Sentence:  <WSD>Very</WSD> small concentrations of these hydrides should be detectable ; in interstellar gas , concentrations as low as * * f molecules / * * f may be sufficient , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
Meaning of the word marked with <WSD> tags:  used as intensifiers; `real' is sometimes used informally for `really'; `rattling' is informal