I got a litre of liquid ketamine with a 50mg/ml concentration and have been cooking it out to powder. The first 4 cooks I did were 100ml at a time, in the oven at just under 100°C, and the yield was just over 5g each time (like 5.3g or 5.5g). The last two cooks suddenly yielded MUCH less. A 100ml one gave me 3.8g, and a 200ml one gave me only 4.4g.

I have considered that the first few batches were not dehydrated properly, but this latest 4.4g instead of 10g can't possibly be a difference of water weight. I have considered that the first few cooks that were over 5g might make up this difference, but no, there definitely wasn't an extra ~6g.

In total, I've got about 30g out of the litre so far, with just another 100ml left in the bottle.

The liquid has been stored in a plastic bottle, in a cool cupboard. I got it mid-May, so it's been around for about 5 months (no idea how long it had been in the bottle before I got it).

Any ideas why the concentration seems to be decreasing? Is there a chance that the last 100ml is suddenly going to yield the 'missing' 20g that one would expect from a litre of this concentration?