Atomic absorption line widths

Earlier an experiment was described in which atomic absorption lines were observed using a hydrogen lamp radiation source. Experimentally it can be shown that these absorption line widths are extremely narrow and can only be isolated under conditions of very high spectral resolution. 

The natural line width is the width of the absorption line not exposed to any broadening effects. It is a hypothetical case because the lines are essentially always broadened by experimental conditions. However the natural line width indicates the lower limit of the absorption lines width. It can be calculated from the uncertainty principle which states that 

Since At is of the order of 10-8 s, it can be calculated that the natural line width is about (10 -s nm). In practice this natural line width is broadened by several effects. These include the Doppler effect caused by the motion of absorbing atoms in their environment (usually a flame). At any given instance 

These and less important effects which cause line broadening may increase the absorption line to values up to (10 3 nm). Although this is a considerable increase over the natural line width it is still very narrow in practice and very difficult to observe with conventional equipment. Consequently it was not practical to use a continuous radiation source such as a hydrogen lamp. Not only would it be very difficult to isolate the absorption line but the total amount of energy radiated by the light source over such a narrow absorption band would be very small and difficult to measure using conventional detectors. 

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FIGURE 12 Influence of line shape on calibration parameters for atomic absorption spectrometry. (A) Comparison of atomic line widths for a hollow cathode lamp versus the atomic absorption line width observed in atmospheric pressure atom cells. (B) The slope and linear dynamic range of the calibration changes from I to N as the emission line width of the hollow cathode lamp becomes broader. 

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. 

The major requirement of the light source for atomic absorption is that it should emit the characteristic radiation (the spectrum) of the element to be determined at a half-width less than that of the absorption line. The natural absorption line width is about 10 4 (A), but due to broadening factors such as Doppler and collisional broadening, the real or total width for most elements at temperatures between 2000 ° and 3000 °K is typically 0.02 — 0.1 A. Hence, a high resolution monochromator is not required. 

Measurement of integrated absorption requires a knowledge of the absorption line profile. At 2000-3000 K, the overall line width is about 10-2 nm which is extremely narrow when compared to absorption bands observed for samples in solution. This is to be expected, since changes in molecular electronic energy are accompanied by rotational and vibrational changes, and in solution collisions with solvent molecules cause the individual bands to coalesce to form band-envelopes (p. 365). The overall width of an atomic absorption line is determined by ... 

Beer s law requires that the linewidth of the radiation source should be substantially narrower than the linewidth of the absorbing sample. Otherwise, the measured absorbance will not be proportional to the sample concentration. Atomic absorption lines are very sharp, with an intrinsic width of only 10 4 nm. 

Gaussian Laser Profile-Voigt Atom Profile. This case turns out to be a better approximation of our experimental situation, i.e., the laser FWHM is fairly broad compared to the absorption line width and the absorption profile of atoms in an atmospheric combustion flame is described by a Voigt profile. Here the laser is assumed to have a Gaussian spectral profile as well as a Gaussian atomic absorption profile. In this case, convolution of two Gaussian functions is still a Gaussian function. Evaluation of the ratio n2/nT, and the fluorescence radiance. Bp, allows determination of the half width of the fluorescence excitation profile, 6X... 

A major breakthrough came in Australia when Alan Walsh1,2 realized that light sources were available for many elements which emitted atomic spectral lines at the same wavelengths as those at which absorption occurred. By selecting appropriate sources, the emission line widths could be even narrower than the absorption line widths (Figure 2). Thus the sensitivity problem was solved more or less at a stroke, and the modern flame atomic absorption spectrometer was bom. 

Secondly, when a CS is used for AAS, it is necessary to utilize a high-resolution monochromator in order to avoid loss of sensitivity and excessive curvature of the calibration function, and also to avoid spectral interferences. Becker-Ross et al. [12] have shown for several elements that the sensitivity continuously increases with increasing resolution until the spectral bandwidth is in the order of the width of the atomic absorption line, and that no further improvement in sensitivity is possible beyond that level. Becker-Ross et al. [13] also determined the half-width of the absorption lines of a large number of elements, and found that a monochromator with a resolution /A of about 100,000 is necessary for HR-CS AAS. Then... 

Atomic absorption spectroscopy (AAS) was practiced in the mid-nineteenth century by passing a small sample into a flame and noting the color of the flame. Compared to molecular absorption, atomic absorption lines are very narrow. The linewidth is defined as the width of the signal at halfheight Ali/i, which for atoms is of the order of 0.002-0.005 nm. Al /2 consists of the natural linewidth plus the Doppler37 linewidth. 

Atomic absorption offers a more practical opportunity for determining isotopic composition than atomic emission. Useful reviews of the possibilities of the technique have appeared in two books [233, 234]. Isotopic analysis is in theory possible provided that highly enriched isotope sources are available, the absorption line width available is less than the isotopic displacement and for a given isotope the nuclear spin hyperfine components must be partially resolved from the other isotopic components of the absorption line. In the simplest possible case, for an element with two isotopes, the lamp is prepared from the first isotope and only this isotope in the atom cell will absorb the radiation. The procedure can then be repeated with a lamp prepared with the second isotope. Effectively this is an extension of the impressive selectivity of atomic absorption, because of the classic lock and key effect, treating the different isotopes as different analytes. 

Flame atomic absorption spectroscopy (AAS) is currently the most widely used of all the atomic methods listed in Table 28-1 because of its simplicity, effectiveness, and relatively low cost. The technique was introduced in 1955 by Walsh in Australia and by Alkemade and Milatz in Holland. The first commercial atomic absorption (AA) spectrometer was introduced in 1959, and use of the technique grew explosively after that. Atomic absorption methods were not widely used until that time because of problems created by the very narrow widths of atomic absorption lines, as discussed in Section 28A-1. 

The widths of atomic absorption lines are much less than the effective bandwidths of most monochromators. 

No ordinary monochromator is capable of yielding a band of radiation as naiTOW as the width of an atomic absorption line (0.002 to 0.005 nm). As a result, the use of radiation that has been isolated from a continuum source by a monochromator inevitably causes instrumental departures from Beer s law (see the discussion of instrument deviations from Beer s law in Section 24C-3). In addition, since the fraction of radiation absorbed from such a beam is small, the detector receives a signal that is less attenuated (that is, P —> Pq) nd the sensitivity of the measurement is reduced. This effect is illustrated by the lower curve in Figure 24-17 (page 733). 

Owing to the line broadening mechanisms, the physical widths of spectral lines in most radiation sources used in optical atomic spectrometry are between 1 and 20 pm. This applies both for atomic emission and atomic absorption line profiles. In reality the spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines. 

The natural line width of an atomic absorption line is extremely narrow an estimate of this width can be obtained from the uncertainty principle ... 

Atomic ahsorplion methods are potentially highly specific because atomic absorption lines are remarkably narrow (0.(K)2 loO.OO.S nm) and because electronic transition energies are unique for each element. On the other hand, narrow line widths erejtte a problem lhat does not normally [Pg.237]

The ability of atomic absorption to distinguish between elements and avoid spectral interferences does not depend on the monochromator. It depends instead on the emission line width of the hollow cathode lamp (typically 0.02A.), and the absorption line width of the element in the flame (typically 0.04A.). These values are far superior to the resolution capabilities of commonly available monochromators. Therefore, the monochromator does not enter directly into the ability of the atomic absorption instrument to give a specific result. See Figure 1.)... 

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. 

Figure 6.4 Width of an atomic absorption line (Zn 213.9 nm line), greatly exaggerated, compared with the emission bandwidth from a continuum source such as a deuterium lamp.

Spectral emission lines in flames are the same width as atomic absorption lines in flames, on the order of 0.01 nm. Why is the spectral bandpass in an atomic emission spectrometer much smaller than that in an AAS ... 

It is important in AA measurements that the emission line width coming from the radiation source is narrower than the absorption line width of the atoms studied. In principle, a high resolution monochromator is not needed to separate the analyte line from the other lines of the spectrum, but in practice, the spectral bandpass of the source should be equal or less than the absorption line width. Otherwise, artificially low absorbance values are obtained leading to reductions in sensitivity. In the AA technique the use of continuum sources (quartz-halogen filament lamps and deuterium and xenon arc lamps) with reasonably priced monochromators is not satisfactory. This is demonstrated in Figure 17. In the case of (A) the emission of radiation is continuous for the whole spectral bandwidth. The energy absorbed by the atoms of the analyte is small in comparison to the whole... 

U Use Equation 7-13 for the resolving power of a grating monochromator to estimate the theoretical minimum size of a diffraction grating that would provide a profile of an atomic absorption line at 500 nm having a line width of 0.002 nm. Assume that the grating is to be used in the first order and that it has been ruled at 2400 grooves/mm. 

Fig. 8. Lanthanide Lm absorption constructed from a superposition of 2p3/2-nd (n = 5 - 00) atomic absorption lines and an arctan curve representing the continuous absorption. The onset of the continuous absorption is fixed at the series limit (n = oo) of the underlying optical spectrum. F indicates the total L ] core level width (3 eV). The resulting spectral shape matches that of the Lm absorption in lanthanide vapors (fig. Ida).

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