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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
Download (direct link): chromatography2003.pdf
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11.3 Environmental Inorganic Analysis
The analysis of inorganic compounds can be carried out by using a variety of analytical techniques, including those based on atomic spectroscopy, X-ray fluorescence spectroscopy, mass spectrometry, electrochemical approaches and chromatography. However, for the purpose of this present section the techniques chosen to be highlighted are those based on atomic spectroscopy, including atomic absorption and atomic emission spectroscopies. Some brief description on the use of an inductively coupled plasma for inorganic mass spectrometry will also be covered.
11.3.1 Atomic Absorption Spectroscopy
Atomic absorption spectroscopy (AAS) is probably the most commonly encountered of the techniques in the laboratory due to the simplicity of the method and
Instrumental Techniques for Trace Analysis
193
Figure 11.7 Block diagram of a typical atomic absorption spectrometer. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.
Figure 11.8 Schematic diagram of a hollow-cathode lamp used in atomic absorption spectroscopy. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.
its low capital cost. The main components of an atomic absorption spectrometer are a radiation source, an atomization cell and a method of wavelength selection and detection (Figure 11.7). The radiation source, or hollow-cathode lamp (HCL) (Figure 11.8), generates a characteristic narrow-line emission of a selected metal.
DQ 11.10
If you were analysing for lead in a sample, which hollow-cathode lamp
would you be using?
Answer
You would, of course, use a lead hollow-cathode lamp.
The atomization cell is the site were the sample is introduced; the type of atomization cell can vary (flame or graphite-furnace) but essentially it causes the metal-containing sample to be dissociated, such that metal atoms are liberated from a hot environment. Such an environment of the atomization cell is sufficient to cause a broadening of the absorption line of the metal. By utilizing the narrowness of the emission line from the radiation source, together with the broad absorption line, means that the wavelength selector only has to isolate the line of interest from other lines emitted by the radiation source (Figure 11.9). This
194
Methods for Environmental Trace Analysis
Hollow-cathode-lamp Absorption Monochromator
emission in flame
Spectral bandpass of monochromator (0.1 nm)
Figure 11.9 Basic principle of atomic absorption spectroscopy - the ‘lock and key’ effect. From Dean, J. R., Atomic Absorption and Plasma Spectroscopy, ACOL Series, 2nd Edn, Wiley, Chichester, UK, 1997. Reproduced with permission of the University of Greenwich.
Figure 11.10 The nebulizer-expansion (mixing) chamber system used for sample introduction in flame atomic absorption spectroscopy.
unique feature of AAS gives it a high degree of selectivity; this process is usually referred to as the ‘lock and key’ effect.
The most common atomization cell is the pre-mixed laminar flame. In this case, the fuel and oxidant gases are mixed prior to entering the burner (the ignition site) in an expansion chamber (Figure 11.10). Two flames are usually
Instrumental Techniques for Trace Analysis Location of flame
195
Figure 11.11 Schematic diagram of the slot-burner used in flame atomic absorption spectroscopy.
used in AAS, i.e. either the air-acetylene flame or the nitrous oxide-acetylene flame. Both are located in a slot burner which is positioned in the lightpath of the HCL (Figure 11.11). The choice of flame is straightforward - the air-acetylene flame (slot length, 100 mm) is the most commonly used, whereas the nitrous oxide-acetylene flame (slot length, 50 mm) is reserved for the more refractory elements, e.g. Al. The latter choice for such elements may well indicate the main characteristic difference of the flames i.e. temperature.
DQ 11.11
From the information already given, which do you think is the hotter flame?
Answer
The nitrous oxide-acetylene flame, with the smaller burner slot length, is
the hotter flame (3150 K) compared to the air-acetylene flame (2500 K).
The introduction of an aqueous sample into the flame is achieved by using a pneumatic concentric nebulizer/expansion chamber arrangement. The nebulizer (Figure 11.12) consists of a concentric stainless-steel tube through which a Pt/Ir capillary tube is located. The sample is drawn up through the capillary by the action of the oxidant gas (air) escaping through the exit orifice that exists between the outside of the capillary tube and the inside of the stainless-steel concentric tube. The action of the escaping air and liquid sample is sufficient to break it up into a coarse aerosol. This action is called the Venturi effect.
DQ 11.12
What do you think are the dual functions of the expansion chamber?
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