Atomic-absorption spectroscopy

In atomic absorption spectrometry (AA) the sample is vaporized and the element of interest atomized at high temperatures. The element concentration is determined based on the attenuation or absorption by the analyte atoms, of a characteristic wavelength emitted from a light source. The light source is typically a hollow cathode lamp containing the element to be measured. Separate lamps are needed for each element. The detector is usually a photomultiplier tube. A monochromator is used to separate the element line and the light source is modulated to reduce the amount of unwanted radiation reaching the detector. 

Atomic absorption is used for the determination of ppm levels of metals. It is not normally used for the analysis of the light elements such as H, C, N, 0, P and S, halogens, and noble gases. Higher concentrations can be determined by prior dilution of the sample. AA is not recommended if a large number of elements are to be measured in a single sample. 

Although AA is a very capable technique and is widely used worldwide, its use in recent years has declined in favor of ICP and XRF methods of analysis. The most common application of AA is for the determination of boron and magnesium in oils. 

Conventional AA instruments will analyze liquid samples only. Dilute acid and xylene solutions are common. The volume of solution needed is dependent on the number of elements to be determined. 

AA offers excellent sensitivity for most elements with limited -interferences. For some elements sensitivity can be extended into the sub-ppb range using flameless methods. The AA instruments are easy to operate with cookbook methods available for most elements. 

Atomic absorption spectroscopy (AAS) is complementary to atomic emission spectroscopy (see Section 3.5.3) and became available for a wide range of atoms in the mid-1950s. 

The main problem in this technique is getting the atoms into the vapour phase, bearing in mind the typically low volatility of many materials to be analysed. The method used is to spray, in a very fine mist, a liquid molecular sample containing the atom concerned into a high-temperature flame. Air mixed with coal gas, propane or acetylene, or nitrous oxide mixed with acetylene, produce flames in the temperature range 2100 K to 3200 K, the higher temperature being necessary for such refractory elements as Al, Si, V, Ti and Be. 

Atomic absorption spectroscopy can sometimes be used for indirect determination of surfactants. 

Anionics may be extracted from water into an organic solvent as the ion pair with bis(eth-ylenediamine)copper(II). The copper is then determined by graphite furnace atomic ab- 

Marcel Dekker, Inc. 270 Madison Avenue, New York, New York 10016 

Cationics can be determined by forming the ion pair with the tetrathiocyanatocobalt(II) or tetrathiocyanatocopper(n) anion and extracting into an organic solvent. The extract is analyzed by atomic absorption spectroscopy, and the cobalt (115) or copper (116) content related to the original concentration of cationic surfactants. 

If the nonionic surfactant is extracted from water into an organic solvent as its potassium tetrathiocyanatozincate(II) complex, its original concentration can be related to the concentration of zinc in the extract, as determined by atomic absorption spectrometry (117) or visible spectrophotometry (118). The gravimetric barium chloride/molybdophosphoric acid method for determination of nonionics has also been adapted to an atomic absorption finish, with the residual molybdenum being determined in the supernate after centrifugation (45). Similarly, the bismuth in the barium/ethoxylated surfactant/tetraiodobismuthate precipitate can be determined by AAS (52). This procedure is discussed with gravimetric analysis. 

3 Atomic absorption spectroscopy (J.Chem.Educ.,S, 1974, 687 752 AnaiChem., 1982,54, 1515, 56(1984), 933A 875A) 

In atomic absorption spectrophotometry, a hollow cathode lamp is used which emits the characteristic line spectrum of the cathode metal. The light from the lamp passes through an atomised mist of the gaseous element and a line of the emitted spectrum (die resonance line) is absorbed. A monochromator then allows this line alone to reach the detector and the narrow absorption band is recorded and/or displayed. Because atoms have no rotational or vibrational levels, transitions from one electronic level to another produces narrow absorption or emission lines. Doppler and pressure broadening vary from 0.01-0.00 Inm. 

The cathode of the lamp which is filled with Ar or Ne at low pressure, sputters when a H.V. is applied to the electrodes. Collision of the noble gas and metal atoms excite the latter then they emit radiation in the visible/u.v. region of the spectrum. The metal compound in the sample to be analysed, dissolved in a suitable solvent, has to be transformed to a mist of gaseous atoms. This is generally achieved by aspirating the solution into a nebuliser where a mist is sprayed in a flame of a flammable gas widi an oxidising gas. The gas mixture may pass through the nebuliser first or it may burn directly. Alternatively, furnace atomisers are used, when smaller volumes of test solutions can be handled. The solution is placed in a horizontal graphite tube or a carbon rod which are heated in an electric furnace. 

Some lamps can be used for several elements. A low pressure mercury lamp can be used for the determination of mercury. However, because of some instability of die lamp output, a double beam spectrophotometer gives more reliable results than single beam instruments. In the former, a rotating sector mirror chopper splits die beam from the lamp into a reference beam and a sample beam, which passes through die burner or atomiser. A mirror combines the two beams which pass through the monochromator. Then the ratio of the intensity of the two pulses is electronically measured, thus elminating any fluctuation of the lamp output. Instruments display absorbance and/or u.v. transmission. 

Atomic absorption spectrophotometry depends on the application of Beer s law (Sec. 2.4.1). It is therefore necessary to construct a calibration plot by preparing a series of standard solutions, the most concentrated of which should give a reading 0.8. 

4 Applications of Atomic Absorption Spectroscopy in Pharmaceutical Analysis 

1 Assay of total zinc in Insulin zinc suspension 

Allan Walsh, in 1955, was the pioneer for the introduction of atomic absorption spectroscopy (AAS), which eventually proved to be one of the best-known-instrumental-techniques in the analytical armamentarium, that has since been exploited both intensively and extensively in carrying out the quantitative determination of trace metals in liquids of completely diversified nature, for instance blood serum-for Ca2+, Mg2+, Na+ and K+ edible oils-Ni2+ beer samples-Cu+ gasoline (petrol)-Pb2+ urine-Se4+ tap-water-Mg2+ Ca2+ lubricating oil-Vanadium (V). 

AAS facilitates the estimation of a particular element in the presence of many other elements efficaciously. In other words, there is absolutely no necessity to separate the test elemenf from the rest thereby not only saving a great deal of time but also eliminating the possibility of various sources of error incurred by these processes. In addition AAS may be used for the estimation of both aqueous and non-aqueous solutions. 

Because of the fact that AAS is free from any cumbersome-sample-preparation, it has proved to be an ideal, dependent and versatile analytical tool for the non-chemists specifically, for instance biologists, clinicians and the engineers, whose interest lies only in the significance of the results. 

---

This same principle, as indicated earlier, is used in atomic absorption spectroscopy and UV absorption. 

The section on Spectroscopy has been expanded to include ultraviolet-visible spectroscopy, fluorescence, Raman spectroscopy, and mass spectroscopy. Retained sections have been thoroughly revised in particular, the tables on electronic emission and atomic absorption spectroscopy, nuclear magnetic resonance, and infrared spectroscopy. Detection limits are listed for the elements when using flame emission, flame atomic absorption, electrothermal atomic absorption, argon ICP, and flame atomic fluorescence. Nuclear magnetic resonance embraces tables for the nuclear properties of the elements, proton chemical shifts and coupling constants, and similar material for carbon-13, boron-11, nitrogen-15, fluorine-19, silicon-29, and phosphorus-31. 

Van Loon, J. C. Analytical Atomic Absorption Spectroscopy. Academic Press New York, 1980. 

A technique is any chemical or physical principle that can be used to study an analyte. Many techniques have been used to determine lead levels. For example, in graphite furnace atomic absorption spectroscopy lead is atomized, and the ability of the free atoms to absorb light is measured thus, both a chemical principle (atomization) and a physical principle (absorption of light) are used in this technique. Chapters 8-13 of this text cover techniques commonly used to analyze samples. 

Atomic absorption, along with atomic emission, was first used by Guystav Kirch-hoff and Robert Bunsen in 1859 and 1860, as a means for the qualitative identification of atoms. Although atomic emission continued to develop as an analytical technique, progress in atomic absorption languished for almost a century. Modern atomic absorption spectroscopy was introduced in 1955 as a result of the independent work of A. Walsh and C. T. J. Alkemade. Commercial instruments were in place by the early 1960s, and the importance of atomic absorption as an analytical technique was soon evident. 

Absorbance profile for Ag and Cr in flame atomic absorption spectroscopy. 

In atomic absorption spectroscopy, the correction of the net absorbance from that due to the sample matrix. 

M HNO3. The concentration of Cu and Zn in the diluted supernatant is determined by atomic absorption spectroscopy using an air-acetylene flame and external standards. Copper is analyzed at a wavelength of 324.8 nm with a slit width of 0.5 nm, and zinc is analyzed at 213.9 nm with a slit width of 1.0 nm. Background correction is used for zinc. Results are reported as micrograms of Cu or Zn per gram of FFDT. 

Scale of Operation Atomic absorption spectroscopy is ideally suited for the analysis of trace and ultratrace analytes, particularly when using electrothermal atomization. By diluting samples, atomic absorption also can be applied to minor and major analytes. Most analyses use macro or meso samples. The small volume requirement for electrothermal atomization or flame microsampling, however, allows the use of micro, or even ultramicro samples. 

The next set of experiments describe suitable applications of atomic absorption spectroscopy. 

Gilles de Pelichy, L. D. Adams, C. Smith, E. T. Analysis of the Essential Nutrient Strontium in Marine Aquariums by Atomic Absorption Spectroscopy, /. Chem. Educ. 1997, 74, 1192-1194. 

Welz, B. Atomic Absorption Spectroscopy. VGH Deerfield Beach, FL, 1985. 

Most potentiometric electrodes are selective for only the free, uncomplexed analyte and do not respond to complexed forms of the analyte. Solution conditions, therefore, must be carefully controlled if the purpose of the analysis is to determine the analyte s total concentration. On the other hand, this selectivity provides a significant advantage over other quantitative methods of analysis when it is necessary to determine the concentration of free ions. For example, calcium is present in urine both as free Ca + ions and as protein-bound Ca + ions. If a urine sample is analyzed by atomic absorption spectroscopy, the signal is proportional to the total concentration of Ca +, since both free and bound calcium are atomized. Analysis with a Ca + ISE, however, gives a signal that is a function of only free Ca + ions since the protein-bound ions cannot interact with the electrode s membrane. 

Detector Detection in FIA may be accomplished using many of the electrochemical and optical detectors used in ITPLC. These detectors were discussed in Chapter 12 and are not considered further in this section. In addition, FIA detectors also have been designed around the use of ion-selective electrodes and atomic absorption spectroscopy. 

This experiment describes a fixed-size simplex optimization of a system involving four factors. The goal of the optimization is to maximize the absorbance of As by hydride generation atomic absorption spectroscopy using the concentration of HCl, the N2 flow rate, the mass of NaBH4, and reaction time as factors. 

A wider range of elements is covered by ICT-AES than by atomic absorption spectroscopy. All elements, except argon, can be determined with an inductively coupled plasma, but there are some difficulties associated with He, Ne, Kr, Xe, F, Cl, Br, O and N. 

The conventional method for quantitative analysis of galHum in aqueous media is atomic absorption spectroscopy (qv). High purity metallic galHum is characteri2ed by trace impurity analysis using spark source (15) or glow discharge mass spectrometry (qv) (16). 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Atomic Absorption Spectroscopy. Mercury, separated from a measured sample, may be passed as vapor iato a closed system between an ultraviolet lamp and a photocell detector or iato the light path of an atomic absorption spectrometer. Ground-state atoms ia the vapor attenuate the light decreasiag the current output of the photocell ia an amount proportional to the concentration of the mercury. The light absorption can be measured at 253.7 nm and compared to estabUshed caUbrated standards (21). A mercury concentration of 0.1 ppb can be measured by atomic absorption. 

Chemical Properties. Elemental analysis, impurity content, and stoichiometry are determined by chemical or iastmmental analysis. The use of iastmmental analytical methods (qv) is increasing because these ate usually faster, can be automated, and can be used to determine very small concentrations of elements (see Trace AND RESIDUE ANALYSIS). Atomic absorption spectroscopy and x-ray fluorescence methods are the most useful iastmmental techniques ia determining chemical compositions of inorganic pigments. Chemical analysis of principal components is carried out to determine pigment stoichiometry. Analysis of trace elements is important. The presence of undesirable elements, such as heavy metals, even in small amounts, can make the pigment unusable for environmental reasons. 

---