1. Are you doing a Hg analysis? I.e., is there supposed to be Hg in, for example, your standards? In this case, it is probably a carry over problem. I believe Hg (like Au) sticks to plastic sample introduction parts and washout times can be longer than expected.
2. If you don't have a reason to expect Hg contamination (e.g., not doing a mercury analysis), then (a) how do you know it's mercury and (b) you should probably be concerned if you have high levels of mercury in your immediate environment.