Monochromators wide bandpass

Effect of bandpass and choice of wavelength on a Beer s law plot. Curve A represents a calibration curve using a narrow bandpass monochromator at k. Curve B represents a calibration curve using a wide bandpass filter at X or a narrow bandpass monochromator at. ... 

Tsuruta, H., Brennan, S., Rek, Z. U., Irving, T. C., Tompkins, W. H., and Hodgson, K. O. (1998). A wide-bandpass multilayer monochromator for biological small-angle scattering and fiber diffraction studies. J. Appl. Crystallogr. 31, 672—682. 

Another requirement of the monochromator is that it should be capable of using wide slits—i.e., wide bandpasses, for elements whose emission spectra are uncomplicated. Zinc, arsenic, selenium, calcium, lead, and the alkalis are examples of such elements. When the bandpass is wider, more energy can be passed through the monochromator and the obtainable precision and detection limits are better. 

As we have already mentioned, atomic absorption lines are very narrow (about 0.002 nm). They are so narrow that if we were to use a continuous source of radiation, such as a hydrogen or deuterium lamp, it would be very difficult to detect any absorption of the incident radiation at all. Absorption of a narrow band from a continuum is illustrated in Fig. 6.4, which shows the absorption of energy from a deuterium lamp by zinc atoms absorbing at 213.9 nm. The width of the zinc absorption line is exaggerated for illustration purposes. The wavelength scale for the deuterium lamp in Fig. 6.4 is 50 nm wide, and is controlled by the monochromator bandpass. If the absorption line of Zn were 0.002 nm wide, its width would be 0.002 x 1/50= 1/25,000 of the scale shown. Such a narrow line would be detectable only under extremely high resolution (i.e., very narrow bandpass), which is not encountered in commercial AAS equipment. 

The geometric slit width is associated with the effective mechanical widths (in mm or )um) at the entrance and exit slits for a given spectral bandpass. The entrance and exit slits, thus, control the portion of the radiation from the source that enters the monochromator and falls on the detector. By use of a wide entrance slit, large amounts of radiation energy reach the detector. In this case, the noise is small compared to the signal, and lower amplification can be employed. When the noise is low, the signal is stable and precise and low detection limits can be measured. The entrance and exit slits should have very similar mechanical dimensions. 

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