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Spectrochemical Analysis by Atomic Absorption and Emission

The Royal Society of Chemistry

Introduction

1 HISTORICAL

The first spectroscopic observation was made by Newton in 1740. He discovered that the radiation of white light splits into different colours when passing through a prism. In the middle of the nineteenth century metal salts were identified by means of their colour in the flame. The first diffraction grating was introduced by Rittenhouse in 1786.

In 1802 Wollaston discovered, in the continuum emission spectrum of the sun, dark lines which were later studied in detail by Frauenhofer. He observed about 600 lines in the sun's spectrum and named the most intensive of them by the letters from A to H. In 1820 Brewster explained that these lines originate from the absorption processes in the sun's atmosphere. Similar observations were made by several researchers in the spectra of stars, flames, and sparks. In 1834 Wheatstone observed that the spectra produced with a spark depended on the electrode material used. Angstrom in turn made the observation that spark spectra were also dependent on the gas surrounding the electrodes. The study of flame spectra became much easier after the discovery of the Bunsen burner in 1856.

Kirchhoff and Bunsen constructed a flame spectroscope in 1859. This new instrument made it possible to study small concentrations of elements which was impossible by the other methods available at that time. They also showed that the lines in the flame spectra originated from the elements and not from the compounds. Applications for this new technique were soon observed in astronomy and analytical chemistry. In the next five years, four new elements (Rb, Cs, Tl, and In) were found by flame emission spectroscopy.

The first quantitative analysis based on the flame emission technique was made by Champion, Pellet, and Grenier in 1873. They determined sodium by using two flames. One flame was concentrated with sodium chloride and the other was fed with the sample solution along a platinum wire. The determination was based on the comparison of the intensities of the flames by dimming the brighter flame with a blue glass wedge.

Diffraction gratings were studied by many scientists in the nineteenth century. By the end of the century gratings were improved markedly thanks to Rowland's studies. In the Rowland spectrograph the slit, grating, and camera were all in the same circle (Rowlands circle).

The main points to note of the spectroscopy of the nineteenth century were:

(i) By sufficient heating the monoatomic gases emit radiation spectra which consist of separate emission lines. Emission spectra of polyatomic gases consists of a number of lines close to each other, while solids and dense gases emit continuum radiation;

(ii) A cool gas absorbs radiation at the same frequencies as it emits radiation. If a continuum emission is directed into a cool gas vapour, the spectrum recorded will contain dark absorption lines or bands;

(iii) The frequencies (or wavelengths) of the lines are characteristic of each atom and molecule, and the intensities are dependent on the concentration.

Both the qualitative (the wavelengths of the lines) and the quantitative analysis (the intensities of the lines) are based on these phenomena.

Emission spectra were first utilized in analytical chemistry as they were simpler to detect than absorption spectra. Flames, arcs, and sparks are all classical radiation sources. Lundegardh first applied a pneumatic nebulizer and an air-acetylene flame. The development of prism and grating instruments was parallel. Photography was employed to detect the spectral lines. The first commercial flame photometers came on the market in 1937.

The wavelength calibration using the red cadmium line at 643.8 nm was performed in 1907. Later the calibration was performed according to the green mercury line at 546.0 nm. In 1960 the new definition for the length of one metre was confirmed to be the wavelength of the krypton-86 line at 605.8 nm multiplied by the factor of 1650732.73. Nowadays one metre is defined as the distance which the light propagates in 1/299792458 seconds in a vacuum.

The basic concepts of atomic absorption spectrometry were published first by Walsh in 1955, this can be regarded as the actual birth year of the technique. At the same time Alkemade and Milatz designed an atomic absorption spectrometer in which flames were employed both as a radiation source and an atomizer. The commercial manufacture of atomic absorption instruments, however, did not start until ten years later. Since then the development of atomic absorption spectrometry has been very fast, and atomic absorption (AA) instruments very quickly became common. The inventions of dinitrogen oxide as oxidant and electrothermal atomization methods have both significantly expanded the utilization field of atomic absorption spectrometry. These techniques increased the number of measurable elements and lowered detection limits. Todays' graphite furnace technique is based on the studies of King at the beginning of the twentieth century.

The use of atomic emission spectrometry expanded markedly when the first commercial plasma atomic emission spectrometers came on the market in the middle of the seventies. The principle of the direct current plasma (DCP) source was reported in the twenties and the first DCP instrument was constructed at the end of the fifties. The first microwave plasma source was introduced in 1950, and the first inductively coupled plasma source was patented in 1963.

Atomic fluorescence in flames was first studied by Nichols and Howes. They reported the fluorescence of Ca, Sr, Ba, Li, and Na in a Bunsen flame in 1923. The first analytical atomic fluorescence spectrometer based on the studies of Bagder and Alkemade, was constructed by Winefordner and his co-workers in 1964.

Alan Gray first suggested the connection of a plasma source and a mass spectrometer in 1975. The direct current plasma jet was first applied in this new technique. Later it was shown that the inductively coupled plasma ( ICP) met the requirements better than the DCP for an ionization source of mass spectroscopic analysis. The pioneering work of ICP-MS was mainly conducted by three research groups (Fassel, Gray, and Date).

Table 1 summarizes important steps of the history of atomic absorption and plasma emission spectrometry.

2 THE PRESENT STATUS OF ATOMIC SPECTROMETRIC METHODS

Atomic absorption, plasma atomic emission, and atomic fluorescence spectrometry are all optical atomic spectrometric techniques developed rapidly during the past years. These methods are based on the measurement of absorption, emission, or fluorescence originated from the free, unionized atoms or atomic ions in gas phase.

Table 2 describes the amount of published papers in atomic spectrometry during the past ten years. A brief survey of the current literature shows that each year fewer papers are dealing with novel AAS instrumentation, techniques, or applications. Instead, most studies are now concerned with plasma emission techniques (especially ICP-AES) or ICP mass spectrometry. Although the number of recent AAS papers is declining, the number of AAS determinations performed each year remains substantial, and the sales of AA instruments remain strong. This is clearly indicating that atomic absorption spectrometry has become a mature analytical technique during its existence of approximately 40 years. Modern atomic absorption instruments are still in principle similar to the instruments from the early days of atomic absorption. The most significant development has occurred in electronics. Microprocessors have markedly simplified working with these instruments. Modern instruments are faster and safer, and the performance with respect to precision and accuracy has improved. The use of an autosampler makes it possible to determine 6 elements in 50 samples in 35 minutes, i.e. about 500 determinations in one hour.

The wide popularity of AAS can be attributed in its simple and convenient use with various methods. In addition, the high sensitivity of graphite furnace AAS is very important in many applications. However, the simultaneous multi-element analysis or qualitative analysis by AAS is an arduous task. These two shortcomings of AAS (as well as many advantages of AAS) derive from the line-like radiation source and the atomizer employed. Because a line-like radiation source (a hollow cathode lamp or an EDL) emits an extremely narrow radiation line that is locked on the resonance lines of the atoms of the analyte element, it provides relatively linear calibration graphs, minimizes spectral interferences, and makes the alignment of the instrumentation and selection of wavelengths easy. However, use of hollow cathode lamps or EDLs, only allows 1 to 3 elements to be determined at the same time, which decreases the rate of multi-element analysis and makes qualitative analysis impractical.

With few exceptions, graphite furnaces and flame atomizers are both limited to use with liquid samples and are capable of effectively atomizing only a fraction of the elements. Graphite furnace determinations require optimization of instrumental conditions for each element ( temperature programme, observation time) in order to obtain optimal results. Thus, multi-element analysis is compromised. In addition, the GF-AAS techniques suffer from inter-element interferences and background absorption which must be overcome.

In order to make the lamp change rapid, various arrangements are offered by the manufacturers. For instance, by a ferris-wheel like turret a sequence of elements can be measured during each turret rotation. Another approach is a combination of a continuum radiation source and a high-resolution spectrometer. However, this combination has not achieved great acceptance. A common problem with continuum sources is their relatively low intensity in the UV region.

Possibilities in continuum AAS include the use of a Fourier transform spectrometer, television-like detectors with an echelle monochromator, a resonance monochromator, and an instrument based on resonance schlieren ( Hook) spectrometry.

Before atomic absorption, atomic emission was used as an analytical method. The intensity of the emitted radiation and the number of emission lines are dependent on the temperature of the radiation source used. A flame is the oldest emission source. It is uncomplicated and its running costs are low. Flame emission is used, especially, for the determination of alkali and alkaline earth metals in clinical samples. Arcs and sparks are suitable radiation sources for multi-channel instruments in laboratories where several elements must be determined in the same matrix at high frequency, like in metallurgical laboratories.

Various plasmas (ICPs, DCPs, MWPs) possess a number of desirable analytical features that make them remarkably useful multi-element atomization-excitation sources. This applies particularly to inductively coupled plasmas. The sample particles experience a gas temperature of about 7000 to 8000 K when they pass through the ICP, and by the time the sample decomposition products reach the analytical observation zone, they have had a residence time of about 2 ms at temperatures ranging downwards from about 8000 to 5000 K. Both the residence time and temperatures experienced by the sample are approximately twice as large as those in a dinitrogen oxide-acetylene flame. ICPs are therefore the most widely used plasma sources.

Plasma AES has several advantages (possibility for the qualitative and simultaneous multi-element analysis, measurements in the vacuum UV region, high sensitivity, low detection limits, less chemical interferences, low running costs) and it has become more and more important for the determination of traces in a great variety of samples. On the other hand, it does not compensate totally for any other instrumental method of analysis, but it compensates for those faults which might exist in other techniques. The complementary nature of plasma AES and AAS capabilities for trace elemental analysis is an important feature of these techniques. Plasma AES exhibits excellent power of detection for a number of elements which cannot be determined or are difficult to determine at trace levels by flame AAS (e.g. B, P, S, W, U, Zr, La, V, Ti) or by electrothermal AAS (B, S, W, U). Thus, optical plasma emission and atomic absorption are not actually alternatives, but in an ideal way complement one another.

Inductively coupled mass spectrometry (ICP-MS) has been undoubtedly a 'hot' analytical technique in the last few years. Since the commercial introduction of ICP-MS instruments (VG Elemental Ltd.).in 1983, approximately 150 of them were installed worldwide during the first five years. A number of conferences have been dedicated to ICP-MS, and many analytical and spectroscopic meetings have included an ICP-MS session or symposium. Now two commercial systems (VG PlasmaQuad and Perkin Elmer Sciex ELAN ICP-MS systems) are available.

ICP-MS is being used in many branches of science. Many desirable analytical characteristics, such as superior detection limits, spectral simplicity, possibility for simultaneous multi-element analysis, and isotope ratio determinations, are reasons for its widespread popularity. However, not even this technique is free from interferences. Particularly, spectral (polyatomic) and non-spectral (suppression and enhancement) interferences cause analysts to consider carefully the sample preparation procedure and finally the matrix. Most of the fundamental research papers published deal with the suppressive and spectral interference effects.

The ICP-MS research has also been recognized as a 'hot' field by the Institute of Scientific Information (ISI). ISI is a general surveyor of all scientific activity. According to its list of 'The 30 Hottest Fields of 1987', ICP-MS was ranked in 16th place. ICP-MS and scanning tunnelling microscopy (19th) were the only analytical techniques represented in this compilation.

Atomic fluorescence has many superior features for trace elemental analysis (spectral simplicity, wide dynamic range, and simultaneous multi-element analysis). However, major practical problems of this technique are connected with the radiation source. Among various radiation sources lasers best meet the requirements for AFS. Atomic fluorescence has not become such a popular technique as plasma atomic emission or plasma mass spectrometry. The analytical applications of AFS have suffered from the lack of commercial instruments. The only commercial atomic fluorescence spectrometer is the Baird Plasma AFS system which consists of pulsed hollow cathode lamps for excitation and an ICP as an atomization cell.

Figure 1 presents the elements which can be determined by AAS and plasma AES methods.

3 TERMS AND DEFINITIONS

A newly developed analytical technique gives rise to new terminology. Existing terms may acquire a specialized meaning and completely new terms have to be invented. In order for people, working with atomic spectroscopy, to communicate with complete understanding several agreements and suggestions have been made by international bodies. The following nomenclatures are dealing with the spectroscopic terms and definitions: IUPAC (International Union of Pure and Applied Chemistry), 'Nomenclature, Symbols, Units, and their Usage in Spectrochemical Analysis', Parts I and II, Pergamon Press, Oxford, 1975; IUPAC, 'Compendium of Analytical Nomenclature', 2nd Edition, Blackwell Scientific Publications Ltd, Oxford, 1987; R.C. Denney, 'A Dictionary of Spectroscopy', 2nd Volume, MacMillan Press, 1982. I n the following text some common terms, definitions, and symbols associated with atomic spectrometry are given.

3.1 General Terms

Atomic absorption spectrometry (AAS). An analytical method for the determination of elements in small quantities. It is based on the absorption of radiation energy by free atoms.

Atomic fluorescence spectrometry (AFS). An analytical method for the determination of elements in small quantities. It is based on the emission of free atoms when the excitation is performed by the radiation energy.

Atomic emission spectrometry (AES). An analytical method for the determination of elements in small quantities. It is based on spontaneous emission of free atoms or ions when the excitation is performed by thermal or electric energy.

Molecular absorption spectrometry with electrothermal vaporization (ETV-MAS). An analytical method for the determination of elements in small quantities. It is based on the absorption of radiation energy by two atomic molecules at elevated temperatures.

Detection limit (DL). The minimum concentration or an amount which can be detected by the analytical method with a given certainty. According to the IUPAC recommendation, DL is the mean value of the blank plus three times its standard deviation.

Instrumental detection limit gives the smallest possible concentration which can be achieved by the instrument. The instrumental detection limit is derived by using the optimum instrumental parameters and the pure solvent (water) as a sample. The instrumental detection limit is useful of comparison of the performance of the different spectrometers.

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Excerpted from Spectrochemical Analysis by Atomic Absorption and Emission by Lauri H.J. Lajunen. Copyright © 1992 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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