Transmittance radiation sources

The fundamental elements of an ultraviolet-absorption analyzer include (a) a radiation source (b) suitable optical filters (c) a sample cell and (d) an output meter. A transmittance measurement is made by calculating the ratio of the reading of the output with die sample in the cell to die reading widi the cell empty (of ultraviolet-absorbing materials). The concentration can be calculated from the known absorptivity of the substances as previously demonstrated by the equations or it may be determined by comparison with known samples. 

Homogeneous irradiation is impossible for high optical density system, radiation The degree of transmittance depends on the radiation source. 

Identification of various crystalline phase in glass-ceramics sample was carried out using powder X-ray diffractormeter with Cu Ka as X-ray radiation source. The microstructure of the glass-ceramic was observed using SEM, the transmittance of the glass-ceramic was learned by UV-VlS-NlR spectrometer and the emission spectrum was investigated by fluorescence... 

A detector must have adequate sensitivity to the radiation arriving from the sample and monochromator over the entire spectral region required. In addition, the source must be sufficiently intense over the wavenumber range and transmittance range. Sources of infrared emission have included the Globar, which is constructed of silicon carbide. There is also the Nemst filament, which is a mixture of the oxides of zirconium, yttrium and erbium. A Nemst filament only conducts electricity at elevated temperatures. Most detectors have consisted of thermocouples of varying characteristics. 

INTERACTIONS BETWEEN SEVERAL RADIATION SOURCES AND CERTAIN POLYMER SURFACES REFLECTANCE - TRANSMITTANCE CHARACTERISTICS... 

It has been found, by the authors, that monochromatic emissive power relationships for different radiation sources are not easily obtained in the literature. For that reason, the various plots of intensity vs. wavelength are included for general information as well as provide a data source for those requiring emissive power information for different radiation systems. Further, Figures 13-30 indicate the relationship of spectral reflectance and spectral transmittance for several transparent or translucent polymers. 

The essential features of an NMR spectrometer shown m Figure 13 5 are not hard to understand They consist of a magnet to align the nuclear spins a radiofrequency (rf) transmitter as a source of energy to excite a nucleus from its lowest energy state to the next higher one a receiver to detect the absorption of rf radiation and a recorder to print out the spectrum... 

The attenuation of electromagnetic radiation as it passes through a sample is described quantitatively by two separate, but related terms transmittance and absorbance. Transmittance is defined as the ratio of the electromagnetic radiation s power exiting the sample, to that incident on the sample from the source, Pq, (Figure 10.20a). 

Finally, values of sx are directly proportional to transmittance for indeterminate errors due to fluctuations in source intensity and for uncertainty in positioning the sample cell within the spectrometer. The latter is of particular importance since the optical properties of any sample cell are not uniform. As a result, repositioning the sample cell may lead to a change in the intensity of transmitted radiation. As shown by curve C in Figure 10.35, the effect of this source of indeterminate error is only important at low absorbances. This source of indeterminate errors is usually the limiting factor for high-quality UV/Vis spectrophotometers when the absorbance is relatively small. 

Determining Concentration by Turbidimetry In turbidimetry the measured transmittance, T, is the ratio of the transmitted intensity of the source radiation, fy, to the intensity of source radiation transmitted by a blank, Iq. 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

For radiant heating it is not sufficient to know just the magnitude of the incident heat flux to determine the temperature rise. We must also know the spectral characteristics of the source of radiation and the spectral properties (absorptivity, a, transmittance, t, and reflectance, f> ) of the material. Recall that... 

The radiation may be due to emissions from a hot source, or to the luminescence, fluorescence or phosphorescence of the sample. An emission spectrum consists of a number of generally very narrow peaks (called spectral lines) occurring at certain wavelengths which are characteristic of the materials contained within the source. The amplitudes of the peaks are related to the abundance or concentration of the materials present. Alternatively, radiation from a source is passed through a sample. In this case the quantity absorbed by the sample at a particular wavelength is again characteristic of the materials present in the sample. This is termed absorption spectrometry and produces spectral transmission lines in the form of equally narrow valleys—or peaks (Fig. 6.42) where the information is expressed in terms of absorbance (si) rather than transmittance (20<57>, and ... 

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... 

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