Sample apertures disks

Figure 15-10 is a schematic of a typical filter fluorom-eler that uses a mercury lamp for fluorescence excitation and a pair of photomultiplier lubes as transducers. The source beam is split near the source into a reference beam and a sample beam. The reference beam is attenuated by the aperture disk so that its intensity is roughly the same as the fluorescence intensity. Both beams pass through the primary filter, with the reference beam then being reflected to the reference photomultiplier tube. The sample beam is focused on the sample by a pair of lenses and causes fluorescence emission. The emitted radiation passes through a second filter and Tutorial Learn more about fluorescence instrumentation. 

XRF is used for the analysis of solid and liquid samples. For quantitative analysis the surface of the sample must be as flat as possible, as will be discussed in the applications section. There are two classes of sample holders, cassettes for bulk solid samples and cells for loose powders, small drillings, and liquids. A typical cassette for a flat bulk solid such as a polished metal disk, a pressed powder disk, a glass or polymer flat is shown in Fig. 8.26(a). The cassette is a metal cylinder, with a screw top and a circular opening or aperture, where the sample will be exposed to the X-ray beam. The maximum size for a bulk sample is shown. The sample is placed in the cassette. For a system where the sample is analyzed face down, the cassette is placed with the opening down and the... 

The samples for the absorption measurements were prepared by grinding both sides of slices of the solid solution to a thickness 30-50ju and polishing on a felt disk. The diameter of the collimating aperture was 3 mm in the measurements on these samples. 

Beam condensers have proven to be very useful when the sample can be prepared as a KBr disk, since devices are available for pressing disks as small as 0.5 mm in diameter. For the examination of very small regions of larger samples, however, beam condensers are less benehcial. Take, for example, the identihcation of an impurity in a polymer him. To examine samples of this type with a beam condenser, the impurity must be mounted immediately behind a pinhole aperture drilled in a metal plate. The smaller the region to be examined, the more difficult it is to mount the pinhole so that it is located exactly over the impurity. Because more and more samples of this type are being examined every day, beam condensers have fallen out of favor and a device with a remote aperture is used instead. This device is the infrared microscope. 

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