Throughout the following sections, all units and dilution factors of rocks and sediments (i.e., materials that are initially solid) are given on a per mass basis, and all units and dilution factors for interstitial waters are given on a per volume basis. This allows the ease of working with these different matrices on a weight vs. volume basis, and also follows standard conventions (e.g., µM in interstitial water data is given in micromoles per liter, that is, per volume).
Acid concentrations in analyte solutions are given in percentage (%) units, as opposed to molarity (M). Although either unit system is appropriate and equally accurate, much of the ICP-AES literature refers to acid concentrations in percentage units, and so that convention is followed here. Concentrated HNO3 acid (16 M) is 70% HNO3. Therefore, preparation of 10% HNO3 (by volume) involves a 7× dilution of concentrated acid (e.g., 1 mL of concentrated acid added to 6 mL of deionized water [DI]).
Concentrations of the 10 major elements in rocks and sediments are commonly given in weight percent units of the oxide (e.g., SiO2), whereas in calibration standards and prime standards purchased from a vendor such as Fisher or VWR the concentrations are sometimes given in µg/mL (ppm) for an element (e.g., Si). Table T1 provides conversion factors between oxide and elemental concentrations. (These are multiplicative factors; to convert SiO2 to Si, multiply by 0.4675.) The conversion from ppm to weight percent (part per hundred) involves division by 104.