The last time I calculated the purity as a function of 'eTtrg' I used tracks with 'zTtrk = (0.2, 0.3)'.  Those plots can be found in the links below.

https://drupal.star.bnl.gov/STAR/blog/dmawxc/update-02012018-run-9-purity
https://drupal.star.bnl.gov/STAR/blog/dmawxc/update-02122018-run-9-purity-star-vs-phenix-data

As a check, I've compared the purity as a function of both 'eTtrg' and 'zTtrk'.  The plots are a little busy (apologies), but there does seem to be differences between the bins of 'zTtrk'.  Only statistical errors are plotted.

And here's the purity with averages over 'eTtrg':