Line-to-background ratio

Ix/Iu is the smallest line-to-background ratio that can still be measured. When the definition is based on a 99.86% probability, it is given by 3 times the relative standard deviation of the background intensities. 

This can be performed successfully both on metal electrodes (e.g. copper) or with graphite electrodes which have been previously impregnated with a solution of Apiezon (e.g. in benzene). In the latter case carriers such as CsCl may be useful so as to increase the line to background ratios as a result of their influence on the current voltage characteristics [354]. The copper arc method has found use in the determination of the rare earths in solutions, which was historically important in radiochemistry related topics [355-357]. 

Figure 11.5 Figure of merit Fm = (integration limits and the line-to-background ratio L/B. TTie curves are normalized to unity at their maxima. (Reprinted by courtesy of EG G ORTEC.)... 

Owing to the long residence times in the excitation zone and the high line-to-background ratios. resulting from the nonequilibrium character of the plasma (gas temperatures of 600-1000 °C... 

WDSs have excellent resolving power, and the peak-to-background ratio of each line is much higher than can be achieved with a crystal detector. With a suitable crystal of large lattice spacing it is possible to detect and count X-rays as soft as boron K or even beryllium K , and this type of spectrometer is widely used when... 

In practical terms, for a 500-dpm peak with a base width of 20 s, a counting efficiency of 70% and a residence time of 12 s, we would observe 70 counts, which would be spread out over the peak width, plus the residence time (a total of 32 s). For a detector that has a 10-cpm background, we would observe 5.3 counts over the same time period of 32 s. The ratio of the observed peak counts to background would be 13 1 and the peak should be clearly visible. However, under the same conditions, a 50-dpm peak would have a count to background ratio of only 1.3 1 and it is doubtful that this peak could be observed in online counting. In the latter situation, off-line radio-... 

At an excitation frequency of 50 MHz, detection limits are better than at 100 MHz (Boumans, 1989). As thallium is easily excitable, optimum signal to background ratios are obtained at low powers (Montaser et al., 1981). The most sensitive emission line in the ICP is probably at 190.864 nm (Barnes, 1980 Boumans, 1989), but vacuum conditions are needed. 

If the X-ray tube power is increased to 400 W and a Pd target tube is used in the geometry shown in Fig. 3 together with a HOPG (highly oriented pyrolithic graphite) crystal, Pd L-lines can be used for the excitation. This will lead to an improvement in the sensitivity, which can be seen in Fig. 4, where a spectrum, measured with a sample with sulfur content of 100 mg/kg, was analyzed. The peak-to-background ratio is improved to 3.7 1 the intensity of the S fluorescence line is increased by a factor of 3 in comparison to the example of direct excitation from above. 

By reflecting and focusing the X-ray beam, a higher flux of excitation radiation is transmitted to the sample. This increases the sensitivity, and lower power X-ray (less than 50 W) tubes can be utilized. A spectrum with a sulfior in oil sample at a sulfur concentration of 100 mg/kg is displayed in Fig. 6. The peak-to-background ratio is improved to 5.5 1, and the intensity of the sulfur fluorescence line is increased by a factor of 2 in comparison to the example of polarization excitation and by a factor of 6 in comparison to the example of direct excitation fi om above. 

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