资料：Atomic absorption spectrometry

Atomic absorption spectrometry (AAS) is an analytical technique that measures the
concentrations of elements. Atomic absorption is so sensitive that it can measure down to parts per billion of a gram (μg dm–3) in a sample. The technique makes use of the wavelengths of light specifically absorbed by an element. They correspond to the energies needed to promote electrons from one energy level to another, higher, energy level.
Atomic absorption spectrometry has many uses in different areas of chemistry.
Clinical analysis. Analysing metals in biological fluids such as blood and urine.
Environmental analysis. Monitoring our environment – eg finding out the levels of various
elements in rivers, seawater, drinking water, air, petrol and drinks such as wine, beer and fruit drinks.
Pharmaceuticals. In some pharmaceutical manufacturing processes, minute quantities of a catalyst used in the process (usually a metal) are sometimes present in the final product. By using AAS the amount of catalyst present can be determined. Industry. Many raw materials are examined and AAS is widely used to check that the major elements are present and that toxic impurities are lower than specified – eg in concrete, where calcium is a major constituent, the lead level should be low because it is toxic.
Mining. By using AAS the amount of metals such as gold in rocks can be determined to see whether it is worth mining the rocks to extract the gold.


How it works
Atoms of different elements absorb characteristic wavelengths of light. Analysing a sample to see if it contains a particular element means using light from that element. For example with lead, a lamp containing lead emits light from excited lead atoms that produce the right mix of wavelengths to be absorbed by any lead atoms from the sample. In  AAS, the sample is atomised – ie converted into ground state free atoms in the vapour state – and a beam of electromagnetic radiation emitted from excited lead atoms is passed through the vaporized sample. Some of the radiation is absorbed by the lead
atoms in the sample. The greater the number ofatoms there is in the vapour, the more radiation is absorbed. The amount of light absorbed is proportional to the number of lead atoms. A calibration curve is constructed by running several samples of known lead concentration under the same conditions as the unknown. The amount the standard absorbs is compared with the calibration curve and this enables the calculation of the lead
concentration in the unknown sample.
Consequently an atomic absorption spectrometer needs the following three components: a light source; a sample cell to produce gaseous atoms; and a means of measuring the specific light absorbed.

The light source
The common source of light is a ‘hollow cathode lamp’ (Fig. 1). This contains a tungsten anode and a cylindrical hollow cathode made of the element to be determined. These are sealed in a glass tube filled with an inert gas – eg neon or argon – at a pressure o

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between 1 Nm–2 and 5 Nm–2. The ionisation of some gas atoms occurs by applying a potential difference of about 300–400 V between the anode and the cathode. These gaseous ions bombard the cathode and eject metal atoms from the cathode in a process
called sputtering. Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the ground state – eg Pb* → Pb + h (Fig. 2). The shape of the cathode concentrates the radiation into a beam which passes through a quartz window, and the shape of the lamp is such that most of the sputtered atoms
are redeposited on the cathode.

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A typical atomic absorption instrument holds several lamps each for a different element. The lamps are housed in a rotating turret so that the correct lamp can be quickly selected.

The optical system and detector A monochromator is used to select the specific wavelength of light – ie spectral line – which is absorbed by the sample, and to exclude other wavelengths. The selection of the specific light allows the determination of the selected element in the presence of others. The light selected by the monochromator is directed onto a detector that is typically a photomultiplier tube. This produces an electrical signal proportional to the light intensity(Fig. 3).
Double beam spectrometers Modern spectrometers incorporate a beam splitter so that one part of the beam passes through the sample cell and the other is the reference (Fig. 4). The intensity of the light source may not stay constant during an analysis. If only a single beam is used to pass
through the atom cell, a blank reading containing no analyte (substance to be analysed) would have to be taken first, setting the absorbance at zero. If the intensity of the source changes by the time the sample is put in place, the measurement will be inaccurate. In the double beam instrument there is a constant monitoring between the reference beam and the light source. To ensure that the spectrum does not suffer from loss of sensitivity, the beam splitter is designed so that as high a proportion as possible of the energy of the lamp beam passes through the sample.
  

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Atomisation of the sample Two systems are commonly used to produce atoms     from the sample. Aspiration involves sucking a solution of the sample into a flame; and electrothermal atomisation is where a drop of sample is placed into a graphite tube that is then heated electrically.
Some instruments have both atomisation systems but share one set of lamps.    Once the appropriate lamp has been selected, it is pointed towards one or other
atomisation system.
Flame aspiration
Figure 5 shows a typical burner and spray chamber.
Ethyne/air (giving a flame with a temperature of 2200–2400 °C) or ethyne/dinitrogen oxide (2600– 2800 °C) are often used. A flexible capillary tube connects the solution to the nebuliser. At the tip of the capillary, the solution is ‘nebulised’ – ie broken into small drops. The larger drops fall out and drain off while smaller ones vaporise in the flame. Only ca 1% of the sample is nebulised.

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Electrothermal atomisation
Figure 6 shows a hollow graphite tube with a platform. 25 μl of sample (ca       1/100th of a raindrop) is placed through the sample hole and onto the platform    from an automated micropipette and sample changer.
The tube is heated electrically by passing a current through it in a pre-programmed series of steps. The details will vary with the sample but typically they might be  30–40 seconds at 150 °C to evaporate the solvent, 30 seconds at 600 °C to drive off any volatile organic material and char the sample to ash, and with a very fast heating rate (ca 1500 °C s-1) to 2000–2500 °C for 5–10 seconds to vaporise and atomise elements (including the element being analysed).
Finally heating the tube to a still higher temperature– ca 2700 °C – cleans it ready for the next sample. During this heating cycle the graphite tube is flushed with   argon gas to prevent the tube burning away. In electrothermal atomisation almost 100% of the sample is atomised. This makes the technique much more sensitive    than flame AAS.

[ 本帖最后由 popshengu 于 2010-8-3 15:26 编辑 ]