Cutoff filters

Fig. 4.18. Detection limit ofTXRF for the residues of aqueous solutions, depending on the atomic number of the analyte element. Three excitations modes were used (a) W-tube, 50 kV Ni-fil-ter, cutoff 35 keV (b) Mo-tube 50 kV Mo filter, cutoff 20 keV (c) W-tube,...

The first second of a stress relaxation step can also show this type of ringing, but it is generally caused by the transducer itself. Thus, the first part of the data may be electronically filtered to remove the transducer ringing by setting a filter cutoff frequency of -40% of the value for the resonant frequency of the transducer and geometry. Some rheometers allow for the measurement of transducer resonant frequency when measuring the geometry inertia. 

Ultrafiltration YM2 filters, cutoff of 2 kDa (Amicon, Danvers, MA) are useful (see Note 8). 

FIGURE 18.12 Effect of multiple layer depth media on filter cutoff behavior and schematic arrangement in a POU filter design. 

Proper selection of the filter medium is more of an art than a science. The filter cutoff must be chosen to capture the smallest particles of interest. Other factors that must be considered are the type of filter (bulk or surface), the required flow rate, and the size of the membrane. These parameters are not independent and the best choice will usually involve trade-offs. Finally, the material from which the filter is made must be considered. It must be selected for compatibility with the intended postfiltration processing. Glass-fiber filters, for example, often have very high blanks for common ions such as chloride and sodium. 

Consider the frequency domain again. The requirement for a sharp cutoff arises because we want to cut off as much noise as possible and the sharper the filter cutoff the closer it can be set to the spectral feature to be saved. It is clear that the filter cutoff needs to be only as sharp as the spectral feature. Therefore, a careful consideration of filter characteristics is most important for high resolution NMR, whereas the RC filter is tolerably good for very broad lines such as those in solids observed by normal NMR. But since the opportunity for some oversampling is much greater for high resolution NMR than in wideline NMR, the absolute necessity for an "ultrasharp" filter is not as great as it is supposed in either case. 

Spatial filter cutoff wavelength should be approximately twice the CMP length. 

Because these filters exhibit a very steep roUoff you can sample at two to three times the filter cutoff frequency. Figure 18.30 shows a trmsfer function of a high-quality antialiasing filter. 

Adsorbant Filter Cutoff ( un) Diameter (cm) Flow Rate (mL min h Column Dimensions Volume Extracted (L) Water Sampling Context and Reference... 

The closer the high-wavenumber end of the spectrum is to the Nyquist frequency, the faster must be the filter cutoff. So-called brick-wall filters have been designed, but they tend to affect the phase of the signal near the Nyquist frequency to a far... 

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