Ultraviolet-Visible-Near Infrared limitation

Electronic Circular Dichroism In contrast to most organic compounds in which CD measurements are limited to the ultraviolet region, most metal complexes possess d-d absorption bands in the more accessible visible and near-infrared regions, allowing for relatively easier application of electronic circular dichroism (ECD) measurements. In fact, the first observation by Cotton of optical rotation measurements through an absorption band and interpretation in terms of differential absorption of the circularly polarized beam was performed on solutions of L-tartrate chromium(III) and copper(II) complexes.100... 

These devices are based on the anisotropic absorption of light. Usually molecular crystals exhibit this property and tourmaline is the classical example for this. For practical purposes, however, micro crystals are oriented in polymer sheets. Polymers containing chromophors become after stretching dichroic polarizers. The devices produced in this manner are called polawids. They have found a broad application in many technologies. Their application in spectroscopy is limited to the near ultraviolet and to the visible and near infrared range of the spectrum. In vibrational spectroscopy polaroids are employed as analyzers only for Raman spectroscopy. 

Phosphors. Infrared radiation can stimulate the emission of visible radiation from certain types of phosphors which have been previously excited by ultraviolet radiation. Becker and Risgin [2.71] have reported that self-activated ZnS crystals have exhibited both visible luminescence and photoconductivity when stimulated by radiation in the 1-3 pm interval. Geusic et al. [2.72] have shown that rare earth phosphors can convert very near infrared (0.93 pm) radiation into the visible with an efficiency as high as 1 %. Although phosphors can be employed to convert images directly, their lack of sensitivity has limited their usefulness. 

Quantitative infrared spectroscopy can provide certain advantages over other analytical techniques. This approach may be used for the analysis of one component of a mixture, especially when the compounds in the mixture are alike chemically or have very similar physical properties (for example, structural isomers). In these instances, analysis using ultraviolet/visible spectroscopy, for instance, is difficult because the spectra of the components will be nearly identical. Chromatographic analysis may be of limited use because separation, of say isomers, is difficult to achieve. The infrared spectra of isomers are usually quite different in the fingerprint region. Another advantage of the infrared technique is that it can be non-destructive and requires a relatively small amount of sample. 

The multiplex advantage is important enough so that nearly all infrared spectrometers are now of the Fourier transform type Fourier transform instruments are much less common for the ultraviolet, visible, and near-infrared regions, however, because signal-to-noise limitations for spectral measurements with these types of radiation are seldom a result of detector noise but instead are due to shot noise and flicker noise associated with the source. In contrast to detector noise, the magnitudes of both shot and flicker noise increase as the radiant power of the signal increases. Furthermore, the total noise for all of the resolution elements in a Fourier transform measurement tends to be averaeed... 

Any equipment that would expose personnel to high levels of visible light and near infrared (400-300 nm) (>1 cd/cm ), infrared (770-3000 nm) (>10w/cm ), or ultraviolet (UV) radiation (180-400 nm) (see table for allowable limits). 

Solid-phase techniques have seen some limited use in the ultraviolet, visible, and near infrared. Wyman has discussed the use of the KBr technique in the ultraviolet... 

The advantages of Fourier transform spectrometry over the use of a scanning monochromator (often referred to as dispersive spectrometry) is fully valid only when the detector noise is independent of the power of the radiation incident on the detector. When the detector is photon shot-noise limited [as it generally is for a photomultiplier tube (PMT), and often is for other sensitive detectors used in the near-infrared, visible, and ultraviolet spectral regions], the noise level is proportional to the square root of the incident power. For a boxcar spectrum, this means that shot noise is proportional to the square root of the number of resolution elements in the spectrum, This disadvantage therefore precisely offsets Fellgett s advantage when continuous broadband sources are employed. It should also be... 

Fiber optics may be purchased that transmit radiation firom the ultraviolet (190 nm) to the infrared (>5 /x.m), but each has a limited range. Plastic and compound glass materials are used for short distances in the visible region, while silica fibers can be used from the UV through the near-IR (2.3 /rm) regions, but they are very costly. Fluoride and calcogenide glasses extend farther into the infrared. 

---