Peak filter, secondary

Secondary peak filter single Signal-to-noise threshold at the back-projected peak positions (see text)... 

To obtain a more monochromatic source peak with lower background than with filters, secondary sources are used. A secondary source can be produced by placing a target element between the X-ray tube and the sample, as shown in Fig. 1.12. The X-ray tube excites the secondary target and the Secondary target fluoresces and excites the sample. The detector as usual detects the X-rays from the sample. Because simple collimation blocks unwanted X-rays, it happens to be a highly inefEcient method. Focusing optics like polycapillary devices (used in microbeam XRF) have been developed so that the beam could be redirected and focused on a small spot (less than 100 pm spot size). 

Another major source of noise is the loop consisting of the output rectifiers, the output filter capacitor, and the transformer secondary windings. Once again, high-peak valued trapezoidal current waveforms flow between these components. The output Alter capacitor and rectifier also want to be located as physically close to the transformer as possible to minimize the radiated noise. This source also generates common-mode conducted noise mainly on the output lines of the power supply. 

Figure 4. GC/NICI-MS ion current profiles for 79Br and 81 Br in secondary effluent and GC-filtered effluent. Ion current ratios were as shown. Note peak labeled was not brominated (no m/z = 81).

Figure 8. Effects of AgNC>3 on GC-NICl ion current profiles of 35Cl and 37 Cl in GAC filter influent (secondary effluent) and effluent (GAC effluent). Peaks labeled in untreated effluents were reduced by AgNC>3. Peaks labeled m in AgNC>3-treated effluents were increased by Ag. (Continued on next page.)...

When analyzing a spectrum, various factors such as the secondary fluorescence, autoabsorption of X-rays by the target and filters, escape peaks, and electronic pile-up have to be taken into account. With all these possible errors, the confidence in the results is near 5% in the case of thick targets for which... 

Fluorescence is an extremely sensitive technique but it is not suitable as a general method to estimate natural DOC content due to the reason that it is impossible to find a reference material that would be common for all different natural waters. Characteristic for different fluorescence studies of NOM/DOM is that they may occasionally be somewhat surprising, contradictory, or laboriously explicable. The main reason for this incoherence is that fluorescence measurements are affected by many environmental factors, e.g., type of solution, pH, ionic strength, temperature, redox potential of the medium, and interactions with metal ions and organics. Several corrections are required to obtain a reliable and comparable spectrum, e.g., instrumental factors, Raman water peak, scattering effects (primary and secondary inner filter effects [31,32]), arbitrary fluorescence units should be standardized (dihydrate of quinine sulfate), etc. 

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