Below detection, limit broadening

Ru(bpy)32+ to the mobile phase was investigated and compared with conventional postcolumn Ru(bpy)32+ addition. The detection limit using oxalate standards with Ru(bpy)32+ in the mobile phase was below 0.1 pM, which was significantly superior to the postcolumn technique. The mobile-phase addition method allowed the instrumentation to be simplified and reduced band broadening caused by postcolumn mixing. 

The volume of the detector cell should have a negligible influence on peak broadening and should, therefore, amount to less than 10% of the elution volume of the narrowest (first) peak. A cell volume of 8 pi is standard. Too small a cell volume impairs the detection limit (as a certain amount of analyte is needed to produce any signal at all). The figure must be well below 8 pi for microHPLC. The cell must have no dead comers which may prevent the peak being fully removed by subsequent eluent. 

Both experiments are applicable for acquisition of 2H and 6Li spectra and for these nuclei only first-order EFG- and CSA-terms in the Hamiltonian are required. For CqS below 750 kHz, both 14N QCPMG and MAS experiments are applicable at 14.1 T but above this Cq limit the hardware demands make it very difficult to employ the QCPMG experiment and single-pulse MAS must be the method of choice for such applications. For larger CqS, indirect detection of either 14N SQ or DQ coherences using rotor-synchronized acquisition is suggested. In this context, the DQ lineshape is not as severely broadened as the SQ lineshape as it is not affected by the first-order quadrupolar Hamiltonian. 

Before discussing these techniques, it is perhaps wise to comment on the practice of calculating active surface areas from crystallite size measurements. This is unreliable for several reasons. First, instrumental techniques such as SEM, TEM, and x-ray line broadening all have their limitations. Monolayer-type dispersions are not detected and ultrasmall crystallites are below the range for accurate determination. Second, shape must be assumed in the calculations. Theoretical and experimental evidence indicates that the most stable shapes are spherclike cubo-octahedra. However, depending on the interaction with the support, crystallites may exist as full spheres, hemispheres, or intermediate structures. For very small sizes, this cannot be resolved easily with TEM. Also, a distribution of sizes and shapes may exist. Calculation of the surface from size measurements is, at its best, an indication of an upper limit. 

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