Soft x-ray microscopy is used for imaging and researching the elemental composition and structure of biological samples and more. With a wavelength range between 4.0 Å (~3.0 keV) and 44 Å (~300 eV), soft x-ray microscopes are capable of achieving spatial resolution of several hundred Å, about 10x better resolution than the maximum of a visible light microscope.
The primary advantages of a soft x-ray microscope are its design simplicity and its ability to form the highest-spatial-resolution images of thick, hydrated biological samples without the time-consuming sample preparation required by electron microscopes in a near-native environment. Also, because biological samples consist largely of hydrogen, carbon, oxygen, and nitrogen (with additional amounts of seven other elements and important trace elements), and have their primary absorption edges (except hydrogen) in the water window between the absorption edge of carbon (284 eV, 4.4 nm) and oxygen (543 eV, 2.3 nm), soft x-ray microscopes provide excellent spectroscopic information and deliver high-contrast images.
The absorption length for soft x-rays in water and major elements in a biological sample.
In a soft x-ray microscope arrangement, soft x-rays from a synchrotron (laser-induced x-rays) are guided along a beamline and a sample is exposed. A zone plate lens is used to form an image of the transmitted x-rays onto a special back-illuminated CCD (one without antireflective coating) and a high-contrast image is captured. In a scanning soft x-ray microscope arrangement, monochromatic soft x-rays from a synchrotron are guided along a beamline and impinge on the zone plate.
The zone plates focus the x-rays to bring them to a very sharp focus (<0.1 µm in diameter) at a single point on the sample. The sample is scanned and the information about the absorption versus position is recorded. This step is repeated several times with different monochromatic x-rays and an image is constructed, pixel by pixel, in the software. Such an approach enables excellent elemental and chemical analyses at very high spectral resolutions, limited merely by synchrotron instruments.
Diagram of main components of soft X-ray microscopy at beam-line XM-1, at Lawrence Berkeley National Laboratory, Berkeley, California, USA .
A table-top soft x-ray microscope based on a long-term stable and nearly debris-free laser plasma from a pulsed nitrogen gas jet target is presented. The microscope operates in the “water window” region at 2.88 nm wavelength. The emitted soft x-ray radiation is focused by an ellipsoidal condenser mirror into the object plane and a sample is imaged using a Fresnel zone plate onto a CCD camera. The spatial resolution of the microscope is about 50 nm demonstrated for a Siemens star test pattern.
V. Cardin, F. Legare et al.