A convenient approach to $10^{-12}~g/g$ ICP-MS limits for Th and U in   Aurubis electrolytic NA-ESN brand copper
  Inductively coupled plasma mass spectroscopy is a powerful technique for measuring trace levels of radioactive contaminants, specifically Th and U, in materials for use in construction of low-background rare-event detectors such as double beta decay and dark matter detectors. I describe here a technique for measuring Th and U contamination in copper using direct acid digestion and dilution, without further chemical processing, achieving results comparable to previous work which utilized more complex chemical pre-concentration techniques. A convenient research-oriented analysis environment is described as well. Results are presented for measurements of three samples from the production line of electrolytically-purified, LME (London Metal Exchange) grade A, NA-ESN Aurubis copper. Purified samples showed levels consistent with zero contamination for both elements, while weak but inconclusive indications of contamination were present for the un-purified anode copper. The best limits achieved are near $1\cdot 10^{-12}$~g/g (95% CL) for both Th and U measured for copper from the cathode of the purification process. 
D.S. Leonard
physics.ins-det hep-ex nucl-ex