Sample Preparation - Oxidation of HCl by HNO3

I have come accross the ICP Operations Guide by Paul Gaines which states that nitric acid matrices lead to oxidation of chloride to molecular Cl2 and are thus not suitable for determination of Cl. I am wondering if you could provide me with the considerations behind this statement or references? I have not found literature stating this process to occur. I plan to measure traces of Cl- in carbonates using the spectrophotometric Hg(SCN)2 method. Hg(SCN)2 forms a chloromercurate complex, liberating SCN, which in turn reacts with ferric iron resulting in a red colour. This method exists in many varieties, including dissolving the ferric iron in nitric acid. How come the Cl- can be measured even when HNO3 is added to the reagents thus being present in the matrix? I wanted to dissolve my samples (carbonates) in nitric acid and was wondering which conditions need to be given to oxidise chloride? Is this only problematic with concentrated nitric acid or at low pH favouring the reduction of nitrate to ammonium?