Preparative HPLC detection tools

All of the aforementioned HPLC purification systems employ detectors other than mass spectrometers as the selection criteria by which peaks are collected. With these systems, however, mass spectrometry is generally employed as a primary structural validation tool. More recently, systems have been described whereby a mass spectrometer has been added to preparative HPLC format to detect compounds of interest for collection. A recent review by Kassel [9] discusses the relative merits of employing MW-triggered versus UV-triggered fraction collection in various environments for the purification of combinatorial libraries. 

The modem HPLC system is a very powerful analytical tool that can provide very accurate and precise analytical results. The sample injection volume tends to be a minor source of variation, although fixed-loop detectors must be flushed with many times their volume in sample to attain high precision. Assuming adequate peak resolution, fluorimetric, electrochemical, and UV detectors make it possible to detect impurities to parts per billion and to quantitate impurities to parts per thousand or, in favorable cases, to parts per million. The major sources of error in quantitation are sample collection and preparation. Detector response and details of the choice of chromatographic method may also be sources of error. 

Olefination of acylsilanes is expected to become a useful method for the preparation of vinylsilanes, which are a powerful synthetic tool . Olefination of the acylsilanes 135 with ynolates provides the -silyl-a,/S-unsaturated esters 136 in high yields with high Z-selectivity (equation 56) . In most cases, the E isomers could not be detected by H NMR and HPLC. This is the first general method for the stereoselective synthesis of tetrasubstituted olefins. 

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