We have been seeing a persistant problem with our intensities dropping as runs progress. We have seen as much as 25% drop in intensity during the run (measured by difference in QC and CCV readings, as well as from one calibration to the next). Our instrument details are as follows:
Varian 715-ES ICP-OES (now owned by Agilent)
Sturman-Masters spray chamber
So far, we have tried changing out torches, spray chambers, nebulizers, and tubing without any success. The instrument is recalibrated every 40 samples, with a QC and CCV after each calibration and before the next recalibration. We have run check samples in various spots throughout the runs, and it appears that the drop is gradual and constant through the entire run. We recently began using an internal standard (Y), which is compensating for the loss, but the loss is still there.
Agilent has been unable to help us, and we would like to avoid a service call if possible. Has anyone else seen this problem? Any suggestions to hellp minimize the effects?