Monochromators slit width

What are the advantages and disadvantages of decreasing monochromator slit width ... 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

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,... 

Why do quantitative and qualitative analyses often require different monochromator slit widths 25-5. Why is iodine sometimes introduced into a tungsten lamp ... 

Fig. 5a-d. NRS and SERS spectra of 9-methylguanine (9-MeGua) 514.5 nm excitation, monochromator slit width 5 cm" . 

Several different types of spectral interferences are possible in analytical atomic fluorescence. If a second, unwanted element emits a fluorescence radiation simultaneously with the analyte element and its wavelength is within the band pass of the monochromator slit width, interference occurs. Not many instances of this type of interference have been identified. Some known examples include cadmium at 2288.0 A and arsenic at 2288.1 A and mercury with iron, thallium, chromium, and magnesium. If such interferences occur, the result will be an erroneous increase in fluorescence signal... 

A second type of spectral interference is due to thermal emission from the flame of the sample. For interference to occur in this case the thermally excited spectral lines also must lie within the spectral band pass of the monochromator slit width, and this also results in an erroneously enhanced signal. In this case, however, the interference can be eliminated by use of a modulated source and an amplifier tuned to the frequency of modulation since the undesired signal will be a steady (dc) signal. 

Table 1 Dependence of the AA measured on the monochromator slit widths...

All fluorescence studies were performed on a Fluorolog-2 spectrofluorometer, Spex Industries (Edison, NJ), equipped with a 450-Watt Xenon arc lamp. The excitation and emission monochromator slit widths were both set at 2 mm. The excitation and emission wavelengths of the various fluorophores used are indicated in Figure 2. All data were acquired at room temperature using quartz cuvettes with sample volumes of 1.5 mL. 

In steady-state PL, the shape of the spectrum is determined by the level of excitation intensity as the defect-related PL often saturates at power densities on the order of to 10 Wcm, and the overall PL spectrum may be skewed in favor of the excitonic emission at higher excitation densities. Similarly, focusing the laser beam and using small monochromator slit widths would also skew the PL in favor of excitonic transitions. In such a case, the chromatic dispersion of the lenses used to collect the PL, as well as the different effective sizes of the emission spots for the ultraviolet (UV) and visible emission attributed in particular to photon recycling process [24], may lead to a noticeable artificial enhancement of the UV (near band edge) over the visible part in the PL spectrum (mainly defect related). Qualitative terms such as "very intense PL attesting to the high quality of the material are omnipresent in the literature on ZnO. In contrast to the wide use of PL measurements, relatively little effort has been made to estimate the absolute value of the PL intensity or its quantum efficiency (QE) for a quantitative analysis. 

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