Chemical frequency selection with

The information in Exhibit 14 has also been used to determine the makeup of the most common chemical mixture associated with eating a balanced diet. If a chemical occurs in at least five of the six food categories, it was selected as a common dietary contaminant and their average frequency in food products was determined. This complex chemical mixture is represented by two banned organochlorine pesticides, two chlorinated organophosphorus pesticides, three organochlorine pesticides, two organophosphorus pesticides, six aromatic petroleum hydrocarbons, and two chlorinated petroleum hydrocarbons. 

Figure 8.2.10 (A) Pulse sequence used for selective excitation of each of the four samples in turn. The RF pulses are frequency-selective and applied at different resonant offsets via phase modulation in the time domain. (B) Normal 1H spectrum of (a) the four samples, and the resulting sub-spectra of 0.5 M (b) 1-propanol, (c) 2-propanol, (d) acetic acid and (e) ethanol in D2O. Since the four spectra are acquired within the relaxation time 77 eff, which is typically set to be3Ti, there is an increase in efficiency by a factor of four. Reprinted with permission from Hou, T., Smith, J., MacNamara, E., Macnaughton, M. and Raftery, D., Anal. Chem., 73, 2541-2546 (2001). Copyright (2001) American Chemical Society...

FIGURE 38. Scheme of resultant 13C NMR spectmm formation from two subtracted spectra with different selective decoupling 29Si frequencies. Reproduced with permission of Collection of Czechoslovak Chemical Communications from Reference 47... 

Only the /lz term leads to cross-peaks by chemical exchange, so the other term will be ignored (in an experiment this is achieved by appropriate coherence pathway selection). The effect of the first part of the sequence is to generate, at the start of the mixing time, Tmix, some z-magnetization on spin 1 whose size depends, via the cosine term, on tl and the frequency, Qv with which the spin 1 evolves during /). The magnetization is said to be frequency labelled. 

In many instances, it is important that some form of chemical selectivity be applied in magnetic resonance imaging so as to distinguish nuclei in one or more specific molecular environment(s). There are many ways of doing this and we discuss here just three. The first option is to ensure that one of the excitation RF pulses is a narrow bandwidth, frequency selective pulse applied in the absence of any gradient [22]. Such a pulse can be made specific to one particular value of the chemical shift and thereby affects only nuclei with that chemical shift. In practice this can be a reasonable method for the specific selection of fat or oil or water in a mixed hydocarbon/water system. 

Localized MR spectroscopy was developed to obtain chemical as well as spatial information. Frequency offsets (caused by the inhomogeneous susceptibility or chemical shift distribution) can be recorded by introducing a further measuring domain, called the spectral-encoding domain. Thus, the dimension of the experiment is increased by one. The spatial distribution of compounds with distinct chemical shifts can be obtained as chemical-shift-selective maps. The spatial distribution of frequency offsets can be acquired by imaging pulse sequence, such as echo-time encoding. The method is suited for the... 

Schwartz and Taylor [43] described a normalized collision energy (NCE) method, in which the optimized amplitude for the resonant supplemental AC dipole is calibrated with a set of pre-selected chemicals (calibrants). They found that the amplitude of the excitation voltage required to fragment their calibrants is substantially linear with ion m/z-value for an individual instrument. With the amplitude of NCE, identical MS/MS results can be observed from instrument to instrument because the error of tr ping frequency calibration with different instruments has been corrected by the NCE process. ... 

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