Dispersive instruments, atomic

Important ongoing developments in HRTEM that are expected to be valuable in catalysis research include the correction of spherical aberrations in electron microscope lenses and monochromatization of the electron beam for improvement of the spatial and spectral resolution. Recently, scanning-TEM (STEM) of atomically dispersed lanthanum atoms on alumina (63) has provided e.x situ aberration-corrected images, but it is noteworthy that there is no technical limitation in applying the correction devices to instruments used for making measurements of samples in reactive environments. 

Microprobe mappings and measurements of minor element concentrations were obtained at the Analytical Centre of the Mineralogical Laboratory of the Natural History Museum (London) using a CAMECA SX 50 instrument equipped with four wave-length dispersive detectors. Atomic force observations were carried out with a Dimension 3000 microscope (Digital Instrument) using the... 

Non-dispersive Instruments. In non-dispersive equipment, the atoms act as their own monochromators. In these instruments element-specific, line-like... 

Dispersive Instruments. In dispersive instruments monochromators are employed for selection of the wavelength. When a line-like radiation source is employed, a monochromator of low resolution is adequate, but for a continuum radiation source a high resolution monochromator is required. In dispersive equipment the exit slit width is narrower than that in non-dispersive equipment. In this way, thermal background emission and stray light originating from the atomizer can be considerably decreased, but at the same time the optical transmission also decreases. The schematic construction of a dispersive AFS instrument is shown in Figure 144. 

When using a crystal spectrometer or an energy dispersive instrument, it is necessary to periodically check the spectral contamination level by inserting a chemically pure low-atomic-number organic sample such as polyethylene (or other similar... 

X-ray fluorescence (XRF) spectroscopy is useful for qualitative elemental analysis of paint samples. It does not require dissolution of the sample and can be applied to dry films. When an energy-dispersive instrument is employed, XRF provides rapid information on the presence of elements of atomic number higher than or equal to 12 (e.g., above magnesium). However, from a quantitative point of view, the sensitivity, accuracy, and reproducibility of XRF measurements is lower than that of flame, electrothermal, or plasma atomic spectrometry. 

With modern detectors and electronics most Enei -Dispersive X-Ray Spectroscopy (EDS) systems can detect X rays from all the elements in the periodic table above beryllium, Z= 4, if present in sufficient quantity. The minimum detection limit (MDL) for elements with atomic numbers greater than Z = 11 is as low as 0.02% wt., if the peaks are isolated and the spectrum has a total of at least 2.5 X 10 counts. In practice, however, with EDS on an electron microscope, the MDL is about 0.1% wt. because of a high background count and broad peaks. Under conditions in which the peaks are severely overlapped, the MDL may be only 1—2% wt. For elements with Z < 10, the MDL is usually around 1—2% wt. under the best conditions, especially in electron-beam instruments. 

Sorption mechanisms of Hg(II) by the nonliving biomass of Potamogeton natans was also elucidated using chemical and instrumental analyses including atomic absorption, electron microscopy, and x-ray energy dispersion analyses. The results showed a high maximum adsorption of Hg(II) (180 mg/g), which took place over the entire biomass surface. Nevertheless, there were spots on the surface where apparent multilayer sorption of Hg(II) occurred. The minimum concentration of Hg(II) in solution that can be removed appears to be about 4-5 mg/L.117... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

Theory Instruments In energy dispersive x-ray fluorescence spectrometry, a sample is bombarded by x-rays that cause the atoms within the sample to fluoresce (i.e., give off their own characteristic x-rays) and this fluorescence is then measured, identified and quantified. The energy of the x-rays identify the elements present in the sample and, in general, the intensities of the x-ray lines are proportional to the concentration of the elements in the sample, allowing quantitative chemical... 

Slurry nebulization has also proved very popular. In this technique, sample (typically 0.25 g) is placed in a 30 ml plastic bottle and 10 g of expanded zirconia beads are added. A dispersant is added and the bottle is sealed and then placed on a mechanical shaker for several hours. During the shaking, the zirconia beads grind the sample into very fine particles. After dilution to a known volume, the slurry may be aspirated directly into an atomic spectrometric instrument. Other methods of slurry preparation also exist, e.g. using a micronizer, but the bottle and bead method is the most common. 

We have used our Single Turnover (STO) reaction sequence to characterize dispersed metal catalysts with respect to the numbers of alkene saturation sites, double bond isomerization sites, and hydrogenation inactive sites they have present on their surfaces (ref. 13). Comparison of the product composition observed when a series of STO characterized Pt catalysts were used for cyclohexane dehydrogenation with those observed using a number of instrumentally characterized Pt single crystal catalysts has shown that the STO saturation sites are comer atoms of one type or another on the metal surface (ref. 10). 

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