Flame atomic absorption spectrometry examples

The choice between flame atomic absorption spectrometry (FAAS) and graphite furnace AAS (GFAAS) will be determined by nature of the specimen. For example FAAS is ideal for serum copper determinations in which concentrations are normally between 12 and 26 //mol/L but GFAAS is needed to measure the concentrations of 0.1 to 0.8/rapid screening test for Wilson s Disease with which urine copper levels are usually greater than 3 tmol/L and can often exceed 10-20 fimo IL. 

A number of indirect flame atomic absorption spectrometry (AAS) methods have been reported for determination of phosphate (Table 8.2). For example, phosphate was determined by measuring molybdenum after solvent extraction of phosphomolybdate [107]. A more recent variation of this method involved flotation of the malachite green-phosphomolybdate ion pair at an aqueous-diethyl ether interface [108]. After dissolution in methanol, molybdenum was determined using flame AAS (nitrous oxide flame) at 313.26 nm. The method was successfully applied to measurement of seawater containing ca. 40 pg P (1.3 pM). 

Flame Atomic Absorption Spectrometry. It is usually considered that about 95% of the observed problems are related to the light system, the nebuliser/ burner and the instrument cleanliness the instrument s optics and electronics rarely fail. For example, most commonly problems accounting for absorbance lower than expected are related to ... 

The need for the determination of metallic constituents or impurities in pharmaceutical products has, historically, been addressed by ion chromatographic methods or various wet-bench methods (e.g. the USP heavy metals test). As the popularity of atomic spectroscopy has increased, and the equipment has become more affordable, spectroscopy-based techniques have been routinely employed to solve analytical problems in the pharmaceutical industry. Table 1 provides examples of metal determinations in pharmaceutical matrices, using spectroscopic techniques, and the reasons why these analyses are important. Flame atomic absorption spectrometry (FAAS), graphite furnace atomic absorption spectrometry... 

Atomic absorption spectrometry is one of the most widely used techniques for the determination of metals at trace levels in solution. Its popularity as compared with that of flame emission is due to its relative freedom from interferences by inter-element effects and its relative insensitivity to variations in flame temperature. Only for the routine determination of alkali and alkaline earth metals, is flame photometry usually preferred. Over sixty elements can be determined in almost any matrix by atomic absorption. Examples include heavy metals in body fluids, polluted waters, foodstuffs, soft drinks and beer, the analysis of metallurgical and geochemical samples and the determination of many metals in soils, crude oils, petroleum products and plastics. Detection limits generally lie in the range 100-0.1 ppb (Table 8.4) but these can be improved by chemical pre-concentration procedures involving solvent extraction or ion exchange. 

The measurement of very low levels of environmental pollutants is becoming increasingly important. The determination of lead, a cumulative toxin, is a good example. The current maximum allowable concentration of lead in British drinking water, before it enters the distribution network, is SO ng ml [29]. Although electrothermal atomization atomic-absorption spectrometry (AAS) can be used to measure this and lower concentrations, it is slow and requires considerable effort to ensure accurate results. Flames can provide simple and effective atom sources, but, if samples are aspirated directly, do not provide sufficient sensitivity. Thus, if a flame is to be used as the atom source, a preconcentration step is required. 

A convenient method is the spectrometric determination of Li in aqueous solution by atomic absorption spectrometry (AAS), using an acetylene flame—the most common technique for this analyte. The instrument has an emission lamp containing Li, and one of the spectral lines of the emission spectrum is chosen, according to the concentration of the sample, as shown in Table 2. The solution is fed by a nebuhzer into the flame and the absorption caused by the Li atoms in the sample is recorded and converted to a concentration aided by a calibration standard. Possible interference can be expected from alkali metal atoms, for example, airborne trace impurities, that ionize in the flame. These effects are canceled by adding 2000 mg of K per hter of sample matrix. The method covers a wide range of concentrations, from trace analysis at about 20 xg L to brines at about 32 g L as summarized in Table 2. Organic samples have to be mineralized and the inorganic residue dissolved in water. The AAS method for determination of Li in biomedical applications has been reviewed . 

Atomic absorption spectrometry (AA). This is a standard laboratory analytical tool for metals. The metal is extracted into a solution and then vaporized in a flame. A light beam with a wavelength absorbed by the metal of interest passes through the vaporized sample for example, to measure zinc, a zinc resonance lamp can be used so that the emission and absorbing wavelengths are perfectly matched. The absorption of the light by the sample is measured and Beer s law is applied to quantify the amount present. 

Figure 1 represents four examples of the evaluation of measurement uncertainty for potassium, calcium, magnesium and glucose using flame photometry, atomic absorption spectrometry and molecular spectrometry (Mg determination with Titan Yellow and glucose determination with glucose oxidase). For the sake of simplicity in Fig. 1, the component of uncertain-... 

Atomic absorption spectrometry with flame (AA-F) or electrothermal atomization furnace (AA-ETA), inductively coupled plasma-emission spectroscopy (ICP-ES), inductively coupled plasma-mass spectrometry (ICP-MS), and high-performance liquid chromatography-mass spectrometry (LC-MS) are state-of-the-art analytical techniques used to measure metals in biological fluids. They are specific and sensitive and provide the cfinical laboratory with the capability to measure a broad array of metals at clinically significant concentrations. For example, ICP-MS is used to measure several metals simultaneously. Photometric assays are also available but require large volumes of sample and have limited analytical performance. Spot tests are also... 

In addition, for speciation coupling of flow injection analysis and column chromatography with flame AAS and also a direct coupling of HPLC with flame AAS, as is possible with high-pressure nebulization, are most powerful. Here the Cr line in the visible region can be used, which makes the application of diode laser atomic absorption spectrometry possible [325]. This has been shown recently by the example of the determination of methylcyclopentadienyl manganese tricarbonyl. 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

Quantitation by external calibration The most common and straightforward method of calibration in atomic absorption spectrometry is the use of an external calibration with suitable standard solutions. It is based on the assumption that the standard solution matches the composition of the sample sufficiently well. This is an assumption that must always be examined with care, since, for example, samples of different viscosity may be aspirated at different rates in flame AAS,... 

For superconductor materials and precursors, high-precision analysis is very important. To this end, flame atomic absorption can be applied, provided that the pre-dsion is optimized, as is possible, for example, by using internal standardization and multichannd spectrometers. It should, however, be remembered that to control the stoichiometric composition, in addition the determination of non-metals such as N and O, which cannot be performed by atomic absorption spectrometry, is compulsory. 

FIGURE 7.20 Examples of sample containers. Left, cuvettes used in UV-VIS spectrophotometry (pathlength is the inner diameter—approximately 1 cm) center, liquid sampling cell used in IR spectrometry (pathlength is thickness of spacer to the left of the pencil tip—approximately 0.1 mm) right, atomic absorption flame (pathlength is the width of the flame—approximately 4 in.). 

Conventional pneumatic nebulizers typically consume sample solution at the rate of ca. 5-8 ml min-1. Thus generally, when flame spectrometry is used on a routine basis, 2-5 ml of sample solution is used per determination. However it is possible to employ much smaller volumes of sample solution.16 Figure 3, for example shows typical atomic absorption signals for the nebulization of 0.01, 0.02, and 0.05 ml of a 1 mg l-1 standard solution, as recorded on a storage oscilloscope, compared with the signal from continuous nebulization. It is clear that only about 0.04 ml of solution is required to obtain the maximum absorbance signal. 

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