Wavelength Dispersion 15-3 Spectrometers

Energy dispersive spectrometers can, in general, collect the spectmm faster and are less expensive than the more sophisticated wavelength dispersive spectrometers. However, they do not have the resolution and cannot separate closely spaced lines as easily as the wavelength dispersive spectrometers. 

Figure 5.5. A wavelength-dispersive spectrometer showing the specimen, crystal and detector all lying on...

Wavelength dispersive X-ray fluorescence spectrometric (xrf) methods, 25 60 Wavelength dispersive spectrometer (WDS), 76 488, 26 433-434 Wavelength dispersive X-ray fluorescence (WDXRF)... 

On the other hand, its poorer resolution and the mediocre signal/noise ratio means it cannot compete with the wavelength dispersive spectrometer (electron microprobc) for quantitative analysis and the analysis of trace elements (the detection limits are generally 10 times higher than in WDS). 

As with X-ray fluorescence, the characteristic X rays are analysed using wavelength dispersive spectrometers (WDS) based on the selective reflection of radiation by a monochromator crystal. The related analytical performance levels are ... 

The essential elements of an electron microprobe are the electron column and the wavelength dispersive spectrometers (Fig. 8.5). The electron column is very similar to that of a scanning electron microscope. Certain elements are added to it (such as a beam controller, viewfinder, etc.) to make it an instrument dedicated to elemental microanalysis. 

Table 6.4 Analyzing Crystals used in Wavelength Dispersive Spectrometers (WDS)...

A Philips PW-14(X) wavelength dispersive spectrometer equipped with four crystals (LiF, Ge, PET, and TIAP) was used for the analysis. The rhodium target tube was operated at 2.5 kW (50 kV and 50 mA). The sample holders were 32 mm in diameter with a copper mask. The elements determined and their respective peak and background times are shown in Table 2. The intensity ratio method was employed using a synthetic monitor specimen. 

The wavelength dispersive spectrometer is the conventional optical instrument in which the photons are spatially dispersed by diffraction by means of a crystal goniometer. For each position of the goniometer, the detector sees only a narrow wavelength band. These spectrometers have good resolution, but can determine only one element at a time. 

The X-ray analysis system for the EMA is a wavelength dispersive spectrometer with gas proportional counter detectors. In the SEM, an energy dispersive X-ray spectrometer with a Si(Li) detector is used. The entire electron and X-ray optical systems are operated under a vacuum of about 10 torr. Modem systems are completely automated with computer control of the instmment parameters, specimen stage movement, data collection and data processing. 

The instrumentation for EM uses the same type of X-ray spectrometers discussed in detail in Chapter 8, with an electron beam as the source and a UHV system that includes the sample compartment. An ED X-ray spectrometer allows the simultaneous collection and display of the X-ray spectrum of all elements from boron to uranium. The ED spectrometer is used for rapid qualitative survey scans of sample surfaces. The wavelength dispersive spectrometer has much better resolution and is used for quantitative analysis of elements. The WD spectrometer is usually equipped with several diffracting crystals to optimize resolution and to cover the entire spectral range. The electron beam, sample stage, spectrometer, data collection, and processing are all under computer control. 

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