Radiation source for AAS

Although most of the radiation sources for AAS are LSs, the great advances in detector technology, especially the development of solid-state array detectors and charge-coupled devices (CCDs), have led to the successful application of continuous sources (CSs) for AAS. A modern CS is based on a conventional xenon short-arc lamp that has been optimized to run in the so-called hot-spot mode.9 This discharge mode requires the appearance of a small plasma spot close to the cathode... 

The main radiation source for AAS is the hollow-cathode lamp (HCL). The HCL (Fig. 27.3) emits radiation characteristic of a particular element. The choice of HCL for AAS is simple. For example, if you are analysing for lead, you will need a lead-coated HCL. It is normal to pre-warm the HCL for about 10 min prior to use. This can be done either by using a separate pre-... 

The boosted hollow-cathode lamp A very intense radiation source for AAS is the boosted hollow-cathode lamp. It has been developed from the regular hollow[Pg.438]

It would seem that an ordinary deuterium lamp that emits a continuum spectrum would make a low-cost radiation source for AA provided the source is passed through a monochromator whereby a narrow bandpass of wavelengths can be seleeted. However, in order for Beer s law to be obeyed, the bandwidth at half height, AX 1/2 for the monoehromator should be less than AX1/2 for the analyte of interest—in this ease, the atoms in the vapor phase of metallic elements. Robinson has artieulated the problem this way (104) ... 

Source of Radiation The radiation source for FAAS instrumentation is quite similar to that of other AAS techniques, such as ETAAS or CVG-AAS (CV-AAS and HG-AAS). The one most commonly applied is the line source (LS), which generates a characteristic narrowline emission of a selected element. There are two principal LSs for AAS the hollow cathode lamp (HCL) and the electrodeless discharge lamp (EDL).8... 

Why must HCLs or EDLs be used as the radiation source for most AAS instruments Illustrate schematically an HCL for lead (Pb). How would you make the cathode ... 

Plasma sources were developed for emission spectrometric analysis in the late-1960s. Commercial inductively coupled and d.c. plasma spectrometers were introduced in the mid-1970s. By comparison with AAS, atomic plasma emission spectroscopy (APES) can achieve simultaneous multi-element measurement, while maintaining a wide dynamic measurement range and high sensitivities and selectivities over background elements. As a result of the wide variety of radiation sources, optical atomic emission spectrometry is very suitable for multi-element trace determinations. With several techniques, absolute detection limits are below the ng level. 

When Walsh started to think about using AAS for analytical purposes back in 1952, one of his key conclusions was that, in order to carry out absorption measurements on luminous atomic vapors, it would be necessary to employ a modulated light source and a synchronously tuned detection system, so that any radiation emitted by the sample would produce no signal at the output of the detection system [2]. This modulation principle, using either an AC-operated radiation source or a chopper in the radiation beam, and a selective amplifier tuned to the same modulation frequency, has ever since been applied in all commercially available atomic absorption spectrometers. It has been considered one of the major advantages of... 

This means that HR-CS AAS, due to its special features, does not need any modulation of the source or any selective amplifier. This also means that a potential source of noise has been eliminated, as both AC operation of hollow cathode lamps and the mechanical choppers are contributing to noise in LS AAS. In addition, other problems that are associated with strong emission of the atomizer source in LS AAS - such as the emission noise caused by the nitrous oxide -acetylene flame in the determination of Ba and Ca due to the CN band emission [3] - are equally absent in HR-CS AAS for the same reasons, that is, the higher intensity of the primary radiation source, and the high resolution. 

Again, in HR-CS AAS these problems are essentially nonexistent for the same reasons as given above. Firstly, because of the relatively constant, very intense emission of the primary radiation source, there are no more weak lines that is, the same high SNR will be obtained on all analytical lines, regardless of their spectral origin. The only factors that will have an influence will be the absorption coefficient and the population of the low excitation level in case nonresonance lines are used. Secondly, because of the high resolution of the monochromator, and the visibility of the entire spectral environment of the analytical line in HR-CS AAS, potential spectral interferences can easily be detected, and in addition cannot influence the actual measurement, except in the rare case of direct line overlap. However, even in this case, HR-CS AAS provides an appropriate solution, as discussed in the previous section. 

A major detraction for LS AAS has always been the relatively short linear region of the calibration curves, typically not more than two orders of magnitude in concentration. The limits of the linear working range arise from stray radiation and the finite width of the emission lines of the radiation source, which is not monochromatic and just three to five times narrower than the absorption profile. With HR-CS AAS, there is no theoretical limit to the calibration range, only the practical limits imposed by the size of the array detector, the increasing possibility of spectral interferences, and the ability to clean the atomizer after extremely high analyte concentrations have been introduced. 

For a CCD detector the absorbance noise is independent of the spectral bandwidth, but it depends on the number of measurement pixels sam and reference pixels ref in such a way that sam should be as small as possible and rel should be larger than sam. The other component that influences the noise is the intensity I of the radiation source, in that the absorbance noise is inversely proportional to the square root of I [12]. As the intensity of the radiation source in CS AAS is in some cases up to two orders of magnitude higher than that of a typical LS for conventional AAS, an improvement in the SNR and limits of detection (LoD) by factors of 3-10 could be expected, unless other factors, such as flame noise, become dominant. The values given in Table 4.1 show that this expectation has in fact been realized for the majority of the elements. 

The typical routine determination of a number of elements in a set of similar sample solutions will therefore no longer be the determination of element A in all the samples, followed by a change of radiation source, wavelength, flame conditions, burner height and so on, and determination of element B in the same samples, and so on, as it is common practice in LS FAAS. In HR-CS FAAS it will rather consist of a calibration of the instrument for all the elements of interest, followed by a determination of all elements in sample 1, all elements in sample 2, all elements in sample 3, and so on. It might be worth mentioning that, at least for a limited number of elements, the total analysis time required for HR-CS FAAS will be even shorter than that for a simultaneous ICP AES measurement, because of the much shorter equilibration time required for a typical AAS burner after changing the sample solution, compared to the spray chambers used in ICP AES. 

Fourthly, as a continuous radiation source is used in HR-CS AAS, any line of the spectrum is available, and even molecular absorption lines can be used for quantitative determination, as shown for the determination of P at PO bands and sulfur at CS bands using FAAS. 

One of the main practical problems with the use of AAS is the occurrence of molecular species that coincide with the atomic signal. One approach to remove this molecular absorbance is by the use of background correction methods. Several approaches are possible, but the most common is based on the use of a continuum source, D2. In the atomization cell (e.g. flame) absorption is possible from both atomic species and from molecular species (unwanted interference). By measuring the absorption that occurs from the radiation source (HCL) and comparing it with the absorbance that occurs from the continuum source (D2) a corrected absorption signal can be obtained. This is because the atomic species of interest absorb the specific radiation associated with the HCL source, whereas the absorption of radiation by the continuum source for the same atomic species will be negligible. 

Apart from the high power of detection, also the realization of the highest analytical accuracy is very important. This relates to the freedom of interferences. Whereas the interferences stemming from influences of the sample constituents on the sample introduction or on the volatilization, ionization and excitation in the radiation source differ widely from one source to another, most sources emit line-rich spectra and thus the risks for spectral interferences in AES are high. In the wavelength range 200-400 nm, as an example, only for arc and spark sources have more than 200 000 spectral lines yet been identified with respect to wavelength and element in the classical MIT Tables. Consequently, spectral interferences are much more severe than in AAS or AFS work. 

Atomic absoiption spectroscopy (AAS) is probably still the most widely employed of all the atomic methods because of its simplicity, effectiveness and relatively low cost. A Tine source of radiation is required for AAS (they do not employ a continuous source of radiation) hence a complete spectrum is not obtained. The sources (which are changed depending on the element of interest) emit certain lines of radiation that have the same wavelength as that of the absorption peak of the analyte of interest. 

Advantages of AES, relative to flame-AAS, include the lack of a requirement for a radiation source. Collisions within the plasma serve to promote analyte atoms to excited state levels. Additionally, this technique is characterised by linearities of response which span three to four orders of magnitude. Limits of detection for ICP-AES are similar to those obtained with flame-AAS (typically within a factor of 3 to 5 - some elements are shghtly less responsive in flame-AAS others slightly more responsive). ICP-AES does require a fairly high resolution monochromator/detection system to scan carefully across analyte emission lines and to be able to resolve them from the other emissions and from the high luminosity of the torch. There are many spectral... 

The radiation source in the case of this flame AAS is a line source hollow cathode lamp (HCL). These are the most commonly used sources in AAS. It can be designed for... 

Instruments for AAS are similar in general design to that shown in Figure 7-la and consisi of a radiation source, a sample holder, a wavelength selector, a detector, and a signal processor and readout. " The sample holder in atomic absorption instruments is the atoml/.er cell that contains the gaseous atomized sample. 

The radiation source used in AAS is an HCL or an EDL, and a different lamp is needed for each element to be determined. Because it is essentially a single-element technique, 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... 

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