Atomisation temperatures

Atomisation method Typical atomisation temperature (K) Basis for method11... 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

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. 

In similar work, Sturgeon et al. [125] compared direct furnace methods with extraction methods for cadmium in coastal seawater samples. They could measure cadmium down to 0.1 pg/1. They used 10 pg/1 ascorbic acid as a matrix modifier. Various organic matrix modifiers were studied by Guevremont [116] for this analysis. He found citric acid to be somewhat preferable to EDTA, aspartic acid, lactic acid, and histidine. The method of standard additions was required. The standard deviation was better than 0.01 pg/1 in a seawater sample containing 0.07 pg/1. Generally, he charred at 300 °C and atomised at 1500 °C. This method required compromise between char and atomisation temperatures, sensitivity, heating rates, and so on, but the analytical results seemed precise and accurate. Nitrate added as sodium nitrate delayed the cadmium peak and suppressed the cadmium signal. 

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. 

Campbell and Ottaway [672] have described a simple and rapid method for the determination of cadmium and zinc in seawater, using atomic absorption spectrometry with carbon furnace atomisation. Samples, diluted 1 + 1 with deionised water, are injected into the carbon furnace and atomised in an HGA-72 furnace atomiser under gas-stop conditions. A low atomisation temperature... 

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. 

Conventional and multivariate methods were used to establish the best pyrolysis and atomisation temperatures and the chemical modifier (centred full factorial designs). A comparison is presented... 

Fe in water-methanol were simplex optimised (ashing temperature, ashing time), ramp and atomisation temperature)... 

Problems in the direct determination of cadmium in soil extracts by graphite furnace atomic absorption spectrometry are overcome by the use of a low atomisation temperature of 1200 °C (mini-furnace or high heating rate of > 2000 °C/s), the addition of molybdenum, hydrogen peroxide and nitric acid as a matrix modifier, and accurate optimisation of the instrumental parameters. 

Disproportionate decomposition of molecular species at high concentration. This results in a lower proportion of free atoms being available at higher concentrations for a constant atomisation temperature. 

Purge gas Dry time Dry temperature Pre-atomisation heating time Pre-atomisation heating temperature Atomisation time Atomisation temperature... 

The temperature programme used was dry for 20 s (110°C), ramp 15 s for a 15s char (550°C) and ramp 9s for a 9s atomisation (2500°C). With determinations in chloride media, the ash or char stage is particularly critical in furnace atomisation, as hydrogen chloride may be formed which aids the removal of chloride. This might otherwise interfere by vaporising the analyte as the chloride before the atomisation temperature is reached. In more open rod-type systems this mechanism is not available, and chloride interferences may be more severe [4]. 

WAVELENGTHS AND ATOMISATION TEMPERATURES FOR DETERMINATION OF TRACE METALS IN ACIDS AND AMMONIA [7]... 

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. 

While an increase in atomiser temperatures (when possible) is an obvious way to reduce chemical interference, a second possibility is the use of matrix modifiers. 

Simplex optimisation of the instrumental conditions (ashing and atomisation temperature, modifier concentration, and atomisation ramping time) to determine As. 

The pyrolysis and atomisation temperatures were optimised using 2 factorial design with 16 assays carried out for each analyte. 

The optimisation step was carried out using a two-level full factorial design involving pyrolysis time, temperature, atomisation temperature and modifier mass. 

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