chemical fingerprint

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A unique pattern indicating the presence of a particular molecule, based on specialized analytic techniques such as mass- or x-ray-spectroscopy, used to identify a pollutant, drug, contaminant, or other chemical in a test sample.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry the unique combination of analytical results from a number of techniques that identifies a particular compound in a sample