Hi,

The sample can be diluted with HNO3 and spiked with internal standards. It is not mandatory to use microwave for sample digestion. You should add oxygen to the nebulizer gas to remove carbon soot deposited on the sampler cone.

I hope this would help,

If you're not going to digest then you probably should consider using known addition as your quantification method to compensate for the matrix.

its better to digest your samples with the mixture of nitric acid and hydrogen peroxide in 2:1 ......

Acidify???  You need to perform complete mineralization with nitric acid (H2O2 opcional), with the proper addition of an ISTD mix.