Sentence:  Very <WSD>small</WSD> 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:  limited or below average in number or quantity or magnitude or extent