Standard Solution for Cation

When I was in research with USDA I made my own standard solutions to include the solvent system in the sample prep. Tip: make your stock solution slightly high initially, then dilute to precise concentration after checking against reference standard. This offsets slight variance in commercial lots of reagent chemicals and standard errors in the weight measurement.

If I analyze phosphorus in the soil sediment with this equipment, will I get phosphate as the result?

The instrument response is in terms of total P. It does not differentiate between the different forms. The plasma will excite all P the same. You differentiate by choosing a solvent that will selectively extract phosphate and then make up your calibration solution based on the gravmetric factor for converting P to phosphate. This sounds like agricultural soil testing, my field of specialty, in which you are primarily interested in how much P is available for uptake by plants. If you are working with pollution information, and want to know the total phosphorus in the sample, base your calibration standards on the weight of P without correcting to phosphate.