MiniCLEAN is a single-phase liquid argon dark matter experiment. During the
initial cooling phase, impurities within the cold gas ($<$140 K) were monitored
by measuring the scintillation light triplet lifetime, and ultimately a triplet
lifetime of 3.480 $\pm$ 0.001 (stat.)..
$\pm$ 0.064 (sys.) $\mu$s was obtained,
indicating ultra-pure argon. This is the longest argon triplet time constant
ever reported. The effect of quenching of separate components of the
scintillation light is also investigated.