High-resolution monochromators

A monochromatic beam of X-rays with about 1 eV bandwidth is produced by the standard beamline equipment, the undulator and the high-heat-load premonochromator being the most important parts among them. Further monochromatiza-tion down to approximately the millielectronvolt bandwidth is achieved with the high-resolution monochromator. The width of a band of a millielectronvolt, however, is much more than the inherent linewidth of the Fe y-radiation, F 10 eV, or the full range of hyperfine-split Mossbauer lines, A m 10 eV. Yet, NFS is detectable because the coherent excitation of the nuclei is caused in the... 

The major requirement of the light source for atomic absorption is that it should emit the characteristic radiation (the spectrum) of the element to be determined at a half-width less than that of the absorption line. The natural absorption line width is about 10 4 (A), but due to broadening factors such as Doppler and collisional broadening, the real or total width for most elements at temperatures between 2000 ° and 3000 °K is typically 0.02 — 0.1 A. Hence, a high resolution monochromator is not required. 

It essentially requires a separate lamp for each element to be determined and this serious lacuna is usually overcome either by using a line-source with the introduction of flame or by using a continuous source with the introduction of a very high resolution monochromator,... 

Figure 3 shows the excitation spectrum of CaO doped with Eu2+ when fluorescence from all sites in the sample was monitored. The dye laser was scanned over the possible absorption lines and each time the wavelength matched a transition on any site, the fluorescence intensity increased and gave a line. Figure 4 shows the same procedure on the same crystal except now a high resolution monochromator was used to monitor the fluorescence that occurred at a wavelength characteristic of a specific site. Now, one sees increases in the fluorescence only when the dye laser matches an absorption line of the same site that has the fluorescence line being monitored. 

Sensitivity could, of course, be improved significantly by using a very high resolution monochromator, which could isolate regions of the spectrum of 0.005... 

As explained in Chapter 1, section 7, unless a very high resolution monochromator, e.g. an echelle monochromator, is used to isolate a very narrow (< ca. 0.005 nm) band of light from a continuum spectrum prior to absorbance measurement, the sensitivity will be very poor.1,2 Although there are occasional reports of analysis by flame AAS using continuum sources such as xenon arc lamps, these are invariably from research laboratories. The vast majority of reported applications use single element line sources, and more than 99% of these applications use hollow cathode lamps. 

Secondly, when a CS is used for AAS, it is necessary to utilize a high-resolution monochromator in order to avoid loss of sensitivity and excessive curvature of the calibration function, and also to avoid spectral interferences. Becker-Ross et al. [12] have shown for several elements that the sensitivity continuously increases with increasing resolution until the spectral bandwidth is in the order of the width of the atomic absorption line, and that no further improvement in sensitivity is possible beyond that level. Becker-Ross et al. [13] also determined the half-width of the absorption lines of a large number of elements, and found that a monochromator with a resolution /A of about 100,000 is necessary for HR-CS AAS. Then... 

Obviously, such a high-resolution monochromator requires active wavelength stabilization in order to avoid drift problems. This has been accomplished through an internal neon lamp, mounted on an adjustable stand in front of the intermediate slit between the pre- and echelle-monochromator, so that it can be moved into the beam automatically if necessary. The neon lamp emits many relatively narrow lines in the 580-720 nm range, and, in the absence of any pre-selection, these are separated by the echelle grating into various superimposed orders. This means that without pre-dispersion at least two neon lines for every grating position surely fall on the detector, and can be used for stabilization. The precision of this stabilization is only limited by the stepper motor for grating adjustment, and is better than one-tenth of a pixel width (see Welz et al. [10]). 

A. F. Silva, D. L. G. Borges, B. Welz, M. G. R. Vale, M. M. Silva, A. Klassen, U. Heitmann, Method development for the determination of thallium in coal using solid sampling graphite furnace atomic absorption spectrometry with continuum source, high-resolution monochromator and CCD array detector, Spectrochim. Acta, 59B (2004), 841. 

Figure 2 Experimental arrangement for measurements of the Fe nuclear resonance at the Advanced Photon Source (APS). In the standard fill pattern, electron bunches with a duration of 100 ps are separated by 153 ns. X-ray pulses are generated when alternating magnetic fields in the undulator accelerate these electron bunches. The spectral bandwidth of the X-rays is reduced to 1 eV by the heat-load monochromator and to 1 meV by the high-resolution monochromator. At the sample, the flux of the beam is about 10 photons/s. APD indicates the avalanche photodiode used to detect emitted X-rays. The lower right inset illustrates that counting is enabled only for times weU-separated from the X-ray pulse, so that only delayed photon emission resulting from decay of the nuclear excited state contributes to the experimental signal...

Following the heat-load monochromator, the X-ray bandwidth is narrowed to approximately 1 eV and centered on the nuclear resonance energy (14.4 kev for Fe). The high-resolution monochromator further reduces the X-ray bandwidth to about 1 meV and motorized scanning of this monochromator tunes the energy over a range (typically within 100 meV of the resonance) adequate to explore excitation or annihilation of vibrational quanta. The X-ray flux at the sample is about 10 photons/s ( 10 tW), which is very low compared to typical milliwatt beam powers in laser-based Raman experiments see Vibrational Spectroscopy). Additional X-ray optics may reduce the beam size. The cross section of the beam at the sample point is currently about 0.5 x 0.5 mm at station D of beam line 3ID at APS. 

Iron also has a very complex spectrum and most other points that have been made in regard to manganese apply to this metal. The choice of the most sensitive among the many absorption lines again was made with the help of the photographic technique by Allan (A7). The strongest line is that at 2483.3 A at which sensitivity limits of 0.1 ppm have been obtained, but the line at 3720 A still permits the detection of iron at the 1 ppm level. Especially narrow slit width and high resolution monochromators are necessary for optimal results, because the resonance... 

Forward optic spectrophotometers are either singlebeam or double-beam spectrophotometers. The singlebeam instruments can be either very simple or expensive depending on the sophistication desired or needed. Simple single-beam instruments have poor stability and excessive drift. These advantages are eliminated in systems equipped with a high-resolution monochromator with adjustable slits, controlled by microprocessors for rapid data acquisition and evaluation of data. 

The resolution and selectivity in ICP emission comes primarily from the monochromator. As a result, a high-resolution monochromator can isolate the analyte spectral line from lines of concomitants and background emission. It can thus reduce spectral interferences. In atomic absorption spectrometry, the resolution comes primarily from the very narrow hollow cathode lamp emission. The monochromator must only isolate the emission line of the analyte element from lines of impurities and the fill gas, and from background emission from the atomizer. A much lower resolution is needed for this puipose. 

Advantages of AES, relative to flame-AAS, include the lack of a requirement for a radiation source. Collisions within the plasma serve to promote analyte atoms to excited state levels. Additionally, this technique is characterised by linearities of response which span three to four orders of magnitude. Limits of detection for ICP-AES are similar to those obtained with flame-AAS (typically within a factor of 3 to 5 - some elements are shghtly less responsive in flame-AAS others slightly more responsive). ICP-AES does require a fairly high resolution monochromator/detection system to scan carefully across analyte emission lines and to be able to resolve them from the other emissions and from the high luminosity of the torch. There are many spectral... 

The cathode ray tubes are scanned in a raster like a television picture. Each scan line is modulated into a series of dots called picture elements (abbreviated to pixels or pels) and each character is built up from these pixels. It soon became possible to manipulate the pixels individually so that as well as characters, dots, lines and shapes could be displayed on the screen. Microcomputers are now available with graphics capabilities rivalling those found on mainframe systems, but at a fraction of the cost. Clearly, more memory locations have to be put aside for graphics displays. For example, compare the text (character) display of the IBM Color/Graphics display with its high-resolution monochrome graphics mode. The 80-character mode puts 25 rows of 80 characters on the screen. Each character is stored in two bytes - one for the character itself and one for its attributes , that is, colour. 

Figure 7. Comparison of two measured muscovite rocking curves for the (006) Bragg reflection acquired by using (a, open circles) an undulator synchrotron beamline with a Si(l 11) monochromator at Er = 7.4 keV, and (b, solid circles) Cu Kax X-rays from a Cu fixed-anode source (Er = 8.04 keV ) followed by a Si(lll) four-bounce high-resolution monochromator and high-resolution diffractometer. The difference between the two-curves is primarily due to the size of the incident beam slit, which is 0.2 mm x 0.05 mm for (a) and 2 mm x 1 mm for (b). Only a source with the brightness of the undulator could be slitted down to such a small size as for (a) and still has sufficient flux on the sample for XSW measurements. Also shown is the best fit to the rocking curve of (a) (solid line). At 7.4 keV, the Darwin width of Si(l 11) is 37 / -ad and that of the muscovite (006) is 20 /rrad.

A more common type of spectral interference in either emission or absorption measurements arises from the occurrence of band emission-spectra due to molecular species in the flame. (In fact, many elements can be measured by means of the band spectra of the molecules they form in certain flames.) Calcium and strontium, for example, exist partially as molecular hydroxides and oxides in a flame and emit bands in the vicinity of both the sodium and lithium resonance lines. When the alkaline-earth/alkali-metal ratio is high, the interference can become serious, unless a high-resolution monochromator is used. 

It is important in AA measurements that the emission line width coming from the radiation source is narrower than the absorption line width of the atoms studied. In principle, a high resolution monochromator is not needed to separate the analyte line from the other lines of the spectrum, but in practice, the spectral bandpass of the source should be equal or less than the absorption line width. Otherwise, artificially low absorbance values are obtained leading to reductions in sensitivity. In the AA technique the use of continuum sources (quartz-halogen filament lamps and deuterium and xenon arc lamps) with reasonably priced monochromators is not satisfactory. This is demonstrated in Figure 17. In the case of (A) the emission of radiation is continuous for the whole spectral bandwidth. The energy absorbed by the atoms of the analyte is small in comparison to the whole... 

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. 

There are disadvantages to the Raman technique. Many samples contain florescent impurities or are inherently fluorescent. This fluorescence leads to baekground radiation, which often makes the observation of Raman spectra difficult. However, methods have been devised, for example using high resolution monochromators, which help circumvent this problem. 

As radiation sources in AAS, those line sources are mainly used that emit the spectral lines of one or more elements. Line sources make it possible to use conventional instead of high-resolution monochromators, as the monochromator only has to isolate the line of interest from other lines (mainly lamp fill gas lines). Hollow cathode lamps and electrodeless discharge lamps are the main types of lamps employed. 

Other sources of background include spectral line (nonanalyte atomic fluorescence) and spectral band (molecular fluorescence) interferences. Spectral line interferences are caused by the presence of another element that can absorb source radiation and emit fluorescence sufficiently close to the analyte wavelength to be collected by the detection system. Spectral band interferences involve the absorption of source light by a molecule whose fluorescence is collected by the detection system. Nonanalyte atomic fluorescence and molecular fluorescence are minimized by the use of a narrow line source and a nonresonance transition. This is in contrast to AES, where spectral interferences are sufficiently severe that a high-resolution monochromator is required. 

In summary, there seems to be no fluorimetric alternative to excel low-temperature techniques in analytical information throughput if chemical separations are to be avoided. Massive use in the environmental field has so far been restricted by the need to use fairly sophisticated devices such as high-resolution monochromators, cryostats, and, perhaps, dye lasers. On the other hand, a number of commercially available spectrofluorimeters provide inadequate spectral resolution for use in low-temperature spectroscopic techniques. However, given the high selectivity of such techniques, their use in the characterization of complex environmental samples will continue to grow in the foreseeable future. 

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