Two major facts were already known about DNA. The first was that it is contained in the nucleus of every living cell. The second was that, in addition to sugar (2-deoxyribose) and phosphate, DNA is composed of two bases: pyrimidines, of which there are two types (cytosine and thymine), and purines, of which there are also two types (adenine and guanine). In addition, two important experimental methods involving paper chromatography and ultraviolet light absorption had recently been developed.
To test the idea that DNA might be a primary constituent of the gene, Chargaff performed a series of experiments. He fractionated out nuclei from cells. He then isolated the DNA from the nuclei and broke it down into its constituent nucleic acids. Then, using paper chromatography, he separated the purines and the pyrimidines. This was done on the basis of the solubility of the substances being analyzed (a piece of chromatography paper is dipped into the solution and the different components of the solution travel different distances up the paper: the most soluble component travels the farthest up, to the driest section of the paper, and so on). He next exposed the separate components of the solution to ultraviolet light. Because each base absorbs light of a different, "characteristic" wavelength, he was able to determine how much of which bases are present in DNA.
Read more: http://www.answers.com/topic/erwin-chargaff#ixzz37w1J5giV