Line to background intensity 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. 

Fig. 20 shows the observed interstellar molecular lines of various isotopic species of formaldehyde, H2CO, as detected by Gardner et al., 1971. This particular line, the lowest asymmetry-doublet transition 110 — lu, is seen in absorption in the continuum radiation of the strong radio source Sgr B2, which is located behind the molecular gas cloud. Frequency is plotted along the abscissa and the ordinate is intensity, expressed in the ratio of line-to-continuum antenna temperatures. For all three formaldehyde isotopes the continuum temperature is Tc T >b Tex- This is the case because the formaldehyde molecules are in approximate equilibrium with the microwave background... 

Quantitative analysis is used to determine the mass concentration of one or more elements in a given area of the sample. It is based on measuring, for each element, the " it-ratio, known as the apparent concentration and defined as the ratio of the net intensity of the radiation emitted by the sample to the net intensity of the same radiation emitted by a pure standard, these intensity levels being measured under the same electron bombardment and radiation detection conditions (the net intensity is defined as the difference between the total intensity measured on the peak of the characteristic line of clement A and the background level measured on each side of the peak). If the standard used is complex, corrections must be made to this reference for the measured intensity ratio reference converted to apparent concentration of a fictive pure standard. 

Figure 3. MAS spectrum at I g = 1.01 kHz, shown on two different vertical scales. Spinning sidebands are labeled according to their corresponding centerbands. Centerbands are indicated by asterisks. Baseline curvature is due to probe background. The ratios of sideband to cenlerband intensities are greater for C70 lines than for Cjo lines, indicating that the rotation of C70 molecules is more anisotropic.

A powder pattern of iron is made with Cu Ka radiation. Assume that the background is due entirely to fluorescent Ka. radiation from the specimen. The maximum intensity (measured above the background) of the weakest line on the pattern is found to be equal to the background intensity itself at that angle. If the film is covered with aluminum foil 0.0015 in. thick, what will be the ratio of / ax for th S line to the background intensity ... 

When the ratio of peak intensity to background for a particular fluorescent line... 

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. 

The principle function of a continuum source background corrector is depicted in Figure 76. The exit slit of the monochromator separates the resonance line of the analyte (half-width about 0.002 nm) from the emission spectrum of the line-like radiation source, and a band of radiation from the continuum spectrum of the deuterium lamp equivalent to the bandpass of the slit (usually 0.2 to 0.7 nm). The intensity of the hollow cathode lamp (/hd) is equalized to the intensity of the deuterium lamp (/ i) before the determination. When the ratio /di//hd = 1, no reading shows on the display. When a... 

A further complication is encountered where the analyte line may be partially overlapped by a second line with a peak at [ [Fig. 9.3(b)]. The best solution in this case is to estimate the contribution of i at p, either by calibration or by means of a blank, and to determine the intensity ratio of i at Op and By assuming that this ratio Ki is constant, the background at Op can be taken as Kjli. In the case of energy-dispersive spectrometers where peak areas are normally used, a simple mathematical solution based on area interference can be employed. However, in practice it is more common to employ a computer-applied least-squares fitting technique (see Chap. 6). 

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