1. A method of preparing a petroleum sample having an inorganic contaminant species for use with one or more of elemental and isotopic signature analysis, comprising:
(a) adding a first demulsifier to the petroleum sample, said first demulsifier having a known concentration of a specified hydrocarbon-soluble elemental species referred to as a species of interest;
(b) separating the petroleum sample into one or more intermediate organic fractions and one or more inorganic fractions;
(c) mixing the one or more intermediate organic fractions with (i) a solvent in which the inorganic contaminant species is soluble and the solvent has a known concentration of the species of interest, and (ii) a second demulsifier having a known concentration of the species of interest;
(d) separating the one or more intermediate organic fractions into one or more prepared organic fractions and one or more solvent-based fractions; and
(e) determining at least one of an elemental signature and an isotopic signature from the one or more prepared organic fractions.
2. The method of claim 1, wherein one or more of the known concentrations of the species of interest in the first demulsifier, the second demulsifier, and the solvent are substantially zero.
3. The method of claim 1, further comprising, before separating the petroleum sample, spiking the petroleum sample with one or more of an inorganic standard and an organic standard, each said standard having a known concentration of a respective ma