Dear Sir. Concerning your issue about the determination of metals in HEPES buffer by the ICP. The ICP-AES analysis was performed with GemTip Cross-Flow nebulizer equipped with a Scott double pass spray chamber with a segmented array, charge-coupled device detector. Plasma gas flow rate was 15 liters/min, auxiliary gas flow rate was 0.5 liters/min, and nebulizer gas pressure was 0.8 × 105 pascal. The software used was ICP Win Lab Version 3. The samples were diluted to approx.10 μM in Chelex-treated 20 mM HEPES, pH 7.5, and 50 mM NaCl. For some samples 5% glycerol (v/v) was used to minimize precipitation; our previous studies of standard metal solutions demonstrated that this concentration of glycerol did not affect quantitation by ICP-AES. For more details, I think the following below link may help you in your analysis:

http://www.mcponline.org/content/4/6/827.full

Thanks

This might depend on which type of ICP-MS instrument you used. The new instrument introduced to the market in the recent years has significantly improved the techniques to solve the problem on carbon deposition, your solution can be directly analysed. But the old instruments  might not able to analyze such a high carbon content solution.

 The instrument is Optima 8000 from Perkin Elmer. I read from some papers that they add some concentrated HNO3 to acidify such samples before running them by ICP, will that work?

Isam is correct.  It is the front end of the setup that is relevant to the application.  In extremis you could evaporate with HNO3 and H2O2.  Be sure to matrix match the standards.