Sentence:  With this purified conjugate the best staining procedure consisted of treating the sections with 1 : 10 dilution of * * f for 30 minutes , washing with PBS for 15 minutes , staining with * * f for 30 minutes , and washing with <WSD>PBS</WSD> for 15 minutes . 
Meaning of the word marked with <WSD> tags:  a solution containing a phosphate buffer