I am trying to view Ca at roughly 1ppb in brine samples on a 7900 using UHMI and Ca40 in H2 mode. Does anyone have experience with this?

Hi Paul, as I understand your brine samples have a lot of Na content and for this reason your are running the Plasma Condition as uHMI, so this means a lot of Argon dilution before your sample reaches the plasma region. This is definitely a good choice as without uHMI your cone/lens system will be very fast covered from your Na content, resulting on extreme drift. As you understand Ca 40 has a known interference from Ar-40 and of course there is plenty of Argon in your system from your plasma. H2 Mode is the best choise as H2 neutralizes the Ar(40)+ interference and leaves the Ca signal almost unaffected. But to be honest it is really difficult to remove 100.0% of the Ar-40 interference and for this reason I would expect a slightly increasing BEC on the Ca-40 calibration. I suppose is close to 20ppb. Can you check please your Ca-40 calibration and the DL and BEC values? That is the limit of your method you cannot report / measure lower concentrations than your BEC. To be honest 20ppb Detection Limit for Ca is a very nice DL, especially in uHMI mode.

Hope this helped,