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Ultrastructural and elemental imaging of biological specimens by soft X-ray contact microscopy.

Soft X-ray contact microscopy offers a means of visualizing unstained as well as stained biological materials at better than 6 nm resolution. Unlike light and transmission electron microscopy, which rely on stains to increase specimen contrast, soft X-ray imaging is dependent upon the differential absorption of incident soft (1-10nm wavelength) X-rays by the endogenous elements within the specimen. The advantages of using soft X-rays for imaging are: 1) reduced specimen damage during exposure; 2) ability to image hydrated specimens at atmospheric pressure; 3) ability to image specimens ranging in thickness from less than 40 nm to as much as 10 microns; and 4) ability to map the elemental composition of the specimen through observation of the differential absorption of properly chosen incident x-ray wave lengths. This paper explains the principles of the image formation and demonstrates the use of soft X-ray contact microscopy with biological samples which could not readily be imaged in their natural form using conventional electron microscopy methods.