grating monochromators can simultaneously monitor several absorption features. These devices were first used for uv—vis spectroscopy. Infrared coverage is limited (see Table 3), but research continues to extend the response to longer wavelengths. Less expensive nir array detectors have been appHed to on-line process instmmentation (125) (see Photodetectors). 

Examples of nir analysis are polymer identification (126,127), pharmaceutical manufacturing (128), gasoline analysis (129,130), and on-line refinery process chemistry (131). Nir fiber optics have been used as immersion probes for monitoring pollutants in drainage waters by attenuated total internal reflectance (132). The usefulness of nir for aqueous systems has led to important biological and medical appHcations (133). 

Sivakesava et al. also used Raman (as well as FT-IR and NIR) to perform a simultaneous on-line determination of biomass, glucose, and lactic acid in lactic acid fermentation by Lactobacillus casei.2 Partial least squares (PLS) and principal components regression (PCR) equations were generated after suitable wavelength regions were determined. The best standard errors were found to be glucose, 2.5 g/1 lactic acid, 0.7 g/1 and optical cell density, 0.23. Best numbers were found for FT-IR with NIR and Raman being somewhat less accurate (in this experiment). 

Hagman and Sivertsson discussed the work performed at Pharmacia and Upjohn52 on monitoring and controlling bioprocesses. They followed the protein production-derived form Chinese hamster cells (CHO-cells) in a 500-1 reactor over a 3-month period. The diagrams of the flow cell and pumping/NIR system are displayed, and the logic behind the work outlined. External and on-line calibrations were performed. 

Standardization The instrument response function can vary from analyzer to analyzer. If calibration transfer is to be achieved across all instrument platforms it is important that the instrument function is characterized, and preferably standardized [31]. Also, at times it is necessary to perform a local calibration while the analyzer is still on-line. In order to handle this, it is beneficial to consider an on-board calibration/standardization, integrated into the sample conditioning system. Most commercial NIR analyzers require some form of standardization and calibration transfer. Similarly, modem FTIR systems include some form of instrument standardization, usually based on an internal calibrant. This attribute is becoming an important feature for regulatory controlled analyses, where a proper audit trail has to be established, including instrument calibration. 

In some manufacturing process analysis applications the analyte requires sample preparation (dilution, derivatization, etc.) to afford a suitable analytical method. Derivatization, emission enhancement, and other extrinsic fluorescent approaches described previously are examples of such methods. On-line methods, in particular those requiring chemical reaction, are often reserved for unique cases where other PAT techniques (e.g., UV-vis, NIR, etc.) are insufficient (e.g., very low concentrations) and real-time process control is imperative. That is, there are several complexities to address with these types of on-line solutions to realize a robust process analysis method such as post reaction cleanup, filtering of reaction byproducts, etc. Nevertheless, real-time sample preparation is achieved via an on-line sample conditioning system. These systems can also address harsh process stream conditions (flow, pressure, temperature, etc.) that are either not appropriate for the desired measurement accuracy or precision or the mechanical limitations of the inline insertion probe or flow cell. This section summarizes some of the common LIF monitoring applications across various sectors. 

On-line MRS contributed to process control by quantitatively predicting the moisture content. MRS also provided process understanding within this study where the authors note that PCA was able to discriminate amongst various hydration states. The target end point of 5-8% w/w HjO was chosen because the trihydrate was the desired form. Using on-line NIRS to monitor this drying process was ideal since end point was determined in real-time prior to the formation of an undesired lower hydrate form. ... 

Figure 13.1 Schematic of the media milling apparatus and the on-line NIR implementation for particle size analysis. Reprinted with permission from Higgins et al. (2003). Copyright 2003 American Chemical Society.

In 2005, Rantanen et al. used on-line NIRS to monitor a laboratory-scale high shear wet granulation process. The study was a central-composite circumscribed design consisting of 17 runs and 3 variables ... 

Given that the introduction of solvent obviously produces wet granules, the wet granulation process includes a drying step. Drying typically occurs in a fluid bed dryer. But there are other options, such as microwave vacuum dryers. In 1994, White applied on-line NIRS to monitor moisture content and predict drying end point in two TK Fielder microwave dryers. The NIR spectral data were correlated to off-line Karl Fischer measurements which resulted in a standard error of prediction equal to 0.6% when the samples... 

Given that NIRS is a rapid, nondestructive technique and that most pharmaceutical blend constituents absorb in the NIR, it has been applied off-line to assess blend homogeneity. In the mid-1990s scientists at Pfizer utilized on-line NIRS to successfully characterize powder blending. ... 

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