Wavelength selection devices

Two wavelength selectors are used in flame OES, monochromators and filters. 

Monochromators. These consist of slits and dispersion elements and are described in Chapters 2 and 6. The common dispersion element in modern flame atomic absorption and emission spectrometers is a diffraction grating. 

The detectors in common use for these systems are the PMT or solid-state detectors such as CCDs and charge injection devices (CIDs). PMTs and photodiode arrays (PDAs) are discussed in Chapter 5. More detailed discussion of solid-state detectors is covered in Section 7.2. 

The simplest and most inexpensive way to select certain portions of the electromagnetic spectmm is with a filter. There are two major types, absorption filters and interference filters. Absorption filters can be as simple as a piece of colored glass. In Section 2.1, we discussed how blue glass transmits blue wavelengths of the visible spectmm but absorbs red and yellow wavelengths. This is an example of an absorption filter for isolating 

A monochromator consists of a dispersion element, an entrance slit and an exit slit, plus lenses and mirrors for coUimating and focusing the beam of radiation. The function of the dispersion element is to spread out in space, or disperse, the radiation faffing on it according to wavelength. The two most common types of dispersion elements are prisms and gratings. You are probably already familiar with the abUity of a prism to disperse white fight into a rainbow of its component colors. 

Diffraction Gratings. UV, visible, and IR radiation can be dispersed by a diffraction grating. A diffraction grating consists of a series of closely spaced parallel grooves cut 

Dispersion of light at the surface of a grating occurs by diffraction. Diffraction of light occurs because of constructive interference between reflected light waves. The path of one wave is shown in Fig. 2.20. Parallel waves can be envisioned on adjacent grooves. Constructive interference or diffraction of light occurs when 

Resolution Required to Separate Two Lines of Different Wavelength 

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A diode laser spectrometer. In this case, if the laser is a tunable laser, there are only two critical components the tunable laser and the detector. Typically, the enabling technology is the laser, which in this mode acts as the light source and the wavelength selection device. 

The basic instrumental needs for chiroptical methods are virtually the same as for other spectroscopic methods, namely, a stable unpolarized illuminating source of sufficient intensity, a wavelength-selection device, sample holder, and detector polarizing elements are essential. Because the only parameter measured in polarimetry and ORD is rotation, the polarizing elements are common to both. A monochromatic source, such as an Na or Hg lamp, is all that is required for polarimetry. Deuterium or halogen lamps are of sufficient intensity for ORD, but highly intense (150 50 W) Xe arc lamps are needed for CD. 

One attractive feature of chemiluminescence for analytical uses is the simple instrumentation. Since no external source of radiation is needed for excitation, the instrument may consist of only a reaction vessel and a photomultiplier tube. Generally, no wavelength selection device is needed because the only source of radiation is the chemical reaction. 

A blank or additive interference produces an effect that is independent of the analyte concentration. These effects could be reduced or eliminated if a perfect blank could be prepared and analyzed under the same conditions. A spectral interference is an example. In emission spectroscopy, any element other than the analyte that emits radiation within the band-pass of the wavelength selection device or that causes stray light to appear within the band-pass causes a blank interference. 

In chemiluminescence (CL) measurements the analyte is mixed with suitable reagents to cause a reaction to occur in which an intermediate or product in an excited electronic state is formed. The flash of light produced is quantitated as a peak height or area. Metal analytes are usually determined by their enhancement of a blank CL reaction. In some cases the analyte can be the CL precursor. The technique is suitable for trace determination of a wide variety of species with often good selectivity. Typically no wavelength selection device is employed and radiation at all wavelengths is directed to a PMT since all the CL arises from the reaction of interest. 

The wavelength selection device in most instruments is a monochromator. The monochromator consists of an entrance slit, a dispersive device, a collimator, and an exit slit. The slits are narrow planar apertures that are used to isolate a narrow... 

As in almost all spectroscopic methods, the instrumentation for infrared or Raman spectroscopy consists of a radiation source, a monochromator or wavelength-selection device of some type, a sample holder, and a detector. 

Flame OES can be performed using most modem atomic absorption spectrometers (discussed in Chapter 6). No external lamp is needed since the flame serves as both the atomization source and the excitation source. A schematic diagram of a flame emission spectrometer based on a single-beam atomic absorption spectrometer is shown in Fig. 7.2. For measurement of the alkali metals in clinical samples such as serum or urine, only a low-resolution filter photometer is needed because of the simplicity of the spectra. The filter photometer is discussed in Section 7.1.1.2. Both instmments require a burner assembly, a flame, a wavelength selection device, and a detector. 

In a nonresonance fluorescence transition, the photons involved in absorption and fluorescence processes have different wavelengths (Figure IB). The particular transition shown in Figure IB is called Stokes direct-line fluorescence, which is frequently used for AFS with laser excitation. Nonresonance transitions have the advantage that a wavelength selection device can be used to distinguish between fluorescence and scattered source radiation. 

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