The other possibility..one that may be more likely is that I measure the stock solution with pipettes and a pipette pump. The two latest batches were both made with one of the pipettes being a new one. Perhaps the one it replaced was contaminated. I do my best to really clean them but they're hard to clean. Perhaps that had residue from my 510 Pyro days. The two pipettes are marked and consistently used for only the A or the B solution.