7800 ICP-MS SIC resulted in high Sb 121 signal

Have you checked that your stock standards are not contaminated and tried a second source of SIC. Do you run He in addition to nogas and does it only fail for nogas but pass in He? 

I've also seen Sb stick around in the tubing, autosampler probe, and spray chamber and result in hits when running higher Cl matrices, so I recommend cleaning all the sample lines (rinse protocol below) followed by a spray chamber soak in dilute aqua regia and seeing if it helps.

Rinse protocol for cleaning out sample lines:

Prepare fresh solutions so make in small batches. Do not leave solutions uncapped when not in use. For the first time, remove the PTFE nebulizer connector and drain into a beaker to prevent troublesome elements from being introduced into the cones/lenses. These solutions can be aspirated through the system, but it’s better to assure the lines are cleaned prior to aspiration.
Step 1: 1:1:4 - NH4OH:H2O2:H2O
Step 2: 1:1:4 - HCl:H2O2:H2O 

ArBr is a potential interferent, monitor and 79 and 81.  They are about 50/50 abundance.  See what counts you see in DI, then introduce your SIC solution.  If it goes way up, then ArBr could be causing this. 

If memory serves, there is Cl- in the SIC solution so KrCl could be a potential interferent.  You can check if your Argon is contaminated with Kr by monitoring masses 82, 83, and 84 while running DI water.  82 and 83 are about the same abundance, 84 is about 5 times higher.  If the counts are in that ratio, you have Kr in your Argon.  Unfortunately gas companies do not look for Kr in their purity check. This is a very common issue with liquid Argon, try a cylinder of HP (99.99% purity) Argon and see if that reduces the Kr counts.

Although 200.8 drinking waters does not allow for reporting from gas modes, try measuring the SIC solution in He mode to see if it is a polyatomic interference.  If the counts go down to near blank levels while monitoring 121amu, then you have a poly atomic interference, if not, it could be contamination.  Do you use an internal standard solution that contains Telurium?  If not, monitor both 121 and 123 for Sb.  They have very similar abundances.  If the results compare, then there is Sb contamination.  If you run Te ISTD this will be hard to check as you will need to rinse the ISTD line for a long time to wash out all the Te.  Again, compare the SIC at masses 121 and 123 amu to what they read in your blank to see if the SIC solution is contaminated.