How to analyse chemical composition

An elemental analysis of living tissue and of a sample of the earth's crust gives similar lists: every element found in the crust is also found in living tissue.

The difference is in proportion. Carbon and hydrogen are far more abundant, relative to other elements, in living matter than in the crust. By weight, carbon is 18.5% of the human body but 0.03% of the crust; hydrogen is 9.5% against 0.14%.

To find the organic compounds, a living tissue (a vegetable or a piece of liver) is ground in trichloroacetic acid (Cl3CCOOH) with a mortar and pestle, and the slurry is strained through cheesecloth or cotton.

This gives two fractions. The filtrate is the acid-soluble pool, in which thousands of organic compounds have been found. The retentate is the acid-insoluble fraction.

A single compound is identified by extracting it, separating it from everything else until it is pure, and then using analytical techniques to work out its molecular formula and probable structure.

All the carbon compounds obtained from living tissue can be called biomolecules.

Inorganic content is found by a destructive test. A weighed piece of fresh tissue (wet weight) is dried to remove all water (dry weight) and then burnt fully. The carbon compounds leave as CO2 and water vapour, and what remains is ash, which contains inorganic elements such as calcium and magnesium.

Inorganic compounds such as sulphate and phosphate also appear in the acid-soluble fraction. Elemental analysis gives the elements; analysis for compounds gives the organic and inorganic compounds present.