Near-IR Instruments

A sophisticated ultraviolet/visible/near-IR instrument has a wavelength range of 185 to 3000 nm. What are its wavenumber and frequency ranges Calculate the frequency in hertz and the energy in joules of an X-ray photon with a wavelength of 2.35 A. 

Infrared spectroscopy instrumentation is almost as widely varying as the applications. Mid-infrared instrumentation today consists almost exclusively of Fourier transform instruments. Although dispersive mid-IR instruments are still used, only a few manufacturers still produce these instruments. Dispersive mid-IR instruments have found a niche in the process monitoring field. Near-infrared instrumentation, on the other hand, is still dominated by dispersive instruments. Recently, some manufacturers have begun to offer near-IR instruments with Fourier transform mechanics and optics. 

Sampling, sample handling, and storage and sample preparation methods are extensively covered, and modern methods such as accelerated solvent extraction, solid-phase microextraction (SPME), QuEChERS, and microwave techniques are included. Instrumentation, the analysis of liquids and solids, and applications of NMR are discussed in detail. A section on hyphenated NMR techniques is included, along with an expanded section on MRI and advanced imaging. The IR instrumentation section is focused on FTIR instrumentation. Absorption, emission, and reflectance spectroscopy are discussed, as is ETIR microscopy. ATR has been expanded. Near-IR instrumentation and applications are presented, and the topic of chemometrics is introduced. Coverage of Raman spectroscopy includes resonance Raman, surface-enhanced Raman, and Raman microscopy. 

Until the 1980s, the most widely used instruments for IR measurements were dispersive spectrophotometers. Now, however, this type of instrument has been largely displaced for mid- and far-lR measurements by Fourier transform spectrometers because of their speed, reliability, signal-to-noise advantage, and convenience. Dispersive spectrometers are still used in the near-lR where they are often extensions of UV-visible instruments, but many dedicated near-IR instruments are of the Fourier transform-IR (FTIR) type. 

The idea of a spectrometer dedicated to near-IR spectroscopy is relatively recent, and the early commercial near-IR instruments were simply UV-visible (or mid-infrared) spectrometers fitted with an additional detector and occasionally a second grating blazed for the near-IR. This equipment provided the basis for the pioneering work of Kermit Whetzel and Wilbur Kaye, who were largely responsible for laying the foundation of analytical near-IR spectroscopy. The development of modern near-IR instrumentation was spurred by research at the US Department of Agriculture Karl Norris discovered that no commercial spectrometer of the time could provide diffuse reflectance measurements of the quality he required, and developed his own computerized near-IR spectrometer... 

Davies AMC (ed) Journal of Near-Infrared Spectroscopy. (This journal regularly includes research articles reporting refinements and advances in near-IR instrumentation.)... 

Multichannel dispersive IR instruments utilize a multichannel detector (Fig. 3.2). A multichannel detector is an array of many detector elements. Using a multichannel detector improves the signal-to-noise ratio proportional to the square root of the number of detector elements. The use of a multichannel detector has improved the performance of near IR instrumentation. 

Photomultipliers are used as detectors in the single-channel instruments. GaAs cathode tubes give a flat frequency response over the visible spectrum to 800 nm in the near IR. Contemporary Raman spectrometers use computers for instrument control, and data collection and storage, and permit versatile displays. 

Principles and Characteristics Both mid-IR (2.5-50 p.m) and near-IR (0.8-2.5 p.m) may be used in combination to TLC, but both with lower sensitivity than UV/VIS measurements. The infrared region of the spectrum was largely ignored when the only spectrometers available were the dispersive types. Fourier-transform instruments have changed all that. Combination of TLC and FTIR is commonly approached in two modes ... 

It is a spectroscopic technique, hence the optical properties of the film can be probed over the entire spectral range of the instrument, typically UV to near-IR. 

While the field of near-IR sensing is frequently regarded as having reached its (scientific) limits, with advances restricted to minor progress in instrumentation and data evaluation procedures, interesting developments are reported in particular in the field of near-IR spectral imaging. 

The majority of currently deployed IR sensors operate in the near-IR. Although near-IR sensors suffer from limited selectivity and sensitivity due to the relatively unstructured broadband absorptions in this frequency range, the easy availability of waveguides and other instrumentation give this spectral range a significant advantage over the mid-IR. Main application areas involve substance identification and process control. 

Similar to IR sensors, process analysis is the prevalent application area. Due to the applicability of standard VIS instrumentation, Raman probes have been used for more than two decades65, 66. Typically, Raman probes are applied where near-IR probes fail and hence are in direct competition to mid-IR probes. 

Some commercially available instruments, in addition to visible spectrophotometers, can also perform measurements in the UV and near IR regions of the spectrum. 

The phenomenon of fluorescence has been synonymous with ultraviolet (UV) and visible spectroscopy rather than near-infrared (near-IR) spectroscopy from the beginning of the subject. This fact is evidenced in definitive texts which also provide useful background information for this volume (see, e.g., Refs. 1-6). Consequently, our understanding of the many molecular phenomena which can be studied with fluorescence techniques, e.g., excimer formation, energy transfer, diffusion, and rotation, is based on measurements made in the UV/visible. Historically, this emphasis was undoubtedly due to the spectral response of the eye and the availability of suitable sources and detectors for the UV/visible in contrast to the lack of equivalent instrumentation for the IR. Nevertheless, there are a few notable exceptions to the prevalence of UV/visible techniques in fluorescence such as the near-IR study of chlorophyll(7) and singlet oxygen,<8) which have been ongoing for some years. 

In this chapter, we review the instrumentation presently available for studying near-IR fluorescence. This includes modern semiconductor devices such as diode lasers and photodiode detectors and also more conventional devices such as discharge lamps and photomultipliers which are traditionally more usually associated with the study of UV/visible fluorescence. Throughout the chapter emphasis will be placed on the novel red/near-IR aspects of instrumentation and we will assume that the reader has a knowledge of the basics of steady-state and time-resolved techniques to the level consistent with Volume 1 of this series. 

Where appropriate we will illustrate the instrumentation with applications demonstrating performance. However, to begin with we will review the red/near-IR implementations of the major system techniques and associated kinetics already in widespread use in fluorescence spectroscopy. 

In this section we will review the application of near-IR system instrumentation to the most commonly encountered fluorescence measurements such as steady-state spectra, excited state lifetimes, anisotropy, microscopy, multiplexing, high-performance liquid chromatography (HPLC), and sensors. 

Other near-IR applications which use similar pulse and phase instrumentation as used in lifetime measurements include optical time-domain reflectometry(25) and photon migration in tissue. 26 ... 

Farrens and Song<40) have replaced the original spark source with a picosecond diode laser in a multiplexed dual wavelength T-formatfluorometer.(41)With an overall instrumental response width of ca. 300 psec full-width half-maximum (FWHM), near-IR fluorescence lifetimes as low as 75 psec in the case of l,l -diethyl-4,4 carbo-cyanine iodide (DCI) (excitation 660 nm) and decay components as low as 48 psec in the case of 124 kDa oat phytochrome (excitation 752 nm) were reported. 

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