ICP-MS 7800 obtained 0.6ppb detection limit for Zinc, is any way to reduce detection limit?


we still got very high Detection limit of 0.6 ppb for Zinc after we rinsed the Digi Tube with 2% HNO3. Does anybody know how to reduce the detection limit? 

we use 2% HNO3 to rinse the Digi Tube, then fill the tube with 2% HNO3, and let them stand overnight with a lid covering. next day , we created the following calibration curve, but still have high background which lead to 0.6ppb. 

we assume there are possibly two reasons to introduce this high background: 

1. water quality. we use water for injection from Baxter. please see the below picture. we want to know what the background reading if milli Q water is used. 

2. another is the instrument setting. we set the instrument setting: 

Thank you


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