Continuous source-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... 

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. 

Multichannel spectrometers which would have a large number of measurement channels and allow the simultaneous determination of a large number of elements, as is done in atomic emission spectrometry, have as yet not found a way into AAS. However, work over a number of years with high-intensity continuous sources and... 

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. 

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. 

Atomic fluorescence has the advantage, compared to AAS, that with a continuous source, several elements may be determined simultaneously. There are, however, problems due to scattered radiation and quenching, but detection limits are lower than for AAS. 

The term protein usually refers to crude protein (CP measured as N content x 6.25) in requirement tables. Protein is required in the diet as a source of amino acids (AAs), which can be regarded as the building blocks for the formation of skin, muscle tissue, feathers, eggs, etc. Body proteins are in a dynamic state with synthesis and degradation occurring continuously therefore, a constant, adequate intake of dietary AAs is required. An inadequate intake of dietary protein (AAs) results in a reduction or cessation of growth or productivity and an interference with essential body functions. 

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. 

The AAS method has several limitations. For the trace elements, particularly the colorants cobalt and nickel, the dilution factor required for analyses of 12 elements by continuous nebulization places these elements close to the detection limits for flame AAS. More accurate data on these and other trace elements are necessary before conclusions can be drawn on the source minerals used to impart color. Phosphorus, a ubiquitous minor component of medieval stained glass, has not been determined by AAS in the course of this work, but has the potential to provide key information on sources of plant ash. A full understanding of the colorant role of the transition metal elements is not possible on the basis of analysis alone UV-visible spectroscopy, electron spin resonance spectrometry, and Mossbauer spectroscopy, for example, are necessary adjuncts to achieve this aim. The results of the application of these techniques and the extension of the AAS method to trace element determination by pulse nebulization and furnace atomization will be addressed in future reports. 

X-ray powder diffraction patterns were obtained on oriented film specimens [7] (2 to 45° 2 , Philips PW 1120, monochromatized CuKa radiation, continuous peak registration). BET surface area and the pore volume distribution were determined from Nj adsorption-desorption isotherms at 77 K (degassing at 393 K, lO" mbar, 5h Sorptomatic 1900, Carlo Erba Instruments). The IR-spectra were recorded on KBr wafers [4] with a Specord 80M spectrometer. The XPS (X-ray photoelectron spectroscopy) spectra were obtained with VG ESCALAB 200 MKII spectrometer equipped with a twin anode AIKa source (1486 eV). The thermogravimetric (TGA) analyses were carried out with a Setaram TG 85 thermobalance at a heating rate of 6 K min in a helium flow of 30 ml min . The chromium content of the samples was determined by EPMA (JEOL 840 scanning electron microscope) with energy dispersive spectrometer (EDS, Tracer Northern) and by AAS (atomic absorption spectroscopy, Perkin Elmer 3030) analyses. 

Fig. 5. (A) AA at 694 nm in TSF-I particles poised at 200 mV and excited by 50-ps flashes at 694.3 nm [(trace (a)] and AA measured in TSF-I particles with P700 pre-oxidized by background illumination [trace (b)] (B) AA measured in TSF-I particles poised at -625 mV (note the different scales for the time axes) (C) AA at 694 nm measured in TSF-I particles (a) poised at 200 mV, (b) P700 pre-oxidized by continuous illumination, and (c) heat treated to inactivate the bound iron-sulfur centers (D) AA (red-region) measured in TSF-I particles 150-ps and 800-ps after the flash 30-ps flashes at either 708- or 689-nm were used [see data-point code in the inset) Note the different absorbance scales used. (E) AA (450-600 nm) induced by 30-ps, 689-nm flashes (F) solid trace is the difference between AA measured at 150-ps and 800 ps the dashed trace is the in vitro difference spectrum for Chl-a anion radical, shifted toward the red by -25 nm. Figure source (A) and (B) from Shuvalov, Ke and Dolan (1979) Kinetic and spectral properties of the intermediary eiectron acceptor A, in photosystem I. Subnanosecond spectroscopy. FEBS Lett 100 6 (C)-(F) from Shuvalov, Klevanik, Sharkov, Kryukov and Ke (1979) Picosecond spectroscopy of photosystem I reaction centers. FEBS Lett 107 314, 315.

---