Monochromator

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. 

The reciprocal linear dispersion dX/dx) is the function of the geometric slit width S) and spectral bandpass (AA ,) of the monochromator  

Prisms Prisms are used to disperse IR, visible, and UV radiation. The most common prisms are constructed of quartz for the UV region, silicate glass for the visible and near-IR (NIR) region. 

The symbols used are as follows / = angle of incidence, 0 = angle of diffraction (or reflectance), p = blaze angle of the grating, d = grating spacing. (Modified from Dean, J.A. and Rains, T.C., eds.. Flame Emission and Absorption Spectrometry, Vol. 2, Marcel Dekker, Inc., New York, 1971. Used with permission.) 

The deuterium lamp is a discharge light source in which deuterium is sealed in a bulb. These lamps require a large and complex power supply, which makes them more expensive than halogen lamps. However, deuterium lamps represent one of the few continuous-spectrum light sources that are stable in the UV range. Typically, a deuterium lamp will have a short emission wavelength of 400 nm, or less. 

The window material limits use of the lamp at the short wavelength end of the spectrum. 

The main objective of a monochromator is to divide and transmit a narrow portion of the optical signal that has been selected from a vrider range of wavelengths available at the input. In its simplest form, the monochromator is composed of two slits (entrance and exit) and a dispersion element (prism or diffraction grating). The main purpose of the entrance slit is to define the geometric properties 

The presence of a prism in the spectrophotometer causes the light to be dispersed into a rainbo v. At the exit slit, the different colors of visible light are intense as each arrives at a separate point in the exit slit plane in this way a series of images of the entrance slit becomes focused on the plane. Rotation of the dispersion element then causes the band of colors to move relative to the exit slit, so that the desired entrance slit image becomes centered on the exit slit (Horiba http //www.horiba.com/us/en/scientific/products/optics-tutorial/monochroma-tors-spectrographs/ c375 2). 

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The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

If the sample is placed in the path of the infrared beam, usually between the source and the monochromator, it will absorb a part of the photon energy having the same frequency as the vibrations of the sample molecule s atoms. The comparison of the source s emission spectrum with that obtained by transmission through the sample is the sample s transmittance spectrum. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

An electron prisin , known as an analyser or monochromator, is created by tlie field between the plates of a capacitor. The plates may be planar, simple curved, spherical, or toroidal as shown in Figure Bl.6.2. The trajectory of an electron entering the gap between the plates is curved as the electron is attracted to the positively biased (iimer) plate and... 

The other type of x-ray source is an electron syncluotron, which produces an extremely intense, highly polarized and, in the direction perpendicular to the plane of polarization, highly collimated beam. The energy spectrum is continuous up to a maximum that depends on the energy of the accelerated electrons, so that x-rays for diffraction experiments must either be reflected from a monochromator crystal or used in the Laue mode. Whereas diffraction instruments using vacuum tubes as the source are available in many institutions worldwide, there are syncluotron x-ray facilities only in a few major research institutions. There are syncluotron facilities in the United States, the United Kingdom, France, Genuany and Japan. 

The construction of a typical monochromator is shown in Figure 10.12. Radiation from the source enters the monochromator through an entrance slit. The radiation is collected by a collimating mirror, which reflects a parallel beam of radiation to a diffraction grating. The diffraction grating is an optically reflecting surface with... 

Effect of the monochromator s slit width on noise and resolution for the ultraviolet absorption spectrum of benzene. The slit width increases from spectrum (a) to spectrum (d) with effective bandpasses of 0.25 nm, 1.0 nm, 2.0 nm, and 4.0 nm. 

Radiation exits the monochromator and passes to the detector. As shown in Figure 10.12, a polychromatic source of radiation at the entrance slit is converted at the exit slit to a monochromatic source of finite effective bandwidth. The choice of... 

Typical grating monochromator with inset showing the dispersion of the radiation by the diffraction grating. 

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. 

An instrument for measuring absorbance that uses a monochromator to select the wavelength. 

Infrared instruments using a monochromator for wavelength selection are constructed using double-beam optics similar to that shown in Figure 10.26. Doublebeam optics are preferred over single-beam optics because the sources and detectors for infrared radiation are less stable than that for UV/Vis radiation. In addition, it is easier to correct for the absorption of infrared radiation by atmospheric CO2 and 1420 vapor when using double-beam optics. Resolutions of 1-3 cm are typical for most instruments. 

The emission spectrum from a hollow cathode lamp includes, besides emission lines for the analyte, additional emission lines for impurities present in the metallic cathode and the filler gas. These additional lines serve as a potential source of stray radiation that may lead to an instrumental deviation from Beer s law. Normally the monochromator s slit width is set as wide as possible, improving the throughput of radiation, while being narrow enough to eliminate this source of stray radiation. 

Multielemental Analysis Atomic emission spectroscopy is ideally suited for multi-elemental analysis because all analytes in a sample are excited simultaneously. A scanning monochromator can be programmed to move rapidly to an analyte s desired wavelength, pausing to record its emission intensity before moving to the next analyte s wavelength. Proceeding in this fashion, it is possible to analyze three or four analytes per minute. 

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