Multiple pulse techniques, carbon

Two recent studies have examined the NMR spectra of coal macerals and lithotypes respectively. Retcofsky and VanderHardt (12) reported the aromaticities of the vitrinite, exinite, micrinite, and fusinite from Hershaw hvAb coal using non-spinning cross-polarization techniques. The fa values of 0.85, 0.66, 0.85, and 0.93 -0.96 for these macerals demonstrate clear variations between the materials at a given rank. Gerstein et. al. (13) used carbon-13 CP/MAS proton combined rotation and multiple pulse spectroscopy (CRAMPS) to examine Iowa vitrain (Star coal) and a Virginia vitrain (Pocahontas 4 coal) with aromaticities of 0.71 and 0.86 respectively. 

The poor editing accuracy of spin-echoes in the presence of a wide range of J values and the inability to fully characterise all carbon multiplicities are the major limitations of these techniques. More complex variations on the pulsed J-modulated spin-echo are to be found that do allow a complete decomposition of the carbon spectra into C, CH, CH2 and CH3 sub-spectra [21] and which also show greater tolerances to variations in Jch [22]. Likewise,... 

The detection of the current generated by reaction at the surface of (usually) carbon fiber or copper microelectrodes at a fixed voltage is capable of low detection limits for electroactive compounds using amperometry, Table 8.14. Several approaches that allow the full possibilities of multiple electrode and pulsed amperometric detection (established techniques in liquid chromatography (section 5.7.4)) have been proven for capillary electrophoresis [508,511]. These methods are not widely used, possibly due to a lack of commercial products and support. Potentiometric detection with polymer-coated wire microelectrodes containing relatively non-specific ion exchange ionophores was used for the detection of low-mass anions or cations [510,511]. 

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