Atomization Flames, Furnaces, and Plasmas

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 Flames were used for decades, but they have been replaced by the inductively coupled plasma and the graphite furnace.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. 

Droplets entering the flame evaporate then the remaining solid vaporizes and decomposes into atoms. Many elements form oxides and hydroxides in the outer cone. Molecules do not have the same spectra as atoms, so the atomic signal is lowered. Molecules also emit broad radiation that must be subtracted from the sharp atomic signals. If the flame is relatively rich in fuel (a rich flame), excess carbon tends to reduce metal oxides and hydroxides and thereby increases sensitivity. A lean flame, with excess oxidant, is hotter. Different elements require either rich or lean flames for best analysis. The height in the flame at which maximum atomic absorption or emission is observed depends on the element being measured and the flow rates of sample, fuel, and oxidizer.6 

In flame spectroscopy, the residence time of analyte in the optical path is 1 s as it rises through the flame. A graphite furnace confines the atomized sample in the optical path for several seconds, thereby affording higher sensitivity. Whereas 1—2 mL is the minimum volume of solution necessary for flame analysis, as little as 1 pL is adequate for a furnace. Precision is rarely better than 5-10% with manual sample injection, but automated injection improves reproducibility to —1%. 

Furnaces offer increased sensitivity and require less sample than a flame. 

A sample can be preconcentmted by injecting and evaporating multiple aliquots in the graphite furnace prior to analysis.8 To measure traces of As in drinking water, a 30-pL 

The most common fuel-oxidant combination is acetylene and air, which produces a flame temperature of 2 400-2 700 K (Table 20-1). When a hotter flame is required for refractory elements (those with high boiling points), acetylene and nitrous oxide is usually the mixture of choice. The height above the burner head at which maximum atomic absorption or emission is observed depends on the element being measured, as well as flow rates of sample, fuel, and oxidant. These parameters can be optimized for a given analysis. 

Graphite is a form of carbon. It burns at high temperature in air  

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In atomic spectroscopy, a substance is decomposed into atoms in a flame, furnace, or plasma. (A plasma is a gas that is hot enough to contain ions and free electrons.) Each element is measured by absorption or emission of ultraviolet or visible radiation by the gaseous atoms. To measure trace elements in a tooth, tiny portions of the tooth are vaporized (ablated) by a laser pulse1 and swept into a plasma. The plasma ionizes some of the atoms, which pass into a mass spectrometer that separates ions by their mass and measures their quantity. 

Fundamental requirements for an atomic absorption experiment are shown in Figure 21-2. Principal differences between atomic and ordinary molecular spectroscopy lie in the light source (or lack of a light source in atomic emission), the sample container (the flame, furnace, or plasma), and the need to subtract background emission. 

Atomic absorption The process by which unexcited atoms in a flame, furnace, or plasma absorb characteristic radiation from a source and attenuate the radiant power of the source. 

Tunable lasers (preferentially dye lasers and diode lasers) are used as primary sources for atomic absorption spectroscopy with various atomizers such as flames, furnaces, or plasmas LAAS laser atomic absorption spectrometry CRS cavity ring-down spectroscopy... 

Numerous methods have been pubUshed for the determination of trace amounts of tellurium (33—42). Instmmental analytical methods (qv) used to determine trace amounts of tellurium include atomic absorption spectrometry, flame, graphite furnace, and hydride generation inductively coupled argon plasma optical emission spectrometry inductively coupled plasma mass spectrometry neutron activation analysis and spectrophotometry (see Mass spectrometry Spectroscopy, optical). Other instmmental methods include polarography, potentiometry, emission spectroscopy, x-ray diffraction, and x-ray fluorescence. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

Besides flame AA and graphite furnace AA, there is a third atomic spectroscopic technique that enjoys widespread use. It is called inductively coupled plasma spectroscopy. Unlike flame AA and graphite furnace AA, the ICP technique measures the emissions from an atomization/ionization/excitation source rather than the absorption of a light beam passing through an atomizer. 

Different analytical techniques such as ICP-OES (optical emission spectrometry with inductively coupled plasma source), XRF (X-ray fluorescence analysis), AAS (atomic absorption spectrometry) with graphite furnace and flame GF-AAS and FAAS, NAA (neutron activation analysis) and others, are employed for the trace analysis of environmental samples. The main features of selected atomic spectrometric techniques (ICP-MS, ICP-OES and AAS) are summarized in Table 9.20.1 The detection ranges and LODs of selected analytical techniques for trace analysis on environmental samples are summarized in Figure 9.15.1... 

Elements that form very stable diatomic oxides are incompletely atomized at the temperature of the flame or furnace. The spectrum of a molecule is much broader and more complex than that of an atom, because vibrational and rotational transitions are combined with electronic transitions (Section 18-5). The broad spectrum leads to spectral interference at many wavelengths. Figure 21-26 shows a plasma containing Y and Ba atoms as well as YO molecules. Note how broad the molecular emission is relative to the atomic emission. 

The relative precision of electrothermal methods is generally in the range of 5% to 10%, compared with the 1% or better that can be expected for flame or plasma atomization. Furthermore, furnace methods are slow and typically require several minutes per element. Still another disadvantage is that chemical interference effects are often more severe with electrothermal atomization than with flame atomization. A final disadvantage is that the analytical range is low, usually less than two orders of magnitude. Consequently, electrothermal atomization is ordinarily applied only when flame or plasma atomization provides inadequate detection limits or when sample sizes are extremely limited. 

The atom reservoir can be a flame, furnace or also a plasma discharge at atmospheric and at reduced pressure. 

Among the various types of atomic spectroscopy, only two, flame emission spectroscopy and atomic absorption spectroscopy, are widely used and accepted for quantitative pharmaceutical analysis. By far the majority of literature regarding pharmaceutical atomic spectroscopy is concerned with these two methods. However, the older method of arc emission spectroscopy is still a valuable tool for the qualitative detection of trace-metal impurities. The two most recently developed methods, furnace atomic absorption spectroscopy and inductively coupled plasma (ICP) emission spectroscopy, promise to become prominent in pharmaceutical analysis. The former is the most sensitive technique available to the analyst, while the latter offers simultaneous, multielemental analysis with the high sensitivity and precision of flame atomic absorption. 

A very recent volume edited by Berthed (2002) is on countercurrent chromatography - the support-free liquid stationary phase. Ebdon et al. (1987) review directly coupled liquid chromatogramphy-atomic spectroscopy. The review by Uden (1995) on element-specific chromatographic detection by atomic absorption, plasma atomic emission and plasma mass spectrometry covers the principles and applications of contemporary methods of element selective chromatographic detection utilizing AA, AES and MS. Flame and furnace are considered for GC and HPLC, while MIP emission is considered for GC and ICPAES for HPLC. Combinations of GC and HPLC with both MIPAES and ICPAES are covered and supercritical fluid chromatographic (SFC) and field flow fractionation (FFF) are also considered. 

Atomic absorption spectrometry (AAS) has been widely used to determine biological materials for aluminium content. Flame techniques, even with the hotter nitrous oxide acetylene flame, do not perform as well as the graphite furnace methods. However, flame atomic absorption methods have been used to analyze brain and cerebrospinal fluid (Krishnan et al., 1972), rat tissues (Mayor et al., 1980), food, urine and feces (Clarkson et al., 1972). heart muscle (Chipperfield et al., 1977), and plasma and tissues (Berlyne et al., 1972 Berlyne et al., 1970 Weinberger et al.. 1972). 

The detection limits of the old methods for the determination of arsenic (10) were too high to determine arsenic in uncontaminated biological samples. With the invention of instrumental techniques, such as flame atomic absorption (emission) spectrometry, graphite furnace atomic absorption spectrometry, neutron activation analysis, inductively coupled plasma atomic emission spectrometry, and inductively coupled plasma mass spectrometry, the ubiquity of arsenic in our environment was proven. The improvement of the analytical techniques has changed the reputation of arsenic from a poisonous substance to an essential trace element at least for warm-blooded animals (11). An arsenic requirement for humans cannot be deduced from these animal experiments. In recent literature, there are certainly more hints that arsenic might be an essential trace element for humans, but there is still a lot of future research work necessary to prove this. 

Several different electrothermal atomizers such as carbon rod, graphite ribbon, graphite furnace, and metal loop atomizers, have been designed for AFS measurements. In general, the electrothermal atomization method is time-consuming and expensive with respect to flame atomization. It is, thus, appropriate to use electrothermal atomization only when the sample amount or analyte concentration restrict the use of flames or plasmas. 

Back extraction is desirable when the organic solvent used cannot be introduced directly into the atomizer (flame, graphite furnace, or plasma). In electrothermal atomization, organic solvents are spread within the graphite tube and because of this the sensitivity is often better for aqueous solutions. The reproducibility of the determination is usually worse in organic solvents than in aqueous solutions. 

The iron released may be measiu ed using a variety of methods based on two main techniques atomic absorption spectrometry (AAS) (flame or furnace) and ultraviolet (UV)-visible spectrophotometry. Flame AAS provides the reference method for determination of plasma iron. Protein precipitation with TCA is followed by centrifugation and measurement of iron in the supernatant by the absorption at 248.3 nm in an air-acetylene flame. While atomic absorption methods are routinely used for urine iron measurements, the need to remove protein and any hemoglobin contamination restricts the use of this technique in routine clinical chemistry for plasma iron. Electrothermal atomization AAS methods are typically used for determination of iron in tissues although inductively coupled plasma (ICP) is becoming more widely available. 

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