High Sulphur background on QQQ method

We developed and validated a QQQ-ICP-MS method for S analysis last year. We have not used the QQQ feature of our 8900 for many months and now struggle to reduce the background to the levels to anywhere close to what we have seen before. Additionally, measurements of a single solution analyse multiple times are very erratic, for example one sample can read twice the concentration of the previous measurement. Any ideas? 

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