Direct flame atomisation

The sensitivity difference between direct flame atomisation and furnace atomisation has been bridged via the general method of atom trapping as proposed by Watling [37]. A silica tube is suspended in the air-acetylene flame. This increases the residence time of the atoms within the tube and therefore within the measurement system. Further devices such as water-cooled systems that trap the atom population on cool surfaces and then subsequently release them hy temporarily halting the coolant flow are sometimes employed. The application of atom-trapping AAS for the determination of lead and cadmium has been discussed by Hallam and Thompson [38]. 

Atomic absorption spectrometry has been applied to the analysis of over sixty elements. The technique combines speed, simplicity and versatility and has been applied to a very wide range of non-ferrous metal analyses. This review presents a cross section of applications. For the majority of applications flame atomisation is employed but where sensitivity is inadequate using direct aspiration of the sample solution a number of methods using a preconcentration stage have been described. Non-flame atomisation methods have been extensively applied to the analysis of ultra-trace levels of impurities in non-ferrous metals. The application of electrothermal atomisation, particularly to nickel-based alloys has enabled the determination of sub-part per million levels of impurities to be carried out in a fraction of the time required for the chemical separation and flame atomisation techniques. 

The methods already described have illustrated the wide applicability of flame atomisation techniques to the analysis of non-ferrous alloys. The introduction of electrothermal atomisation has enabled the direct determination of sub-part per million levels of impurities. The presence of very low levels of lead, bismuth and other low melting point metals is known to have a deleterious effect on the metallurgical properties of nickel alloys. 

The column effluent is directly introduced into the mixing chamber of the flame atomiser. Since the transport efficiency of flame AAS is usually only 5—10%, this explains why the sensitivity of this type of coupling is limited, particularly in comparison with GC-AAS. 

Flames and plasmas can be used as atomisation/excitation sources in OES. Electrically generated plasmas produce flame-like atomisers with significantly higher temperatures and less reactive chemical environments compared with flames. The plasmas are energised with high-frequency electromagnetic fields (radiofrequency or microwave energy) or with direct current. By far the most common plasma used in combination with OES for analytical purposes is the inductively coupled plasma (ICP). 

The radiation from the hoUow cathode lamp and the deuterium lamp are directed alternately through a chopper to the (flame or graphite furnace) atomiser. The rotating chopper wheel allows the radiation of either source to pass alternately while the absorption is detected for either of the two beams (Fig. 12.15). The absorbance of the deuterium lamp is then subtracted from the absorbance of the element-specific line source. 

The combination of flame AAS and gas chromatography represents probably the first example of a hyphenated technique used for speciation analysis [30]. It is particularly favorable since the analytes arrive at the detector already in the gaseous state. Interfacing is straightforward, and in most cases a simple heated transfer line is used to direct the analytes to the atomiser. However, despite its apparent simplicity, the design of such a transfer line is critical since peak broadening due to dead volumes, cold spots, or lack of chemical inertness have to be avoided. 

Burner flame is impinging directly on temperature sensor. Thermowell is damaged due to flame. Check burner installation, tertiary air distributor, and atomisation of fuel oil which can cause this problem. Provide additional view glasses on furnace shell to observe the flame. Clean strainer in fuel oil pipe. 

As described previously, vapour introduction approaches are by far the most common application of atomic fluorescence. Despite this, mention of other methods should be made. If a conventional nebulizer and spray chamber assembly (see AAS section) is used, it is possible to introduce liquid samples directly to the atom cell. In circumstances such as these, it is necessary to use more robust air-acetylene or nitrous oxide-acetylene flame, or perhaps an ICP. The use of an ICP as an atom cell for AFS measurements has led to the development of a number of different techniques, e.g. ASIA, an acronym for atomiser, source, inductively coupled plasmas in AFS. This technique uses a high-powered ICP as a source and a low-powered ICP for the atom cell. It has been found that ICP-AFS yields linear calibrations over 4—6 orders of magnitude and is more sensitive than ICP-AES. 

The cathode of the lamp which is filled with Ar or Ne at low pressure, sputters when a H.V. is applied to the electrodes. Collision of the noble gas and metal atoms excite the latter then they emit radiation in the visible/u.v. region of the spectrum. The metal compound in the sample to be analysed, dissolved in a suitable solvent, has to be transformed to a mist of gaseous atoms. This is generally achieved by aspirating the solution into a nebuliser where a mist is sprayed in a flame of a flammable gas widi an oxidising gas. The gas mixture may pass through the nebuliser first or it may burn directly. Alternatively, furnace atomisers are used, when smaller volumes of test solutions can be handled. The solution is placed in a horizontal graphite tube or a carbon rod which are heated in an electric furnace. 

---