Ultramicroscopy 5 (1980) 521-523 © North-Holland Publishing Company LETTER TO THE EDITOR CHEMICAL MEASUREMENTS OF RADIATION DAMAGE IN ORGANIC SAMPLES AT AND BELOW ROOM TEMPERATURE R.F. EGERTON Physics Department, University of Alberta, Edmonton, Canada T6G 2Jl Received 11 June 1980 Electron-irradiation damage provides a basic limi- tation to the spatial resolution obtainable in electron- microscope imaging and microanalysis of biological specimens [1,2]. Because some stages in the damage process are likely to be temperature-dependent, one possible means of reducing electron-beam degradation is to maintain the specimen at cryogenic temperatures during examination. Using loss of structural order (as judged from an electron diffraction pattern) as the criterion of damage, the observed increase in radia- tion resistance below room temperature has been somewhat variable [3-7]. On the other hand, using total mass loss as a criterion of radiochemical decom- position, the temperature dependence has appeared more substantial [8,9]. In the present studies, elec- tron energy-loss spectrometry (EELS) is used to monitor the escape of individual chemical elements from amorphous polymer films as a function of elec- tron exposure and irradiation temperature. Specimens were mounted on 400-mesh copper grids and irradiated by 80 keV electrons in a JEM 100B microscope fitted with a specially constructed specimen holder (details to be published) which could be cooled to below 100 K by circulation of liquid nitrogen. At the dose rates (<0.4 mA cm -2) and sam- ple thicknesses (<50 nm) used in the experiments, temperature rise due to the incident beam should be negligible [10]. Energy-loss spectra were obtained from a magnetic spectrometer [11], the collection semi-angle (determined by an objective aperture) being a = 8.4 mrad. The spectra were stored and anal- ysed using a Tracor 1710 MCA programmed for light- element microanalysis [12]. Concentrations of car- bon, nitrogen and oxygen were measured from the ~ I at°ms cm-2xlOl6 ' 10 ~, D.~7 = 6 mC cm -2 i ,,, s • f o-o o Q ° • • • -',<-'- D .37 = • ~ 2 mC cm "2 I D I l~dose (C cm -z) 2- I I I I I .02 .04 0 0 0 "D rl T atoms cm -2 xlO16 , I i .02 0 00÷ 0 a a+ C dose (C cm -z) A A A ~-N A I , I .04 .06 Fig. 1. Amount of carbon (square data points), nitrogen (triangles) and oxygen (circles) per unit area of thin collodion samples during irradiation by 80 keV electrons at room temperature (left) and with liquid-nitrogen cooling (on the right). 521