  The Jiangmen Underground Neutrino Observatory will build the world's largest
liquid scintillator detector to study neutrinos from various sources. The 20 kt
liquid scintillator will be stored in a $\sim$600 t acrylic sphere with 35.4 m
diameter due to the good light transparency, chemical compatibility and low
radioactivity of acrylic. The concentration of U/Th in acrylic is required to
be less than 1 ppt (10$^{-12}$ g/g) to achieve a low radioactive background in
the fiducial volume of the JUNO detector. The mass production of acrylic has
started, and the quality control requires a fast and reliable radioassay on
U/Th in acrylic. We have developed a practical method of measuring U/Th in
acrylic to sub-ppt level using the Inductively Coupled Plasma Mass Spectrometer
(ICP-MS). The U/Th in acrylic can be concentrated by vaporizing acrylic in a
class 100 environment, and the residue will be collected and sent to ICP-MS for
measuring U/Th. All the other chemical operation is done in a class 100 clean
room, and the ICP-MS measurement is done in a class 1000 clean room. The
recovery efficiency is studied by adding the natural nonexistent nuclei
$^{229}$Th and $^{233}$U as the tracers. The resulting method detection limit
(MDL) with 99% confidence can reach 0.02/0.06 pg $^{238}$U/$^{232}$Th /g
acrylic with $\sim$75% recovery efficiency. This equipment and method can not
only be used for the quality control of JUNO acrylic, but also be further
optimized for the radioassay on other materials with extremely low
radioactivity, such as ultra-pure water and liquid scintillator.
