Spectroscopic instruments

Most spectroscopic measurements involve the use of an appropriate combination of source, dispersive device, and detector to analyze the absorption or emission spectrum of a sample. If only the wavelength or frequency of the radiation is measured, the resultant instrument is called a spectrometer. If the instrument provides a measure of the relative intensity associated with each wavelength, it is called a spectrophotometer, but this fine distinction is often ignored. Absorption spectra are often characterized by the transmittance Tat a given wavelength this is defined by 

Another way of describing spectra is in terms of the absorbance A, where 

The absorbance is related to the path length d of the sample and the concentration c of absorbing molecules by the Beer-Lambert law, 

Visible-Ultraviolet Spectrophotometers. Many commercial spectrophotometers are suitable for the experiments in this text. Often, for instructional purposes, greater insight and more flexibility can be obtained by assembly of an instrument from modular components, available from such sources as Thermo Oriel, Optometries USA, Ocean Optics, or other companies. More commonly an integrated instrument is used to provide greater reliability under use by relatively large numbers of students of varying backgrounds. 

Optical diagram of Varian-Cary models 219 and 2000 series double-beam spectrophotometers (available as models 400 and 500 from Varian). 

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The very first spectroscopic instruments, from Newton s prism and pinhole to Frauenhofer s simple spectroscope, were constructed to observe luminescence. Even though the great sensitivity of luminescence detection seemed to promise that luminescence would become an important tool for chemical analysis, the fact is that absorption spectroscopy was the first spectroscopic technique to be widely used. At first glance, this may seem surprising since absorption spectroscopy is inherently less sensitive and had to await the development of more complex instrumentation, especially, electronically amplified detection. 

A chroaatogreuB provides information regarding the complexity (numlser of components), quantity (peak height or area) and identity (retention par uleter) of the components in a mixture. Of these parameters the certainty of identification based solely on retention is considered very suspect, even for simple mixtures. When the identity can be firmly established the quantitative information from the chromatogram is very good. The reverse situation applies to spectroscopic techniques which provide a rich source of qualitative information from which substance Identity may be inferred with a reasonable degree of certainty. Spectroscopic Instruments have, however, two practical limitations it is often difficult to extract quantitative... 

Direct measurement of soil is most often carried out on air-dried soil and involves spectroscopic instruments and methods. For example, X-ray dispersion (XRD), X-ray fluorescence (XRF), infrared (IR) spectroscopy,... 

Atomic absorption and inductively coupled plasma spectrometers are metal-selective spectrometers used for organic metal analysis. The connection of these spectroscopic instruments to a liquid chromatograph is relatively simple. Chromatograms of alkylmercury3 and aminoplatinum analytes4 are shown in Figures 2.8 and 2.9, respectively. 

Carl Zeiss, Inc. also describes a spectrofluorometer system for process monitoring," but it does not currently appear as a standard marketed product on their web site. HORIBA Jobin Yvon also markets a fluorescent process analyzer, but it is a laser-induced time-domain based measurement system tailored for uranium or equivalent analysis." Finally, while numerous miniature spectrofluorometers are also available (Carl Zeiss, StellarNet Inc., Ocean Optics and Avantes), they are not packaged and configured for process applications. Although there is an established need and continued growing interest in realtime process spectrofluorometry, relative to conventional process spectroscopic instruments such as NIR, UV-vis and Raman, commercial process spectrofluorometers are currently available on a very limited basis. 

E. Bouveresse, D.L. Massart and P. Dardenne, Calibration transfer across near-infrared spectroscopic instruments using Shenk s algorithm effects of different standardisation samples. Anal. Chim. Acta, 297, 405 16 (1994). B.G. Osborne and T. Feam, Collaborative evaluation of universal cahbrations for the measurement of protein and moisture in flour by near-infrared reflectance, /. Food TechnoL, 18, 453 60 (1983). 

The application of atomic spectroscopic instruments as element-specific detectors in chromatography has been reviewed by van Loon More recently, Krull has extensively reviewed their use in high pressure liquid chromatography (HPLC). Atomic spectrometry has found wide acceptance in the field of liquid chromatography because, in most cases, the fractions can be directly analysed after elution from the column. However, it is possible to use the technique for the analysis of solid samples without first dissolving the matrix. This is particularly useful after electrophoresis, where the fractions are fixed either in a gel or on paper. Kamel et al. have shown that it is possible to cut the appropriate sections and insert them into the carbon furnace for analysis. The disadvantage of this approach is that the precision is usually poorer (about 10%) and it is difficult to calibrate the instrument. Nevertheless, this approach is very useful if it is used for qualitative speciation. 

Cost-Effective Spectroscopic Instrumentation for the Physical Chemistry Laboratory 190... 

Unlike the common spectroscopic instruments, the GC detector ordinarily must complete its analysis in a few seconds,... 

Check alphabetical index for a wide range of spectroscopic instruments that are based upon numerous materials-euergy relations. Also see Analysis (Chemical). 

Gives basic information on the measurements of chlorophylls in various spectroscopic instruments. 

Several new methods and instruments based on infrared spectroscopy are being developed for food applications. Advances in spectroscopic instruments and data analysis have enabled the rapid and nondestructive analysis of cheese parameters in just a few seconds (e.g., Nicolet Antaris FT-NIR by Thermo Electron Corp.). Another recent development is the miniaturization of FTIR instrumentation, which would enable onsite analysis, while the cheese is being produced. The TruDefender FT handheld FTIR by Ahura Scientific, Inc. (Fig. 5.7) is a portable handheld spectrometer that could be applied to food analysis. With numerous developments in FTIR spectroscopy and several potential food analysis applications still unexplored, there is great research potential in this technique that could benefit the industry and research institutions. 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Bouveresse, E., Massart, D.L. and Dardenne, P., Calibration Transfer Across Near-Infrared Spectroscopic Instruments using Shenk s Algorithm Effects of Different Standardisation Samples Anal. Chim. Acta 1994, 297, 405—116. 

In THE PAST DECADE, IMPROVEMENTS IN infrared spectroscopic instrumentation have contributed to significant advances in the traditional analytical applications of the technique. Progress in the application of Fourier transform infrared spectroscopy to physiochemical studies of colloidal assemblies and interfaces has been more uneven, however. While much Fourier transform infrared spectroscopic work has been generated about the structure of lipid bilayers and vesicles, considerably less is available on the subjects of micelles, liquid crystals, or other structures adopted by synthetic surfactants in water. In the area of interfacial chemistry, much of the infrared spectroscopic work, both on the adsorption of polymers or proteins and on the adsorption of surfactants forming so called "self-assembled" mono- and multilayers, has transpired only in the last five years or so. 

These components are assembled in different ways to produce several instrument designs. Here we consider two general types of spectroscopic instruments ... 

Probably the most neglected field is that of metal borate complexa-tion in solution. Early proposals of ionic species were based on rather dubious evidence, and a great deal of new experimental work is required. With more sophisticated spectroscopic instruments becoming available, both this phenomenon and the related topic of polyborate ions in solution will be easier to observe. 

When observed using spectroscopes (instruments for forming and examining optical spectra) the Type-0 stars show prominent hydrogen and helium emission and few or no emission lines of the other elements. The next hottest... 

The goal of methods that standardize instrument response is to find a function that maps the response of the secondary instrument to match the response of the primary instrument. This concept is used in the statistical analysis procedure known as Procrustes analysis [97], One such method for standardizing instrument response is the piecewise direct standardization (PDS) method, first described in 1991 [98,100], PDS was designed to compensate for mismatches between spectroscopic instruments due to small differences in optical alignment, gratings, light sources, detectors, etc. The method has been demonstrated to work well in many NIR assays where PCR or PLS calibration models are used with a small number of factors. 

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