Digital BW, The Print

I recently returned from vacation and fired up my printer (1280). 
I'm using it with the Piezotone Selenium inks. What happened next 
was a surprise. The inks appeared to be warm and somewhat brown in 
tone. I ran nozzle checks and found all was in order. My question is 
whether it is possible for the inks to deteriorate in the pre- 
filled carts? I have checked the new prints against some made 
several months ago and they are much warmer and appear brown in tone 
compared to the original prints I made with the same carts months 
ago.

Thanks,

Barry