Imaging with the Brookhaven Scanning Transmission X-Ray Microscope

Abstract

Current x-ray microscopes offer the possibility of imaging unstained, unfixed, unsectioned, micron thick, wet biological specimens with a resolution approaching 30nm. Because the absorption of x-rays depends primarily on the elemental composition of the specimen, the images lend themselves to quantitative analysis of sample structure. The Scanning Transmission X-ray Microscope at Brookhaven National Lab uses a zone plate to form a focused probe through which the sample is scanned. The modulation transfer function of the microscope has been measured and agrees well with calculations based on the zone plate construction. This knowledge permits recovery of the intrinsic contrast of object features using image deconvolution. Current zone plates provide a Rayleigh resolution of 50 nm, and the ability to see 30 nm objects. This resolution has been achieved in practice for images of test specimens and dried biological specimens, where high doses can be tolerated. Single images of wet V. faba chromosomes show 80 nm resolution. Subsequent images show that specimens were degraded by radiation damage following the initial exposure. Many researchers have used the Brookhaven STXM for problems in neurobiology, protein transport, elemental imaging, and chromosome structure.