Sentence:  In the last few years , the application of chromatographic and other modern techniques to the problem of isolating TSH has led to further <WSD>purification</WSD> ( Bates and Condliffe , 1960 ; Pierce , Carsten and Wynston , 1960 ) . 
Meaning of the word marked with <WSD> tags:  the act of cleaning by getting rid of impurities