Radiant density

Particular problems of photochemical engineering are related to the scaling-up of photoreactors. This is mainly due to problems of lamp technology related to the variations of the radiant exitance M with the increase of the lamp s geometry and electrical input power. Thus, to carry out a reasonable scaling-up and optimization of photoreactors the radiant exitance M or the radiant density (expressed as the ratio of radiant power P to the arc length I of the lamp in W cm , see Tab. 4-1) of the lamps used must be fixed (Braun et al., 1993 a). This, however, is a challenge for the manufacture of lamps. 

The intensity per unit of wavelength (radiant density Bx) is obtained by multiplying with 4-nc/X1. Accordingly, for a hydrogen plasma the intensity of the Bremsstrahlung is given by ... 

A continuous source and a continuously measuring detector here the signal is proportional to the mean radiant density of the source B/ve and the measurement time to, the shot noise Ns (being of) is proportional to the signal fJ/ve and to, and... 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

In most instruments, the radiant flux is modulated periodically. This can be achieved by modulating the current of the primary source or with the aid of a rotating sector (g) in the radiation beam. Accordingly, it is easy to differentiate between the radiant density emitted by the primary source and that emitted by the flame. Both single beam and dual-beam instruments (see also Fig. 77) are used. In the latter the first part of the radiation of the primary source is led directly into the monochromator, whereas the second part initially passes through the flame. In this way fluctuations and drift can be compensated for insofar as they originate from the primary radiation source or the measurement electronics. Furthermore, the spectrometer can be provided with equipment for a quasi-simultaneous measurement of the line and background absorption [253]. 

Furthermore, the radiant density of the D2 lamp in a large part of the spectrum is fairly low. Hence, the procedure limits the number of analytical lines which can be used and the number of elements that can be determined. As the spectral radiance of the D2 lamp is generally low as compared with that of a hollow cathode lamp, the latter must be operated at a low radiant output (low current), which means that detector noise limitations and poor detection limits are soon encoun-terd. Finally, as work is carried out with two primary radiation sources, which are difficult to align as they have to pass through the same zone of the atom reservoir, this may lead to further systematic errors. 

Cu, oo describes the influence of the radiation source, (dfJy/dl) is the spectral radiation density for the background intensity, B0 is the radiant density for an analytical line at the concentration c = 1 and A7L is the physical width of the analysis line. The second term (Ai) describes the influence of the spectral apparatus. 

A simplified model for the fluorescence process can be drawn up for two levels [662]. When a two-level system is considered (Fig. 125) and excitation is expected to occur only as a result of absorption of radiation with radiant density pv, without any contributions from collision processes to the excitation, the population of the excited level (n2) can be given by ... 

When the absorption of radiation increases up to a certain value A2i and fe21 become negligible. Then n2 = nT/2 and becomes independent of the radiant density of the exciting radiation and a state of saturation is reached. This situation can be realized when lasers are used as primary sources. 

The radiating level may often be somewhat below the level to which pumping occurs and the transition between the two levels can be radiationless and take place through heat losses. Also the fluorescence transition sometimes does not end at the ground level. With two levels resonance fluorescence occurs, where stray radiation from the exciting radiation limits the power of detection. In the case of three-level systems there is non-resonant fluorescence, where this limitation does not apply. However, here the radiant densities are much lower. Therefore, populating the excited level will only be sufficiently successful when using very intensive primary sources such as tunable lasers. 

In atomic fluorescence work, the radiant density of the primary source can vary with ... 

As primary sources, continuous sources such as a tungsten halogenide or a deuterium lamp can be used. They have the advantage that multielement determinations are possible. However, because of the low radiant densities saturation is not obtained and the power of detection is not fully exploited. With line sources such as hollow cathode sources and electrodeless discharge lamps much higher radiances can be obtained. Even ICPs into which a concentrated solution is introduced can be used as a primary source, through which multielement determinations become possible. 

Fairly short reach due to the reduction of radiant density flux wifh disfance. 

A continuous source and a continuously measuring detector here the signal is proportional to the mean radiant density of the source and the measurement... 

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