Off-bead analysis

For off-bead analysis, coupling between chromatographic separation and mass spectrometric detection has proven especially powerful. The combination between high performance liquid chromatography (HPLC) and electrospray ionisation mass spectrometry has the advantage that purity of product mixtures can be coupled on-line with the product identification. 

Non-chemical Factors Affecting the Completion of Solid-phase Reactions 507 Tab. 12.1 Comparison of on-bead and off-bead analysis. 

The loading of resin 60 and the success of each step of its synthesis was determined by an off-bead analysis involving the removal of the reaction products from the resin through an oxidative hydrolysis of the sulfoximine group at the S—N bond with formation of the corresponding sulfones by a method we have described recently [42]. 

The use of fast, reliable, sensitive on-bead methods circumvents the drawbacks to off-bead analysis outlined above. The modification of common analytical techniques has provided the SP chemist with valuable and often preferred alternatives to off-bead methods for SPS reaction monitoring. 

Destructive methods, where the analytical sample is consumed by the analysis, and nondestructive methods will be presented and their qualitative or quantitative nature will be discussed. They will be divided into off-bead methods, where the resin-bound reaction product(s) are cleaved from the support with subsequent analysis of the cleavage solution, and on-bead methods, where single or multiple beads are analyzed directly. 

It seems likely that the use of MAS NMR-related analytical techniques for quantitative SPS analysis will become more widespread in the future. Recent experiments have demonstrated the technical feasibility of improving the sensitivity (182) using also macrobeads (183) the integration of this with other techniques such as MALDI-TOF and the complete off-bead characterization intermediates and final products will provide fast, reliable, and complete information about the structure, purity, and yield of any product obtained through SPS. This will have a major impact on the assessment phase of the chemistry for SPS, which will, in turn, have a profound influence on combinatorial chemistry. These and related issues will be analyzed in depth in Chapters 6-8. 

Fig. 11.6. Flow cytometric analysis of surface water from points at 1.5 mile intervals off shore from Cape Flatteras, North Carolina. Forward scatter and orange autofluorescence identify two Synechococcus populations with different phycoerythrin content. Beads were used to calibrate the number of cells present. From Chisholm et al. (1986).

Accordingly, a 0-1 molar solution of phthalic anhydride in benzene (100 ml.) was pyrolyzed at 690° under nitrogen at a steady rate of 30 ml/hr (Fields and Meyerson, 1965). The pyrolysis tube was Vycor, filled with Vycor beads contact time was 11 5 sec, which gave a 58% conversion of the phthalic anhydride. Acetylene was steadily evolved, along with carbon dioxide and carbon monoxide in a 1 1 ratio these were identified in the gas stream by mass-spectral analysis of samples taken at regular intervals. The benzene was distilled off and the products boiling over 180° (2 g) were analyzed by mass spectrometry on a Consolidated Model... 

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