Generally, the purifying rebaudioside A from a mixture of stevia glycosides of the stevia plant is similar with that of extracting stevioside.

The following brief manufacturing process of high purity Reb-A (98%) is from China manufacturer, Qufu Xiangzhou (1):

The leaves of stevia rebaudiana bertoni are first dried, crushed, and extracted in hot water. The extract is a dark solution containing stevia glycosides, along with impurities such as leaf pigments and pectin.

Treated the extract with ferric chloride and calcium oxide to precipitate impurities, and follow filtration. Passed the filtrate through an adsorption resin to trap the steviol glycosides.

Subsequently, wash the resin with ethanol to release steviol glycosides and then pass the ethanol extract through ion exchange resin for desalination, and then go through a decolorization process with activated carbon. Then follows the process of drying, concentration and decolorization. Finally, obtain rebaudioside A by spray drying.

As other stevia glycosides have very similar chemical structures – the same backbone steviol, so it is difficult to obtain high purity reb A.

http://www.chinafooding.com/Stevia