Basic instrumentation for AAS

For a population of n excited atoms, the emitted light intensity depends upon the number of atoms dn that return to the ground state during the interval of time dt (dn/dt = kn). Since n is proportional to the concentration of the element in the hot zone of the instrument, the emitted light intensity which varies as dn/dt, is itself proportional to the concentration  

This expression is only valid for low concentrations in the absence of selfabsorption or of ionization. As previously suggested, to conduct an analysis by flame emission, the response of the instrument requires a calibration with a series of standards. 

If no sample is present in the flame, the detector will receive all of the light intensity 7q emitted by the source within this spectral interval selected by the 

CHAPTER 13 - ATOMIC ABSORPTION AND FLAME EMISSION SPECTROSCOPY 

1 Hollow cathode lamps (HCL) and electrodeless discharge lamps (EDL) 

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Flame atomic absorption was until recently the most widely used techniques for trace metal analysis, reflecting its ease of use and relative freedom from interferences. Although now superceded in many laboratories by inductively coupled plasma atomic emission spectrometry and inductively coupled plasma mass spectrometry, flame atomic absorption spectrometry still is a very valid option for many applications. The sample, usually in solution, is sprayed into the flame following the generation of an aerosol by means of a nebulizer. The theory of atomic absorption spectrometry (AAS) and details of the basic instrumentation required are described in a previous article. This article briefly reviews the nature of the flames employed in AAS, the specific requirements of the instrumentation for use with flame AAS, and the atomization processes that take place within the flame. An overview is given of possible interferences and various modifications that may provide some practical advantage over conventional flame cells. Finally, a number of application notes for common matrices are given. 

The concentration of inorganic components in forage crops varies according to crop maturity, temperature, and soil pH and composition. The analyses of mineral content can reveal soil or management deficiencies as well as optimum harvest time for proper crop management. Actual mineral analyses are used to determine the amount of mineral supplementation to be added to an animal ration for proper nutritional balance. Reference methods of analysis include inductively coupled argon plasma (ICP), atomic absorption spectroscopy (AAS), and x-ray fluorescence spectroscopy (XRF). These techniques are well established for the analysis of mineral elements in whole-plant material. The exact procedures for sample preparation and analysis are well documented. Copies of the procedures may be obtained from instrument manufacturers or are readily found using basic texts for each analytical technique. 

Flame AA The power in a flame AA system is basically used for the hollow cathode lamps and the onboard microprocessor that controls functions like burner head position, lamp selection, photo multiplier tube voltage, grating position, etc. A typical instrument requires less than 1000 watts of power. If it is used for 1000 h per year, it will be drawing less than 1000 kW total power, which is about 150 per year. 

A conventional monochromator (either Ebert, Czerny-Turner, Littrow or Echelle) may be used (see AAS sections for details), but some of the more basic instrumentation uses interference filters. These are optical filters that remove large bands of radiation in a nondispersive way. A dispersion element such as a prism or a grating is therefore not required. Only a relatively narrow band of radiation is allowed to pass to the detector. The disadvantage with such devices is that they are not particularly efficient and hence much of the fluoresced light is lost. An alternative development is the multi-reflectance filter. This is shown diagramatically in Figure 3, and has the... 

The basic instmment used for atomic emission is very similar to that used for AA with the difference that no primary light source is used for atomic emission. One of the more critical components for atomic emission instruments is the atomization source (Grove, 1971) because it must also provide sufficient energy to excite the atoms as well as atomize them. 

Atomic absorption spectroscopy of VPD solutions (VPD-AAS) and instrumental neutron activation analysis (INAA) offer similar detection limits for metallic impurities with silicon substrates. The main advantage of TXRF, compared to VPD-AAS, is its multielement capability AAS is a sequential technique that requires a specific lamp to detect each element. Furthermore, the problem of blank values is of little importance with TXRF because no handling of the analytical solution is involved. On the other hand, adequately sensitive detection of sodium is possible only by using VPD-AAS. INAA is basically a bulk analysis technique, while TXRF is sensitive only to the surface. In addition, TXRF is fast, with an typical analysis time of 1000 s turn-around times for INAA are on the order of weeks. Gallium arsenide surfaces can be analyzed neither by AAS nor by INAA. 

Almost any standard AAS instrument can be used for basic work in applied geochemistry, but there are a number of features which can improve the scope and speed of analysis. 

Figure 3-12 shows the basic components of an AA spectrophotometer. The basic component of such an instrument is the hollow-cathode lamp made of the metal of the substance to be analyzed and is different for each metal analysis. In some cases, an alloy is used to make the cathode, resulting in a multielement lamp. 

The detection limit depends basically on the technique for transferring mercury from the sample into ground-state atoms in the gas phase, and on the fraction of the atoms, which can be simultaneously introduced in the light path of the AAS instrument. The conversion into the ground-state atoms is achieved by thermal decomposition or by chemical reduction. 

For the basic AA (citrulline, homocitrulline, ornithine), glycine and arginine, data are acquired in the multiple reaction monitoring (MRM) mode by monitoring specific transitions with specific collision energies as optimized for the specific instrument. 

In general this is not a cmcial limitation for basic investigations especially in simultaneous HR-CS F AAS, as it will be shown in Chapter 7. But the question of the appropriate detector is still open to design a sufficiently qualified instrument, which offers specifications comparable to competitive analytical methods, a topic that will be discussed in Chapter 9. 

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