Where is my antimony going?

I work in a trace metals laboratory operating an ICP-OES. I've been having some trouble with the recovery of antimony in a weak $\ce{HCl}$ solution post-digestion.


The antimony is part of a mix of elements in ~$\ce{5\% HNO3}$, 1 mL of which is added to a beaker with ~$\pu{50 mL}$ of $\ce{0.5N HCl}$, then heated at a low temperature for ~10-15 minutes. There is a filtering process afterward using simple glass fibre filters. I've been getting between 60-75% recovery of the antimony, and at least 80-90% recovery of the other elements.


I've been able to find some information regarding issues with antimony and $\ce{HCl}$, and possibly with natural light affecting the stability of the solution, but nothing definitive or on-topic.


Any ideas?


