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 HCl solution post-digestion.
The antimony is part of a mix of elements in ~5% HNO3, 1 mL of which is added to a beaker with ~50 mL of 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 HCl, and possibly with natural light affecting the stability of the solution, but nothing definitive or on-topic.