Atomic atomisation efficiency

Physical (transport) interferences. This source of interference is particularly important in all nebulisation-based methods because the liquid sample must be aspirated and transported reproducibly. Changes in the solvent, viscosity, density and surface tension of the aspirated solutions will affect the final efficiency of the nebulisation and transport processes and will modify the final density of analyte atoms in the atomiser. 

Cadmium, copper and zinc associated with various proteins have been studied by means of an ion chromatograph coupled to a flame AAS (Ebdon et al., 1987). The design of the interface meant that the nebuliser of the AAS could be eliminated, thus avoiding the low efficiency of the nebulisation. Effluent from the HPLC was collected as discrete aliquots on a series of rotating platinum spirals that entered the flame atomiser. An atom trap (tube in flame) was included to increase the sensitivity of the detector by allowing the analyte to remain for a longer period in the optical path. 

Sample introduction is probably one of the most important stages for reproducible measurements and is related to the efficiency of sample uptake to the plasma source. Normally samples are introduced in solution form and in latter years sample introduction as solids and gases directly or from GC columns is now commonly employed on a routine basis where applicable. Selection for the best sample introduction method needs careful consideration, keeping in mind that the properties of the atomiser will dictate its design and operation. For adequate thermal dissociation, volatilisation, excitation and atomisation of aerosol particles, the efficiency of nebulisation will determine the sensitivity and reproducibility of analyte response. The following requirements must be considered when analysing samples using atomic emission methods ... 

In using atomic spectroscopy analysis the sample introduction is an extension to sample preparation. To understand the limitations of practical sample introduction systems it is necessary to reverse the train of thought, which tends to flow in the direction of sample solution > nebulisation > spray chamber > excitation > atomisation. An introduction procedure must be selected that will result in a rapid breakdown of species in the atomiser to give reproducible results irrespective of the sample matrix. In designing an FI A system to carry out atomic emission and to generate efficient free atom production for excitation the following criteria must be adhered to as closely as possible ... 

Sample preparation is again a key step in the analysis. The sample to be analysed is usually in solution in order to be efficiently introduced and atomised in the flame or plasma. For many solid samples, such as dyed/printed textiles, this will involve digestion in strong acid followed by ashing in a furnace (to break down organics and drive off carbon and hydrogen). After ashing the sample is taken up in some acid and diluted to volume prior to atomic analysis. 

A number of flameless atomisation techniques have been developed to (i) increase the efficiency of production of analyte atoms within the optical beam of the instrument and/or (ii) exploit the volatility of analyte transformation products. 

Atomic absorption spectrometry requires that the species under investigation prevails in the gaseous and atomic state so that absorption of free atoms can be observed. The two most common methods for the production of atoms in the gas phase make use of thermal energy to vaporise and atomise the analyte. The sample transfer efficiency, i. e. how much of the sample is reaching the actual atomisation zone. 

The most widely used atomiser for hydride generation is the heated quartz T-tube atomiser with a typical diameter of 10 mm and a length of 100—150 mm, making it compatible with the optical path of most AA spectrometers. The quartz tube is electrically heated to 700—1000 °C which permits one to optimise the atomisation temperature for each element. The quartz tube may either have open ends, or these ends are sealed by removable quartz windows, and holes at the extreme ends of the quartz tube provide the gas flow outlets. This set-up increases the residence time of the atoms in the light path and thus improves sensitivity. With continued use the performance of the quartz tube atomiser invariably deteriorates in terms of sensitivity and precision. This is attributed both to devitrification of the inner surface of the quartz tube to a less inert modification, and to contamination of the inner atomiser surface by deposition of small particles and droplets that were not efficiently removed by the gas—liquid phase separator. 

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