General Spectroscopy - Poor precision for Arsenic and Selenium by ICP-OES

Poor precision is frequently related to transient signal intensities and/or peak related effects such as interferences or poor signal-to-noise ratios. Transient signals can result from timing settings during sample uptake, etc., but if you observe good RSD values for other elements then this is unlikely to be the cause. Therefore, it's possible that either interferences or low relative signal intensities are the issue, and we'd recommend a close examination of the As and Se spectra. The shape of the peak and baseline (blank) spectra should indicate if interferences or signal/noise ratio effects are the root cause, Additionally, it's often helpful to examine multiple lines when troubleshooting, and comparison of your preferred lines with other As and Se emission lines might provide a simple explanation for the results you're observing.