References—continued repeated information

The characterization of HDS catalysts has been the sub ject of a large number of papers, and virtually all the surface techniques and analytical tools available today, as well as powerful theoretical methods, have been extensively employed in order to tackle this exceedingly complicated problem [see e.g. ref. 15]. Tlie mass of information thus obtained has been interpreted in terms of several different models that have been evolving over the years into a rather sophisticated and well founded picture however, in spite of all the data available and of over seven decades of industrial practice, the exact nature and the structure of the catalytically active HDS sites of standard catalyst formulations continue to be the subject of controversy and frequent speculation. A great deal of the published work in this area has been devoted to the study of unpromoted catalysts in both calcined and sulfided forms, and this has resulted in the clarification of several important aspects nevertheless, for the sake of brevity, our description will concentrate essentially on the promoted Co-Mo catalysts in their sulfided forms, which are the ones most frequently used for practical purposes. Many excellent reviews widely cover the various theories and models which have been put forward for HDS active sites (see e.g. refs. 14, 15, and references therein) and thus there is no need to repeat that information at length here. 

Properties of an analytical method including (im)precision, accuracy and repeatability are commonly studied and reported in validation studies and are properly defined in [14-16]. Those definitions are not repeated here. An international vocabulary containing sound scientific definitions from metrological experts [17] is referred to for additional information. For example, the expression analyte is commonly used in chemistry meaning the compound to be measured. However, one measures the quantity of the compound and therefore sound science should call it a measurand . Here we will continue to use the expression analyte. 

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