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Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities Beatriz Fernández, Lara Lobo, and Rosario Pereiro, University of Oviedo, Oviedo, Spain © 2018 Elsevier Inc. All rights reserved.

Introduction Fundamentals Instrumentation Instrument Components Light sources Atomizer Wavelength selector Detector Light modulator Background corrector Other components Spectrometer Configurations Interferences and Analytical Performance Characteristics Interferences Spectral interferences Non-spectral interferences Analytical Figures of Merit Applications Acknowledgments References

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Introduction Atomic absorption spectrometry (AAS) is a technique in which free gaseous atoms absorb electromagnetic radiation at a specific wavelength to produce a measurable signal. The absorption signal is proportional to the concentration of those free absorbing atoms in the optical path. Therefore, for AAS measurements the analyte must be first converted into gaseous atoms, usually by application of heat to a cell called atomizer. The type of atomizer defines the two main AAS-based analytical techniques: flame AAS (FAAS) that provides analytical signals in a continuous fashion and electrothermal AAS (ETAAS) delivering analytical signals in a discontinuous mode (2–4 min per sample). In both cases, liquid (or dissolved) samples are easily introduced into the analyzer, as an aerosol in the case of FAAS or as well-defined low microliter volumes in ETAAS. Furthermore, the coupling of hydride generation and cold vapor methods allow the introduction of analytes in the atomizer as a gas phase.1 Also, especially in ETAAS, direct elemental analysis of solids without previous dissolution is feasible.2 The AAS physical phenomenon was noted by Wollaston and Fraunhofer and explained by Kirchoff and Bunsen in the 19th century when they observed dark lines in the solar spectrum. Despite these early studies, AAS was mostly restricted to astrophysical applications until the 1950s. The analytical application of AAS was considerably delayed because of the apparent need for very high resolution to make quantitative measurements (typical atomic absorption lines may be narrower than 0.002 nm, while monochromators capable of isolating spectral regions narrower than 0.1 nm are rather expensive). In 1955 Walsh (Australia) overcame this obstacle with a light source emitting narrow lines. The use of the hollow cathode lamp (HCL) as light source allowed the demonstration of the analytical applicability of AAS measuring the light absorbed by atomic vapor in a flame. The idea was pursued independently by Alkemade in The Netherlands and Walsh in Australia, their works being published in 1955. Nowadays, AAS is routinely employed for elemental analysis of about 70 elements of the periodic table.3 Fundamentals, key components of atomic absorption spectrometers and analytical performance characteristics of AAS-based techniques are briefly described below.

Fundamentals The basic processes in optical atomic spectrometry involve the outer electrons of the atomic species. From Planck’s law (Eq. 1), the relationship between the optical spectrum (atomic lines) of an element and the energy level transitions of the valence electrons can be understood. E ¼ hu ¼ hc=l

(1)

being h the Planck’s constant, u frequency, c velocity of light and l wavelength.

Encyclopedia of Analytical Science, 3rd edition

https://doi.org/10.1016/B978-0-12-409547-2.14116-2

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Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

However, atomic lines are not purely monochromatic since the spectral lines are broadened mainly due to Doppler and Lorentz effects (it has to be noted that there is also a natural broadening due to Heisenberg’s uncertainty principle, though generally this natural broadening is very small as compared to the other broadening phenomena). An atomic absorption line is the result of an electron transition from a lower to a higher state of the atom produced by a photon of adequate energy (i.e., frequency, wavelength) which matches the energy required for such transition (Fig. 1). As the population of the excited levels is generally very small compared with that of the ground state (i.e., the lowest energy state of a given atom), absorption is greatest in lines resulting from transitions from the ground state (resonance lines). In routine analysis by AAS, light of a proper wavelength and a given intensity Io passes through the cloud of free atoms of the analyte contained in the absorption volume of path length L inside an atomizer. The analyte free atoms absorb electromagnetic radiation of this specific wavelength. The intensity I (also called the transmitted light) emerging from the atomizer will depend on Io, the length of the optical path L, the concentration of the analyte atoms and the particular atomic transition measured. In absence of interferences the amount of light energy absorbed by the analyte atoms present in the optical path will be Io  I. The relationship between I and Io (Eq. 2) follows the Lambert-Beer law: I ¼ Io eku L

(2)

being ku the absorption coefficient, which depend on the wavelength and the concentration of absorbing atoms of the analyte. In order to have a linear relationship between the measured signal and the concentration (Eq. 3), the absorbance value is defined as: Abs ¼ log Io =I

(3)

Thus, by combining Eqs. (2) and (3), it is obtained a linear dependence between absorbance and ku (Eq. 4): Abs ¼ log eku L ¼ 0:4343ku L

(4)

From classical dispersion theory it can be shown that ku is in practical terms proportional to the concentration of analyte atoms in the atomizer (No), as Eq. (5) shows: Abs ¼ constant LN o

(5)

That is, this formula is similar to the well-known Lambert-Beer law in classical colorimetry or spectrophotometry in liquid solutions: Abs ¼ eL concentration

(6)

Instrumentation Instrument Components An atomic absorption spectrometer consists of four main parts (see Fig. 2): (i) a lamp (generally emitting spectral lines), (ii) an atomizer (either a flame or an electrothermal device) where gaseous atoms from the sample are produced, (iii) a wavelength selector to isolate the specific light absorbed, and (iv) a detector with the corresponding electronics and readout system to detect and quantify the intensity of the light passing through the exit slit of the wavelength selector. Additionally, a lamp light modulator and a background corrector are also customarily required. As can be seen in Fig. 2, the light source, the atomizer and the wavelength selector are located in a straight-line configuration.

Light sources The most common lamps used in AAS are line sources, emitting a narrow-line spectrum of the element of interest, in particular the HCL and the electrodeless discharge lamp (EDL). Moreover, laser diodes have been proposed also as light sources in AAS.4 The HCL is a bright and stable line emission source commercially available for many elements. However, HCLs present problems of low emission intensity and short lamp lifetimes for some volatile elements (e.g., As, Se, Hg). For such elements EDLs with a radiofrequency (or microwave) coil surrounding the bulb with the metal (or salt of the element) are used. Because the gas pressure and temperature are low, Lorentz and Doppler broadening effects are small in HCLs and EDLs. Hence, both lamps can

E1 υ=

E1 - Eo h

Eo Fig. 1 Photon absorption mechanism. Horizontal lines represent different energy levels in an atom. Eo is the term used for the lowest energy level, which is referred to as the ground level (all practical absorption measurements originate from atoms in the ground state).

Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

(i) Lamp

(ii) Atomizer

(iii) Wavelength selector

L

3

(iv) Detector, electronics & readout

sample

Io

I

Fig. 2 Schematic view of the main components of an instrument for AAS including the basic components: (i) lamp, (ii) atomizer, (iii) wavelength selector, and (iv) detector, electronics and readout system. Light is pulsed (depicted as dashed lines) with a chopper (not depicted in the scheme) or, more commonly, by electronically pulsing the lamp.

produce extremely narrow atomic lines, narrower than absorption line widths in flames and electrothermal atomizers (see Fig. 3). The stable, intense and narrow-line emission of HCL and EDL sources at the center of the absorption profiles (Walsh’s absorption) guarantees high analyte specificity and good detection limits, even in combination with low-resolution monochromators. On the other hand, in recent years it has been proposed the combination of a high intensity source emitting a continuum with a high resolution spectrometer. Such type of instrument is commercially available for FAAS and ETAAS.5 The hollow cathode lamp A HCL consists of a cylindrical hollow cathode (containing the element of interest) and an anode within an enclosed glass tube filled with and inert gas at low pressure. High-purity neon or argon (1–5 Torr) is used as filler gas. By applying a potential difference of about 300–400 V between the anode and the cathode, gas ionization takes place and a plasma between the two electrodes is formed. At the low pressure, gas ions (e.g., Arþ, Neþ) impact with the cathode and metal atoms are ejected (“sputtering” process) towards the plasma. Through collisions in the plasma, the sputtered metal atoms become excited and radiation at the characteristic wavelengths of the element is emitted through relaxation to their ground state (Fig. 4). Boosted HCLs, with a second discharge to further excite the sputtering atoms, aiming at increasing the output from the HCL, are available. Multi-element HCLs are also commercialized; however, some of them are subjected to certain limitations such as sensitivity compromises. Light sources emitting a continuum An instrument for atomic absorption measurements based on a high intensity light source emitting a continuum (a xenon short-arc lamp) in combination with a high resolution double-echelle spectrometer and an array detector is also commercially available. Such combination offers interesting advantages like multi-elemental capabilities, possibilities for more reliable background correction since the entire spectrum around the analytical line can be measured, extension of the dynamic range (combining measurements at the center and the wings of the absorption line) and analysis of elements for which line a radiation source is not available.6

Atomizer Two heat sources are most frequently used in AAS to produce atoms: a flame (chemical combustion energy) and an electrothermal (electrical energy) atomizer. Some commercial instruments have pre-aligned flame and furnace atomizers, making feasible to select any of them by software means. Flame The analysis of liquid samples by FAAS generally requires the aspiration and nebulization of the solution, the elimination of large droplets and the final introduction of fine droplets into the burner. Pneumatic nebulizers using a jet of compressed gas (the nebulization gas, typically the oxidant) are frequently used. They produce droplet diameters in the range 1–50 mm. Next, a premix chamber is located where large droplets of the nebulized sample solution condense (about 90% of the sample droplets) and drain out while the remaining fine droplets (about 10% of the droplets) mix with the fuel and oxidant gases before they enter the burner head which supports the flame. The slot of premix burners is long and thin. Once in the flame, a sequence of complex processes begins: desolvation of the droplets, vaporization of the solid or molten particles, dissociation of molecular species to form atoms, and possible ionization of atoms. Flames obtained with a premix chamber are very stable and show a structure consisting of defined cones. The appearance and the size of these regions is governed by type and ratio of “fuel/oxidant” used. The part of the flame to be measured depends on the analyte. Both the temperature of the flame and its oxidizing or reducing characteristics are of importance. Several combinations of fuel and oxidant can be employed to produce a flame in FAAS. Nowadays, the most widely used gas combinations for FAAS are airacetylene (2100–2400 C) and nitrous oxide-acetylene (2600–2900 C). The hotter and more reducing N2OdC2H2 flame is very useful for the analysis of “refractory” elements, like Al or Si, that tend to form heat-stable oxides in the air-acetylene flame. Electrothermal atomizer Electrothermal atomizers based on tubular small-size graphite tubes (3–5 cm in length and a few mm in diameter) provide high vaporization and atomization efficiencies, well-controlled thermal and chemical environment, require low volume (microliters) of sample and offer interesting possibilities for direct solid analysis. A determination by ETAAS starts by dispensing a small,

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Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

Intensity

Spectral bandpass of the monochromator

Width of absorption line in the atomizer Width of HCL emission

Wavelength Fig. 3 Importance of line widths in AAS. In practice, the typical spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines.

Power + supply –

anode

Ar or Ne at 1 - 5 torr

cathode

–

Light output

Mo

Ne+ Ne+ M*

light

Fig. 4 Basic diagram of a hollow cathode lamp, with an enlarged view depicting sputtering, excitation and emission processes.

well-known, volume of sample into the atomizer. The sample is then subjected to a multi-step temperature program, sequentially producing drying of the sample, pyrolysis and atomization (the concentration of the analyte atoms in this last step depends on the concentration of the analyte and the volume of sample introduced in the atomizer). This cycle ordinarily ends with a higher temperature clean out step. Then, the furnace cools down for the next ETAAS analysis. The combined use of an autosampler, a L’Vov platform, a suitable background corrector, appropriate matrix modifiers, pyrolytically coated graphite tubes, rapid heating of the furnace and fast electronics to detect the absorbance has allowed the development of accurate and highly sensitive ETAAS analytical methods.

Wavelength selector The most common wavelength selectors used in AAS are monochromators, based on Czerny-Turner, Ebert and Littrow designs. In recent years, echelle optics is being incorporated to commercial AAS instruments. The employment of an echelle configuration in combination with an intense continuous lamp brings about important capabilities such as measurement of the spectral background close to the line and simultaneous multi-elemental analysis.

Detector The quantification of the intensity of the light passing the exit slit of a wavelength selector requires a proper transducer. For such purpose a photomultiplier tube (PMT) is usually employed. The interesting properties of the PMT are numerous: large wavelength coverage, large dynamic range, high amplification gain and low noise. However, there is a current trend towards the replacement of PMTs by multichannel detectors with many individual, small adjacent detectors called pixels. Within this context, charge transfer device detectors are currently used, based either on charge-coupled device or charge-injection device technology.

Light modulator The light source should be modulated to provide a means of avoiding the continuous emission from the atomizer. This can be achieved mechanically (e.g., with a rotating chopper located between the light source and the atomizer) or electronically (pulsing the lamp). Synchronous detection will eliminate the unmodulated signal emitted by the atomizer and will measure only the modulated signal coming from the lamp. However, care has to be taken because light absorbed or scattered by other atoms or molecules will give rise to a background absorption which will summed up to the specific absorption of the analyte.

Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

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Background corrector Absorption by molecular species and light scattering from solid or liquid particles originated from the sample can give rise to a noticeable unspecific background. To accurately measure the specific absorbance due to the analyte it is necessary to subtract the background from the total absorbance. A common, rather inexpensive, background correction approach is based on the incorporation in the spectrometer of a continuum emission light source such as the deuterium lamp.7 Unfortunately, the deuterium background corrector cannot accurately correct neither for high absorbance nor for structured molecular background. Another background correction strategy is based on pulsing alternatively the HCL at low and then at very high current8: during the low current part of the cycle the line is narrow and the analyte and background are measured together; however, during the high current pulse, the lamp emission lines broaden and mainly the background is probed. This method is simple and does not require additional components, but again cannot correct for structured molecular background. Finally, a powerful background correction method is based on the splitting by a magnetic field of the spectral lines of an atom and the polarization of those lines, that is, is based on the Zeeman effect. Such type of background corrector allows measuring background signals at the analytical wavelength,9 thus permitting the correction of structured molecular background.

Other components A variety of accessories are commercialized with AAS instruments, like for example: autosamplers with functions such as dilution and automatic dosing of modifiers, modular systems for cold vapor and hydride operation (either in batch or flow mode), direct solid samplers in ETAAS, automatic burner rotation systems to obtain different light-path distances and therefore an extension of linear ranges, flame microsamplers for small sample volumes, etc.

Spectrometer Configurations In those cases where only a single light beam is used (see Fig. 2), it is required first a blank reading to set absorbance to zero (Abs ¼ log Io/I). Unfortunately, the intensity of the lamp may not remain constant during the whole analysis and, therefore, the final measured absorbance will be inaccurate. To overcome this problem, double beam spectrometers are used. Double beam spectrometers incorporate a beam splitter in the light path before the atomizer, in such a way that a part of the light beam from the lamp passes through the sample cell (i.e., atomizer) and the other is used as reference (see Fig. 5). A beam recombiner (see location in Fig. 5) alternatively passes light from the atomizer (sample beam) and from the reference beam. The alternating light signals (either from the sample beam or from the reference beam) generates alternating electrical signals, allowing for a continuous comparison between the beam passing through the gaseous sample within the atomizer and the reference beam.

Interferences and Analytical Performance Characteristics Interferences Interferences in AAS can be divided in two broad groups: spectral and non-spectral.

Spectral interferences Spectral interferences in AAS are principally due to molecular absorption and scattering in the light path produced by unvaporized solvent droplets, undissociated matrix components and formed molecular species from the sample. Background absorption is a problem especially serious in ETAAS which could limit the reliability of the analysis. Although the addition of chemical modifiers can help to decrease such interferences, in common practice it is required to resort to instrumental background correctors.

1

Reference beam Wavelength selector Sample beam

Lamp

Beam splitter

Detector, electronics & readout

Atomizer Beam recombiner

Fig. 5 Schematic view of a double beam spectrometer for atomic absorption measurements. A beam splitter producing two beams (the “sample beam” passing through the atomizer and the “reference beam”) is located before the atomizer. A beam recombiner is located after the atomizer. The different components are denoted in the figure.

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Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

Non-spectral interferences Non-spectral interferences are those affecting the production or the availability of analyte absorbing atoms. In FAAS, physical interferences are caused by differences in the physical properties (viscosity, density, etc.) between the dissolved sample and the calibration standards and they can be avoided by using the same solvent for sample and standards. Chemical interferences are produced by formation of low volatility compounds containing the analyte. Selection of appropriate flame conditions (e.g., a nitrous oxide-acetylene flame offering higher temperatures and a reducing atmosphere to decompose and/or prevent the formation of refractory oxides, should be used for analytes which react with flame gases forming stable oxides and hydroxides), matching of standard solutions to the sample, or addition of some special substances reducing interferences are strategies commonly used to reduce chemical interferences in FAAS. Moreover, a noticeable fraction of alkali and alkaline earth elements may be ionized in the flame, giving rise to a decreased amount of atoms of the analyte. In ETAAS, non-spectral interferences are very important and can be due to different processes, including diffusion of the analyte from the optical path before reaching the atomization temperature (e.g., formation of gaseous compounds with the analyte, co-volatilization or thermal expulsion of the analyte together with rapidly expanding matrix gases, etc.) and occlusion of the analyte. They hinder the formation of free analyte atoms during the atomization step. Such interferences can be minimized using strategies like L’Vov platforms, use of appropriate chemical modifiers10 and the standard addition method.

Analytical Figures of Merit Quantitative AAS analytical methods are based on calibration curves, which in principle are linear. However, deviations from the Lambert-Beer law at high concentrations occur and analytical linear ranges in AAS are not too large (about three orders of magnitude above the corresponding limits of quantification). This is due to the fact that not all radiation reaching the detector has been absorbed to the same extent by the analyte atoms, and this is particularly noticeable at high analyte concentrations. Ionization interferences in flames give rise to an upward curvature at high concentrations, because a larger fraction of the analyte atoms are ionized at lower concentrations. To overcome this type of interference an “ionization suppressor” can be added (e.g., an easily ionizable element), providing a large amount of electrons to the flame thus suppressing ionization of the analyte. On the other hand, incomplete dissociation of analyte molecules leads to a curvature towards the concentration axis. The detection limits (DLs) achieved in FAAS greatly depend on the element analyzed. Additionally, DLs are subjected to the quality of the instrumentation. Thus, here just some approximate values will be given. DLs obtained by FAAS are of the order of mg L1 (ppm) while these figures are much lower by using ETAAS: typical DLs in ETAAS are usually lower than 1 mg L1 (ppb). Under optimized conditions, precision achievable by FAAS lies in the range of 0.5%–2%. Precision in the case of ETAAS is of the order of 0.5%–5%, provided that particular attention is paid to control sample contamination risks.

Applications The continued use of flame-based atomic spectrometric instruments can be attributed to the fact that they are very robust and comparatively cheap, offer high sample throughput and the corresponding analytical methods are well established and validated. The number of analytical applications of FAAS countless. In fact, many applications of FAAS are commonly carried out even for educational purposes, like determination of calcium in foodstuffs.11 Calcium is one of the elements most frequently analyzed by FAAS. The Ca ionization that occurs in the air-acetylene flame can be controlled by the addition of an alkali salt to samples and standards. Besides, 0.1%–1.0% La (or Sr) or 1% EDTA is usually added to avoid reduction of calcium sensitivity due to the presence of elements which give rise to stable oxy-salts (e.g., Si, Al, or P). This last type of chemical interferences can be also overcome by using a nitrous oxide-acetylene flame. ETAAS analytical methods are more complicated and the sample throughput is lower compared to FAAS. However, when high sensitivity is needed, the atomic absorption technique of choice is ETAAS. Some examples are the lead determination in whole blood and urine12 and the aluminum determination in serum and urine,13 just to name a few. Lead is a toxic element whose dangerous consequences affect children particularly. Low concentrations of aluminum in serum are associated to several diseases as renal insufficiency, encephalopathy, microcytic anemia, etc. Different approaches have been proposed for those determinations by ETAAS. The addition of appropriate chemicals to reduce interferences is crucial. In particular, in recent years a lot of research has been dedicated to the development of successful permanent modifiers, for example, iridium,12 ruthenium,13 or mixtures like Zr þ Ir, W þ Ir W þ Rh.10 Finally, it should be mentioned that special effort has been devoted to the development of clean methodologies to avoid contamination of the low concentrations analyzed. Nowadays, the inductively coupled plasma (ICP) either with detection by optical emission spectrometry (OES) or by mass spectrometry (MS) are competitive techniques of AAS.14 ICP-OES is a popular technique for multielemental analysis, though the instrument cost and the running expenses are much higher than those needed for AAS. In those cases where very high sensitivity is required, ICP-MS is the technique of choice for multielemental analysis. However, atomic absorption instruments, though in general are single-element techniques, are still widely employed for inorganic elemental analysis in clinical, pharmaceutical, environmental, agriculture, food technology, geochemical/mining and industrial routine applications. Extensive information on analytical methodologies and protocols for elemental determinations by AAS are available and often provided by the instrument manufacturers.

Atomic Absorption Spectrometry: Fundamentals, Instrumentation and Capabilities

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Acknowledgments Lara Lobo acknowledges financial support through program Juan de la Cierva-Incorporación (IJCI-2015-25801) and Beatriz Fernandez acknowledges her research contract RYC-2014-14985 through the Ramón y Cajal Program, both from the Spanish Ministry of Economy and Competitiveness.

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