  Recently Berkovits has constructed a picture raised, compound field $b_B$
which is used to compute higher loop amplitudes in the pure spinor approach of
superstrings. On the other hand, in the twisted and gauge fixed, superembedding
approach with $n=2$ world-sheet (w.s.) supersymmetry that reproduces the pure
spinor formulation, a field $b$ appears quite naturally as the current of one
of the two twisted charges of the w.s. supersymmetry, the other being the BRST
charge. In this paper we study the relation between $b$ and $b_B$. We shall
show that $bZ$, where $Z$ is a picture raising operator, and $b_B$ belong to
the same BRST cohomological class. This result is of importance since it
implies that the cumbersome singularity which is present in $b$, is in fact
harmless if $b$ is combined with $Z$.
