Absorption lines measurement

Fig. 1.9 (a) Water overtone absorption line measured with unmodulated (b) and with phase-modulated single-mode diode laser... 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

When using the DIAL method to measure the concentration of a molecule with discrete absorption the wavelengths of the two laser beams, on and off a narrow absorption line, must be very similar (less than 1 nm separation) so that the background absorption and backscatter is the same for both. 

Technology has been introduced for on-line estimation of the kappa number based on absorption of ultraviolet light (35). This breakthrough ia optical sensor technology permits closed-loop feedback control of digesters from on-line measurement of the kappa number. 

Fig. 40. Flole-burning spectra of thioindigo in benzoic acid crystal at 1.35 K. The scanning laser frequency cu is measured with respect to the burning laser frequency cUb is detuning of the burning laser frequency relative to the center of absorption line.

It would appear that measurement of the integrated absorption coefficient should furnish an ideal method of quantitative analysis. In practice, however, the absolute measurement of the absorption coefficients of atomic spectral lines is extremely difficult. The natural line width of an atomic spectral line is about 10 5 nm, but owing to the influence of Doppler and pressure effects, the line is broadened to about 0.002 nm at flame temperatures of2000-3000 K. To measure the absorption coefficient of a line thus broadened would require a spectrometer with a resolving power of 500000. This difficulty was overcome by Walsh,41 who used a source of sharp emission lines with a much smaller half width than the absorption line, and the radiation frequency of which is centred on the absorption frequency. In this way, the absorption coefficient at the centre of the line, Kmax, may be measured. If the profile of the absorption line is assumed to be due only to Doppler broadening, then there is a relationship between Kmax and N0. Thus the only requirement of the spectrometer is that it shall be capable of isolating the required resonance line from all other lines emitted by the source. 

The method involves the irradiation of a sample with polychromatic X-rays (synchrotron radiation) which inter alia promote electrons from the innermost Is level of the sulfur atom to the lowest unoccupied molecular orbitals. In the present case these are the S-S antibonding ct -MOs. The intensity of the absorption lines resulting from these electronic excitations are proportional to the number of such bonds in the molecule. Therefore, the spectra of sulfur compounds show significant differences in the positions and/or the relative intensities of the absorption lines [215, 220, 221]. In principle, solid, liquid and gaseous samples can be measured. 

A laser-based detection system for NO2 gas (which is an industrial hazard and common environmental pollutant) was developed by Koybayashi et al.4. This was achieved by splitting light, from an Ar-ion multi-line laser, into two paths, one passing through a measurement gas, and the other being transmitted directly to the measurement unit as a reference signal. The detection unit contained two filters to separate the two chosen laser lines, and these were then detected on separate optical receivers. One of these chosen laser lines coincided with a strong absorption line in the NO2... 

Figure 9. C02 Detection variation of modulation Index (m), with optical filter centre wavelength and bandwidth. The broad range of absorption lines causes a very complex variation of modulation indices when using narrow filters (not all peaks at narrow filter bandwidths are shown, as this would obscure the behaviour with wider filter bandwidths). Reference and measurement cells are assumed to he of 1 m length and contain 100% C02 gas at 1 Bar/20 °C.

The vibrational frequency of the special pair P and the bacteriochlorophyll monomer B have also been extracted from the analysis of the Raman profiles [39,40,42,44,51]. Small s group has extensively performed hole-burning (HB) measurements on mutant and chemically altered RCs of Rb. Sphaeroides [44,45,48-50]. Their results have revealed low-frequency modes that make important contribution to optical features such as the bandwidth of absorption line-shape, as well as to the rate constant of the ET of the RCs. 

The intensity of an absorption line in the spectra of atoms or molecules can be measured and it follows the empirical law called the Beer-Lambert law. Consider the amount of light entering a cylinder full of hydrogen gas, as shown in Figure 3.1. If the radiation is absorbed by the hydrogen gas, the amount of light emerging from the opposite end of the cylinder is reduced. 

Characteristic infrared absorption lines have been identified for various hydrogen-acceptor and hydrogen-donor complexes (see Chapter 8), and the strength of such a line in any given specimen is a measure of the quantity of the complex present. However, depth resolution is crude, and masking by free-carrier absorption is sometimes a problem. Raman lines have also been seen (see Chapter 8) and in principle should be capable of detecting species that are not infrared active however, the sensitivity is low, and the most interesting and presumably abundant species, an H2 complex, has not yet been detected in this way. 

With the advent of 8- to 10-metre class telescopes in the 1990s, it became possible to approach the primordial deuterium problem more directly, by measuring the isotopic component in absorption lines of hydrogen due to intervening gas along... 

Nonmetals cannot generally be determined by direct measurement of atomic absorption in a flame because their absorption lines occur in the vacuum ultraviolet region where gases of the flame and atmosphere absorb strongly. Some can be determined by absorption of metastable lines. For example, phosphorous can be determined by the atomic absorption of a metastable line at 2135 A. A number of indirect methods for the determination of nonmetals have been described 19). 

Measurement of integrated absorption requires a knowledge of the absorption line profile. At 2000-3000 K, the overall line width is about 10-2 nm which is extremely narrow when compared to absorption bands observed for samples in solution. This is to be expected, since changes in molecular electronic energy are accompanied by rotational and vibrational changes, and in solution collisions with solvent molecules cause the individual bands to coalesce to form band-envelopes (p. 365). The overall width of an atomic absorption line is determined by ... 

To make accurate measurements of the integrated absorption associated with such narrow lines requires that the linewidth of the radiation source be appreciably smaller than that of the absorption line. In practice, this could be achieved with a continuum source only if expensive instrumentation of extremely high resolving power were used, and it is doubtful whether conventional photomultiplier detectors would be sufficiently sensitive at the resulting low radiation intensities. An alternative arrangement is to... 

Interferences in atomic absorption measurements can arise from spectral, chemical and physical sources. Spectral interference resulting from the overlap of absorption lines is rare because of the simplicity of the absorption spectrum and the sharpness of the lines. However, broad band absorption by molecular species can lead to significant background interference. Correction for this may be made by matrix matching of samples and standards, or by use of a standard addition method (p. 30 et seq.). 

Kamat et al. [1.85] described a process by which the water content in freeze dried sucrose is measured by infra-red spectroscopy (A = 1000 to 2500 nm) and a newly developed fiber optic (Fig. 1.98). However, whether an interpretation of the absorption lines with respect to the water content is possible must be investigated from product to product. The location of the lines and their relative intensity can prevent their necessary discrimination. For pure... 

Fig.27. )P and absorption spectra of an MOPPV LB film. Open and solid circles show value of x(3) of the MOPPV LB film in the direction parallel and perpendicular to the dipping direction. Open triangles show x of an MOPPV cast film. Solid and broken lines show the absorption spectra measured by the polarized light parallel and perpendicular to the dipping direction, respectively. Dotted line shows an absorption spectrum of the MOPPV cast film.

In the method described by Willie et al. [167] atomic absorption measurements were made with a Perkin-Elmer 5000 spectrometer fitted with a Model HGA 500 graphite furnace and Zeeman effect background correction system. Peak absorbance signals were recorded with a Perkin-Elmer PRS-10 printer-sequencer. A selenium electrodeless lamp (Perkin-Elmer Corp.) operated at 6W was used as the source. Absorption was measured at the 196.0nm line. The spectral band-pass was 0.7nm. Standard Perkin-Elmer pyrolytic graphite-coated tubes were used in all studies. 

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