Dynamic range problem

We normally avoid protonated solvents, because the very intense solvent peak will obscure nearby protons, and the dynamic range problem will also... 

The identification of these 123 compounds (see Table I) was made possible only by the synergistic application of several analytical techniques. For example, the very high concentrations of a few compounds in most of the samples (e.g., no. 6,10,46, 81), precluded identification of many of the minor components during GCMS analysis. This dynamic range problem was solved, at least qualitatively, by HPLC followed by mass spectrometry. 

Pulse sequences for proton detection must transfer magnetic information concerning the heteronucleus, for example its chemical shift, to the nucleus prior to proton detection. Simultaneously they should suppress the resonance that does not contain any information about the heteronucleus, i.e. the one from the nuclei not coupled to any half-spin tin nuclei, so as to both avoid dynamic range problems and simplify the spectrum. 

Isokinetic flow can be achieved if the sampling system monitors the ambient wind velocity and adjusts the pumping speed as appropriate. Unfortunately, this system exacerbates the dynamic-range problem because the system samples faster in high winds, when the aerosol concentration tends to be higher. 

Relative water intensity when compared with concentrations of metabolites of interest, + + + indicates that few metabolite signals can be observed without water suppression due to dynamic range problems. 

McCoy et a/. have proposed a method termed ERASER based on self-refocused and hard rectangular pulses. The scheme does not generate phase roll. Moy et alP have demonstrated the use of frequency shifted, self-refocused top hat pulses to observe amide resonances, that they termed selective-excitation-corrected spectroscopy (SelECSy). This method, although not providing exceptional water suppression, overcomes the dynamic range problem and produces uniform amide proton excitation... 

FAAS is the oldest version of AAS. It works with liquid samples which after nebu-lization are mixed with acetylene and introduced in a flame atomizer burner with air-acetylene or N20-acetylene flame. A single measurement can be completed within 10 s. Theoretically the method is applicable to 60-70 elements and due to its low cost, selectivity and simple operation is preferred whenever the concentration of the determined elements is within its possibilities. The sensitivity of FAAS is of the same order of magnitude as of ICP-AES and for some elements even worse which in recent years has lead to the replacement of FAAS with the multielement ICP-AES. To increase the sensitivity of FAAS usually preconcentration procedures are applied before measurement. Otherwise FAAS permits direct measurements in the low mg/kg range of a number of metals in soils and sediments, six to ten elements in plants and in natural waters. It is sensitive enough for the direct determination of Al., Ba, Ca, Cu, Fe, K, Na, Mg, Mn and Zn in different environmental materials (alkaline metals are determined by flame atomic emission spectrometry). Due to the narrow dynamic range problems with the accuracy appear (e.g. Djingova et al., 1991) and very often dilutions are necessary which decreases relative sensitivity and increases the possibilities for errors. 

To address the solvent dynamic range problem, one or multiple signals from the solvent are selectively suppressed in the NMR spectrum. Solvent suppression is not a perfect solution. Compound peaks that are proximal to the solvent are also completely or partially suppressed. Similarly, hydrogens that readily exchange with water are also equally suppressed with the water solvent peak. Additionally, solvent suppression causes artifacts and streaking in 2D NMR spectra. This streaking may obscure cross peaks that fall near the solvent chemical shift in either spectral dimension. The commonly used solvent suppression NMR techniques, such as, PRESAT, WET (Smallcombe and Patt,... 

Hen egg yolk is a rich source of PLs, where the individual PLs are present in totally different amounts. For instance, PC comprises about two-thirds of the total PL, while PI constitutes less than 1%. Consequently, the hen egg yolk extract serves as an excellent example to investigate dynamic range problems. 

In the late 1970s and early 1980s, a number of attempts to couple these techniques were carried out. However, these studies suffered from the low sensitivity of the NMR spectrometer systems then available. Also, because of dynamic-range problems, there was a need to use expensive deuterated solvents for the HPLC because the solvent suppression methods in use at the time could not cope with fully protonated solvents. The reduction of HPLC-NMR to routine use was slow in developing and not practically achieved until technical improvements in electronics, higher magnetic fields strengths, advanced solvent suppression sequences, and improved instrumental sensitivity, made it feasible to interface an HPLC directly to an NMR spectrometer. 

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