Empirically nitric acid has been found the most effective already decades ago. Remember that nitrates are well soluble in water and chlorides and sulfates often not.

If nitric acid is not available you can use sulfuric acid (H2SO4) and hydrogen peroxide (H2O2) for determination Major (Ca, P, Na, K, .....) and trace elements (Zn, Cu,....) and good luck

Moustafa Ghandour

If you use sulfuric acid ...use it concentrated or diluted with a certain concentration ??

Of course concentrated acid as: 0.2g air dried sample + 10 ml H2SO4 (conc.) heating then add carefully about 5 ml H2O2 (one time or two time 2.5 ml per one). and finally you will get stable colorless solution will be diluted to 100 ml in volumetric flask using distilled water and you should make a blank (all previous procedure but without sample). Good luck

Please check Vicente et al. (2004). J Sci Food Agric 84:2035–2040.

DOI: 10.1002/jsfa.1908