Sentence:  Very 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 <WSD>line</WSD> . 
Meaning of the word marked with <WSD> tags:  a single frequency (or very narrow band) of radiation in a spectrum