Principles Techniques And Limitations Of Near Infrared Spectroscopy PdfBy Canan O. In and pdf 17.05.2021 at 16:28 10 min read
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Near Infrared Spectroscopy: fundamentals, practical aspects and analytical applications. It is addressed to the reader who does not have a profound knowledge of vibrational spectroscopy but wants to be introduced to the analytical potentialities of this fascinating technique and, at same time, be conscious of its limitations. Essential theory background, an outline of modern instrument design, practical aspects, and applications in a number of different fields are presented.
- Novel method for shark age estimation using near infrared spectroscopy
- Multi-time-point analysis: A time course analysis with functional near-infrared spectroscopy
- Fourier-transform infrared spectroscopy
Continuous wave near infrared spectroscopy CW NIRS provides non-invasive technology to measure relative changes in oxy- and deoxy-haemoglobin in a dynamic environment. NIRS parameters that measure O 2 delivery and capacity to utilise O 2 in the muscle have been developed based on response to physiological interventions and exercise.
Novel method for shark age estimation using near infrared spectroscopy
Near Infrared Spectroscopy: fundamentals, practical aspects and analytical applications. It is addressed to the reader who does not have a profound knowledge of vibrational spectroscopy but wants to be introduced to the analytical potentialities of this fascinating technique and, at same time, be conscious of its limitations.
Essential theory background, an outline of modern instrument design, practical aspects, and applications in a number of different fields are presented. This work does not intend to supply an intensive bibliography but refers to the most recent, significant and representative material found in the technical literature. Keywords: near-infrared spectroscopy, chemometrics, instrumentation, analytical applications. Introduction and Historical Overview.
Near Infrared Spectroscopy NIR is a type of vibrational spectroscopy that employs photon energy hn in the energy range of 2.
This energy range is higher than necessary to promote molecules only to their lowest excited vibrational states through a fundamental vibrational transition and lower than typical values necessary for electron excitation in molecules except for some rare earth compounds.
The analytical methods resulting from the use of the NIR spectroscopic region reflect its most significant characteristics, such as: fast one minute or less per sample , non-destructive, non-invasive, with high penetration of the probing radiation beam, suitable for in-line use, nearly universal application any molecule containing C-H, N-H, S-H or O-H bonds , with minimum sample preparation demands.
The combination of these characteristics with instrumental control and data treatment has made it possible to coin the term Near-Infrared Technology. However, in order to have such a type of spectroscopy and to evolve it into a useful analytical technique, it was first necessary to discover the radiation in the NIR energy range, which is invisible to the naked eye.
However, this did not constrain the clever mind of the German born, English scientist, Frederick William Herschel. Sir Herschel was an astronomer and musician who, in between composing and discovering the planet Uranus, also marked his place in science by running an experiment to find out the contribution of each of the colours arriving from dispersed white sunlight in increasing the temperature of the substances exposed to them.
The experiment is cited as an example of scientific insight because Herschel did not stop probing what happen with the temperature when he reached the end of the visible red colour region of the dispersed light. Contrary to common sense, he continued observing what happened with the temperature, placing the thermometer beyond that point. Surprisingly, he found the temperature still rises.
Herschel used blackened bulb thermometers and glass prisms which are transparent to short wave NIR radiation and reported his achievement by referring to this region as "calorific rays" found beyond the red. This region was later named infrared, using the Greek prefix "infra" which means "below". The first non-visible part of the electromagnetic spectra had then been reported to exist by the year Although NIR radiation had been detected before that of the medium infrared, it was this latter spectral region that first gained wide acceptance after the work initiated in by Coblentz, who was the first researcher to obtain absorbance spectra of pure substances and verify their usefulness for the identification of organic functional groups.
While medium infrared spectroscopy gained wide acceptance and constantly received both theoretical and instrumental progress, NIR spectroscopy was neglected by spectroscopists who, for a long time, could not find any additional attractive information in that spectral region which was occupied by broad, superimposed and weak absorption bands. In fact, this early ostracism, despite its potentialities, of the NIR spectral region was clearly demonstrated in a paper by Wetzel whose suggestive title is "Near-Infraed Reflectance Analysis - Sleeper Among Spectroscopic Techniques", 4 published in Nevertheless, the decade of the eighties has marked the "boom" of the technique.
From to the total number of papers published dealing with NIR was about , while the following decade saw a number greater than Such productivity led to the appearance, in , of the Journal of Near Infrared Spectroscopy, the first periodical entirely dedicated to this field. Pioneering work on the analytical exploitation of the NIR spectral region had been developed since , when the determination of water in gelatin by employing its stronger absorption in the NIR region was described.
Although the paper by Kubelka and Munk 13 on diffuse reflection had been published before any practical application of NIR had been proposed, the significance of this type of measurement technique passed unnoticed by the analytical spectroscopists for a long period. However, efforts towards the development of instrumental methods for application to agriculture, initiated by the Department of Agriculture of the USA, would end with the recognition of the diffuse reflectance measurement mode as the most suitable for the kind of sample in which they were interested.
At this point the figure of Karl Norris emerges as the most prominent person in the history of NIR spectroscopy. Karl Norris initiated his work with NIR by searching for new methods for the determination of moisture in agricultural products, first by extracting the water in methanol 14 and soon afterward by suspending ground seeds in CCl 4.
The first results of such an approach were published in 15 and republished in a special issue of the Journal of Near Infrared Spectroscopy honouring Karl Norris in Simultaneously, it paved the way to a more ambitious achievement, that was the use of diffuse reflectance as a non-destructive measurement in the NIR region, which makes it possible to work with the sample directly, without any pre-treatment. Further work by Ben-Gera and Norris consolidates the approach of using NIR spectroscopy and originated one of the most cited paper in the field.
After the door had been opened and the potentialities demonstrated by the pioneers, NIR spectroscopy encountered fast development impelled mainly by instrumental spectrophotometer improvements associated with spectral data acquisition and their treatment micro-computer and which, to a great extent depended on the new discipline of Chemometrics, 18 which supplies the tools for gathering information and its wise use.
A recent review, has been published emphasising one of the main qualities of NIR spectroscopy: its rapid-response as an analytical tool. In Brazil, the first contributions to the field of applied analytical NIR spectroscopy can be traced back to Possibly, other pioneering uses of NIR in Brazil were carried out. However, these have not been reported in a traceable publication. The attribution of the year of for the first Brazilian contribution to NIR is in agreement with the contents of the preface of the first edition of the book by Williams and Norris on NIR technology where is it can be found that " For an analytical chemist trained in classical methodology, such as the author and many of his contemporary colleagues, perhaps one of the most significant and motivating examples is the replacement of a wet chemical procedure the Kjeldhal method for determination of protein in commodities wheat, corn and soybean, for example , by the direct NIR method.
The Kjeldhal method, as every analytical chemist well knows, is a quite robust and accurate method for determination of protein in a multitude of samples, mainly commodities. The method is based on the digestion of the ground grain in concentrated sulphuric acid containing a catalyst such as mercury or selenium.
After an hour or so the digested product is made alkaline and submitted to a distillation. The distilled ammonia is collected in standard boric acid solution and, finally, the excess acid is titrated with standard sodium hydroxide solution.
This cumbersome and time consuming procedure has been replaced by the NIR spectroscopic method which can directly determine the protein content of the whole or ground grain, through diffuse reflectance, in less than one minute, generating virtually no hazardous residue.
On the other hand, it is also wise, in order to be impartial, to alert the reader that, as marvellous it can appear at first glance, NIR technology is and will be always heavily dependent on the existence of good and acceptable reference methods as the Kjeldhal method. That is because at the learning stage modelling stage the direct method based on NIR needs to be able to identify the spectral characteristics or which combination of those characteristics are to be correlated for, in the above example, determining protein content in grain.
The difference between failing or succeeding in this task is greatly dependent on the quality of the reference values associated with the samples in the training set.
Nevertheless, once the learning stage is concluded, the final result is perhaps the closest that our present technology is able to produce of an ideal analytical method. In order to access the origin of a NIR spectra, to be able to interpret it and have an important tool to guide in analytical method development, one should be familiar with the fundamentals of vibrational spectroscopy.
The NIR spectrum originates from radiation energy transferred to mechanical energy associated with the motion of atoms held together by chemical bonds in a molecule.
Although many would approach method development in a purely empirical way, knowledge of the theory can help to look at the important wavelengths and quicker optimisation of the modelling stage. Vibrational spectroscopy.
At ambient temperature most of the molecules are in their fundamental vibrational energy levels. Atoms or group of atoms participating in chemical bonds are displacing one in relation to the other in a frequency that is defined by the strength of the bond and the mass of the individual bonded atoms or their groups.
The amplitudes of these vibrations are of a few nanometers and will increase if some energy is transferred to the molecule. This energy can be transferred from a photon of a given wavelength l , for which the energy E p can be given by:.
The classical mechanical model for a diatomic molecule. The simplest classical model employed to have a didactic insight on the interaction of radiation and matter in the NIR spectral region depicts a diatomic molecule as two spherical masses m 1 and m 2 connected with a spring with a given force constant k. The molecular vibration can be described by a simplified model supposing a harmonic oscillator for which the potential energy V , as a function of the displacement of the atoms x , is given by:.
Figure 1A shows the behaviour of the potential energy as a function of atom displacement from the equilibrium minimum energy position. This first approach is useful to understand the concept of vibrational energy. However, it fails when a microscopic system such as molecules is being considered. The failure arises from the fact that molecular systems can not assume the continuous energy profile predicted by the classical "balls-on-spring" model. The molecular system can only have some discrete energy levels E u defined by quantum mechanics by the equation:.
In the classical model this frequency is defined by:. Furthermore, for this model, the difference of energy between two adjacent states is always the same see Fig.
The energy of the electromagnetic radiation that is absorbed in order to promote the molecule to an excited level should match the difference between two adjacent energetic levels. Therefore, the photon energy must be. Figure 2 shows the effect of photon absorption on the energy and amplitude of vibration.
The classical analogue to this behaviour is the concept of resonance. In this concept, the physical characteristics of a "string" stretched between two supporting points, such as its linear density and the force by which it is stretched, will define its natural frequency of vibration as a guitar string does. The amplitude of this natural vibration therefore, its energy can be increased by exposing the string to an acoustic wave propagating in the air, with the same frequency, produced, for example, by a distant stroked string with the same characteristics.
The first string undergoes no energy change if the acoustic wave frequencies and the natural frequency do not match each other. Similarly, only radiation of a certain frequency and wavelength can excite the vibrational levels of molecules.
However, this model fails in the molecular world because it is not a quantum model. In the "string world" , the energy they can obtain from the exciting mechanical wave can increase continuously while a "quantum string" is able to vibrate at only a given frequency and at only some pre-defined amplitude.
Although the harmonic model can help understanding vibrational spectroscopy, it produces some disappointing restrictions for NIR spectroscopy because it can not permit transitions where Du is greater than 1.
Also, the vibrations in the harmonic model are independent and their combinations would not exist under the restrictions imposed by the model. Nevertheless, both overtones and combination bands exist. The anharmonic model. Figure 1B shows a more realistic mechanical model for a diatomic molecule. The molecule is still approximated by two balls connected with a spring.
However, the model considers some non-ideal behaviours of the oscillator which account for repulsion between electronic clouds when the atomic nuclei approach notice how the potential energy rises fasten than in the harmonic model and a variable behaviour of the bond force when the atoms move apart from one another.
In fact, in a real molecule, the over displacement "strengthening of the spring" of the atomic nuclei will cause molecule bond rupture with consequent dissociation of the atoms. A complex function of the potential energy is assumed to describe the last effect which can be approximated by using higher order terms of displacement, as depicted in the equation.
A function that approximates the anharmonic behaviour of a diatomic molecule is the Morse function that describes the potential energy of the molecule using the equation:. Applying quantum mechanics to the Morse equation results in the vibrational levels being described by the equation:.
These two types of bands are the most common absorption bands in the NIR spectral region. It also predicts that the separation between two adjacent energy levels decreases with u, the vibrational quantum number. Under the assumptions of the anharmonic model, the vibrations are no longer independent of each other and can interact with one another. Therefore, the total vibrational energy E u contains cross-terms from more than one vibration in the molecule:. Anharmonicity can also be present in the electrical properties of a molecule.
Specifically, it affects its dipole moment which, in an anharmonic model, does not have a linear dependence with the interatomic distance. This kind of anharmonicity can provide the way for overtones and combination bands to occur even if no mechanical deviation of the harmonic model is observed for a given system.
Origin and intensity of a NIR absorption band. So far, it is possible to understand from theory that radiation of a given frequency, capable to supply exactly the energy between two vibrational levels or of their overtones or combinations of two or more vibrations, can be absorbed by the molecule and can produce excitation to a higher vibrational energy level.
The match of radiation energy with the energy difference between two vibrational levels causes a selective response of the molecular system to the incident radiation. It means that in a given wavelength range, some frequencies will be absorbed, others that do not match any of the energy differences possible for that molecule will not be absorbed while some will be partially absorbed.
Multi-time-point analysis: A time course analysis with functional near-infrared spectroscopy
In the data analysis of functional near-infrared spectroscopy fNIRS , linear model frameworks, in particular mass univariate analysis, are often used when researchers consider examining the difference between conditions at each sampled time point. However, some statistical issues, such as assumptions of linearity, autocorrelation and multiple comparison problems, influence statistical inferences when mass univariate analysis is used on fNIRS time course data. In order to address these issues, the present study proposes a novel perspective, multi-time-point analysis MTPA , to discriminate signal differences between conditions by combining temporal information from multiple time points in fNIRS. In addition, MTPA adopts the random forest algorithm from the statistical learning domain, followed by a series of cross-validation procedures, providing reasonable power for detecting significant time points and ensuring generalizability. Using a real fNIRS data set, the proposed MTPA outperformed mass univariate analysis in detecting more time points, showing significant differences between experimental conditions. The data set and all source code are available for researchers to replicate the analyses and to adapt the program for their own needs in future fNIRS studies. Functional near-infrared spectroscopy fNIRS is a noninvasive tool for recording hemodynamic activity along the scalp time-locked to response events.
Marcelo V. Fernanda S. Costa c. The aim of this study was to quantitatively determine the olanzapine in a pharmaceutical formulation for assessing the potentiality of near infrared spectroscopy NIR combined with partial least squares PLS regression. The method was developed with samples based on a commercial formulation containing olanzapine and seven excipients. The method was validated in the range from 1.
This chapter provides a review on the state of art of the use of the visible near-infrared vis-NIR spectroscopy technique to determine mineral nutrients, organic compounds, and other physical and chemical characteristics in samples from agricultural systems—such as plant tissues, soils, fruits, cocomposted sewage sludge and wastes, cereals, and forage and silage. Currently, all this information is needed to be able to carry out the appropriate fertilization of crops, to handle agricultural soils, determine the organoleptic characteristics of fruit and vegetable products, discover the characteristics of the various substrates obtained in composting processes, and characterize byproducts from the industrial sector. All this needs a large number of samples that must be analyzed; this is a time-consuming work, leading to high economic costs and, obviously, having a negative environmental impact owing to the production of noxious chemicals during the analyses. Therefore, the development of a fast, environmentally friendly, and cheaper method of analysis like vis-NIR is highly desirable. Our intention here is to introduce the main fundamentals of infrared reflectance spectroscopy, and to show that procedures like calibration and validation of data from vis-NIR spectra must be performed, and describe the parameters most commonly measured in the agricultural sector. Developments in Near-Infrared Spectroscopy.
Fourier-transform infrared spectroscopy
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Fourier-transform infrared spectroscopy FTIR  is a technique used to obtain an infrared spectrum of absorption or emission of a solid, liquid or gas. An FTIR spectrometer simultaneously collects high-resolution spectral data over a wide spectral range.
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