How are solar system abundances determined

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of 

Our current level of understanding about the original composition of the solar system is the result of a boot-strap effort involving more than a century of gathering compositional data about the Sun, stars, other astronomical obj ects, the Earth, other planets, and meteorites establishing relationships between objects and developing increasingly sophisticated models of stellar nucleosynthesis and chemical and physical processes that operated in the early solar system. 

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