Wavelengths selection

A wavelength selector that uses either absorption, or constructive and destructive interference to control the range of selected wavelengths. 

Band of radiation exiting wavelength selector showing the nominal wavelength and effective bandpass. 

A wavelength selector that uses a diffraction grating or prism, and that allows for a continuous variation of the nominal wavelength. 

Effect of the monochromator s slit width on noise and resolution for the ultraviolet absorption spectrum of benzene. The slit width increases from spectrum (a) to spectrum (d) with effective bandpasses of 0.25 nm, 1.0 nm, 2.0 nm, and 4.0 nm. 

Typical grating monochromator with inset showing the dispersion of the radiation by the diffraction grating. 

In order to plot the absorption spectrum of a compound or complex ion, we must be able to carefully control the wavelengths from the broad spectrum of wavelengths emitted by the source so that we can measure the absorbance at each wavelength. Additionally, in order to perform quantitative analysis by Beer s law, we need to be able to carefully select the wavelength of maximum absorption, also from this broad spectrum of wavelengths, in order to plot the proper absorbance at each concentration. These facts dictate that we must be able to filter out the unwanted wavelengths and allow only the wavelength of interest to pass. 

FIGURE 8.2 An illustration of how a red-colored glass filter isolates the red region of the visible spectrum. 

The word monochromator is derived from the Latin language, mono meaning one and chromo meaning color. It is a device more sophisticated than an absorption filter that isolates the narrow band of wavelengths from visible and ultraviolet sources. 

interchangeable colored glass filters used in a flow injection analyzer for different analytes. Notice the value of the wavelength inscribed on the edge of each filter. Right, Steve Kruse of the City of Lincoln, Nebraska, Wastewater Treatment Plant Laboratory prepares to change the filter in the instrument as he prepares to test for a different analyte in wastewater samples. 

FIGURE 8.4 Rotation of the dispersing element directs different wavelengths to emerge from the exit slit. 

The simplest monochromatization tool that can be used in powder diffractometry is a P-filter (also see Chapter 2, section 2.3.2.1). Filter materials have their K-absorption edges just above the wavelength of the strongest Kp spectral line of the anode of the x-ray tube, and their performance is based on how completely they absorb the characteristic impurity wavelengths and how well they transmit the desired parts of the characteristic x-ray spectrum. For example, to eliminate (absorb) nearly all the p-component of a copper anode (X, = 1.39 A) but to transmit most of the 

On a downside, high quality crystal monochromators are relatively expensive, and they also reduce the intensity of characteristic x-rays by a factor of two to three. It is worth noting that when the monochromator is made from a low quality single crystal or when it is improperly aligned, the resulting reduction of the transmitted intensity may be severe. 

The primary decision, and often the most difficult, is the choice of a suitable chromophore which will provide information pertinent to the problem under consideration. In the case of intrinsic chromophores this must be a choice of system, namely the one in which the chromophore occurs and in which it exhibits the desired properties. For example, the carotenoid pigments which record the light-dependent membrane potential associated with photosynthesis. 

Such pigments occur in many photosynthetic membranes, but their use as molecular voltmeters is most easily accomplished in preparations, known as chromatophores, from certain photosynthetic bacteria such as Rhodobacter sphaeroides or Bhodopseudomonas capsulata. Again, studies of oxygen consiunption and delivery in vivo require highly vascularized tissues which contain many mitochondria, for example, brain, liver or cardiac muscle (although in the latter case the myoglobin contribution cannot be resolved from that of haemc obin). 

In each case, however, the oxygen carriers tend to mask the much less abundant mitochondrial pigments. It is not always easy to resolve contributions from endogenous pigments in cell suspensions as the cell density required to give a suitable output is often too large to be compatible with provision of an adequate oxygen supply. 

Whether the final decision is to use an endogenous or an added chromo- 

Instrument compatiMity. Only chromophores with characteristics compatible with the instruments available are suitable. The main features to consider are excitation wavelength, availability of appropriate emission filters and the sensitivity of the detector. For example, many dyes tvith red fluorescence are usually adequately excited by 543 nm laser light and detected via the rhodamine filter sets provided vwth commercial fluorescence microscopes likewise dyes with green fluorescence are suited to the fluorescein set up. 

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Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

Laser Photochemistry. Photochemical appHcations of lasers generally employ tunable lasers which can be tuned to a specific absorption resonance of an atom or molecule (see Photochemical technology). Examples include the tunable dye laser in the ultraviolet, visible, and near-infrared portions of the spectmm the titanium-doped sapphire, Tfsapphire, laser in the visible and near infrared optical parametric oscillators in the visible and infrared and Line-tunable carbon dioxide lasers, which can be tuned with a wavelength-selective element to any of a large number of closely spaced lines in the infrared near 10 ]lni. 

In an attempt to shed some light on the wavelength selection Datye and Langer [139] considered finite amplitude perturbations of the local wavelength. This type of approach was used in a somewhat refined version by... 

Prepare a benzene-toluene mixture by placing 0.05 mL of each liquid in a 25 mL graduated flask and making up to the mark with methanol. Take 1.5 mL of this solution, place in a lOmL graduated flask and dilute to the mark with methanol this solution contains benzene at the same concentration as solution 5, and toluene at the same concentration as solution 5. Measure the absorbances of this solution at the two wavelengths selected for the Beer s Law plots of both benzene and toluene. Then use the procedure detailed in Section 17.48 to evaluate the composition of the solution and compare the result with that calculated from the amounts of benzene and toluene taken. 

It is important to note that there are many publications which discuss optimal ways of selecting individual spectral wavelengths for use with ILS. Much of this work comes from the near infrared (NIR) community. It provides many examples of the power of intellegent wavelength selection. Unfortunately these methods often require more computional time and power than is convenient. 

Mark, H. "A Monte Carlo Study of the Effect of Noise on Wavelength Selection during Computerized Wavelength Searches", Appl. Spec. 1988 (8) 1427-1440. 

The presentation in this paper concentrates on the use of large-scale numerical simulation in unraveling these questions for models of two-dimensional directional solidification in an imposed temperature gradient. The simplest models for transport and interfacial physics in these processes are presented in Section 2 along with a summary of the analytical results for the onset of the cellular instability. The finite-element analyses used in the numerical calculations are described in Section 3. Steady-state and time-dependent results for shallow cell near the onset of the instability are presented in Section 4. The issue of the presence of a fundamental mechanism for wavelength selection for deep cells is discussed in Section 5 in the context of calculations with varying spatial wavelength. 

Figure 2. UV light-induced deposition of silver nanoparticles (al-a3) and wavelength-selective visible light-induced dissolution of silver nanoparticles (bl-b3).

W.R. Hruschka, Data analysis wavelength selection methods, pp. 35-55 in P.C. Williams and K. Norris, eds. Near-infrared Reflectance Spectroscopy. Am. Cereal Assoc., St. Paul MI, 1987. P. Geladi, D. McDougall and H. Martens, Linearization and scatter-correction for near-infrared reflectance spectra of meat. Appl. Spectrosc., 39 (1985) 491-500. 

D. Jouan-Rimbaud, D.L. Massart, R. Leardi, et al.. Genetic algorithms as a tool for wavelength selection in multivariate calibration. Anal. Chem., 67 (1995) 4295 301. 

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