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Chromatografy Methods for Environmental - Ando D.J.

Ando D.J. Chromatografy Methods for Environmental - Wiley publishing , 2003. - 265 p.
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The occurrence of molecular absorbance and scatter in AAS can be overcome by the use of background correction methods. Various types of correction procedures are common, e.g. continuum source, Smith-Hieftje and the Zeeman effect. In addition, other problems can occur and include those based on chemical, ionization, physical and spectral interferences.
11.3.2 Atomic Emission Spectroscopy
The instrumentation used for atomic emission spectroscopy (AES) consists of an atomization cell, a spectrometer/detector and a read-out device. In its simplest form, flame photometry (FP), the atomization cell consists of a flame (e.g.
Instrumental Techniques for Trace Analysis
air-natural gas), while the spectrometer comprises an interference filter followed by a photodiode or photoemissive detector. Flame photometry is used for the determination of, for example, potassium (766.5 nm) or sodium (589.0 nm).
Most modern instruments for AES use an inductively coupled plasma (ICP) as the atomization cell. The ICP is formed within the confines of three concentric glass tubes or plasma torch (Figure 11.15). Each concentric glass tube has an entry point, with the intermediate (plasma) and external (coolant) tubes having tangentially arranged entry points and the inner tube consisting of a capillary tube through which the aerosol is introduced from the nebulization/spray chamber. Located around the outer glass tube is a coil of copper tubing through which water is recirculated. Power input to the ICP is achieved through this copper load or induction coil, typically in the range 0.5-1.5 kW at a frequency of 27 or 40 MHz. The inputted power causes the induction of an oscillating magnetic field whose lines of force are axially orientated inside the plasma torch and follow
Sample aerosol
Figure 11.15 Schematic diagram of an inductively coupled plasma located within its torch, as employed in atomic emission 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.
Methods for Environmental Trace Analysis
elliptical paths outside the induction coil. At this point in time, no plasma exists. In order to initiate the plasma, the carrier gas flow is first switched off and a spark added momentarily from a Tesla coil, which is attached to the outside of the plasma torch by means of a piece of copper wire. Instantaneously, the spark, a source of ‘seed’ electrons, causes ionization of the argon carrier gas. This process is self-sustaining so that argon, argon ions and electrons co-exist within the confines of the plasma torch but protruding from the top in the shape of a bright white luminous ‘bullet’. This characteristic bullet shape is formed by the escaping high-velocity argon gas causing air entrainment back towards the plasma torch itself. In order to introduce the sample aerosol into the confines of the hot plasma gas (7000-10000 K) the carrier gas is switched on; this punches a hole into the centre of the plasma, thus creating the characteristic ‘doughnut’ or toroidal shape of the ICP. In the conventional ICP system, the emitted radiation is viewed laterally, or side-on. Therefore, the element radiation of interest is ‘viewed’ through the luminous plasma.
DQ 11.14
Do you think that viewing the plasma side-on will have any detrimental effect?
As you are viewing the elemental radiation side-on you must also be raising the level of background radiation observed. Some modern instruments allow the plasma to be viewed end-on, thereby reducing the background radiation.
The most common method of liquid sample introduction in AES is via a nebulizer. The type of nebulizer used in modern instruments has not altered significantly since its first usage, despite of its inefficiency. Most nebulizers have transport efficiencies of between 1-2%.
DQ 1.15
How would you define transport efficiency?
Transport efficiency is defined as the amount of the original sample solution that is converted to an aerosol and then transported into the plasma source.
The basis of the nebulizer is to convert an aqueous sample into an aerosol by the action of a carrier gas. In order to produce an aerosol of sufficient particle size, ideally of <10 ^m to avoid substantial cooling/extinguishing of the plasma, it is necessary to present the generated aerosol into a spray chamber. The latter
Instrumental Techniques for Trace Analysis
has the advantage of further reducing the original aerosol particle size towards the ideal by providing a surface for collisions and/or condensation. The generation of condensation represents part of the inefficiency of the nebulizer/spray chamber sample-introduction system.
Light emitted from the plasma source is focused onto the entrance slit of a spectrometer by using a convex lens arrangement. The spectrometer is required to separate the emitted light into its component wavelengths. In practice, depending on the requirements of the analyst and the capital cost of the instrument, two options are available. The first involves a capability to measure one wavelength, corresponding to one element at a time, while the second allows multi-wavelength or multi-element detection. The former is called sequential analysis or sequential multi-element analysis if the system is to be used to measure several wavelengths one at a time, while the latter is termed simultaneous multi-element analysis. The typical wavelength coverage of a spectrometer for AES is between 167 nm (Al) to 852 nm (Cs). After wavelength separation has been achieved, it is obviously necessary to ‘view’ the spectral information. This is most commonly achieved by using either a photomultiplier tube (PMT) or a charge-coupled device (CCD).
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