The technique of invariant mass spectroscopy has been used to measure, for the first time, the ground state energy of neutron-unbound $^{28}textrm{F},$ determined to be a resonance in the $^{27}textrm{F} + n$ continuum at $2underline{2}0 (underline{5}0)$ keV. States in $^{28}textrm{F}$ were populated by the reactions of a 62 MeV/u $^{29}textrm{Ne}$ beam impinging on a 288 $textrm{mg/cm}^2$ beryllium target. The measured $^{28}textrm{F}$ ground state energy is in good agreement with USDA/USDB shell model predictions, indicating that $pf$ shell intruder configurations play only a small role in the ground state structure of $^{28}textrm{F}$ and establishing a low-$Z$ boundary of the island of inversion for N=19 isotones.
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