Spectral bandwidth defined

In a polychromator PDA spectrometer it was previously mentioned that there is no exit slit - all diffracted wavelengths (after order sorting) fall onto the diode-array detector. In this case what defines the analyser spectral bandwidth Each individual detector element integrates the signal falling onto it, and allocates that signal to a specific wavelength. In... 

An important parameter of a spectrometer is the relative bandwith RB. It is defined by the spectral bandwidth of the spectrometer relative to the linewidth to be recorded. 

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... 

Figure 3-7 Spectral characteristics of a sharp-cutoff filter (a) and a wide-bandpass filter (b). The narrow-bandpass filter (c) is obtained by combining filters a and b. The spectral bandwidth of filter c (distance n-m) is defined as the width in nanometers of the spectral transmittance curve at a point equal to one half of maximum transmittance.

The spectral bandwidth of a spectrophotometer is measured by use of a mercury-vapor lamp, which shows a number of sharp, well-defined emission lines between 250 and 580 nm. The apparent width of an emission band at half-peak height is taken to be the spectral bandwidth of the instrument (see Figure 3-7). The spectral bandwidth may also be calculated from the manufacturer s specifications. Interference filters with spectral bandwidths of 1 to 2nm are available and may be used to check those instruments with a nominal spectral bandwidth of 8 nm or more. 

Knowledge of the atomic spectra is also very important so as to be able to select interference-free analysis lines for a given element in a well-defined matrix at a certain concentration level. To do this, wavelength atlases or spectral cards for the different sources can be used, as they have been published for arcs and sparks, glow discharges and inductively coupled plasma atomic emission spectrometry (see earlier). In the case of ICP-OES, for example, an atlas with spectral scans around a large number of prominent analytical lines [329] is available, as well as tables with normalized intensities and critical concentrations for atomic emission spectrometers with different spectral bandwidths for a large number of these measured ICP line intensities, and also for intensities calculated from arc and spark tables [334]. The problem of the selection of interference-free lines in any case is much more complex than in AAS or AFS work. 

In contrast to nonresonant two-photon excitation, the cross-section for resonant two-photon excitations is relative large if the atoms are excited via a strong resonance transition. To achieve resonant two-photon excitation, however, two tunable lasers are necessary with sufficiently narrow spectral bandwidths. The laser beams intersect the absorbing volume in CO- or counter-propagating direction. If the first laser is tuned to the Doppler profile of the lower transition, atoms are excited with a well-defined velocity component in beam direction, whereas the second laser probes the population density of the excited atoms within this velocity group (Figure 4). The basic arrangement for isotope-selective analysis makes use of two absorption volumes, which are intersected by... 

Extinction spectra of single silver nanodisks have been reported [49]. The spectra shows an extinction band in the visible to near-infrared region that shifts toward the longer wavelength with the increment of the aspect ratio defined as (diameter)/(thickness), similar to the gold nanorods. The spectral bandwidth is broader than the gold nanorods. Since the sample is a single nanoparticle, the broad... 

In contrast, the natural bandwidth is an intrinsic property of the sample, independent of the instrument bandwidth, and is defined as the width (in nm) at half the height of the sample absorption peak, as shown in Figure 11. For example, the value for the natural bandwidth of the 340 nm peak of NADH is 58 nm, whereas for most cytochromes at room temperature the natural bandwidths in the a-region are of the order of 10 nm. It is easy to conceive that having too broad a spectral bandwidth would result in an apparent decrease of sample absorption. This is because the incident light would contain a large fraction of radiation with wavelengths poorly absorbed by the sample. 

FiG. 17. An optimum spectral bandwidth will fully define an absorption band without introducing unnecessary noise. The a and P bands of reduced cytochrome c are scanned with several spectra band widths. A wide value of 20 nm (1) has completely distorted the separation and the peak heights of the two bands. Narrowing the spectral bandwidth to 10 nm (2), 5 nm (3) and 1 nm (4) shows a sequential improvement in band definition. However, adjustment of the spectral bandwidth to a narrow 0.08 nm (5) significantly increases the noise level with no noticeable improvement in peak height or band separation of the two bands (10). 

We have already discussed quantum-beat spectroscopy (QBS) in connection with beam-foil excitation (Fig.6.6). There the case of abrupt excitation upon passage through a foil was discussed. Here we will consider the much more well-defined case of a pulsed optical excitation. If two close-lying levels are populated simultaneously by a short laser pulse, the time-resolved fluorescence intensity will decay exponentially with a superimposed modulation, as illustrated in Fig. 6.6. The modulation, or the quantum beat phenomenon, is due to interference between the transition amplitudes from these coherently excited states. Consider the simultaneous excitation, by a laser pulse, of two eigenstates, 1 and 2, from a common initial state i. In order to achieve coherent excitation of both states by a pulse of duration At, the Fourier-limited spectral bandwidth Au 1/At must be larger than the frequency separation ( - 2)/ = the pulsed excitation occurs at... 

The spectral bandwidth (AA) back-reflected has a narrow bandwidth defined by the materials birefringence (An). AA is typically of the order of 10 nm. 

The incremental formula is pretty easy to use it requires only multiplication and division. Everything needed is defined. It applies, however, only to very special situations infinitesimally small collectors, emitters, and spectral bandwidths. What do we do if we find ourselves wifh a large collector, or a wide field of view (FOV), or a bandpass that is not limited Our equation requires extension in four ways, which can often be treated separately. These are described here and illustrated in Figure 2.4. 

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