Heating, atomic spectroscopy

Chemical Analysis. The presence of siUcones in a sample can be ascertained quaUtatively by burning a small amount of the sample on the tip of a spatula. SiUcones bum with a characteristic sparkly flame and emit a white sooty smoke on combustion. A white ashen residue is often deposited as well. If this residue dissolves and becomes volatile when heated with hydrofluoric acid, it is most likely a siUceous residue (437). Quantitative measurement of total sihcon in a sample is often accompHshed indirectly, by converting the species to siUca or siUcate, followed by deterrnination of the heteropoly blue sihcomolybdate, which absorbs at 800 nm, using atomic spectroscopy or uv spectroscopy (438—443). Pyrolysis gc followed by mass spectroscopic detection of the pyrolysate is a particularly sensitive tool for identifying siUcones (442,443). This technique rehes on the pyrolytic conversion of siUcones to cycHcs, predominantly to [541-05-9] which is readily detected and quantified (eq. 37). 

In atomic spectroscopy, analyte is atomized in a flame, an electrically heated furnace, or a plasma. Flames were used for decades, but they have been replaced by the inductively coupled plasma and the graphite furnace. We begin our discussion with flames because they are still common in teaching labs. 

Figure 21-6 An electrically heated graphite furnace for atomic spectroscopy (—38 mm long, in this case). [Courtesy Instrumentation Laboratory, Wiirrington, MA.]...

The fact that excited atoms give off specific colors and not a rainbow of colors suggested to Niels Bohr, a Danish physicist, that electrons are permitted in only certain locations within the atom. These locations are called energy levels. Each element behaves in its own unique way when excited by heat or electricity and produces a very specific pattern of lines of color called the atomic spectrum of that element (Figure 8.5). This unique chemical fingerprint is the foundation of atomic spectroscopy, a method of analysis used by forensic and medical laboratories to identify elements... 

The physico-chemical properties of the analytes and the way they reach the detector have made atomic spectroscopy the detection technique of choice in most instances. A heated quartz cell or a similar device is connected directly to the gas outlet of the separation cell [26]. The use of an atomic fluorescence detector has provided methods for selenium [25,27] and mercury [28,29] that possess excellent analytical features and use inexpensive instruments. On a less affordable level are ICP emission [30] and atomic emission cavity spectrometers [31]. 

An atomic spectroscopy method for the analysis of vitamin B in the presence of other vitamins such as B2, B6, B12, nicotinamide, and vitamin C can be carried out after reacting it with a Pb2+ salt in a basic solution (NaOH). The test involves measuring the unreacted lead using ICP-OES in solution after centrifugation. The sulphur in the vitamin is quantitatively precipitated as PbS after heating to 85°C. The difference between the unreacted lead in solution and the precipitated PbS can be used quantitatively to determine the level of vitamin B in pharmaceutical preparations or in natural products. This technique was studied extensively by Hassan [10] and compared favourably with other techniques giving recoveries close to 100%. 

Burners Sources of heat for laboratory operations or for flame atomic spectroscopy. 

Decomposition involves the liberation of the analyte (metal) of interest from an interfering matrix by using a reagent (mineral/oxidizing acids or fusion flux) and/or heat. The utilization of reagents (acids) and external heat sources can in itself cause problems. In elemental analysis, these problems are particularly focused on the risk of contamination and loss of analytes. It should be borne in mind that complete digestion may not always be required as atomic spectroscopy frequently uses a hot source, e.g. flame or inductively coupled plasma, which provides a secondary method of sample destruction. Therefore, methods that allow sample dissolution may equally be as useful. 

I shall begin with atomic spectroscopy. When an element is vaporized and heated, one or more of the electrons of an atom may be ejected from its normal distribution and briefly hang above the atom before collapsing back into its normal cloud... 

As mentioned earlier, optical atomic spectroscopy is only able to analyze solution sample. As a result, ceramic powders to be tested should be made into solution. The solution is then broken into line droplets and vaporized into individual atoms by heating, which is the step critical to the precision and accuracy of the analysis. Flame is generally used to vaporize the solution, which is therefore also known as flame atomic absorption spectrometry or flame AA. 

Figure 20-5 (a) Electrically heated graphite furnace for atomic spectroscopy. Sample is injected through the port at the top. LVov platform inside the furnace is heated by radiation from the outer wall. Platform is attached to the wall by one small connection hidden from view. [Courtesy Perkin-Elmer Corp., Norwalk, CT.] (b) Heating profile comparing analyte evaporation from wall and from platform. 

For the analysis of ceramic powders by optical atomic specfroscopy, a portion of the powder has to be converted into individual atoms. In practice, this is achieved by dissolving the powder in a liquid to form a solution, which is then broken into fine droplets and vaporized into individual atoms by heating. The precision and accuracy of optical atomic spectroscopy are critically dependent on this step. Vaporization is most commonly achieved by introducing droplets into a flame (referred to as flame atomic absorption spectrometry or flame AA). Key problems with flame AA include incomplete dissociation of the more refractory elements (e.g., B, V, Ta, and W) in the flame and difficulties in determining elements that have resonance lines in the far ultraviolet region (e.g., P, S, and the halogens). While flame AA is rapid, the instruments are rarely automated to permit simultaneous analysis of several elements. 

Spectroscopic determination of atomic species can only be carried out in the gas phase, where the individual atoms or ions are well separated. Consequently, the first step in the process is atomization, where the sample is volatilized (heated to the gas phase) and decomposed to produce an atomic gas. The differences between the various atomic spectroscopy techniques available, largely lie in the different ways of doing this. The most widely used method is flame atomization, where the sample is decomposed in a flame (a sophisticated version of the common flame test), but other common methods (Table 5.2) are... 

Atomic absorption spectroscopy is an alternative to the colorimetric method. Arsine is stiU generated but is purged into a heated open-end tube furnace or an argon—hydrogen flame for atomi2ation of the arsenic and measurement. Arsenic can also be measured by direct sample injection into the graphite furnace. The detection limit with the air—acetylene flame is too high to be useful for most water analysis. 

The first reaction pathway for the in situ formation of a metal-carbene complex in an imidazolium ionic liquid is based on the well loiown, relatively high acidity of the H atom in the 2-position of the imidazolium ion [29]. This can be removed (by basic ligands of the metal complex, for example) to form a metal-carbene complex (see Scheme 5.2-2, route a)). Xiao and co-workers demonstrated that a Pd imida-zolylidene complex was formed when Pd(OAc)2 was heated in the presence of [BMIMjBr [30]. The isolated Pd carbene complex was found to be active and stable in Heck coupling reactions (for more details see Section 5.2.4.4). Welton et al. were later able to characterize an isolated Pd-carbene complex obtained in this way by X-ray spectroscopy [31]. The reaction pathway to the complex is displayed in Scheme 5.2-3. 

Instead of employing the high temperature of a flame to bring about the production of atoms from the sample, it is possible in some cases to make use of either (a) non-flame methods involving the use of electrically heated graphite tubes or rods, or (b) vapour techniques. Procedures (a) and (b) both find applications in atomic absorption spectroscopy and in atomic fluorescence spectroscopy. 

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