Electron probe x-ray analysis of single ferritin molecules.

Single molecules and groups of two or three ferritin molecules were subjected to electron probe x-ray microanalysis in a transmission electron microscope equipped with a liquid nitrogen cooled stage. Significant Fe Kalpha peaks were generated during 100-sec counts when single ferritin molecules were excited with a probe current of 0.35 nA/60 nm spot, less than the maximal current available in a thermionic gun. There was a linear relationship between the number of ferritin molecules analyzed and count rates. The experimental results are compared to the theoretically calculated Fe Kalpha yields and to the results of Isaacson and Johnson [(1975) Ultramicroscopy I, 33-52] with electron energy loss analysis. We conclude that current state of the art electron probe x-ray analysis can realize the theoretically predicted sensitivity of the method, and estimate 0.9 X 10(-19) g of Fe as the minimal mass detectable with maximal (thermionic) probe current during a 100-sec count and with 95% confidence.