The atomiser

The most commonly used atomiser is the chemical flame, based upon the combination of a fuel gas (e.g., acetylene) with an oxidant (e.g., air or nitrous oxide). The sample solution is introduced into the flame using a nebuliser in which the passage of the oxidant creates a partial vacuum by the venturi effect and thus the sample solution is drawn up through a capillary. Thus, an aerosol is produced having a wide variety of droplet sizes. This process is shown in Fig. 2. 

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. 

In the premix or laminar flow system the sample aerosol, oxidant and fuel are mixed in an inert chamber such that the larger droplets of sample are broken up or drained off before entering the flame. In this way a quieter and more stable flame is produced which is supported on a, typically, 10 cm path-length burner head. Because only the fine mist and evaporated sample reaches the flame an even burning takes place. This produces better atomisation and reduces interferences. If the nebuliser is adjustable, and most will 

Proper alignment of the flame in the light path is obviously important. The instrument should provide a sturdy mount for the sampling system and should provide for vertical, horizontal and rotational control. Quite clearly 

The flame is a chemical reaction which takes place in the gas phase. The ideal flame for atomic absorption would generate the correct amount of thermal energy to dissociate the atoms from their chemical bonds. The most commonly used flames are aii -acetylene and nitrous oxide—acetylene. The choice of oxidant depends upon the flame temperature and composition required for the production of free atoms. These temperatures vary the molecular or chemical form of the element. Air and acetylene produce flame temperatures of about 2300°C and permit the analysis by atomic absorption of some thirty or so elements. The nitrous oxide—acetylene flame is some 650°C hotter and extends the atomic absorption technique to around 66 elements. It also permits the successful analysis of most elements by flame atomic emission, in many cases at fractional parts per million levels, providing adequate spectral resolution is available. 

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J ct Spra.y, The mechanism that controls the breakup of a Hquid jet has been analy2ed by many researchers (22,23). These studies indicate that Hquid jet atomisation can be attributed to various effects such as Hquid—gas aerodynamic interaction, gas- and Hquid-phase turbulence, capillary pinching, gas pressure fluctuation, and disturbances initiated inside the atomiser. In spite of different theories and experimental observations, there is agreement that capillary pinching is the dominant mechanism for low velocity jets. As jet velocity increases, there is some uncertainty as to which effect is most important in causing breakup. 

Rotary atomisation produces the most uniform atomisation of any of the aforementioned techniques, and produces the smallest maximum particle sise. It is almost always used with electrostatics and at lower rotational speeds the electrostatics assist the atomisation. At higher rotational speeds the atomisation is principally mechanical in nature and does not depend on the electrical properties of the coating material. If the viscosity of a coating material is sufficiendy low that it can be deUvered to a rotary atomiser, the material can generally be atomised. The prime mover is usually an ak-driven turbine and, provided that the turbine has the requked power to accelerate the material to the angular velocity, Hquid-dow rates of up to 1000 cm /min can be atomised using an 8-cm diameter beU. 

Difficult introduction of a solid sample into the atomiser... 

Most cations and anions cause interference on the atomisation of selenium. Bismuth and antimony depress selenium adsorption. 

Arsenic lowered somewhat the peak temperature in atomisation profile for selenium. Copper tends to suppress the interferences of diverse elements on atomisation of selenium. However, the interferences from large concentrations of diverse elements and matrices were not improved even in the presence of copper at the atomisation step. Therefore, the separation of selenium from matrices was recommended. 

Selenium is extracted as diethyldithiocarbamate complex from the solution containing citrate and EDTA [5]. Ohta and Suzuki [6] found that only a few elements, such as copper, bismuth, arsenic, antimony, and tellurium, are also extracted together with selenium. They examined this for effects of hundredfold amounts of elements co-extracted with the selenium diethyldithiocarbamate complex. An appreciable improvement of interferences from diverse elements was observed in the presence of copper. Silver depressed the selenium absorption in the case of atomisation of diethyldithiocarbamate complex, but the interference of silver was suppressed in the presence of copper. The atomisation profile from diethyldithiocarbamate complex was identical with that from selenide. 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

Soo [96] determined picogram amounts of bismuth in seawater by flameless atomic absorption spectrometry with hydride generation. The bismuth is reduced in solution by sodium borohydride to bismuthine, stripped with helium gas, and collected in situ in a modified carbon rod atomiser. The collected bismuth is subsequently atomised by increasing the atomiser temperature and detected by an atomic absorption spectrophotometer. The absolute detection limit is 3pg of bismuth. The precision of the method is 2.2% for 150 pg and 6.7% for 25 pg of bismuth. Concentrations of bismuth found in the Pacific Ocean ranged from < 0.003-0.085 (dissolved) and 0.13-0.2 ng/1 (total). 

Lundgren et al. [132] showed that the cadmium signal could be separated from a 2% sodium chloride signal by atomising at 820 °C, below the temperature at which the sodium chloride was vaporised. This technique has been called selective volatilisation. They detected 0.03 xg/l cadmium in the 2% sodium chloride solution. They used an infrared optical temperature monitor to set the atomisation temperature accurately. 

Campbell and Ottaway [136] also used selective volatilisation of the cadmium analyte to determine cadmium in seawater. They could detect 0.04 pg/1 cadmium (2pg in 50 pi) in seawater. They dried at 100 °C and atomised at 1500 °C with no char step. Cadmium was lost above 350 °C. They could not use ammonium nitrate because the char temperature required to remove the ammonium nitrate also volatilised the cadmium. Sodium nitrate and sodium and magnesium chloride salts provided reduced signals for cadmium at much lower concentrations than their concentration in seawater if the atomisation temperature was in excess of 1800 °C. The determination required lower atomisation temperatures to avoid atomising the salts. Even this left the magnesium interference, which required the method of additions. 

Figure 5.5. Zeeman profiles of a seawater sample (Sandy Cove N.9) and Sb profiles. The first pair of profiles represents a single 12 xl aliquot, the second pair, two aliquots, and the third pair, three aliquots. The modifier was 200 xg (NH4)2HP04,8% HN03, and 5 ng Mg(N03)2. The char temperature was 550 °C, and the atomisation temperature 1600 °C. Source [135]...

Ohta and Suzuki [397] investigated the electrothermal atomisation of lead for accurate determination of lead in water samples. Thiourea served to lower the atomisation temperature of lead and to eliminate the interferences from chloride matrix. The addition of thiourea also allowed the accurate determination of lead irrespective of its chemical form. The absolute sensitivity (1% absorption) was 1.1 x 10 12 g of lead. The method permits the direct rapid determination of lead in water samples including seawater. 

A graphite furnace procedure has been described [674] for the direct determination of iron, chromium, and manganese in seawater and estuarine waters in which the interference normally associated with the presence of sodium chloride is eliminated. The technique requires only very small sample volumes (10-20 il) for the atomisation stage. The reproducility of the method was very good. Sensitivities of 0.4,0.2, and 0.07 xg/l and precisions of determination of 4.5,3, and 11% (at 2 xg/l level) were obtained for iron, manganese, and zinc. 

Figure 5.18 is an absorbance versus time plot obtained by Hoenig and Wollast [681] for the determination of trace metals in seawater. It shows the absorbance profiles of the desired elements as a function of the atomisation temperature. The scale starts with cadmium, for which the absorption signal appears around 400 °C, followed by lead (756 °C), copper (1000 °C), manganese (1200 °C), nickel (1300 °C), and chromium (140 °C). 

The palladium and magnesium nitrates modifier has a substantial equalising effect on the atomisation temperature of the nine elements investigated. The optimum atomisation temperature for all but one element (thallium) is between 1900 and 2100 °C. This means that all elements can be determined at a compromise atomisation temperature of 2100 °C with a minimum sacrifice in sensitivity. Such uniform conditions for as many elements as possible are of vital importance if simultaneous multielement furnace techniques are envisaged. Moreover, in conventional graphite furnace AAS, uniform conditions for a number of elements can greatly facilitate and simplify daily routine analysis. 

The extracts were then atomised and fed into the ROTARC reactor for high temperature treatment. In the first case the atomised extract was mixed with the torch gas (Argon) only. It was a pure pyrolysis, which was effective in the sooting of the reactor walls and it was making the scrubber fluid dirty. The disadvantage of the pure pyrolysis process confirmed our theoretical considerations on thermal destruction of PCB s presented in [9]. To avoid sooting, we fed steam into the reaction chamber in the amount of 10% above the stoichiometry. In this case, which we call the wet pyrolysis , we obtained the destruction efficiency of oil- PCB s at least 99.99%. The offgas analysis on the concentration of oil-PCB s were below the detection limit 0.2 ppm. 

The performance of a spray dryer or reaction system is critically dependent on the drop size produced by the atomiser and the manner in which the gaseous medium mixes with the drops. In this context an atomiser is defined as a device which causes liquid to be disintegrated into drops lying within a specified size range, and which controls their spatial distribution. 

Another application based on a polymer s increased ease of flow in the presence of ultrasound is the atomisation of polymers. Here the particles are fed through an extruder then atomised (Fig. 5.51). The size of droplets (D) are governed by Eq. 5.44, where X is the surface tension, p is the density and F is the exciting frequency. 

If the water were to be injected into a cold engine cylinder, the flash steam would immediately condense and there would be no pressure rise. To overcome this problem, the cylinder head and walls are heated and supply additional heat to the wet steam entering the cylinder. The atomised water droplets experience extremely high collision rates with the cylinder walls because of the explosive effect of the flash process. The tiny size of the droplets, coupled with high collision rates ensure rapid absorption of heat allowing them to be quickly converted to steam which is then heated further to superheat. 

It is far from certain that such a high-voidage spray zone can exist within a fluidized bed a spray zone, with a submerged nozzle, would require a jet to be blown in the dense phase by the atomising air. Work by Rowe et al. (1979) and by Smith and Nienow (1982), using X-ray... 

A variety of methods are available for the production of glass beads. These generally involve the atomisation of molten glass or the melting of fine glass powder. A variety of surface treatments are used, mainly of the silane type. A wide p article size range is available, but the finer sizes (3 0 micron and below) are most used in thermoplastics. 

For these techniques, a dissolved sample is usually employed in the analysis to form a liquid spray which is delivered to an atomiser e.g. a flame or electrically generated plasma). Concerning optical spectrometry, techniques based on photon absorption, photon emission and fluorescence will be described (Section 1.2), while for mass spectrometry (MS) particular attention will be paid to the use of an inductively coupled plasma (TCP) as the atomisation/ionisation source (Section 1.3). The use of on-line coupled systems to the above liquid analysis techniques such as flow injection manifolds and chromatographic systems will be dealt with in Section 1.4 because they have become commonplace in most laboratories, opening up new opportunities for sample handling and pretreatment and also to obtain element-specific molecular information. 

For quantitative purposes in AAS, a magnitude called transmittance (T) which relates, for a given wavelength, the intensity (measured by the detector) of the light source (Zq) and the intensity not absorbed which has passed through the atomiser or transmitted light (7) is used ... 

The amount of light absorbed is a function of the so-called absorption coefficient (A ) and of the optical pathlength in the atomiser cell (ft) k depends on the frequency of the selected analytical line and on the concentration of the analyte absorbing atoms. The general absorbance law (Lambert Beer Bouguer law) relates transmittance (and so measured intensities I and If) to k and b through the following equation ... 

The parameter used in the analytical calibrations by AAS is absorbance (A), which is linearly related to k (that is, at a given 2, with the atomic concentration of the analyte in the atomiser) and with the length of the optical path ... 

Vo being the analyte atom density (number of atoms per unit volume) in the ground state in the atomiser. The relationship between the atom concentration per unit volume (iVy A o) and the concentration of the analyte in the sample, C, is linear under fixed working conditions for a given line of the analyte. Therefore, we can establish a linear relationship between absorbance and C ... 

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