By April 1950, Gutowsky and Charles J. Hoffman were able to observe proton resonance shifts for compounds containing fluorine nuclei, using both inorganic and organic samples. Gutowsky reported the discovery of resonance shifts of nuclei within a covalently bonded molecule, an effect observed by others in nitrogen compounds with ionic bonding. Comparing results from a variety of samples, Gutowsky and his group improved the accuracy of their instrument through careful procedures, checks of the instrument, repeated tests, cross-checking using other instruments and techniques, and comparisons with outside data. In September, when members of his group observed a double resonance in PF3 where it was not predicted, it was at first assumed that it was a result of impurities or an incomplete reaction in preparation. However, their observations held up under stringent testing, and other researchers were independently reporting related results: Walter D. Knight (Brookhaven National Laboratory), William C. Dickinson (MIT), and Warren Proctor and Fu Chun Yu (Stanford). Gutowsky looked to molecular structure and theory for explanations of what became known as "chemical shift".