7800 ICP-MS SIC resulted in high Sb 121 signal

Question

Using standard 6020 Interference Check A solution with a 0.05 - 500 ppb calibration range for Antimony determination (mass 121 No gas mode) , SIC meauserement gave a 0.7 ppb signal (~15x LLOQ).
That issue has happened for quite some time now (months actually, unfortunatelly I wasn't there when this started to happen) but It has never been an issue before (values were consistently below 0.1 ppb), does anyone have some suggestion what kind of interference could it be? Only found some ArBr interference on mass 121 but seems quite unlikely

Tom Herzog · Answer

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.