Spectroscopic Methods Applicable to Different Sample Sizes

Low cross-linked polystyrene resins (1% divinylbenzene) is probably the most popular solid support. These resins swell to 2-6 times their original volume depending on the solvent used. Swollen resin, after removal of solvent and without excessive drying, remains in a rubbery state and can be easily flattened for FTIR study in the transmission mode. The support-bound compound should be washed free of reagent and solvent. 

SPECTROSCOPIC METHODS APPLICABLE TO DIFFERENT SAMPLE SIZES 

Ten milligrams of resin contains approximately 50,000-70,000 beads ( 50 im diameter). Sample sizes used in analyses via vibrational spectroscopy range from single beads to 10-mg aliquots. 

FTIR Microspectroscopy.3 A microscope accessory coupled to a liquid-nitrogen-cooled mercury-cadmium-telluride (MCT) detector can be used to obtain an IR spectrum. This is possible in both the transmission and reflectance modes. Several beads are spread on an IR-transparent window (NaCl, KBr, diamond) and possibly flattened via a hand-press or a compression cell. The IR beam is focused on a single bead using the view mode of the microscope. The blank area surrounding the bead is isolated using an adjustable aperture, and a spectrum is recorded using 32 scans ( 1 min). A nearby blank area of the same size on the IR transparent window is recorded as the background. 

Analyses of sample sizes of approximately 100 beads are convenient at the reaction optimization stage in solid-phase organic syntheses. As in singlebead analyses, reactions in progress can be followed continually using microscale analysis methods. Several readily available spectroscopic accessories that facilitate such analyses are described below. 

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SPECTROSCOPIC METHODS APPLICABLE TO DIFFERENT SAMPLE SIZES 221... 

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