Width monochromator bandwidth

Decreasing the monochromator exit slit width decreases the bandwidth of radiation and decreases the eneigy reaching the detector. Thus, resolution of closely spaced absorption bands is achieved at the expense of decreased signal-to-noise ratio. For quantitative analysis, a monochromator bandwidth that is 1/5 of the width of the absorption band (measured at half the peak height) is reasonable. 

We will concentrate here on correction using a continuous emission lamp. The method consists of measuring, alternatively, the atomic absorption from the line of the element and the non specific absorption from a continuous spectrum lamp, over an range centred on the line and defined by the monochromator bandwidth. As this is much greater than the width of the line being analysed, we can consider that the second measurement corresponds solely to continuous (non specific) absorption. Continuous spectrum lamps used to correct the background arc ... 

For quantitative analysis, use a wavelength of maximum absorbance so that small errors in the wavelength do not change the absorbance very much. Choose a monochromator bandwidth (controlled by the exit slit width in Figure 19-5) small enough that it does not distort the band shape, but not so small that the spectrum is too noisy. Widening the slit width distorts the spectrum. In the lowest trace, a monochromator bandwidth that is one-fifth of the width of the sharp absorption bands (measured at half the peak height) prevents distortion. [Courtesy M. D. Seltzer, Michelson Laboratory, China Lake, CA.)... 

Equation 10.1 has an important consequence for atomic absorption. Because of the narrow line width for atomic absorption, a continuum source of radiation cannot be used. Even with a high-quality monochromator, the effective bandwidth for a continuum source is 100-1000 times greater than that for an atomic absorption line. As a result, little of the radiation from a continuum source is absorbed (Pq Pr), and the measured absorbance is effectively zero. Eor this reason, atomic absorption requires a line source. 

The resolution of a monochromator is the smallest frequency interval the instrument can separate. The limiting resolution is the bandwidth measured at half height when scanning across an infinitely narrow intense source 22). As already mentioned, the broader excitation line width of Ar+ lasers (0.15 to 0.25 cm-1) compared to that of the He-Ne lasers (0.05 cm-1) means a lower resolution limit when the Ar+ laser is used as a Raman source. 

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 synchronous spectra (SF) were collected in the 260-460 nm excitation wavelength range using bandwidth of AA=20 nm between the excitation and emission monochromators. All SF and emission spectra were recorded with a 10 nm slit width on both monochromators. The scan speeds of spectra were 500 nm/min. 

Bandwidth is the width of the wavelength band that is allowed to exit a monochromator. The narrowness of this band is called the resolution. High resolution corresponds to a very narrow bandwidth and vice versa. 

The proportionality factor k depends on several parameters, in particular on the optical configuration for observation (i.e. the solid angle through which the instrument collects fluorescence, which is in fact emitted in all directions) and on the bandwidth of the monochromators (i.e. the entrance and exit widths see Chapter 5). 

Furthermore, if one uses the wavelength modulation technique, which is strictly not surface sensitive but only enhances sharp structures and as the bandwidth of a grating monochromator decreases with decreasing energy, the low frequency peak will appear broad for reasonable slit widths. The... 

The luminescence emission spectrum of a specimen is a plot of luminescence intensity, measured in relative numbers of quanta per unit frequency interval, against frequency. When the luminescence monochromator is scanned at constant slit width and constant amplifier sensitivity, the curve obtained is the apparent emission spectrum. To determine the true spectrum the apparent curve has to be corrected for changes of the sensitivity of the photomultiplier, the bandwidth of the monochromator, and the transmission of the monochromator with fre-... 

M3, which compensates for aberrations and yields an excellent quality of image b) Czerny-Turner design, similar in conception, incorporating two spherical mirrors. M and jf/4 c) design with a concave grating (7( allowing simultaneous dispersion and focusing of the radiation. The spectral bandwidth of these monochromators depends on the width of the entrance and exit slits, F, and F2-... 

The wider the exit slit in Figure 20-5, the wider the band of wavelengths selected by the monochromator. We usually measure slit width in terms of the bandwidth of radiation selected by the slit. Instead of saying that a slit is 0.3 mm wide, we might say that the bandwidth getting through the slit is 1.0 nm. 

The linear dispersion. This is the spectral width in nm transmitted through the exit slit of the monochromator. It is given in units of nm/mm. For example 5 nm/mm would mean that the nominal bandwidth of the monochromatic light is 5 nm if the slit width is 1 mm. This defines the purity of the monochromatic light. 

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... 

In some assays it is necessary to specify the minimum desirable resolution, since changes in the spectral bandwidth (or monochromator slit-width) can seriously affect the observed absorbance of sharp peaks. The British Pharmacopoeia (1980) requires that the spectral bandwidth employed should be such that further reduction does not lead to an increase in measured absorbance. This is particularly important for drugs that have aromatic or strongly-conjugated systems, e.g. diphenhydramine,... 

The widths of the entrance and exit slits of the monochromator(s), which determine the optical bandwidth ( /l) at a given wavelength A should be narrow compared to the width of the spectral features. Absorption coefficients of narrow peaks measured with too large slit widths will be too small ... 

Slits help focus light onto the monochromators and the detector they regulate the wavelength range that excites and is emitted by the sample. Smaller slit widths are more selective, producing a narrower range of spectral bandpass (bandwidth at half the peak transmittance), but a decrease in transmitted light is noted and, therefore, there is a decrease in sensitivity. There are two types of slits fixed and variable. 

Wavelength repeatability is a measure of the precision of wavelength measured. The bandwidth refers to the width of an emission band (from the monochromator) at half peak height. This value, normally provided by the manufacturer is accepted. Using a mercury vapor lamp one can also check the spectral width. A number of well defined emission lines at 243.7, 364.9, 404.5, 435.8, 546.1, 576.9, and 579 nm can be used to check spectral bandwidth. However, the accuracy of the absorbance measured is dependent on the ratio of spectral bandwidth to the normal bandwidth (NEW) of the absorbing species. Most active pharmaceutical compounds have a normal bandwidth of approximately... 

The widths of atomic absorption lines are much less than the effective bandwidths of most monochromators. 

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