In 1977–1978 and again in 1981, the EPA funded projects for the construction of classification trees to recognize the presence of certain elements in compounds through the examination of their mass spectra. The EPA, as part of its regulatory function, collects numerous samples of air and water containing unknown compounds and tries to determine the presence of toxic substances. According to McLafferty: "The fragment ions indicate the pieces of which the molecule is composed, and the interpreter attempts to deduce how these pieces fit together in the original molecular structure. In such correlations have been achieved for the spectra of a variety of complex molecules." The critical element of the bromine tree was the construction of a set of questions designed to recognize bromine hallmarks. If bromine occurs in combination with chlorine, then since chlorine (weight 35) has an isotope of weight 37 that occurs 24.5 percent of the time, there is a different theoretical ratio vector.