Higher-twist quark-mass contributions to deep-inelastic scattering
  In this letter we extend the factorization procedure of the deep-inelastic hadron tensor, proposed by Qiu, to include non-zero quark masses. The manifest gauge invariance of both soft and hard parts is preserved. Using a so-called spurion to generate the quark-mass terms, the simple parton-model interpretation is also kept. The calculation of the deep-inelastic transverse-spin structure function $g_2$ is used to illustrate the algorithm. 
D. Boer and R.D. Tangerman