Principles Techniques And Limitations Of Near Infrared Spectroscopy Pdf


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Continuous wave near infrared spectroscopy CW NIRS provides non-invasive technology to measure relative changes in oxy- and deoxy-haemoglobin in a dynamic environment.

Multi-time-point analysis: A time course analysis with functional near-infrared spectroscopy

Fourier-transform infrared spectroscopy FTIR [1] 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. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time.

The term Fourier-transform infrared spectroscopy originates from the fact that a Fourier transform a mathematical process is required to convert the raw data into the actual spectrum. The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a monochromatic light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength. This is how some UV—vis spectrometers work, for example. Fourier-transform spectroscopy is a less intuitive way to obtain the same information.

Rather than shining a monochromatic beam of light a beam composed of only a single wavelength at the sample, this technique shines a beam containing many frequencies of light at once and measures how much of that beam is absorbed by the sample.

Next, the beam is modified to contain a different combination of frequencies, giving a second data point. This process is rapidly repeated many times over a short time span. Afterwards, a computer takes all this data and works backward to infer what the absorption is at each wavelength.

The beam described above is generated by starting with a broadband light source—one containing the full spectrum of wavelengths to be measured. The light shines into a Michelson interferometer —a certain configuration of mirrors, one of which is moved by a motor. As this mirror moves, each wavelength of light in the beam is periodically blocked, transmitted, blocked, transmitted, by the interferometer, due to wave interference.

Different wavelengths are modulated at different rates, so that at each moment or mirror position the beam coming out of the interferometer has a different spectrum. As mentioned, computer processing is required to turn the raw data light absorption for each mirror position into the desired result light absorption for each wavelength.

The processing required turns out to be a common algorithm called the Fourier transform. The raw data is called an "interferogram". The first low-cost spectrophotometer capable of recording an infrared spectrum was the Perkin-Elmer Infracord produced in The lower wavelength limit was chosen to encompass the highest known vibration frequency due to a fundamental molecular vibration.

Measurements in the far infrared needed the development of accurately ruled diffraction gratings to replace the prisms as dispersing elements, since salt crystals are opaque in this region. More sensitive detectors than the bolometer were required because of the low energy of the radiation.

One such was the Golay detector. An additional issue is the need to exclude atmospheric water vapour because water vapour has an intense pure rotational spectrum in this region. Far-infrared spectrophotometers were cumbersome, slow and expensive. The advantages of the Michelson interferometer were well-known, but considerable technical difficulties had to be overcome before a commercial instrument could be built. Also an electronic computer was needed to perform the required Fourier transform, and this only became practicable with the advent of mini-computers , such as the PDP-8 , which became available in In a Michelson interferometer adapted for FTIR, light from the polychromatic infrared source, approximately a black-body radiator, is collimated and directed to a beam splitter.

Light is reflected from the two mirrors back to the beam splitter and some fraction of the original light passes into the sample compartment. There, the light is focused on the sample. On leaving the sample compartment the light is refocused on to the detector. The difference in optical path length between the two arms to the interferometer is known as the retardation or optical path difference OPD.

An interferogram is obtained by varying the retardation and recording the signal from the detector for various values of the retardation. The form of the interferogram when no sample is present depends on factors such as the variation of source intensity and splitter efficiency with wavelength.

This results in a maximum at zero retardation, when there is constructive interference at all wavelengths, followed by series of "wiggles". The position of zero retardation is determined accurately by finding the point of maximum intensity in the interferogram.

When a sample is present the background interferogram is modulated by the presence of absorption bands in the sample. Commercial spectrometers use Michelson interferometers with a variety of scanning mechanisms to generate the path difference. Common to all these arrangements is the need to ensure that the two beams recombine exactly as the system scans. The simplest systems have a plane mirror that moves linearly to vary the path of one beam. In this arrangement the moving mirror must not tilt or wobble as this would affect how the beams overlap as they recombine.

Some systems incorporate a compensating mechanism that automatically adjusts the orientation of one mirror to maintain the alignment. Arrangements that avoid this problem include using cube corner reflectors instead of plane mirrors as these have the property of returning any incident beam in a parallel direction regardless of orientation. Systems where the path difference is generated by a rotary movement have proved very successful.

One common system incorporates a pair of parallel mirrors in one beam that can be rotated to vary the path without displacing the returning beam.

Another is the double pendulum design where the path in one arm of the interferometer increases as the path in the other decreases. A quite different approach involves moving a wedge of an IR-transparent material such as KBr into one of the beams. Increasing the thickness of KBr in the beam increases the optical path because the refractive index is higher than that of air. One limitation of this approach is that the variation of refractive index over the wavelength range limits the accuracy of the wavelength calibration.

The interferogram has to be measured from zero path difference to a maximum length that depends on the resolution required. In practice the scan can be on either side of zero resulting in a double-sided interferogram.

Mechanical design limitations may mean that for the highest resolution the scan runs to the maximum OPD on one side of zero only. The interferogram is converted to a spectrum by Fourier transformation. This requires it to be stored in digital form as a series of values at equal intervals of the path difference between the two beams. This can trigger an analog-to-digital converter to measure the IR signal each time the laser signal passes through zero.

Alternatively, the laser and IR signals can be measured synchronously at smaller intervals with the IR signal at points corresponding to the laser signal zero crossing being determined by interpolation. The result of Fourier transformation is a spectrum of the signal at a series of discrete wavelengths. The range of wavelengths that can be used in the calculation is limited by the separation of the data points in the interferogram.

The shortest wavelength that can be recognized is twice the separation between these data points. For example, with one point per wavelength of a HeNe reference laser at 0. Because of aliasing any energy at shorter wavelengths would be interpreted as coming from longer wavelengths and so has to be minimized optically or electronically. The wavelengths used in calculating the Fourier transform are such that an exact number of wavelengths fit into the length of the interferogram from zero to the maximum OPD as this makes their contributions orthogonal.

This results in a spectrum with points separated by equal frequency intervals. The separation is the inverse of the maximum OPD. This is the spectral resolution in the sense that the value at one point is independent of the values at adjacent points. Most instruments can be operated at different resolutions by choosing different OPD's.

Instruments for routine analyses typically have a best resolution of around 0. The point in the interferogram corresponding to zero path difference has to be identified, commonly by assuming it is where the maximum signal occurs. This so-called centerburst is not always symmetrical in real world spectrometers so a phase correction may have to be calculated.

The interferogram signal decays as the path difference increases, the rate of decay being inversely related to the width of features in the spectrum. If the OPD is not large enough to allow the interferogram signal to decay to a negligible level there will be unwanted oscillations or sidelobes associated with the features in the resulting spectrum. To reduce these sidelobes the interferogram is usually multiplied by a function that approaches zero at the maximum OPD.

This so-called apodization reduces the amplitude of any sidelobes and also the noise level at the expense some reduction in resolution. For rapid calculation the number of points in the interferogram has to equal a power of two.

A string of zeroes may be added to the measured interferogram to achieve this. More zeroes may be added in a process called zero filling to improve the appearance of the final spectrum although there is no improvement in resolution. Alternatively, interpolation after the Fourier transform gives a similar result.

There are three principal advantages for an FT spectrometer compared to a scanning dispersive spectrometer. Another minor advantage is less sensitivity to stray light, that is radiation of one wavelength appearing at another wavelength in the spectrum.

In dispersive instruments, this is the result of imperfections in the diffraction gratings and accidental reflections. In FT instruments there is no direct equivalent as the apparent wavelength is determined by the modulation frequency in the interferometer.

The interferogram belongs in the length dimension. Much higher resolution can be obtained by increasing the maximal retardation. This is not easy, as the moving mirror must travel in a near-perfect straight line. The use of corner-cube mirrors in place of the flat mirrors is helpful, as an outgoing ray from a corner-cube mirror is parallel to the incoming ray, regardless of the orientation of the mirror about axes perpendicular to the axis of the light beam. In Connes measured the temperature of the atmosphere of Venus by recording the vibration-rotation spectrum of Venusian CO 2 at 0.

The throughput advantage is important for high-resolution FTIR, as the monochromator in a dispersive instrument with the same resolution would have very narrow entrance and exit slits. FTIR is a method of measuring infrared absorption and emission spectra. For a discussion of why people measure infrared absorption and emission spectra, i. The output is similar to a blackbody.

Mid-IR spectrometers commonly use pyroelectric detectors that respond to changes in temperature as the intensity of IR radiation falling on them varies. These detectors operate at ambient temperatures and provide adequate sensitivity for most routine applications.

To achieve the best sensitivity the time for a scan is typically a few seconds. Cooled photoelectric detectors are employed for situations requiring higher sensitivity or faster response. Liquid nitrogen cooled mercury cadmium telluride MCT detectors are the most widely used in the mid-IR. With these detectors an interferogram can be measured in as little as 10 milliseconds.

Very sensitive liquid-helium-cooled silicon or germanium bolometers are used in the far-IR where both sources and beamsplitters are inefficient. However, as any material has a limited range of optical transmittance, several beam-splitters may be used interchangeably to cover a wide spectral range. For the mid-IR region the beamsplitter is usually made of KBr with a germanium-based coating that makes it semi-reflective.

Using Near-Infrared Spectroscopy in Agricultural Systems

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.

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. One of the challenges of the twenty-first century is to achieve a more productive agriculture, while improving the safety and quality of food.


infrared (NIR) spectroscopy (NIRS). This review summarizes the most recent literature about the principles, techniques, advantages, limitations, and applications.


Fourier-transform 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.

Multi-time-point analysis: A time course analysis with functional near-infrared spectroscopy

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Novel method for shark age estimation using near infrared spectroscopy

The system can't perform the operation now. Try again later. Citations per year. Duplicate citations. The following articles are merged in Scholar.

Marcelo V. Fernanda S. Costa c.

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Fourier-transform infrared spectroscopy FTIR [1] 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. This confers a significant advantage over a dispersive spectrometer, which measures intensity over a narrow range of wavelengths at a time. The term Fourier-transform infrared spectroscopy originates from the fact that a Fourier transform a mathematical process is required to convert the raw data into the actual spectrum. The most straightforward way to do this, the "dispersive spectroscopy" technique, is to shine a monochromatic light beam at a sample, measure how much of the light is absorbed, and repeat for each different wavelength.

 - Вы же учились в колледжах. Ну, кто-нибудь.

Чем глубже под землю уходил коридор, тем уже он становился. Откуда-то сзади до них долетело эхо чьих-то громких, решительных шагов. Обернувшись, они увидели быстро приближавшуюся к ним громадную черную фигуру. Сьюзан никогда не видела этого человека раньше. Подойдя вплотную, незнакомец буквально пронзил ее взглядом.

 - Давай я тебе помогу. - Ах ты, пакостник. - Не знаю, что ты такое подумала.

Створки давили на плечо с неимоверной силой. Не успел Стратмор ее остановить, как она скользнула в образовавшийся проем. Он попытался что-то сказать, но Сьюзан была полна решимости. Ей хотелось поскорее оказаться в Третьем узле, и она достаточно хорошо изучила своего шефа, чтобы знать: Стратмор никуда не уйдет, пока она не разыщет ключ, спрятанный где-то в компьютере Хейла. Ей почти удалось проскользнуть внутрь, и теперь она изо всех сил пыталась удержать стремившиеся захлопнуться створки, но на мгновение выпустила их из рук.

 Понятия не имею. Я уже говорила, что мы ушли до их прибытия. - Вы хотите сказать - после того как стащили кольцо.

Он опустил руку и отвернулся, а повернувшись к ней снова, увидел, что она смотрит куда-то поверх его плеча, на стену. Там, в темноте, ярко сияла клавиатура. Стратмор проследил за ее взглядом и нахмурился Он надеялся, что Сьюзан не заметит эту контрольную панель. Эта светящаяся клавиатура управляла его личным лифтом.

Все глаза были устремлены на нее, на руку Танкадо, протянутую к людям, на три пальца, отчаянно двигающихся под севильским солнцем. Джабба замер. - О Боже! - Он внезапно понял, что искалеченный гений все это время давал им ответ.

 - В одном из ваших мозговых штурмов. - Это невозможно. Я никогда не распечатываю свои мозговые штурмы. - Я знаю. Я считываю их с вашего компьютера.

 Что еще за второй ключ. - Тот, что Танкадо держал при. Сьюзан была настолько ошеломлена, что отказывалась понимать слова коммандера. - О чем вы говорите. Стратмор вздохнул.

 Похоже, что-то стряслось, - сказала Сьюзан.  - Наверное, увидел включенный монитор. - Черт возьми! - выругался коммандер.

Я позвоню и все объясню. Мне в самом деле пора идти, они связи, обещаю.

4 Comments

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27.03.2021 at 14:12 - Reply

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Florismart B.
02.04.2021 at 03:42 - Reply

NIRS is a noninvasive and relatively low-cost optical technique that is becoming a widely used instrument for measuring tissue O2 saturation, changes in hemoglobin volume and, indirectly, brain/muscle blood flow and muscle O2 consumption.

Stetasdonsi
03.04.2021 at 17:16 - Reply

This review summarizes the most recent literature about the principles, techniques, advantages, limitations, and applications of NIRS in exercise physiology and.

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03.04.2021 at 18:34 - Reply

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