  The discovery of naturally occurring long-lived isomeric states (t_1/2 > 10^8
yr) in the neutron-deficient isotopes 211,213,217,218Th [A. Marinov et al.,
Phys. Rev. C 76, 021303(R) (2007)] was reexamined using accelerator mass
spectrometry (AMS). Because AMS does not suffer from molecular isobaric
background in the detection system, it is an extremely sensitive technique.
Despite our up to two orders of magnitude higher sensitivity we cannot confirm
the discoveries of neutron-deficient thorium isotopes and provide upper limits
for their abundances.
