Scanning x-ray fluorescence microscopy is analogous to scanning electron microscopy. Maps of the distribution of chemical elements are produced by scanning the specimen with a very small x-ray beam while collecting the XRF spectrum. Our goal is to perform such scanning microscopy with resolution in the range of <1 to 10 μm, using standard laboratory x-ray tubes. In order to increase the radiation flux on the specimen, we are investigating mirror optics in the Kirkpatrick-Baez (K-B) configuration, K-B optics uses two curved mirrors mounted orthogonally along the optical axis. The first mirror provides vertical focus, the second mirror provides horizontal focus. We have used two types of mirrors: synthetic multilayers and crystals. Multilayer mirrors are used with lower energy radiation such as Cu Kμ. At higher energies such as Ag Kct, silicon wafers are used in order to increase the incidence angles and thereby the photon collection efficiency. In order to increase the surface area of multilayers which reflects x-rays at the Bragg angle, we have designed mirrors with the spacing between layers graded along the optic axis in order to compensate for the changing angle of incidence. Likewise, to achieve a large reflecting surface with silicon, the wafers are placed on a specially designed lever arm which is bent into a log spiral by applying force at one end. In this way, the same diffracting angle is maintained over the entire surface of the wafer, providing a large solid angle for photon collection.

X-ray Microfluorescence (XRMF) analysis uses a finely collimated beam of X-rays to excite fluorescent radiation in a sample (Nichols & Ryon 1986). Characteristic fluorescent radiation emanating from the small interaction volume element is acquired using an energy dispersive detector placed in close proximity to the sample. The signal from the detector is processed using a computer-based multi-channel analyzer.

XRMF imaging is accomplished by translating the sample through the small X-ray beam in a step or continuous raster mode. As the sample is translated, a pixel by pixel X-ray intensity image is formed for each chemical element in the sample. The resulting digitized image information for each element is stored for subsequent processing and/or display. The images, in the form of elemental maps representing identical areas, may be displayed and color coded by element and/or intensity and then overlayed for spatial correlation.

The present study of parameters affecting the performance of an X-ray microfluorescence system has shown how such systems use X-ray beams with effective spot sizes less than 100 micrometers to bridge the gap in analytical capabilities between predominately surface micro analytical techniques such as SEM/EDX and bulk analytical methods such as standard XRF analysis. The combination of XRMF spectroscopy with digital imaging allows chemical information to be obtained and mapped from surface layers as well as from layers or structures beneath the sample surface. Simultaneously, it provides valuable high resolution chemical information in a readily interpreted visual form which displays the homogeneity within a given layer or structure. XRMF systems retain the advantages of minimal sample preparation, non-destructive analysis and high sensitivity inherent to XRF methods.

In the EDXRF analysis of minor and trace elements in a variety of matrices, the use of a polarized x-ray source incident on a sample will provide minimum detection limits three to five times lower than the use of non-polarized sources (1,2). There are various methods of producing monochromatic polarized x-rays for specimen excitation (3,4,5,6 ). Such x-ray sources may produce the lowest detection limits for a single element or a narrow range of elements. However, if one is interested in simultaneously analyzing a broad range of elements, a polychromatic source is desired (7,8,9). We present here a new method for producing broad-band polarized x-rays.

Spectrographic analysis of light elements by soft and ultrasoft fluorescent x-rays has become a useful technique for many applications of elemental analysis, using single crystals, soap multilayers and a combination of total reflection and filtering.

Instead of the wavelength dispersive method based on Bragg reflection which provides high resolution combined with low reflecting intensity, monochromatization combining total reflection by a selected mirror and an appropriate filter offered an alternative approach in order to increase measurable intensity with reasonable spectral resolution.

Recently, the use of synthetic multilayers, which are prepared by sputter/evaporation techniques, has been introduced for the detection of soft and ultrasoft x-rays. Studies on the use of these new wavelength dispersing devices have been conducted and it has been found that the reflectivity of these devices is very high compared with single crystals and soap multilayers and that their resolving power is fairly good.

The use of polarized x-rays to excite fluorescence spectra with decreased backgrounds and improved detection limits is reaching a mature state of development. With bulk, low-Z specimens, polarized x-ray sources have produced detection limits which are ~℩ to 3 times lower than are obtained with the best unpolarized photon sources. Based upon experience and the known properties of larger solid angle geometries, further significant to dramatic improvements are anticipated.

Energy dispersive x-ray fluorescence is an established and versatile tool for measuring major and trace elements in virtually any kind of solid or liquid specimen. The usefulness of the method could he extended even further if the time of analysis for multicomponent samples could he reduced. In other words, we desire to analyze a wide range of elements with detection limits at least as good as obtained when the excitation conditions are optimized for a specific element or narrow range of elements. A major impediment to achieving this goal when analyzing bulk, low-Z materials is the scatter of source radiation into the detector by the specimen being analyzed. The adverse effects of the scattered radiation are its contribution to the background signal (i.e., “noise“) and its overwhelming contribution to the limited counting rate of the system electronics.

Polarized x-radiation produced by scatter at 90° from boron carbide is shown to be a superior excitation source for the measurement of trace elements by energy dispersive x-ray fluorescence. In a close-coupled system which uses a high power x-ray tube as the primary source, the losses due to the geometric requirements and scattering efficiency of the polarizer can be more than compensated. With the system described here which uses a Mo anode x-ray tube, detection limits for the element between K and Sr in NBS orchard leaves are approximately 2 to 4.5 times lower using polarized excitation in comparison to direct excitation, and about <1 to 3 times lower in comparison to secondary excitation.