Monochromator, fluorescence atomic

Lamp Flame Monochromator Detector Atomic Fluorescence... 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The instrumentation required for atomic fluorescence measurements is simpler than that used for absorption. As the detector is placed so as to avoid receiving radiation directly from the lamp, it is not strictly necessary to use a sharp-line source or a monochromator. Furthermore, fluorescence intensities are directly related to the intensity of the primary radiation so that detection limits can be improved by employing a high-intensity discharge lamp. 

The absorption measurement via observation of the total fluorescence has advantages when the probe cannot be placed inside the laser cavity. It is not necessary to employ any monochromator or spectrograph. The spectral resolution limit, which is set by the finite Doppler width of the absorbing gas and which is already far lower than the resolution of most spectographs, may be drastically reduced by using an atomic or molecular beam perpendicular to the laser beam. 

In some flame AFS systems, interference filters and solar blind photomultipliers have been used to reduce the background, but usually a conventional monochromator is used. As in AAS, the source signal is modulated so that the atomic fluorescence can be distinguished from atomic emission. 

Electrothermal atomizers are also suitable for AFS as, when an inert gas atmosphere is used, quenching will be minimized. In the nuclear, electronic, semiconductor and biomedical industries where detection limits have to be pushed as low as 1 part in lO (or 0.1 pg g- in the original sample), electrothermal atomization with a laser as excitation source (LIF-ETA) may be used. Figure 6.5 shows schematically a common way of observing the fluorescence in LIF-ETA. The fluorescence signal can be efficiently collected by the combination of a plane mirror, with a hole at its centre to allow excitation by the laser, positioned at 45° with respect to the longitudinal axis of the tube and a lens chosen to focus the central part of the tube into the entrance slit of the fluorescence monochromator. 

The laser atomic fluorescence excitation and emission spectra of sodium in an air-acetylene flame are shown below. In the excitation spectrum, the laser (bandwidth = 0.03 nm) was scanned through various wavelengths while the detector monochromator (bandwidth = 1.6 nm) was held fixed near 589 nm. In the emission spectrum, the laser was fixed at 589.0 nm, and the detector monochromator wavelength was varied. Explain why the emission spectrum gives one broad band, whereas the excitation spectrum gives two sharp lines. How can the excitation linewidths be much narrower than the detector monochromator bandwidth ... 

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.]... 

Krause et a/.123-125 have recently reported a series of measurements of the spin-orbit relaxation of the alkali metals in their first excited states (2P). The technique, for example for atomic caesium with AE = 554 cm-1, consists of irradiating the metal vapour with light from a monochromator to excite only one of the 2P states. The vapour pressure of the metal is controlled at 10-6 torr to avoid imprisonment of the resonance radiation. The components of the fluorescence light are measured with a photomultiplier by isolating the 2P - 2S lines with interference filters. In the presence of added gases which cause the transitions... 

An atomic fluorescence spectrometric determination of selenium was first reported by Dagnall et al. [185] using a dispersive spectrometer equipped with an air-propane flame, giving a detection limit of 0.25 xg/ml of selenium on aspiration of aqueous solutions using a pneumatic nebuliser. Fluorescence from the 204 nm selenium resonance line was observed when the flame was irradiated by radiation from a selenium electrodeless discharge lamp, the optical axis of which was aligned at 90 °C to the optical axis of the monochromator. 

If the flame background emission intensity is reduced considerably by use of an inert gas-sheathed (separated) flame, then an interference filter may be used rather than a monochromator, to give a non-dispersive atomic fluorescence spectrometer as illustrated in Figure 14.36-38 Noise levels are often further reduced by employing a solar blind photomultiplier as a detector of fluorescence emission at UV wavelengths. Such detectors do not respond to visible light. The excitation source is generally placed at 90° to the monochromator or detector. Surface-silvered or quartz mirrors and lenses are often used to increase the amount of fluorescence emission seen by the detector. 

The stability of the wavelength setting of a monochromator can be a problem in high resolution spectrometry. This difficulty has been overcome by the use of the resonance monochromator (S24), consisting of a hollow cathode lamp modified to produce only an atomic vapor. The vapor is irradiated with the light to be analyzed and fluorescence occurs at the resonant wavelength of the cathode element. The intensity of the fluorescence is proportional to the component of that wavelength in the primary radiation. 

In the case of atomic absorption and atomic fluorescence the selectivity is thus already partly realized by the radiation source delivering the primary radiation, which in most cases is a line source (hollow cathode lamp, laser, etc.). Therefore, the spectral bandpass of the monochromator is not as critical as it is in atomic emission work. This is especially true for laser based methods, where in some cases of atomic fluorescence a filter is sufficient, or for laser induced ionization spectrometry where no spectral isolation is required at all. 

Fig. 126. Experimental set-up for laser induced atomic fluorescence work, (a) Flame, ICP, etc., (b) dye laser (c) pumping laser, (d) photomultiplier, (e) monochromator, (f) boxcar integrator, (g) data treatment and display.

The first laser beam can be amplitude modulated ( lkHz) with an optical chopper (Fig. 2), which modulates the concentration of the excited metal atoms, M. Because the excited metal atoms have a much higher reactivity, the concentration of product molecules is also modulated. The modulated fluorescence excited by the second laser is then detected by a PMT and a lock-in amplifier. A monochromator or an optical filter is used to analyze the emission and to control the optical bandwidth detected by the PMT. This photochemical modulation and synchronous detection of the fluorescent signal is a very powerful technique for increasing the signal-to-noise (S/N) ratio. 

There are two typical spectroscopic experiments. In the first, both lasers are in resonance with atomic (laser 1) and molecular (laser 2) transitions and the monochromator (Fig. 2) is scanned to record the laser-induced fluorescence. In the second type of experiment, the monochromator is used as a filter and is not scanned while the second laser wavelength is changed. This second type of experiment is called a laser excitation scan since a fluorescent signal is detected by the PMT only when laser 2 is in resonance with a molecular transition. In this case, the scanning laser can be broadband for survey work or single mode for high-resolution experiments. 

A dispersive system for atomic fluorescence measurements cotisisis of a modulated source, an alomi/.er (flame or nonflame), a monochromator or an interference filter system, a detector, and a signal processor and readoul. Wilh the exception of the source, most of these components are similar to those discussed in earlier parts of this chapter. 

In theory, no monochromator or lilter should be necessary for alomic fluorescence measurements when an LDI. or hollow-cathode lamp serves as the excitation source becau.se the emitted radiation is, in principle, that of a single element and will thus excite onlv atoms of that element. A nondispersive system then could be... 

The atomic absorption method for determining the concentration of metallic elements has now gained wide acceptance. Instrumentation is relatively inexpensive and simple to use. Analytical interferences are less prevalent than with most other techniques means of recognizing and combating the interferences that do exist are described. The article discusses the basic principles of atomic absorption and also describes the fundamental design and modern improvements in the major components of instrumentation hollow-cathode lamps, burners, photometers, and monochromators. Atomic absorption is compared with some of its rival techniques, principally flame emission and atomic fluorescence. New methods of sampling and the distinction between sensitivity and detection limit are discussed briefly. Detection limits for 65 elements are tabulated. 

If the resonance detector is well-designed, the vast majority of the magnesium atoms are unexcited. The resonance lines from the magnesium hollow cathode lamp will cause the magnesium atoms in the resonance detector to fluoresce. Some of this fluorescence will fall on a photomultiplier detector placed at right angles to the optical path. The intensity of fluorescence is proportional to the intensity of emission. Non-resonant lines from the lamp or from the flame will have no effect on the resonance detector. Therefore, a system of narrow bandwidth is produced without the requirement of a monochromator. 

The schematic for atomic fluorescence instruments is shown in Figure 27. As before, the sample is atomized in the flame. However, unlike atomic absorption, the sample in the flame is illuminated by a line emission source which is at an angle to the path between the flame and the monochromator. As the atoms in the flame absorb radiation from the lamp, they fluoresce at the resonance or a higher wavelength, with an intensity proportional to the concentration of the element of interest. 

The various instruments used for the measurement of atomic fluorescence have been similar to each other in principle and optical design. In most studies, the source of excitation, of whatever type, has been focused on the flame the fluorescence, usually at a right angle, has been focused on the entrance slit of the monochromator. The detector in all studies has been a photomultiplier tube, the output of which has been amplified and recorded. Figure 1 is a block diagram of the apparatus used successfully in our laboratory (5) it is quite similar to one described by Winefordner... 

Many of the photochemical advances that have occurred during the past two decades have followed from the development of microwave discharge vacuum ultraviolet light sources, emitting either intense monochromatic atomic resonance radiation at fixed wavelengths (determined by nature rather than for utility) or over broad continua produced by the fluorescent decay of rare gas excimers which provide tunable sources after passage through a vacuum monochromator (but at the cost of reduced intensity). 

All types of nonresonance fluorescence, particularly direct-line fluorescence, can be analytically useful sometimes it is more intense than resonance fluorescence, and it offers the advantage that scattering of the exciting radiation can be eliminated from the fluorescence spectrum by removing it with a filter or a monochromator. Self-absorption problems (absorption of the emitted radiation by the sample atoms) can also be avoided by measuring fluorescence at a nonresonance line that is not also absorbed. 

The basic instrumentation for atomic-fluorescence spectroscopy is shown in Figure 10.13. The source is placed at right angles to the monochromator so that its radiation (except for scattered radiation) does not enter the monochromator. The source is chopped to produce an AC signal and minimize flame-emission interference. As in molecular fluorescence (Chap. 9), the intensity of atomic fluorescence is directly proportional to the intensity of the light impinging on the sample from the source. 

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