  The sensitivity of our liquid argon purity measurement -around 0.03%- leads
us to refine the tools used classically in this field. First we introduce an
analytical form describing the spectrum of a 207Bi source for different values
of purity and ionisation chamber gap. Second we analyse a surprising new
effect: the variation of the ionisation yield of this source with the liquid
argon temperature. Third we use our data to refine the charge collection models
which relate electron recombination and attachment cross-sections to the
electrical field.
