High-resolution continuous source atomic absorption

High-Resolution Continuous Source Atomic Absorption Spectrometry... 

A primary source is used which emits the element-specific radiation. Originally continuous sources were used and the primary radiation required was isolated with a high-resolution spectrometer. However, owing to the low radiant densities of these sources, detector noise limitations were encounterd or the spectral bandwidth was too large to obtain a sufficiently high sensitivity. Indeed, as the width of atomic spectral lines at atmospheric pressure is of the order of 2 pm, one would need for a spectral line with 7. = 400 nm a practical resolving power of 200 000 in order to obtain primary radiation that was as narrow as the absorption profile. This is absolutely necessary to realize the full sensitivity and power of detection of AAS. Therefore, it is generally more attractive to use a source which emits possibly only a few and usually narrow atomic spectral lines. Then low-cost monochromators can be used to isolate the radiation. 

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. 

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... 

The continuous source is quite useful for certain purposes if it is intense and a monochromator of high resolution is available. Photographic recording of absorption spectra can be made in the same manner as arc or spark emission spectra are recorded. In this manner atomic absorption spectra are readily available for the study of a number of spectral absorption lines, in contrast to the single-line absorption usually obtained with a hollow cathode source. 

To a hmrted extent, atomic absorption spectrometry can also be used for multielement determinations. Several manufacturers introduced systems with multilamp turrets, where different lamps can be held under pre-heated conditions. Here, rapid switching from one lamp to another enables sequential multi-element determinations to be made by flame atomic absorption, for a maximum of around five elements. Simultaneous determinations are possible with multi-element lamps, however, the number of elements that can be brought together and used as a hollow cathode lamp with a sufficiently stable radiation output and lifetime is rather limited. The use of continuous sources facilitates flexible multi-element determinations for many elements in principle. It is necessary to use high-resolution spectrometers (e.g., echelle spectrometers) with multi-channel detection. CCDs of-... 

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