Interesting question. I would suspect that plant components (individual mineral nutrients, sugars, etc.) would be too variable to use as estimators of dry mass. Maybe what you can do is determine the fresh weight/dry weight ratio of a few plants and the estimate plant biomass (i.e., dry weight) from the fresh weight of your plants. If you do this, you must be careful that you handle the plants in such a way as to minimize errors due differences in water loss among plant samples.

I agree with Juan :)

Thanks for the answer. The problem is that i didn't weigh the seedlings before ICP analysis...so i wolud like to understand if i could have an estimation of their weights by analyzing the ICP data.