  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
$2\underline{2}0 (\underline{5}0)$ 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.
