<P> In addition, the isotopic composition of an element can vary depending on its source, hence its contribution to the mass of even a pure stoichiometric compound may vary . This variation is used in geochemical dating since astronomical, atmospheric, oceanic, crustal and deep Earth processes may concentrate lighter or heavier isotopes preferentially . With the exception of hydrogen and its isotopes, the effect is usually small, but is measurable with modern instrumentation . </P> <P> An additional note: many natural polymers vary in composition (for instance DNA, proteins, carbohydrates) even when "pure". Polymers are generally not considered "pure chemical compounds" except when their molecular weight is uniform (monodisperse) and their stoichiometry is constant . In this unusual case, they still may violate the law due to isotopic variations . </P>

Law of multiple proportions vs law of definite composition