Polychromator

J. S. Mills and P. Smith, eds.. Cleaning, Ketouching and Coatings, Technology and Practice for Easel Paintings and Polychrome Sculpture, The International Institute for Conservation of Historic and Artistic Works, London, 1990. 

In 1979, a viable theory to explain the mechanism of chromium electroplating from chromic acid baths was developed (176). An initial layer of polychromates, mainly HCr3 0 Q, is formed contiguous to the outer boundary of the cathode s Helmholtz double layer. Electrons move across the Helmholtz layer by quantum mechanical tunneling to the end groups of the polychromate oriented in the direction of the double layer. Cr(VI) is reduced to Cr(III) in one-electron steps and a colloidal film of chromic dichromate is produced. Chromous dichromate is formed in the film by the same tunneling mechanism, and the Cr(II) forms a complex with sulfate. Bright chromium deposits are obtained from this complex. 

Figura 3 Grating spectrometers commonly used for ICP-OES (a) monochromator, in which wavelength is scanned by rotating the grating while using a singie photomultiplier tube (PMT) detector (b) polychromator, in which each photomultiplier observes emission from a different wavelength (40 or more exit slits and PMTs can be arranged along the focal plane) and (c) spectrally segmented diode-array spectrometer.

Direct-reading polychromators (Figure 3b) have a number of exit slits and photomultiplier tube detectors, which allows one to view emission from many lines simultaneously. More than 40 elements can be determined in less than one minute. The choice of emission lines in the polychromator must be made before the instrument is purchased. The polychromator can be used to monitor transient signals (if the appropriate electronics and software are available) because unlike slew-scan systems it can be set stably to the peak emission wavelength. Background emission cannot be measured simultaneously at a wavelength close to the line for each element of interest. For maximum speed and flexibility both a direct-reading polychromator and a slew-scan monochromator can be used to view emission from the plasma simultaneously. 

Polychroismtis, m. polychroism, pleochroism. polychromatisch, a. polychromatic, polychrome. PolychromsMure,/. polychromic acid, polycyclisch, a. polycyclic. 

Chromium plating from hexavalent baths is carried out with insoluble lead-lead peroxide anodes, since chromium anodes would be insoluble (passive). There are three main anode reactions oxidation of water, reoxidation of Cr ions (or more probably complex polychromate compounds) produced at the cathode and gradual thickening of the PbOj film. The anode current density must balance the reduction and reoxidation of trivalent chromium so that the concentration reaches a steady state. From time to time the PbOj film is removed as it increases electrical resistance. 

The molecular structure of the anchored Cr(VI) has been a strong point of discussion in the literature, and several molecular structures (monochromate, dichromate, polychromates) have been proposed (see Scheme 3). The nature of the silica support, the chromium loading, and the activation method can all influence the chemical state of the supported chromium. 

Scheme 3 Anchoring reaction of chromate on a silica support. Adjacent surface hydroxyl groups are consumed and chromiimi attaches to the siuface by oxygen linkages, either in mono-, di- or polychromate forms...

Figure 2.5 A tip-enhanced near-field Raman microscope which we have developed. The microscope is based on AFM for control of the metallic nano-tip, an inverted optical microscope for illumination/collection of the light field and a polychromator for measurement of the Raman signal.

Present address Polychrome GmbH, Seesener Str. 11, D-37520 Osterrode, Germany. 

Chromium zeolites are recognised to possess, at least at the laboratory scale, notable catalytic properties like in ethylene polymerization, oxidation of hydrocarbons, cracking of cumene, disproportionation of n-heptane, and thermolysis of H20 [ 1 ]. Several factors may have an effect on the catalytic activity of the chromium catalysts, such as the oxidation state, the structure (amorphous or crystalline, mono/di-chromate or polychromates, oxides, etc.) and the interaction of the chromium species with the support which depends essentially on the catalysts preparation method. They are ruled principally by several parameters such as the metal loading, the support characteristics, and the nature of the post-treatment (calcination, reduction, etc.). The nature of metal precursor is a parameter which can affect the predominance of chromium species in zeolite. In the case of solid-state exchange, the exchange process initially takes place at the solid- solid interface between the precursor salt and zeolite grains, and the success of the exchange depends on the type of interactions developed [2]. The aim of this work is to study the effect of the chromium precursor on the physicochemical properties of chromium loaded ZSM-5 catalysts and their catalytic performance in ethylene ammoxidation to acetonitrile. 

Proteins in Colour Layers of Easel Paintings, Murals, Polychrome Statues, etc. 

Colombini M.P., Fuoco R., Modugno F., Menicagli E., Giacomelli A., GC MS characterisation of proteinaceous and lipid binders in UV aged polychrome artefacts, Microchemical Journal, 2000,67,291 300. 

F. Ronca, Protein Determination in Polychromed Stone Sculptures, Stuccoes and Gesso Grounds, Studies in Conservation, 39, 107 120 (1994). 

A. Casoli, P.C. Musini, G.Palla, Characterization of Binding Media from Ancient Polychrome Sculptures by means of GC/MS, Chromatographia, 42, 421 430 (1996). 

Polychromator, 23 144 Polyclonal antibodies DAS-conjugate, 14 145 versus monoclonal antibodies,... 

Gosser, D. C., Ohnersorgen, M.A., Simon, A.W., and Mayer, J. W. (1998). PIXE analysis of Salado polychrome ceramics of the American Southwest. Nuclear Instruments and Methods in Physics Research B 136-138 880-887. 

Labtam Plasma Scan 8440 Simultaneous (polychromator or with optional monochromator for sequential) 60-70... 

FIGURE 9.21 An illustration of the optical path of an ICP instrument that utilizes the simultaneous direct reading polychromator for the measurement of spectral lines. 

The advantage of ICP is that the emissions are of such intensity that it is usually more sensitive than flame AA (but less sensitive than graphite furnace AA). In addition, the concentration range over which the emission intensity is linear is broader. These two advantages, coupled with the possibility of simultaneous multielement analysis offered by the direct reader polychromator design, make ICP a very powerful technique. The only real disadvantage is that the instruments are more expensive. See Workplace Scene 9.3. 

C. D. Tran and R. J. Furlan, Acoustic-optic tunable filter as a polychromator and its application in multidimensional fluorescence spectrometry, Anal. Chem. 64, 2775-2782(1992). 

The use of a linear detector array in the image plane of a polychromator in place of the fluorescence monochromator in Figure 12.1 enables the parallel data accumulation of complete fluorescence spectra. Silicon photodiode arrays, operated in a CCD mode(34) are the most widely used detector elements. The spectral response of the diodes enables fluorescence to be detected from the near-UV up to ca. 1100 nm with a peak response in the near-IR. Up to 8192 elements are now available commercially in a single linear array at low cost. However, the small length of each element (ca. 10 [im) presently limits sensitivity and hence cylindrical lens demagnification is often necessary. 

Streak cameras and multianode microchannel plate photomultipliers (MCP-PMs) interfaced to a polychromator also permit multiwavelength fluorescence decay measurements, the spectral response of both being determined by the photocathode composition. 

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