Subject size-based selectivity

Reversed-phase liquid chromatography shape-recognition processes are distinctly limited to describe the enhanced separation of geometric isomers or structurally related compounds that result primarily from the differences between molecular shapes rather than from additional interactions within the stationary-phase and/or silica support. For example, residual silanol activity of the base silica on nonend-capped polymeric Cis phases was found to enhance the separation of the polar carotenoids lutein and zeaxanthin [29]. In contrast, the separations of both the nonpolar carotenoid probes (a- and P-carotene and lycopene) and the SRM 869 column test mixture on endcapped and nonendcapped polymeric Cig phases exhibited no appreciable difference in retention. The nonpolar probes are subject to shape-selective interactions with the alkyl component of the stationary-phase (irrespective of endcapping), whereas the polar carotenoids containing hydroxyl moieties are subject to an additional level of retentive interactions via H-bonding with the surface silanols. Therefore, a direct comparison between the retention behavior of nonpolar and polar carotenoid solutes of similar shape and size that vary by the addition of polar substituents (e.g., dl-trans P-carotene vs. dll-trans P-cryptoxanthin) may not always be appropriate in the context of shape selectivity. 

All of the above particulate investigations were based on mini-radiocarbon measurement techniques, with sample masses typically in the range of 5-10 mg-carbon. This constituted a major advantage, because it was practicable to select special samples (given region, source impact, sediment depth) and to further subject such samples to physical (size) or chemical separation before 14C measurement. This type of "serial selectivity" provides maximum information content about the samples and in fact it is essential when information is sought for the sources or atmospheric distributions of pure chemical species, such as methane or elemental carbon. 

There are different strategies for the selection of sub-matrices for evolving type factor analyses. The classical, original mode has been presented so far. The most important alternative procedure is based on a moving window of fixed size. In other words, a window of a pre-defined number of consecutive spectra is moved along the columns of the matrix Y. Each window is subjected to SVD, the singular values are stored and their logarithms are plotted. 

Each potential split selected at the step 3 can be the subject of a design procedure, which can be of shortcut type or based on rigorous simulation. For some particular or innovative separators the sizing algorithm might not be available, but this fact should not hinder the procedure. It suffices that the feasibility of the task can be guaranteed by an equipment supplier, or this is put on the list of equipment for which suppliers are searched. 

Sample sizes for the sub-studies have been calculated based on a two-sided alpha error of 0.1 and a power of 0.8, but have also been tempered by considerations of logistical feasibility. Preliminary piloting of the instrument confirms these estimates. In particular, the simulation exercise has been considered relatively selectively for a smaller number of subjects and target groups, largely because of anticipated logistical constraints. 

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