Mobility filters

Although the timescale and the principle of mobility filters are readily outlined in this way, the validity of the representation is limited, because relaxation times depend on the spectral densities of molecular motion at more than one frequency, which is neglected in Fig. 7.1.3. In addition, the spectral densities relevant to NMR of condensed matter... 

Fig. 10.3.7 Pulse sequence for spin-diffusion imaging with ID spatial resolution [Wei8] and effect of mobility filters, (a) The magnetization source is selected by the dipolar filter which suppresses the magnetization in the sink. During the spin-diffiision time the magnetization dif ses from the source to the sink, (b) The dipolar filter selects magnetization from chain segments which are highly mobile and intermediately mobile. By use of a lineshape filter the signal loss is analysed only for the mobile components. IP(Tc) is the probability for a particular correlation time to arise in the sample. It is essentially the spectral density of motion.

Of particular importance for detection of chemical or physical change in polymer materials are mobility filters, which are sensitive to differences in the numbers of molecules within a given window of correlation times. Within reasonable approximation such filters are relaxation filters. Here, Tj filters are sensitive to differences in the fast motion regime while T2 and Tip filters are sensitive to the slow motion regime. Which time window is of importance can be seen from Fig. 5.7 [101]. It shows a double-logarithmic plot of the mechanical relaxation strengths Hi(t) for two carbon-black filled styrene-butadiene rubber (SBR) samples as a function of the mechanical relaxation time T. They have been measured by dynamic mechanical relaxation spectroscopy. In terms of NMR, the curves correspond to spectral densities of motion. But the spectral densities relevant to NMR are mainly those referring... 

The exception to the rule IMS must be interfaced to MSs through a pinhole leak is when they are interfaced to some type of ion-trapping MS. Because ion traps collect ions for several seconds, IMS mobility resolution is lost. Thus, ion traps such as Paul traps, linear traps, Orbitraps, and FTICR instruments require a mobility-filtering type of IMS. When drift tube IMS instruments are interfaced to trapping MSs, a two ion gate system must be employed to select specific mobilities to trap in the MS. 

Differential mobility spectrometry (DMS) and FAIMS find thdr niche as mobility filters for MSs. They were first interfaced to MS by Guevermont et aL they demonstrated an... 

Perhaps the most spectacular of the early results that demonstrated the potential of FAIMS as a mobility filter for MS was in the detection of nine chlorinated and brominated haloacetic acids at the part-per-triUion levels in drinking water. - The selectivity and efficiency of ion transmission through the FAIMS into the MS improved the detection limits of these compounds by three to four orders of magnitude over conventional ESI-MS methods. 

Miller, R.A., Zahn, M., Longitudinal field-driven ion mobility filter and detection system. US Patent 6,815,669, 2004. 

An important class of magnetization filters are mobility filters which select magnetization based on the time scale of segmental motions ((19), and references therein). The parameters for discrimination are the amplitude and characteristic frequency or the correlation time tc of molecular motions. The effect a filter exerts on a NMR signal can be represented by the filter transfer function. Examples are given in Figure 30 (163,164) with transfer function for filters, which select magnetization based on the time scale of molecular motion. 

After disposal of the decontamination devices (abrasion tools for concrete, washing facilities) and contaminated equipment (mobile filtering stations, vacuum cleaner, decent water collecting facilities etc.), the plant is free of artificial radioactive nuclides generated during the former plant operation. This will be assessed by measurement, documented and confirmed by the authorities. Based on this documentation, the AVR plant will be released from the obligations under atomic law. 

The DMA is a narrow band mobility filter that needs to be scanned to cover a desired mobility range. It therefore has a duty cycle limitation similar to that of a gated IMS instrument coupled to a steady ion source. For a pulsed ion source such as MALDI,(5 ) however, pulsed IMS would use the available ions more efficiently than the DMA, and the duty cycle would be determined by the repetition rate of the source. For a given mobility, however, the DMA has near unit transmission and is far more efficient than gated IMS with steady ion sources. 

FAIMS has been referred to as an ion mobility filter instead of an ion mobility analyzer. ( ) Both FAIMS and T-wave IMS systems are relatively low-resolution ion mobility separation devices. Thus the primary focus of this chapter is on the DTIMS, which can have sufficient resolution to separate ions in complex mixtures based on ion mobility alone. 

The ability of the custom-built dual gate/ion mobility quadrupole ion trap mass spectrometer equipped with electrospray ionization (ESI-DG-IM-QIT-MS) to perform mobility-filtered ion selection prior to mass analysis has been evaluated with peptides in the positive ion mode and carbohydrates in the negative ion mode. The separation of a disaccharide melibiose [a-D-Gal-(l-6)-D-Glc] and a trisaccharide rafflnose [a-D-Gal-(l-6)-a-D-Glc-(l-2)-P-D-Fru] as chlorine adducts in the negative... 

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