Also known as "multi-acid digestion" or “near-total digestion”, this method uses a combination of HNO3 (nitric acid), HF (hydrofluoric acid), HClO4 (perchloric acid) and HCl (hydrochloric acid). Some laboratories differ in the specific acids used but the most important acid is HF because it dissolves silicate minerals. Four acid digestion quantitatively dissolves nearly all minerals in the majority of geological samples however, some refractory minerals may only be partially digested (eg. garnet, barite, rare earth oxides, columbite-tantalite, titanium, tin and tungsten). Some elements can also precipitate or volatilize during digestion. Fusion techniques are generally preferred for accurate quantification of Al, Ba, Cr, Hf, Mo, Mn, Nb, Pb, Si, Sn, Ti, Ta, W, Zr, As, Sb, Se and Te.

An interval for which we can assign a 95% probability that the Recommended Value lies within; alternatively, we can say that 95% of all confidence intervals determined in a like manner (from different samples of the population) will include the Recommended Value (it is important to note that this is not equivalent to two standard deviations of the Recommended Value).

(where r = 0.95 & 1- a = 0.99) may be defined as meaning that 99% of the time at least 95% of samples will have values lying between the specified interval. Put more precisely, this means that if the same number of samples were taken and analysed in the same manner repeatedly, 99% of the tolerance intervals so constructed would cover at least 95% of the total population, and 1% of the tolerance intervals would cover less than 95% of the total population.