I have a problem with high mass element (> 200amu) sensitivity on the 8800 in O2 mode. Instrument was optimised during start-up, with good mass sensitivity (> 200 Mcps/ppm on mass 205). I normally run LA ICP-MS, so batches are tuned with dry plasma. All methods in NoGas mode show good sensitivity, with little drift in mass sensitivity over several hours of analysis. Switching to O2 mode usually results in ~5x lower counts, but sensitivity was > 100x lower for mass 205, while masses lower than 200amu showed the usual sensitivity in O2 mode. This seem to coincide with cleaning the cones and lenses, but since the sensitivity was fine in NoGas mode for all masses, I do not expect a problem there ...
No amount of batch tuning solved the problem, power cycled the instrument, updated the software (MH 4.5), then sensitivity was back to normal. Started a 7 hour sequence, but after only 1 hour mass 205 sensitivity was at 50%, after 6 hours it was 5x lower. This decrease was worse for higher masses, with 209 30x less, 232 (mass shifted to 248) was 80x less and 238 (mass shifted to 254) was 600x less.
Can anyone share some ideas on what to look for?