Sentence:  Very small concentrations of these hydrides should be detectable ; in interstellar gas , concentrations as low as * * f molecules / * * f may be <WSD>sufficient</WSD> , as compared to the * * f hydrogen atoms * * f required for detection of the 21 - cm line . 
Meaning of the word marked with <WSD> tags:  of a quantity that can fulfill a need or requirement but without being abundant