Observations are made of the time variation of the intensities of spectral lines emitted by small quantities of impurity ions in a deuterium discharge in ZETA. A vacuum ultra-violet monochromator which covers the wave-length range 500-2000 Å is used. The observations are interpreted in terms of temporal ionization distributions determined for a partially contained plasma model. The observations are consistent with the model and it is shown that plasma containment in ZETA is violated by two processes: an escape of plasma to the wall represented by a loss rate coefficient λ(sec^-1), and an injection of atoms in a neutral or low state of ionization represented by an injection coefficient Λ(sec^-1). The value of λ is effectively independent of the stabilizing magnetic field strength B_z, whereas the injection coefficient varies considerably with this parameter. Both λ and A are closely correlated with ε, the energy input per unit mass of gas, and over the range of conditions investigated they show a linear increase with ε. The ion containment time 1/λ is about 100 microseconds at ε = 3 keV/m_p and at this condition most of the energy input is carried to the walls by the escaping plasma. The net effect of the processes of loss and injection controls the variation of plasma line density, and in general this shows a decrease with time or `pump-out'. This is measured and related to other discharge phenomena.