Hi Richard,
Ba contamination (easiest to see as the major Ba138 isotope just on the edge of the Xe area) is often cell-associated, in my experience. I think in many cases this comes from the cells having "seen" Ba-containing buffers and such, perhaps even during initial processing (eg, during PBMC processing and such). Even if you switch to Ba-free buffers after, you might not be able to get rid of all of it.
Here's a recent example:
Not too bad....I've certainly seen far worse. But you can see some additional Ba associated with the cell, particularly in the bottom cell event (relative to the background, non-cell-event level).
Unfortunately, I don't know of any way to retroactively get rid of initial Ba contamination. As far as I can tell, it just keeps diffusing out with each wash. Therefore, you can *decrease* it, but not completely remove it....similar to Iodine levels sometimes seen in PBMCs from the Ficoll-Paque step (usually highest on Monocytes, in my experience).
In short, though, I don't know how much difference it makes: we don't us the 138 channel (or any other Ba isotope channel) for any reagent. And as long as your level is comparatively low, you don't have to worry about M+16 spillover into the 154 channel.....as long as you're monitoring Ba138, you can detect any potential problem.
Mike