Near-Infrared Spectroscopy

This chapter describes the introduction to NIR spectroscopy. The discovery of infrared region is mentioned first, and then, the definition of NIR region and characteristics of NIR spectroscopy are explained. Finally, the brief history of NIR spectroscopy is outlined.

This chapter describes the principles and characteristics of NIR spectroscopy. It is divided into two subchapters, 2–1. Characteristics and advantages of NIR spectroscopy: In this subchapter some emphasis is put on the versatility of NIR spectroscopy. Some examples of NIR spectra are explained 2–2. Principles of NIR spectroscopy based on quantum mechanics: To understand principles of NIR spectroscopy, principles of IR spectroscopy are described using quantum mechanics first, and then detailed explanation about molecular vibrations-fundamentals, overtones and combinations is given. Anharmonicity is mentioned briefly.

When light interacts with a single particle, there are three possible outcomes: absorption, scattering, or transmission. In spectroscopy, one measures the remission from and/or transmission through a macroscopic sample. Such a sample might contain countless locations at which there is a change in refractive index, each of which gives rise to scattered light. This fact poses a challenge in building theoretical models applicable to spectroscopy: even if our theoretical understanding of single interactions is very good, the number of individual interactions is typically too big to make accounting for all of them realistic. This chapter presents an overview of modeling strategies that can be of use in near infrared spectroscopy. Recognizing that no one approach is uniformly applicable, care is taken to call attention to assumptions made in each modeling approach and limitations that are imposed by these assumptions.

This chapter is concerned with the introduction to spectral analysis in the NIR spectroscopy. It consists of two major parts, conventional spectral analysis and spectra pretreatments. In the former, various conventional spectral analysis methods such as group frequency analysis, derivative spectra, difference spectra, spectral analysis based on perturbation, comparison of a NIR spectrum with the corresponding IR spectrum, and isotope exchange experiments are explained. In the latter part smoothing, derivative methods, multiplicative scatter correction (MSC), standard normal variate (SNV), centering methods, and normalization are described.

In this chapter, the quantum mechanical basis for computational studies of near-infrared spectra (NIR) is discussed. Since this topic is rarely covered in detail in the literature, the necessary prerequisites are provided as well, which include (i) the coordinate frame for the description of molecular vibrations, (ii) methods for the determination of the vibrational potential, (iii) the principles of the harmonic approximation, and (iv) its role as the foundation for methods taking anharmonic effects into account. The details of various anharmonic approaches in quantum vibrational spectroscopy are discussed, including methods based on the vibrational self-consistent field (VSCF) approach, vibrational perturbation theory (VPT) as well as one- and multidimensional grid-based methods. The merits and pitfalls of these approaches are critically assessed from the perspective of applications in NIR spectroscopy. Selected examples from recent literature are included to demonstrate how these methods can be applied to solve practical problems in spectroscopy. The aim of this chapter is to provide a comprehensive presentation of the topic aimed at a spectroscopic audience, while remaining accessible and focused on the key details. Although primarily intended for readers interested in NIR spectroscopy, the essential information provided in this chapter represents a fundamental perspective on quantum vibrational absorption spectroscopy and is useful for a more general readership as well.

Two-dimensional (2D) correlation spectroscopy is a well-established method for analysis of perturbation-induced spectral changes in various kinds of data. Due to selective correlation of the peaks and resolution enhancement, it provides useful information on the dynamics of spectral changes and enables more reliable band assignments. The generalized 2D correlation approach permits to apply various kinds of perturbations and makes possible for correlation between data obtained from different experiments (hetero-correlation). At the beginning of this chapter are shown the basic principles of 2D correlation spectroscopy together with the rules for interpretation of the synchronous and asynchronous spectra. Next, we report new developments in this method like sample–sample correlation spectroscopy and perturbation–correlation moving-window 2D correlation spectroscopy. Finally, are shown selected examples of successful applications of 2D correlation spectroscopy for study of interactions and molecular structure.

This chapter is a primer on the use of multivariate data analysis—or chemometrics—to near-infrared spectra. The extraordinary synergy between near-infrared spectroscopy and the data analysis methods called chemometrics has led to a green analytical revolution in practically all areas of life sciences and related industries for quality control and process monitoring. The near-infrared spectroscopy method is nondestructive, rapid and environmentally friendly. However, the most unique advantage of near-infrared spectroscopy is that it can measure samples remotely and unbiased, as is, i.e., solids and liquids without interfering with the sample or sample preparation. The success of near-infrared spectroscopy would not have been possible without the chemometric data processing. This chapter gives an overview, including tricks of the trade, of the most common chemometric techniques for analysis of near-infrared spectral ensembles illustrated by downloadable data examples.

The emergence of handheld spectrometers in the past decade marked a significant turning point in the evolution of the practical applications of near-infrared (NIR) spectroscopy. Miniaturized sensors enabled a new and previously unattainable spectrum of applications of NIR spectroscopy. Nonetheless, several issues connected with the use of miniaturized spectrometers have become apparent. In contrast to a matured design of a FT-NIR benchtop spectrometer, the handheld devices are much less uniform and incorporate various novel technologies. These compact technologies result in different performance of miniaturized spectrometers, with narrower spectral regions or lower resolution over which they operate. For this reason, current research focus is on thorough systematic evaluation of the applicability limits and analytical performance of these devices in a variety of applications. This chapter aims to present a comprehensive information on the principles of the technology and application potential of miniaturized NIR spectrometers.

What type of components does a NIR spectrometer consist of? How do these parts determine the performance of the instruments? The measurement targets of NIR spectroscopy span a wide variety from transparent liquids to opaque solid samples, and as described in Chap. 8, the NIR spectrometers are characterized by a wide variety of device specifications and shapes. Consequently, what are the criteria for choosing a spectrometer? In the first half of this chapter (9.1), the basics of the optics that comprise the NIR spectrometer, such as the light source, spectroscopic element, and detector, are explained. This will allow the reader to understand the specifications, that control the functions of the spectrometer. Next, in the latter half of this chapter (9.2), the measurement method is explained for each sample form, namely liquid, solid, and paste. The most characteristic feature of NIR spectroscopy is the use of diffuse reflected light, and the “interactance” method, which is a unique application. It can be inferred that diffuse reflectance method contributes to the expansion of the range of sample forms that are measurable by NIR spectroscopy.

The hardware of near-infrared (NIR) spectroscopy is almost the same as UV-VIS and infrared spectrometer except the wavelength area. However, the high SN ratio and stability of the instruments are required for a quantitative analysis by NIR spectroscopy, because of the smooth and dull absorption peaks of the NIR spectral shapes. These are realized by the hardware and computer technologies and are special features of the hardware of NIR spectroscopy. It is important to understand what they are when you use or design a near-infrared spectrometer. These aspects of the technologies are described. Instrumental difference also is an important problem in NIR spectroscopy where a calibration is used to predict contents of the matter. In this Chapter, not only the method to avoid the instrumental difference, but also the sources of the instrumental are described. To decrease the instrumental difference, it is crucial to understand why and how the instrumental difference is generated. The information described in this chapter will help you design a new NIR instrument, and a designing process with less effort is also described.

This chapter summarizes the principle and application of time-of-flight (TOF) NIR spectroscopy, which can evaluate the contribution of scattering and absorption of light in samples simultaneously. In order to construct robust calibrations for organic materials by NIR spectroscopy, it is important to evaluate and understand the spectral contribution from light absorption (absorption resulting from harmonics or overtones of the fundamental absorptions of molecular vibrations) and light scattering (mainly due to the cellular structure). In this chapter, we introduce the principle of TOF-NIR spectroscopy and some applications to agricultural, medical area, and forest products.

A general framework for method development based on the analytical quality by design process is presented and applied to the development of near-infrared spectroscopic methods. The framework is particularly well suited to secure stakeholder alignment, setting appropriate expectations and ensuring that resources are spent appropriately. After setting method goals and expectations and confirming feasibility, a risk assessment is performed to identify all the factors that could affect the method. The method is then developed with the intention to mitigate the impact of those risks. The result is a robust method that can be tested and validated if required by the regulatory environment of use. Aspects of method lifecycle are also discussed as method development is only a part of the process of successfully using near-infrared spectroscopic methods in routine commercial applications. Aspects of interface to the process, sample set selection, model optimization, system suitability, and performance monitoring are discussed in the context of building robust methods. The analytical quality by design framework can significantly streamline method development and lifecycle management efforts to ensure a successful deployment and long-term value generation from a NIR spectroscopic method. Continuous improvement ensures method performance over the useful life of the method.

Near-infrared (NIR) spectroscopy is a powerful tool in studies of physicochemical properties of various kinds of samples. In particular, NIR spectroscopy contributed considerable to our understanding of intermolecular interactions (e.g. hydrogen bonding), molecular structure, solvent effect, clustering, phase transitions, kinetics. Because of mechanical and electrical anharmonicity of molecular vibrations, NIR spectra provide unique information not available from the other spectral regions. On the other hand, to elucidate useful information from NIR spectra, more sophisticated methods of data analysis than those applied in mid-infrared (mid–IR, MIR) region are necessary. This chapter presents selected examples demonstrating the variety of problems in the field of physical chemistry that have been studied by NIR spectroscopy.

NIR has been used for decades as an innovative technique in agriculture. There are many benefits, and today, researchers active in agronomy science are not the only ones using NIR extensively in their daily research but also breeders, farmers and agri-processors, using it as an efficient tool for the assessment of a large number of parameters and criteria including detection of contaminants. Undoubtedly, NIRS has demonstrated clear advantages in the analysis of soil, crops, forages, silages and faeces, but also for the analysis of agro-food products such as feed and dairy products. These analyses are no more conducted only at the laboratory level but go more and more to the sample. The new generation of instruments (portable and handheld devices) allow to perform the analyses at the field, farm, orchard or greenhouse level in order to get information to take the right decision at the right moment. This chapter aims to summarise some of these applications and attempts to give the trends of a selection of recently completed or current projects. Readers aiming to delve further into the potential of NIR in agriculture can refer to dedicated books (Williams and Norris in Amer Assn of Cereal Chemists, 312 p, 2001 [1]) or recent reviews (Baeten et al. in Handbook of food analysis, pp 591–614, 2015 [2], Dale et al. in Appl. Spectrosc. Rev. 48(2):142–159, 2013 [3], García-Sánchez et al. in Agricultural systems, pp 97–127, 2017 [4]).

The combination of speed, accuracy and simplicity provided by NIR spectroscopy ensured its use as a preferred quality control tool in the food and beverage industries. These applications are increasingly simplified by the availability of readily available factory calibrations. A challenge receiving increasing attention is that of the detection of food adulteration, and a large effort is being made to evaluate NIR spectroscopy as a suitable method. The recent trend towards miniaturisation of NIR instruments contributes to the technology becoming portable and more affordable. The trust put into NIR spectroscopy as an effective analytical tool in the food industry will remain. In addition, investigations into new and innovative applications to the benefit of the food industry are seen on a daily basis.

Near-infrared spectroscopy (NIRS) is suitable for both the qualification and quantification of organic properties associated with C–H, O–H, or N–H groups. There have been considerable efforts made toward proposing and developing various technologies and devices for the rapid and nondestructive measurement of various samples related to natural materials and environmental sciences. In this chapter, the utilizations of NIRS in the fields of wood material, soil, sediment, waste liquid, atmospheric gas detection, and archeological science will be explained through some representative studies.

Near-infrared spectroscopy (NIRS) enables rapid and nondestructive analyses of the components of an object. NIRS has many applications in agriculture. In particular, it has been combined with information and communication technology (ICT) to facilitate the management of equipment and data, thereby significantly expanding its application range. This chapter discusses the application of ICT, especially in sugarcane production.

Since the first applications of near-infrared spectroscopy in the pharmaceutical industry to today’s in-line and in real-time monitoring and control of manufacturing processes, the technology has come a long way in sensitivity, robustness, and deployability. The pharmaceutical industry is now able to rely on the technology to release medicine to patients without having to sample for off-line analyses. The sensitivity of NIR light to the matrix, which makes it a tool of choice for raw material identification and counterfeit detection, can be an issue for quantitative and qualitative methods as much of the variance has to be captured by the model prior to validation to avoid repeated method updates. Nevertheless, its flexibility of implementation, hardware ruggedness, and wide range of applicability makes it a tool of choice for process understanding, monitoring, and control. In this chapter, an overview of the current usage is provided for the development, understanding, and control of pharmaceutical processes for small molecule drug substances, drug products, and biopharmaceutical materials. A discussion of the regulatory environment and available guidance documents is provided. Finally, the intended method use and the associated method validation requirements are discussed in the context of building fit for purpose methods.

Near-infrared (NIR) spectroscopy occupies a distinct spot as an investigation tool in bioscience. It gained an ultimate value in several areas of application, e.g., in characterization of plant material, examination of body fluids, exploration of the structure and properties of water and biomolecules in aqueous environment. On the other hand, certain limitations of this technique have been apparent and its full potential seems yet to be unveiled. In recent years, key advancements in technology and methods have pushed the frontier of NIR spectroscopy in bio-applications. Trend-setting studies demonstrated the capacity of NIR spectroscopy to excel in previously unattainable scenarios such as in vivo examination of entire organisms. The advent of miniaturized instrumentation enabled a new spectrum of applications in plant-related research. Advancements in data analytical methods decisively pushed the limit in interpretability of NIR spectra, enabling better understanding of NIR spectral features of biomolecules. These advancements were accompanied by a continuous refinement of established approaches. This chapter discussed the established applications, current developments and future prospects of NIR spectroscopy in broadly understood bio-applications.

In recent years, near-infrared (NIR) spectroscopy has seen much progress in instrumentation and measurement techniques. It has been used for monitoring in many fields of analytical spectroscopy. Examples are the characterization of materials from processes of the chemical and pharmaceutical industry in addition to the broad field of food industrial and biotechnological applications. Another important field for NIR spectroscopy is found within the medical sciences with topics such as clinical chemistry, sensing and monitoring of changes of homeostasis of the body, with biofluids and tissues from many organs involved. Here, in vitro laboratory work and in vivo monitoring must be mentioned. Regarding instrumentation, laboratory analyzers are kept firmly in our view, but point-of-care (POC) applications need also to be taken into account. Sensing devices for non-invasive measurements on special parameters such as blood glucose and hemoglobin or information on the redox status of tissues is another broad area with oxygenation of hemoglobin and myoglobin as most important parameters. Absorption measurements are most often carried out with transmission and reflection techniques, but due to the synthesis of new marker substances, also fluorescence measurements in the NIR spectral range become more advanced, especially for imaging and immunoassay developments.

This chapter provides a survey on the current state of the art of in-line analysis by various NIR techniques for process control of two very specialized categories of polymer materials: polymer coatings and textiles from synthetic fibers. In case of coatings, monitoring of the conversion of radiation-curable monomers such as acrylates, methacrylates, cycloaliphatic epoxies and vinyl ethers that is achieved during irradiation is primarily discussed, since the conversion strongly determines application and processing properties of such coatings. Moreover, in-line measurement of the coating thickness (from only a few up to several hundreds of micrometers), the spatial distribution of various parameters of interest across the coatings as well as the characterization of thin printed layers in the printing press are further subjects of the first part. The second part deals with the application of NIR methods for process monitoring and quality control in textile converting. Technical textiles are often subject of special treatment and finishing steps such as impregnation, coating, lamination etc. which have to be controlled in order to ensure adequate processing. NIR techniques have been shown to be an appropriate tool for this problem. In particular, hyperspectral imaging can help to retain the required homogeneity of textile webs or laminates after finishing, e.g., with respect to the application weight of functional finishes or adhesive layers. Furthermore, NIR spectroscopy is used for identification and sorting of textiles with the objective of recycling of the materials. Hence, an overview of the current status of the use of NIR spectroscopic techniques in textile technology is given.

Visualization of the spatial distribution of surface properties is increasingly desired in many fields. Spectral imaging combines spectroscopy with imaging, implying images with three dimensions: two spatial and one spectral. NIR spectral imaging enjoys many of the useful features of NIR spectroscopy such as suitability for nondestructive measurement, in situ analysis and potential for transmission measurements. NIR imaging systems can provide high-speed monitoring and stability. These features are very attractive not only for laboratory-based studies but also for applications in a number of practical fields such as pharmaceutical, medical, engineering, biological and agricultural. NIR imaging technology is still developing by improvement of spectral analysis method; chemometrics and image analysis methods. In this section, we describe the concept of NIR spectral imaging at first and introduce the basic design of NIR imaging devices and the features of newly developed devices. Finally, the potential of NIR imaging for practical situation is demonstrated through several applications reported at recent years.

The concept of process analytical technology (PAT) started around the 1970s with the advent of personal computers in combination with instrumental analytical chemistry. Over the years, increasingly sophisticated and holistic quality management concepts such as quality by design (QbD) were developed and strongly promoted, especially by the American Food and Drug Agency (FDA) around 2002. Recently, the German initiative for the Fourth Industrial Revolution “Industrie 4.0 (i40)” was introduced, which is similar to the US “Industrial Internet Consortium (IIC)” concept or the “Industrial Internet of Things (IIoT).” Another initiative in Asia is the Chinese campaign “Made in China 2025.” The role of PAT in all these concepts is to develop and integrate context-sensitive intelligent sensors to enable understanding of the process at the basic mechanistic (molecular) level in order to achieve knowledge-based production in the future. This contribution starts with a short introduction into the concept of PAT/QbD and other new concepts for the next generation of spectroscopic sensors in the manufacturing industry. The fundamental limitations of spectroscopy in terms of sensitivity and selectivity are discussed, and the need to increase robustness for industrial applications is described. A critical discussion on problems and problem solutions are provided when scattering samples are investigated. An outlook on how to use NIR spectroscopy within the petrochemical industry and how to manage a PAT project complements this chapter.