Exitance narrow band

A monochromator disperses light into its component wavelengths and selects a narrow band of wavelengths to pass on to the sample or detector. The monochromator in Figure 20-2 consists of entrance and exit slits, mirrors, and a grating to disperse the light. Prisms were used instead of gratings in older instruments. 

Notwithstanding this larger flow-cells (10-25 pi) are used to give improved sensitivity at the expense of reduced resolution. As in conventional fluori-metry, scatter of the primary beam in the direction of the detector occurs and must be removed either by filters or monochrometers. Likewise, narrow band pass or cut-off filters or monochrometers are used to select the wavelength of the exiting beam. 

Each spectrometer for sequential or simultaneous multi-element plasma AES measurements is equipped with a monochromator in order to (i) have an adequate wavelength selection, and (ii) collect as much light as possible from a selected spectrum area in the radiation source. A monochromator consists of a) an entrance slit, b) a collimator to produce a parallel beam of radiation, (c) a dispersing element (a prism or a grating), d) a focusing element reforming the specific dispersed narrow bands of radiation, and (e) one or more exit slits to isolate the desired spectral band or bands. 

The sample, which initially exists as a narrow band in the system, is then carried through the precolumn and the analytical column set, where molecular size discrimination occurs. The discriminated sample elutes from the column set and passes through a universal detector, which generates an electrical (millivolt) signal proportional to the instantaneous sample concentration. The sample and mobile phase then exit the detector and are carried to a waste container the electrical signal is transmitted to an integrator, recorder, or computer for display and/or further processing. 

In Fig. 23.17, X5o,3/x5o,3,initial is plotted as a function of the modified dimensionless number We x)ro X using the same results as given in Fig. 23.16. All data points are now located in a narrowed band from which in particular a more uniform critical value for the secondary emulsion drop breakup of [IFei Drop/ llcr 3-5 can be derived (Fig. 23.17). The exception of a less good fitting dataset relates to liquid-cap nozzle flow experiments with the largest nozzle exit diameter of 2.5 mm for which there is a clear tendency of secondary drop breakup at significantly lower... 

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. 

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