Capabilities and limitations of NIR

Before selecting a process analytical technology to implement, it is helpful to understand the capabilities and limitations of the technology. Good introductions to NIR spectroscopy and instrumentation can be found in Chapter 5, and to chemometric methods in Chapter 12. NIR pharmaceutical applications are covered in Chapters 13 and 14. Additional information on NIR techniques and applications can be found in Williams,  

In general, NIR can be used to monitor compounds that contain C-H, 0-H, or N-H bonds. NIR is generally used to monitor bulk composition (components at levels of 0.1% and higher), rather than trace-level composition. However, NIR is very sensitive to water, alcohols, and amines, and under the right circum- 

Note The focus of this book is on multivariate analyzers, but it is worth keeping in mind that there are still a lot of process analyzer applications in the chemical industry that are 

Because of the large investment required to implement NIR process analyzers, there has to be a very large business value to be gained through the analyzer to make the effort worthwhile. This is why it is critical to assess the business value of a potential analyzer at the very beginning of a project. Failure to honestly assess the value (both benefits and 

Rapid (near real time) Indirect (secondary method)  

Multi-component capability Multi-component capability ( interferences) 

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Step 3 Feasibility assessment Detemiine (via literature search, consultation, etc.) suitable candidate methods and assess the feasibility of these methods. The purpose of this step is to collect enough information to accurately define (or scope ) and prioritize the project. The amount of information required may vary considerably, depending on the magnitude of the proposed project. Significantly less information is required to develop a new method for an existing off-line analyzer than is required to justify spending several million dollars on a major installation of on-line analyzers. (The capabilities and limitations of NIR process analyzers are discussed in Section 15.2.4.)... 

Molecular probe dyes for the determination of potassium, lithium, and sodium have been identified. Additionally, an NIR probe selective for potassium has been fabricated. The detection limits of this probe are in the ppm range. Lower detection limits may be achieved by varying the matrix which allows the entrapment of ions. Preliminary data for the detection of lead and cadmium demonstrate the potential capability of these probes for environmental applications. The development of OFMP for the detection of other ions of environmental interest such as Be2+, Hg2+, As3+, and Ni2+ is currently underway. 

Clearweld is a further enhancement of the transmission laser welding process that provides designers with a tool to laser weld clear, coloured or opaque thermoplastics. Clearweld uses materials capable of strong absorption in the NIR spectrum which impart minimal colour. It can be used in number of ways, including, but not limited to, a coating at the joint line, inclusion within the substrate or application as a consumable film. Compared with previous technologies, it offers complete freedom in the choice of colour, reflectivity and transparency of the work pieces at visible wavelengths. 

Kramer etfl/., 1989 Wvalle eta/., 1989). Nitrite reductase mRNA was present in small amounts in uninduced leaves of both species but not in roots of maize. Induction by nitrate increased the NiR mRNA in both species and in both the roots and leaves of maize. Perhaps one of the two NiR genes present in maize is expressed constitutively in leaves (Kramer et al., 1989). The NiR mRNA induction pattern in maize (Kramer et al., 1989) is similar to the NR mRNA induction pattern in barley (Melzer et al., 1989). Nitrite reductase mRNA is rapidly induced by nitrate, reaching a peak within 5 hr and then declining to a lower level even in the presence of continued nitrate. The NiR mRNA appears to be short lived with a half-life as short as 30 min (Kramer et al, 1989). The mechanisms regulating these fluctuations in NiR mRNA are not known at this time. Nitrite was capable of limited induction of NiR mRNA in spinach leaves (Back etal, 1988), although the role of nitrite is equivocal due to Ae possibility of in vivo oxidation of nitrite to nitrate (Aslam et al, 1987). 

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. 

FFS capable of producing a limited number of bands cannot do all things for all people. Yet, because the NIR absorptions of food and food products are broad and... 

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