Atomization, flame promoting

Heat energy from the flame promotes electrons from the 3s subshell to the 3p subshell in the sodium atoms. 

When small amounts of alkali metals, alkaline earth metals, or their salts are introduced into a gaseous flame, flame reactions occur relatively easily at low temperatures.The liberated metal atoms are promoted to excited states and then return to their normal states. Radiation corresponding to the characteristic line spectra of the individual metal atoms is emitted as a result of this energy transition. Colored radiation discernible by the human eye, ranging from red to blue, is dependent on the type of metal atoms, as shown in Table 12AP- i... 

Figure 21-1 also illustrates an atomic fluorescence experiment. Atoms in the flame are irradiated by a laser to promote them to an excited electronic state from which they can fluoresce to return to the ground state. Figure 21-4 shows atomic fluorescence from 2 ppb of lead in tap water. Atomic fluorescence is potentially a thousand times more sensitive than atomic absorption, but equipment for atomic fluorescence is not common. An important example of atomic fluorescence is in the analysis of mercury (Box 21-1). 

Almost all atomic emission is carried out with an inductively coupled plasma, whose temperature is more stable than that of a flame. The plasma is normally used for emission, not absorption, because it is so hot that there is a substantial population of excited-state atoms and ions. Table 21-3 compares excited-state populations for a flame at 2 500 K and a plasma at 6 000 K. Although the fraction of excited atoms is small, each atom emits many photons per second because it is rapidly promoted back to the excited state by collisions. 

The majority of flame analyses in water and effluents presents few problems. A pretreatment on the sample is performed only when necessary, as described earlier. Standards are prepared in the linear range of the analytical curve and blank solutions are also made up. It is preferable to acidify blanks, standards and samples to 1% with hydrochloric acid. Apart from acting as a preservative, it promotes atomisation of the analyte by forming volatile metal chlorides. The atomic absorption instrument is then set up and flame conditions and absorbances are optimised for the analyte. Following this, blanks, standards and samples are aspirated into the flame absorbances are recorded and results calculated. 

For the majority of elements commonly determined in water by AAS, an air—acetylene flame (2300°C) is sufficient for their atomisation. However, a number of elements are refractory and they require a hotter flame to promote their atomisation. Because of this, a nitrous oxide—acetylene flame (3000° C) is used for the determination of these elements. Refractory elements routinely determined in water are aluminium, barium, beryllium, chromium and molybdenum. Chromium shows different absorbances for chromium(III) and chromium(VI) in an air-acetylene flame [15] but use of a nitrous oxide-acetylene flame overcomes this. Barium, being an alkaline earth metal, ionises in a nitrous oxide—acetylene flame, giving reduced absorption of radiation by ground state atoms, however in this case an ionisation suppressor such as potassium should be added to samples, standards and blanks. 

The promotion of the decay of active particles (H atoms and OH-radicals) in the presence of various metal compounds has been repeatedly verified Which metal particles do actually react with the atoms and radicals propagating the chain reactions in a flame Hastie studied flames of SbBrj-inhibited methane and discovered CHjBr and HBr in the preflame zone, and Sb and SbO in the high-temperature reaction zone. Thus, halogen as well as metal are supplied to the gas phase. Both are capable of reacting with the active particles in the combustion reaction, embracing extensive flame areas. According to Ref. the metal reacts via the following route ... 

LEI utilizes a pulsed dye laser to promote analyte atoms to a bound excited state. Laser excitation enhances the thermal (collisional) ionization rate of the analyte atom, producing a measurable current in the flame 12). The laser-related current is detected with electrodes and is a measure of the concentration of the absorbing species. LEI may proceed by photoexcitation (via one or more transitions) and thermal ionization or a combination of thermal excitation, photoexcitation, and thermal ionization. 

Advantages of AES, relative to flame-AAS, include the lack of a requirement for a radiation source. Collisions within the plasma serve to promote analyte atoms to excited state levels. Additionally, this technique is characterised by linearities of response which span three to four orders of magnitude. Limits of detection for ICP-AES are similar to those obtained with flame-AAS (typically within a factor of 3 to 5 - some elements are shghtly less responsive in flame-AAS others slightly more responsive). ICP-AES does require a fairly high resolution monochromator/detection system to scan carefully across analyte emission lines and to be able to resolve them from the other emissions and from the high luminosity of the torch. There are many spectral... 

The maximum flame-retardance is obtained with an antimony to halogen atomic ratio of 1 3 161). An intermediate product of the reaction of Sb203 with a halogen-containing polymer substrate, or with a flame retardant, is an antimony oxyhalide. Pitts found that other oxides may either promote (Fe203, CuO, Ti02) or hinder (ZnO, CaO, MgO) the decomposition of antimony oxyhalides. This, in turn, enhances or diminishes the overall flame-retardant effect161). 

The heat of the flame vaporises the sodium chloride, producing some Na and Cl atoms. Electrons are promoted into the 4th shell in some of the sodium atoms. These electrons then fall back to the ground state, which is the 3s orbital, and energy is given out in the form of light. The flame... 

Atomic absorption spectrometry (AAS) is nowadays one of the most important instrumental techniques for quantitative analysis of metals (and some few metalloids) in various types of samples and matrices. The history of atomic absorption spectrometry dates back to the discovery of dark lines in the continuous emission spectrum of the sun by WoUaston in 1802. The lines are caused by the absorption of the elements in the atmosphere of the sun. His work was taken up and further pursued by Fraunhofer in 1814. In 1860, Kirchhoff and Bunsen demonstrated that the yellow hne emitted by sodium salts when introduced into a flame is identical with the so-caUed D-Hne in the emission spectrum of the sun. However, it took nearly one century before this important discovery was transferred into a viable analytical technique. In 1955, Alan Walsh published the first paper on atomic absorption spectroscopy [4]. At the same time, and independently of Walsh, AUce-made and Wilatz pubhshed the results of their fundamental AAS experiments [5, 6]. But it was the vision of Walsh and his indefatigable efforts that eventually led to the general acceptance and commercialisation of AAS instrumentation in the mid-1960s. Further instrumental achievements, such as the introduction of the graphite furnace and the hydride generation technique, in the second half of the 1960s further promoted the popularity and applicability of the technique. 

---