An Infrared Spectrophotometric Analysis for Natural Rubber in Guayule Shrubs

An accurate assay of rubber content in guayule plant tissues is essential to any genetic improvement or bioregulator program. The simpler, more rapid and less expensive the assay, the better. The several methods of rubber assay that have been used have various limitations. Earlier methods for assaying rubber content in guayule are described in two USDA reports. These are principally gravimetric methods which are tedious and relatively nonselective. One of the better ones consists in bromination of the guayule double bonds, precipitation from solution, collection, drying, and weighing the crystalline rubber bromide. This bromide method is more selective for rubber in the presence of resins than the gravimetric methods where rubber is weighed directly. For multiple and repetitive analyses, it is time consuming and somewhat unpleasant due to the recommended excess of bromine. A gravimetric method reported by Tysdal in 1951 was modified recently. The method consists of grinding guayule shrub in an efficient blender with aqueous alcohol saturated with salt. The resulting slurry is screened to recover resinous rubber particles, which are dried and weighed. Precision is considered adequate for distinguishing plants of widely differing rubber content but not for detecting incremental changes. Another rubber assay of merit is the photometric method of Traub. This volumetric method depends upon a controlled precipitation of rubber particles from solution and the determination of rubber quantity by photometric measurement of the resulting turbidity. It is being used with some modifications on a guayule breeding project at the Los Angeles State and County Arboretum Research Laboratory. The principal improvement consists in extracting the rubber from the plant tissue by homogenizer milling in solvent rather than lengthy soxhlet extractions.