  Levels of trace radiopurity in active detector materials is a subject of
major concern in low-background experiments. Among the radio-isotopes, $\k40$
is one of the most abundant and yet whose signatures are difficult to reject.
Procedures were devised to measure trace potassium concentrations in the
inorganic salt CsI as well as in organic liquid scintillator (LS) with
Accelerator Mass Spectrometry (AMS), giving, respectively, the
$\k40$-contamination levels of $\sim 10^{-10}$ and $\sim 10^{-13}$ g/g.
Measurement flexibilities and sensitivities are improved over conventional
methods. The projected limiting sensitivities if no excess of potassium signals
had been observed over background are $8 \times 10^{-13}$ g/g and $3 \times
10^{-17}$ g/g for the CsI and LS, respectively. Studies of the LS samples
indicate that the radioactive contaminations come mainly in the dye solutes,
while the base solvents are orders of magnitude cleaner. The work demonstrate
the possibilities of measuring naturally-occurring isotopes with the AMS
techniques.
