  It is known the single transverse spin asymmetry in semi-inclusive deep
inelastic scattering can be factorized by a twist-3 distribution function
$T_F$, which contains a gluon field strength tensor. With transverse gluon
included in the power expansion, we find the gluon field strength tensor can be
recovered definitely in soft-gluon-pole contribution at leading order of
$\alpha_s$ expansion. This conclusion holds in Feynman and light-cone gauges.
