I recently started working on this technique and want to start with the metal with the most minor interferences and the least amount of sample preparation. Can you give me a few suggestions? Thank you in advance
Please look at the instrument manufacturer's manuals. This is advisable since not all instruments behave the same way. Also please consider that the matrix can have big effects and a detergent such as Triton X 100 may help.
I'd go for Cs. That circumvents issues with solubility in solutions and is a) not a prominent contamination and b) not prone to spectral interferences from solvents you may use.