Nebulizer-burner Systems

The main function of the nebulizer-burner systems is the conversion of the sample solution first into the aerosol and then into the atomic vapour, after which the absorption measurement is carried out. These systems are the most important part of the AA instrument. Their correct function and efficiency guarantees the success of the analysis. 

Nebulization includes the conversion of the sample solution into a mist or aerosol. In pre-mix spray chambers, mist droplets of the correct size distribution are selected and introduced, with the flame gzises, into the burner in which the atomization takes place. The technique most often employed to produce atomic vapour is the indirect nebulization system described above. The method in which the nebulizer and burner are combined is called a direct nebulization. In this method the sample mist is formed and mixed with the fuel gas at the opening of the burner. Burners employed in the indirect and direct nebulization systems are called pre-mix burners and direct injection burners, respectively. 

The indirect injection burner is safer than the pre-mix burner, because the fuel gas and oxidant are mixed at the orifice of the burner and no danger from explosion hence exists. There is some turbulence in the flame, because the gases are mixed and burned in the same region. In addition, the flame is 

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TABLE 4.1. Comparison of Limits of Detection for Selected Elements, Obtained with LS AAS and HR-CS AAS, Using the Same Nebulizer-Burner System (AAS 6 Vario, Analytik Jena, Germany) and the Same Integration Time of 5 s... 

Nebulizer burner system which converts the test solution to gaseous atoms. The function of nebulizer is to produce a mist or aerosol of the test solution. 

Figure 10.9. Premix nebulizer-burner system. Adapted from G. D. Christian andF. J. Feldman, Atomic Absorption Spectroscopy Applications in Agriculture, Biology, and Medicine, New York Wiley-Inter-science, 1970, p 80, by permission of John Wiley and Sons.

The flame atomic absorption spectrometer is inherently a flow-through detector, with which the sample solutions are continuously fed into the nebulizer-burner system through suction. Despite the relatively large volume of the spray chamber (usually about 1(X) ml) in comparison to the spectrometric flow-cell, the detector was shown to have very little contribution to the dispersion of the injected sample in comparison to other components of the FI system [11]. With careful optimization, as little as 50-80 //I sample may be injected to achieve 80-95% of the steady state signal obtained by conventional sample introduction (see Fig. 2.14). 

Ionization interferences Falsely high results may be obtained because of the high concentrations of sodium and potassium, which are among the more easily ionized elements. As with matrix interferences, they may not be evident with all nebulizer-burner systems and the presence of possible interferences should be investigated with any new instrument. If ionization interference is observed the calibration solutions should be prepared with sodium and potassium at approximately the same concentration as in the test specimens. This applies to biological fluids and to solutions prepared from tissues. 

Figure 4.1.2 is a photograph of a coimterflow burner assembly. The experimental particle paths in this cold, nonreacting, counterflow stagnation flow can be visualized by the illumination of a laser sheet. The flow is seeded by submicron droplets of a silicone fluid (poly-dimethylsiloxane) with a viscosity of 50 centistokes and density of 970 kg/m, produced by a nebulizer. The well-defined stagnation-point flow is quite evident. A direct photograph of the coimterflow, premixed, twin flames established in this burner system is shown in Figure 4.1.3. It can be observed that despite the edge effects.

Nebulizer-bumer system. The purpose of the system is to produce rmiformly fine fog of droplets from the test solution. The burner has a long and narrow slot at the top so that the flame provides a long absorption path for the incident radiation. The fuel-oxidant system used may be acetylene air, acetylene-nitrous oxide, hydrogen-air etc. 

Figure 4.26. Flow system and setup for simultaneous multicomponent gradient scanning by flame photometric FI A embodying standard addition. The sample (5) is initially aspirated into loop L, which upon turning of the valve (V) is propelled forward by pump P through the FI A system and toward the detector (F), the sample being sandwiched between the inert carrier solution (water) and an infinitely long zone of standard carrier solution (SC) F, flame nebulizer-burner T, timer A/, scanning monochromator O, storage oscilloscope and R, X-Y recorder.

The German "Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area, Working Group Analytical Chemistry" has developed a continuous nebulization FAAS method for copper in serum based on a 1 -r- 1 dilution (dilution depends on the applied burner system) with a determination limit of 0.1 mg/L At a mean copper concentration of 1.24 mg/L the within batch precision was 2.4% and the day-to-day precision 3.2% (Winter and Schaller, 1985). Accuracy was checked against certified reference materials. The method was applied on fortified samples from a serum pool of healthy persons and compared by a GF/ AS method developed and tested at the same time (Angerer et al., 1985). The FAAS method is still in successful routine use in a number of German laboratories because of its reliability, simplicity and speed (Schaller, 1993). 

The nebulizer and burner system is probably the most important component of the atomic-absorption or emission spectrophotometer, because it is imperative that neutral (un-ionized) atoms of the test element be presented to the optical system. When the sample solution passes into the flame, it must be in the form of small droplets. The process of breaking down a solution into a fine spray is known as nebulization. Nebulization is generally carried out with the support or oxidant gas. 

Figure 1 Diagram of nebulizer, spray chamber, and burner system components as used in the PerkinElmer AAnalyst 200/400 atomic absorption spectrometer. (Reproduced by kind permission of PerkinElmer Inc. All rights resenred.)...

Figure 6.8 (a) Premix burner system. [ 1993-2014 PerkinElmer, Inc. All rights reserved. Printed with permission, (www.perkinelmer.com).] (b) Schematic nebulizer designs. (Top), modified Babington type (left), concentric (the most common in FAAS) (right) cross-flow type. (From Parsons, M.L., Atomic absorption and flame emission, in Ewing, G.A., ed.. Analytical Instrumentation Handbook, 2nd edn., Marcel Dekker, Inc., New York, 1997. Used with permission.)... 

The nebulizer is mounted in the spray-chamber burner assembly, as shown for the nitrous oxide or air-acetylene burner system (Fig. 32). Air or nitrous oxide are fed to the mixing chamber through the nebulizer. The combustion gas is fed directly to the burner. A self-aspirating concentric nebulizer is often used, but other types, including the Babington and cross-flow types (Section 21.4.1) are also applied. Nebulizers may be made of glass, corrosion-resistant metals such as Pt-Ir. or plastics such as Ryton. Sample solution consumption usually amounts to ca. S miymin in the case of gas flows of 1 -5 L/min. 

Flame photometer or spectrophotometer incorporating nebulizer and burner, filters, prism or grating monochromator, photocell or photomultiplier detection system. 

Schematic diagram of a concentric nebulizer system for a premixed burner.

The surface tension and, to a lesser extent, the viscosity of the sample solution are important factors in nebulization efficiency, since work must be performed in the nebulization step to overcome these properties of the liquid. For this reason, the surface tension and viscosity should be maintained as nearly identical as possible in samples and standards. With reasonably concentrated solutions, this can be done quite simply by diluting the test solution. With less concentrated solutions, it may be necessary to match the matrix composition of samples and standards. Concentrated solutions should also be diluted to avoid encrustation of salts on the nebulizer and burner. The same is true when handling heterogeneous systems such as colloids or solutions high in protein content. 

An FIA system is especially useful in the analysis of solutions containing high levels of solids such as saturated salt solutions or dissolved fusion mixtures. The burner slot or nebulizer will not become blocked since the system is continuously and thoroughly rinsed with the carrier stream after each sample measurement. The FIA system requires less than 400 /zl of sample solution, which is much less than with continuous aspiration. Thus, FIA-FAAS is the preferred technique when only small sample amounts are available. Low sample consumption is also beneficial for routine analysis, since with fully automated sequential multi-element analysis more determinations can be performed with a given sample volume. 

Other laminar flow burners are available. Most of them premix the fuel and oxidant and have a separate nebulizer to break the liquid sample into small droplets. A drain, or chamber, is provided so the larger droplets do not enter the flame but are collected and removed from the system. This... 

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