  Inductively Coupled Plasma Mass Spectrometry (ICP-MS) allows for rapid,
high-sensitivity determination of trace impurities, notably the primordial
radioisotopes $^{238}$U and $^{232}$Th, in candidate materials for
low-background rare-event search experiments. We describe the setup and
characterisation of a dedicated low-background screening facility at University
College London where we operate an Agilent 7900 ICP-MS. The impact of reagent
and carrier gas purity is evaluated and we show that twice-distilled
ROMIL-SpA-grade nitric acid and zero-grade Ar gas delivers similar sensitivity
to ROMIL-UpA-grade acid and research grade gas. A straightforward procedure for
sample digestion and analysis of materials with U/Th concentrations down to 10
ppt g/g is presented. This includes the use of $^{233}$U and $^{230}$Th spikes
to correct for signal loss from a range of sources and verification of
$^{238}$U and $^{232}$Th recovery through digestion and analysis of a certified
reference material with a complex sample matrix. Finally, we demonstrate assays
and present results from two sample preparation and assay methods: a
high-sensitivity measurement of ultra-pure Ti using open digestion techniques,
and a closed vessel microwave digestion of a nickel-chromium-alloy using a
multi-acid mixture.
