Sentence:  This involves the application of a strong magnetic field to the <WSD>radical</WSD> vapor , which shifts the low-frequency spectra to a conveniently high microwave range , where they may be measured with optimum sensitivity . 
Meaning of the word marked with <WSD> tags:  an atom or group of atoms with at least one unpaired electron; in the body it is usually an oxygen molecule that has lost an electron and will stabilize itself by stealing an electron from a nearby molecule