Radiation damage of fluorinated organic compounds measured by parallel electron energy loss spectroscopy

Abstract

The sensitivity of fluorinated organic compounds to radiation damage in the electron microscope was measured by electron energy loss spectroscopy (EELS). Five classes of molecules were investigated with fluorine atoms situated on: (1) an aliphatic chain, (2) an aromatic ring, (3) a trifluoromethyl group on an aromatic ring, (4) a trifluoromethyl group on a heterocyclic ring, and (5) a trifluoromethyl group next to a carbonyl group. The damage dose for fluorine loss was found to depend strongly on the position of the fluorine atoms and on specimen temperature. For poly-substituted fluorine on an aliphatic chain, approximately half of the fluorine was retained at doses in excess of 10 7 e/nm 2. At room temperature molecules containing trifluoromethyl groups on aromatic or heterocyclic rings were much more sensitive to fluorine loss than compounds having fluorine substituted directly on the rings. This behavior is consistent with the relatively low chemical stability of the trifluoromethyl group in these structures. The rapid loss of fluorine at low electron doses ( ≈ 10 4e/nm 2) in the trifluoromethyl aromatic compounds was reduced several orders of magnitude by cooling the specimen to liquid-nitrogen temperatures. An understanding of how specific types of fluorinated compounds suffer damage under the electron beam will be important in their potential application as biochemical or pharmacological probes.