Flame atomiser

This chapter describes the basic principles and practice of emission spectroscopy using non-flame atomisation sources. [Details on flame emission spectroscopy (FES) are to be found in Chapter 21.] The first part of this chapter (Sections 20.2-20.6) is devoted to emission spectroscopy based on electric arc and electric spark sources and is often described as emission spectrography. The final part of the chapter (Sections 20.7-20.11) deals with emission spectroscopy based on plasma sources. 

In general terms, Thomerson and Thompson43 have cited the following disadvantages of flame atomisation procedures ... 

QF Quartz tube flame atomiser chemiluminescence detector ... 

Flame atomisation. A burner is fed with a combustible gas mixture and constructed in a robust fashion to resist possible explosions of the gas (Fig. 14.8). The flame, which has a rectangular base of approximately 10 cm by 1 mm is aligned with the optical axis of the instrument. The sample is aspirated by the Venturi effect into the mixture of combustible gases feeding the flame. 

Robinson, J.W. and Choi, D.S. (1987) Development of a thermospray flame atomiser for AAS improved sensitivity, interfacing with HPLC. Spectrosc. Lett., 20, 375-390. 

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. 

At room temperature and at the temperatures obtained by most flame atomisers the number of atoms in the excited state is a very small fraction of those in the ground state and for absorption purposes can be ignored. We will discuss this relationship later. 

Two basic types of flame atomising systems have been used for atomic absorption. Firstly, the total consumption or turbulent burner system in which the total sample aerosol in the oxidant stream and the fuel gas are fed separately through concentric tubes to the burner jet, where the flame is burned. Considerable turbulence, both optical and acoustic, takes place. On the positive side these burner systems are very simple in construction and thus were cheap to manufacture, did not flash-back and could handle virtually any mixture of gases. However, this system is now obsolete. 

Thus, it can be seen that there are several variables associated with the flame atomiser that must be optimised to achieve the best sensitivity and detection limit. The flame must be correctly positioned with respect to the light path. The fuel oxidant ratio should be investigated to establish the optimum chemical environment for atomisation. The nebuliser and impact bead (where fitted) must be optimised to produce, overall, the best signal-to-noise ratio. 

Flame atomisation systems have some disadvantages, however, which limit their potential and convenience in use. These drawbacks have led many workers to devise techniques for the atomisation of samples for analysis that are not based entirely on nebuliser/flame systems. Some of the possible drawbacks of flames for analytical work are ... 

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. 

Methods given here are written in as general a format as possible to allow, it is hoped, application to a range of atomisers. The analyst may resort to electrothermal atomisation as a result of one of two factors insufficient sample size to allow nebulisation into a flame or insufficient sensitivity when using flame atomisation. Where neither of these factors apply then the flame... 

Acid dissolution is a particularly favourable approach for carbonates and sulphides, where the matrix anion will be removed during the evolution of carbon dioxide or hydrogen sulphide, and for salts of organic acids, where the anion seldom causes interference problems. Conversely, sulphates can cause problems during flame atomisation and chlorides during furnace atomisation ways of dealing with such problems are discussed below. 

The analysis of brines perhaps deserves special mention as the high sodium chloride concentrations are extremely unfavourable for electrothermal atomisation and most troublesome in flame analysis. The preferred approach is probably solvent extraction with either oxine or APDC to remove the trace metals into a small volume of MIBK for flame atomisation or chloroform for electrothermal cells. Care must be taken to avoid interference from chloro-complexes in the extraction, and if this is suspected an ion-association extraction of these complexes might be preferable. 

Atomic absorption spectrophotometry (AAS) is one of the most widely used methods. Either flame atomisation or electrothermal atomisation (ETA) using the graphite furnace is employed. Some useful references are given in the Bibliography. 

The following colorimehic and atomic absorption spectropbotomehic mediods are applicable to serum. The atomic absorption method uses a conventional air-acetylene flame atomiser. 

Numerous procedures for die determination of lead in blood by atomic absorption spectrophotometry have been reported. The first mediod described here uses a nickel cup to contain the sample, a conventional air-acetylene flame atomiser, and a... 

Standard Lead Solutions. Prepare diluted solutions of lead as described in the flame atomisation method, above. 

Flame atomisation is not necessary for the atomic absorption spectrophotomehy of mercury. The cold vapour technique described here employs a reduction vessel (which may be purchased) to produce mercury vapour the vapour is led to a quartz absorption cell within the atomic absorption inshument. The method is applicable to inorganic and organic mercurial compounds in urine. 

The following atomic absorption spectrophoto-metric method is applicable to blood and urine, and uses a conventional air-acetylene flame atomiser. Standard Thalliim Solutions. Dissolve 1.3034g of thallous nitrate (TINO3) in sufficient water to produce 1000 ml. This solution contains 1 mg ofTl in 1 ml. Serially dilute the solution with water to produce solutions containing 0.2, 0.5, 1.0, 2.0, and 4.0 p,g of T1 in 1 ml. 

We are only concerned here with flame atomisation. The sample in solution is introduced through a nebulizer and vaporised in the form of an aerosol in a premixing chamber where it is mixed with combustion gases and an oxidant. This mixture is transferred to the burner where the combustion and atomisation of the sample occurs (Fig. 2.5). 

Commercial plasma atomisers became available in the mid 1970s and offer several advantages over flame atomisers. A plasma is a conducting gaseous mixture containing a significant concentration of cations and electrons. 

Alvarado, J. R. Jaffe, 1998. Tube in flame atomisation away ofenhancing detection limits in flame atomic absorption spectrometry. J. Analytical Atomic Spectrom. 13 37-40. 

Flame atomisation In flame atomisation, the sample solution is introduced into the flame with a particularly designed nebuHser (Fig. 12.6). The function of the nebuHser is to disrupt the continuous sample stream into a mist of fine droplets of typically 5—20 pm diameter which are swept into the mixing chamber. The aerosol is then mixed with the fuel gas and the oxidant gas before reaching the burner head. As a number of physical and chemical reactions, e. g., vaporisation, dissociation, reduction, or oxidation, may occur, it becomes evident that precise control of the operating conditions of flame atomisation is required to obtain stable and sensitive signals. Optimisation of flame atomic absorption measurements has thus the double role of maximising the elemenf s response while minimising the undesired side-reactions. 

However, continuum-source background correction is implemented in many commercial instruments and is particularly useful in combination with flame atomisation or hydride generation. 

Quantitation by the standard addition technique Matrix interferences result from the bulk physical properties of the sample, e.g viscosity, surface tension, and density. As these factors commonly affect nebulisation efficiency, they will lead to a different response of standards and the sample, particularly with flame atomisation. The most common way to overcome such matrix interferences is to employ the method of standard additions. This method in fact creates a calibration curve in the matrix by adding incremental sample amounts of a concentrated standard solution to the sample. As only small volumes of standard solutions are to be added, the additions do not alter the bulk properties of the sample significantly, and the matrix remains essentially the same. Since the technique is based on linear extrapolation, particular care has to be taken to ensure that one operates in the linear range of the calibration curve, otherwise significant errors may result. Also, proper background correction is essential. It should be emphasised that the standard addition method is only able to compensate for proportional systematic errors. Constant systematic errors can neither be uncovered nor corrected with this technique. 

The determination of the different forms (e. g. compounds or complexes) in which an element occurs (often referred to as the speciation of an element and speciation analysis, respectively [28]) is in most cases performed by hyphenated techniques. These are the combination of a high-performance separation technique such as gas or liquid chromatography, or electrophoresis, and an element- or compound-specific detector [29]. While the former provides the separation of the different elemental species prevalent in the sample, the latter brings selective and sensitive detection. In the case of AAS, only the hyphenation with gas and Hquid chromatography, respectively, has gained importance. The combination of atomic absorption spectrometry and electrophoresis has never proven successfiil, obviously due to the incompatibility of the extremely low flow rates of electrophoretic separations with the aspiration volumes of flame atomisers and the difficulties of interfacing the two techniques. 

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. 

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]. 

Trace elements Solutions obtained after digesting samples of about 0.2 g were analysed using either Atomic Absorption Spectrometry (AAS) with graphite furnace atomisation (Perkin-Elmer 4000, HGA 500), or flame atomisation (Varian Techtron), or... 

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