Is it normal that The counts for the analyte and internal standard from the same solution kept going up (ICP-MS 7800)?


I am running ICP-MS to determine Co Assay and also elemental impurities. 

1. in the elemental impurities test, standard internal recovery kept going up from 100-113%. The internal standard was introduced into the system with online tubing. 

2. The first run of  Cobalt Assay, Co, and standard internal counts kept going up from 100%-107% (against the beginning point).  The internal standards were added to the Co solution, so Co and internal standards were introduced into the system in the same way through the peristaltic pump.  The increase rate for Co and internal standards are almost the same.  

3. In The second run of Cobalt, internal standards and Co were introduced into the system as the same as the first run. The trends for Co and internal standards were still there,  but recovery against the beginning point were from 100%-102%. 

My questions are: 

1  Is this trend normal? should it be random in a certain range? 

2. is this trend relative to the introducing system, like a pump, and nebulizer? 

3. if the system has something wrong, what can we do to fix it? 

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