I used atomic  atomic absorption techniques so take my answer with a grain of salt.  Why you want to acidify your sample?  As far as I understand in your instrument the sample get ionized by the plasma.  Successively the ions are separated by magnetic fields and detected.

The only thing I would do, if the instrument doesn't do it already, is to prepare some solutions at different known concentration of your metals to calibrate the instrument.

The follow article is perfect for you research, Cheers.

Article Metal and Metalloid Multi-elementary ICP-MS Validation in Wh...

Thanks for the answers, Fabrizio Benedetti and Hugo Olivares-Rubio!