ICP-MS 7900 Variable Sensitivity

Question

Is this level of variability in the 7/89/205 counts during plasma startup considered normal? 
 
 During install counts were as follows versus specification (converted to local units):

m/z 
 Specification 
 Result

7 
 > 3,000 
 4,896

89 
 > 15,000 
 34,144

205 
 > 9,000 
 25,086

I'm attempting to set a requirement for routine testing but an unsure what's feasible given the variability. If it's not normal, how can I go about controlling this better?

Tom Herzog · Answer

No, that is not normal. The last 2 runs shown (07/20 and 07/24) do not show good mass ratio. If you open the individual results from 07/19, and go to the Meters tab. Compare the IF/BK and Analyzer pressures to what they read on the 07/24 report. Higher vacuum numbers can cause this issue. This could be caused by the cones and seal to the interface. Inspect the cones with a microscope or some other magnification device, holes should be round and not jagged. While cones are out for inspection, look at the graphite gasket that seals the sample cone. It should not have any indentations, cracks, and it should freely spin on the interface. If not, replace the graphite gasket. 
 Next, replace the nebulizer, this is the most common cause of low sensitivity. Pump tubing condition and arm tension is second to the neb. 
 Torch grounding can also be to blame, make sure the platinum torch shield is nice and shiny, not dull and corroded. 
 Please refer to this document found here in the community posts for help on troubleshooting sensitivity issues: www.agilent.com/.../troubleshooting-tool-for-low-sensitivity.pdf