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Principles of Instrumental Analysis 儀器分析

Chapter 8 (p. 215~229) 6th Edition An Introduction to Optical Atomic Spectrometry 1 2 Introduction Three major types of spectrometric methods are used to identify the elements present in samples of matter and determine their concentrations: (1) optical spectrometry, (2) mass spectrometry, (3) X-ray spectrometry.

• In (1) optical spectrometry, discussed in this chapter, the elements present in a sample are converted to gaseous atoms or elementary ions by a process called atomization. The ultraviolet-visible absorption, emission, or fluorescence of the atomic species in the vapor is then measured.

• In (2) atomic mass spectrometry (Chapter 11), samples are also atomized, but in this case, the gaseous atoms are converted to positive ions (usually singly charged) and separated according to their mass-to-charge ratios. Quantitative data are then obtained by counting the separated ions.

• In (3) X-ray spectrometry (Chapter 12), atomization is not required because X-ray spectra for most elements are largely independent of their chemical composition (or physical state) in a sample. Quantitative results can therefore be based on the direct measurement of the fluorescence, . absorption, or emission spectrum of the sample 3 8A OPTICAL ATOMIC SPECTRA p.216

In this section, we briefly consider the theoretical basis of optical atomic spectrometry and some of the important characteristics of optical atomic spectra.

• 8A-1 Energy Level Diagrams The energy level diagram for the outer electrons of an element is a convenient method to describe the processes behind the various methods of atomic spectroscopy. The diagram for sodium shown in Figure 8-1a is typical. Notice that the energy scale is linear in units of electron volts (eV), with the 3s orbital assigned a value of zero. The scale extends to about 5.14 eV, the energy required to remove the single 3s electron to produce a sodium ion, the ionization energy. 4 FIGURE 8-1 Energy level diagrams for (a) atomic sodium and (b) magnesium(I) ion. Note the similarity of lines shown in blue but not in actual wavelengths (Å). 5 Ch8 An Introduction to Optical Atomic Spectrometry P.216 Discussion on Fig. 8-1 p.216 • Horizontal lines on the diagram indicate the energies of several atomic orbitals. • Note that the p orbitals are split into two levels that differ only slightly in energy. • The classical view rationalizes this difference by invoking the idea that an electron spins on an axis and that the direction of the spin may be in either the same direction as its orbital motion or the opposite direction.

• Both the spin and the orbital motions create magnetic fields as a result of the rotation of the charge of the electron. • The two fields interact in an attractive sense if these two motions are in the opposite direction; the fields repel one another when the motions are parallel. • As a result, the energy of an electron whose spin opposes its orbital motion is slightly smaller than that of an electron with spin parallel to its orbital motion.

6 Note: p. 216 Similar difference exist in the d and f orbitals, but their magnitudes are ordinarily so slight as to be undetectable; thus, only a single energy level is indicated for d orbitals in Figure 8- 1a.

The splitting of higher energy p, d, and f orbitals into two states is characteristic of all species containing a single external electron. Thus, the energy level diagram for the singly charged magnesium ion (Mg+), shown in Figure 8-1b, has much the same general appearance as that for the uncharged sodium atom. So also do the diagrams for the di-positive aluminum ion (Al+2) and the remainder of the alkali-metal atoms (Groups-IA, IIA, IIIA).

It is important to note, however, that the energy difference between the 3p and 3s states is approximately twice as great for the magnesium ion as for the sodium atom because of the larger nuclear charge of the magnesium. 7 FIGURE 8-2 Energy level diagram for atomic magnesium (Singlet vs triplet). The width of the lines between states indicates the relative line intensities. Note that a singlet-to-triplet transition is considerably less probable than a singlet-to- singlet transition. Wavelengths are presented in angstroms. 8 Ch8 An Introduction to Optical Atomic Spectrometry P.217 P. 216 The p, d, and f orbitals of the triplet state are split into three levels that differ slightly in energy. These splittings can also be rationalized by taking into account the interaction between the fields associated with the spins of the two outer electrons and the net field that arises from the orbital motions of all the electrons. [scheme next page)

In the singlet state, the two spins are paired, and their respective magnetic effects cancel; thus, no energy splitting is observed.

In the triplet state, however, the two spins are unpaired (that is, their spin moments lie in the same direction.). The effect of orbital magnetic moment on the magnetic field of the combined spins produces a splitting of the p level into a triplet. [Fig. 8-3, next page] This behavior is characteristic of all of the alkaline-earth atoms (Li, Na, Mg, Al..), singly charged aluminum (Al+) and beryllium ions (Be+), and so forth. As the number of electrons outside the closed shell becomes larger, the energy level diagrams become increasingly complex. Thus, with three outer electrons, a splitting of energy levels into two and four states occurs; with four outer electrons, singlet, triplet, and quintet states exist. 9 no energy splitting

triplet state are split into three levels that differ slightly in energy

FIGURE 8-3 Spin orientations in singlet ground and excited states and triplet excited state. [Two outer electrons]

A pair of spin-1/2 particles can be combined to form one of three states of total spin 1 called the triplet, or a state of spin 0 which is called the singlet.

Refs: For more detailed explanation of terminology: Theoretical physics: Singlet state, doublet, triplet state From Wikipedia, the free encyclopedia In theoretical physics, or textbooks for quantum physics. 10 Ch8 An Introduction to Optical Atomic P.217 p. 217 - Line numbers and atomic numbers

Although correlation of atomic spectra with energy level diagrams for elements such as sodium and magnesium is relatively straightforward and amenable to theoretical interpretation, the same cannot be said for the heavier elements, particularly the transition metals. These species have a larger number of closely spaced energy levels; as a consequence, the number of absorption or emission lines can be enormous.

For example, a survey1 has listed the number of lines observed in the arc and spark spectra of neutral and singly ionized atoms for a variety of elements. For the alkali metals, this number ranges from 30 for lithium (Z=3), to 645 for cesium (Cs, Z=55); for the alkaline earths, magnesium has 173, calcium 662 [Z=20], and barium 472 [Z=56]. Typical of the transition series, on the other hand, are chromium, iron, and cerium [Ce, Z=58] with 2277, 2757, and 5755 lines, respectively.

Fewer lines are excited in lower-temperature atomizers, such as flames; still, the flame spectra of the transition metals are considerably more complex than the spectra of species with low atomic numbers.

11 Note that radiation-producing transitions shown in Figures 8-1 and 8-2 are observed only between certain of the energy states. For example, transitions from the 5s or the 4s to the 3s states do not occur, nor do transitions among the various p states (5p to 4p or 3p, etc) or several d states.

Such transitions are said to be “forbidden,” and there are selection rules that permit prediction of which transitions are likely to occur and which are not.

These rules are beyond the scope of this text. 12 Atomic Emission Spectra p. 218 At room temperature, essentially all of the atoms of a sample of matter are in the ground state. For example, the single outer electron of a sodium atom occupies the 3s orbital under these circumstances. Excitation of this electron to higher orbitals can be brought about by the heat of a flame, a plasma, or an electric arc or spark. The lifetime of the excited atom is brief, however, and its return to the ground state results in photon emission. The vertical lines in Figure 8-1a indicate some of the common electronic transitions that follow excitation of sodium atoms; the wavelength of the resulting radiation is also shown.

The two lines at 589.0 and 589.6 nm (5890 and 5896 Å) are the most intense and are responsible for the yellow color (0.589 um) that appears when sodium salts are introduced into a flame. 13 FIGURE 8-4 A portion of the flame emission  Note that the two smaller peaks: spectrum for sodium, 800 568.2nm and 568.82 nm “non- resonance emission lines” [4d to 3p- ppm in naphtha-isopropanol; 1/2 and 3p-3/2] emission, are not oxyhydrogen flame; slit 0.02 seen in the blue curve (which is for mm. Note that the scale is absorption). expanded in the upper trace

Similarly, peak at 500 nm (emission) and flame conditions were is not seen in blue curve (). changed to reveal greater detail for Na lines, but not for molecular bands. Note Note the emission lines (black curves) also that the lines at 589.00 Are broader than the blue lines. and 589.59 nm are off scale in the upper trace.

 568.2nm and 568.82 nm (smaller) are two “non- resonance lines” [4d to 3p] emission.

 Blue-color curve: Atomic absorption of Na. Note the differences between the flame emission and absorption of Na atom.14 Ch8 An Introduction to Optical Atomic Spectrometry P.218 In Fig. 8-4, The large lines at 589.00 and 589.59 (merged into one broader line) are emissions due to excited electrons returning from 3p-1/2 and 3p-3/2 back to 3s.

Emission lines at 568.2nm and 568.82 nm (smaller) are two “non-resonance lines” [from 4d to 3p] emission.

That is they do not return back to ground directly.

The energies from 4d to 3p is higher [2.3 eV] than from 3p to 3s [~2.2 eV].

Therefore, the lines are at lower wavelength.

15 16 Cont’d - Atomic emission spectra p. 218

Figure 8-4 shows a portion of a recorded emission spectrum for sodium. Excitation in this case resulted from spraying a solution of sodium chloride (NaCl) into an oxyhydrogen flame.

Note the very large peak at the far right, which is off scale and corresponds to the 3p to 3s transitions at 589.0 and 589.6 nm (5890 and 5896 Å) shown in Figure 8-1a. The resolving power of the monochromator used was insufficient to separate the peaks. These two lines are resonance lines, which result from transitions between an excited electronic state and the ground state. As shown in Figure 8-1 (electronic energy diagram), other resonance lines occur at 330.2 and 330.3 nm (3302 and 3303 Å) as well as at 285.30 and 285.28 (2853.0 and 2852.8 Å). The much smaller peak at about 570 nm (5682 and 5688 Å) in Figure 8-4 is in fact two unresolved non-resonance lines that arise from the two 4d to 3p transitions also shown in the energy level diagram. 17 Atomic Absorption Spectra In a hot gaseous medium, sodium atoms are capable of absorbing radiation of wavelengths characteristic of electronic transitions from the 3s state to higher excited states. For example, sharp absorption lines at 589.0, 589.6, 330.2, and 330.3 nm (5890, 5896, 3302, and 3303 Å) appear in the experimental spectrum. In Figure 8-1a (next pp), each adjacent pair of these peaks corresponds to transitions from the 3s level to the 3p and the 4p levels, respectively.

*Note that non-resonance absorption due to the 3p to 5s transition is so weak that it goes undetected because the number of sodium atoms in the 3p state is generally small at the temperature of a flame.

Thus, typically, an atomic absorption spectrum consists predominantly of resonance lines, which are the result of transitions from the ground state to upper levels. 18 FIGURE 8-1 Energy level diagrams for (a) atomic sodium and (b) magnesium(I) ion. Note the similarity of lines shown in blue but not in actual wavelengths (Å). [shown again here for discussions] 19 Ch8 An Introduction to Optical Atomic Spectrometry P.216 Atomic Fluorescence Spectra Atoms or ions in a flame fluoresce when they are irradiated with an intense source containing wavelengths that are absorbed by the element. The fluorescence spectrum is most conveniently measured at 90o to the light path. The observed radiation is most often the result of resonance fluorescence involving transitions from excited states returning to the ground state. For example, when magnesium atoms are exposed to an ultraviolet (UV) source, radiation of 285.2 nm (2852 Å) is absorbed as electrons are promoted from the 3s to the 3p level (see Figure 8-2); the resonance fluorescence emitted at this same wavelength may then be used for analysis. In contrast, when sodium atoms absorb radiation of wavelength 330.3 nm (3303 Å), electrons are promoted to the 4p state (see Figure 8-1a). A radiationless transition to the two 3p states takes place more rapidly than the fluorescence- producing transition to the ground state. As a result, the observed fluorescence occurs at 589.0 and 589.6 nm (589020 and 5896 Å). [i.e., non-resonance fluorescence] FIGURE 8-2 Energy level diagram for atomic magnesium. The width of the lines between states indicates the relative line intensities. Note that a singlet-to-triplet transition is considerably less probable than a singlet-to- 21 singlet transition. Wavelengths are presented in angstroms. [Repeated here again] Ch8 An Introduction to Optical Atomic Spectrometry P.217 FIGURE 8-1 Energy level diagrams for (a) atomic sodium and (b) magnesium(I) ion. Note the similarity of lines shown in blue but not in actual wavelengths (Å). [Repeated here again] 22 Ch8 An Introduction to Optical Atomic Spectrometry P.216 Figure 8-5 illustrates yet a third mechanism for atomic fluorescence (plus radiationless decay). Here, some of the thallium atoms [Tl, Z=81], excited in a flame, return to the ground state in two steps, including a fluorescence emission (non-resonance) step producing a line at 5350Å; radiationless deactivation to the ground state quickly follows. Resonance fluorescence at 3776 Å is also observed.

FIGURE 8-5 Energy level diagram for thallium showing the source of two fluorescence lines. 23 24 FIGURE 6-24 Partial energy-level diagrams for a fluorescent organic molecule.

[補充 From Chap. 6]

25 Ch6 An Introduction to Spectrometric Methods P.154 FIGURE 15-2 Partial energy-level diagram for a photoluminescent system. [補充 From Chap. 15] 26 Ch15 Molecular Luminescence Spectrometry P.401 8A-2 Atomic Line Widths p.219 The widths of atomic lines are of considerable importance in atomic spectroscopy. For example, narrow lines are highly desirable for both adsorption and emission work because they reduce the possibility of interference due to overlapping spectra. Furthermore, as will be shown later, line widths are of prime importance in the design of instruments for atomic absorption spectroscopy. For there reasons, it is worthwhile considering some of the variables that influence the width of atomic spectral lines.

FIGURE 8-6 Profile of an atomic line showing definition of the effective line width Δλ1/2.

27 FIGURE 8-7 Cause of Doppler broadening. 28 Ch8 An Introduction to Optical Atomic Spectrometry P.221 8A-3 Effect of Temperature on Atomic spectra p.221 Boltzmann equation:

• Nj/No=(gj/go) exp[-Ej/kT] (Eq. 8-1)

• where Nj and No are the number of atoms in an excited state and ground state, respectively. • K= Boltzmann const. (1.28X10-23 J/K), T (in K),

• Ej : energy difference between excited state and ground state.

• gj/go : statistical factor that are determined by the number of states having equal energy at each state.

• Next: Example 8-2

29 P. 222  Example 8-2. (effect of T on atomic emission) Example 8-2: Calc. the ratio of Na atoms in 3p excited state to those in ground state (3s). ------Boltzmann equation: Nj/No=(gj/go) exp[-Ej/kT] (Eq. 8-1)

k= Boltzmann constant, 1.38x10-23 J/K E= energy diff. between two states of transition (3p to 3s, for example). This energy is equal to the energy difference between 3p and 3s.

gj/go=statistic weight for 3s and 3p quantum (=2,6, respectively)

Atomic emission of wavelength λ=589.3nm (3p to 3s for Na) can be converted to wavenumber (wavenumber=1/λ in cm-1) and then to energy. (E=hν, h=1.986x10-23 Jcm-1).

• Calc’d. results: Nj/No = 0.000172 (2500K), and = 0.000179 (at 2510K)

Interpretation/discussion: •  At 2500K, Na atoms in excited state (3p) are only 0.017% of Na atoms in unexcited state (3s, ground). •  At 2510K, Na atoms in excited state (3p) are increased from 0.017% to 0.018% of Na atoms in unexcited state (3s, ground). 30 Cont’d Discussion: Example 8-2: A temperature increase by 10K (25002510K)

For Atomic Emission; •  10K increase results in 4% increase in the number of excited Na+ ion.

Thus, a temperature increase leads to increase in emitted power (in Atomic emission spectra).

 Therefore, analytical method based on measurement of emission requires close control of atomization temperature. 31 Temperature effect in AA and AF measurements: p. 222 • AA and AF methods are theoretically less dependant on temperature (than AE method), because both measurements are based on initially UNEXCITED atoms, rather than thermally EXCITED ones.

However, temperature actually does exert indirect influence on AA and AF in some ways. For examples: • Increase in T  increases the efficiency of atomization and total number of atoms in vapor.

• Temperature influences the degree of ionization of analyte. (see Ionization Equilibria, p.222) .

• Therefore, reasonable control of flame temperature is also required for AA and AF.

32 8A-4. Band and continuum spectra associated with atomic spectra. P.223 Generally, when atomic line spectra are generated, both band and continuum radiation are produced as well. Band spectra often appear while determining elements by atomic absorption and emission spectrometry. For examples, when solution calcium ions are atomized in a low- temperature flame, molecular absorption/emission bands for CaOH appear in regions of 554 nm (Figure 8-8). Band and continuum radiations are potential sources of interferences.

FIGURE 8-8 Molecular flame emission and flame absorption spectra for CaOH. Atomic emission wavelength of barium is also indicated. 33 B. Techniques and instrumentation

• In previous Part A of this chapter, focuses are placed on theories and general background of atomic spectroscopy.

• In following two parts (Part Band C), discussions are mainly on techniques of atomic spectroscopy: sample preparation, introduction, nebulization, atomization, etc.

34 8B - ATOMIZATION METHODS p. 223 In order to obtain both atomic optical and atomic mass spectra, the constituents of a sample must be converted to gaseous atoms or ionized atoms, which can then be determined by emission, absorption, fluorescence, or mass spectral measurements. As noted earlier, the process by which the sample is converted into an atomic vapor is called atomization. The precision and accuracy of atomic methods are critically dependent upon the atomization step and the method of introduction of the sample into atomization region. The common types of atomizers are listed in Table 8-1. Detailed descriptions of several of these devices are found in Chapters 9, 10, and 11.

35 Next page 36 8C - SAMPLE INTRODUCTION METHODS p.223

Sample introduction has been termed the “Achilles Heel” [i.e., the most fragile step] of atomic spectroscopy because in many cases this step limits the accuracy, the precision, and the detection limits atomic spectrometric measurements. The primary goal of a sample introduction system in atomic spectroscopy is to transfer a reproducible and representative portion of a sample into one of the atomizers list in Table 8-1 with high efficiency and with no adverse interference effects. The ease with which this goal is realized is strongly dependent on the physical and chemical state of the analyte and sample matrix. For solid samples or refractory materials, sample introduction usually is a problem; For solutions and gaseous samples, the introduction is often trivial. For this reason, most atomic spectroscopy studies are usually performed in solutions. 37 8C-1. Introduction of solution samples p. 224 Atomization devices fall into two classes: continuous atomizers and discrete atomizers.

With continuous atomizer, such as plasma or flames, samples are introduced in a steady manner. [Note: Our lab AA uses discrete atomizer by a syringe.] With discrete atomizer, samples are introduced in a discontinuous manner with devices such as syringe. The most common

discrete atomizer is electrothermal atomizer38 . Samples  solutions Samples for atomic spectrometric analysis are most commonly dissolved in an aqueous medium and then introduced into the atomizer by means of a nebulizer that converts the liquid to a fine mist, or aerosol.

Types of Nebulizers - see Table 8-2

39 40 Continuous sample introduction methods. General methods for introducing solution samples into plasma and flames are illustrated in Figure 8-9. Direct and continuous nebulization is most often used.

FIGURE 8-9 Continuous sample- introduction methods. Samples are frequently introduced into plasmas or flames by means of a nebulizer, which produces a mist or spray-Samples can be introduced directly to the nebulizer or by means of flow injection analysis (FIA; Chapter 33) or high-performance liquid chromatography (HPLC; Chapter 28). In some cases, samples are separately converted to a vapor by a vapor generator, such as a hydride generator or an 41 electrothermal vaporizer. Fig. 8-10 The complex process that must occur to produce free atoms or elementary ions are illustrated in Fig. 8-10.

FIGURE 8-10 Processes leading to atoms, molecules, and ions with continuous sample introduction into a plasma or flame. The solution sample is converted into a spray by the nebulizer. The high temperature of the flame or plasma causes the solvent to evaporate, leaving dry aerosol particles.

Further heating volatilizes the vapor particles, producing atomic, molecular, and ionic species. These species are often in equilibrium, at least in localized42 regions. Pneumatic Nebulizers p. 224 Fig. 8-11-a,b,c,d The most common kind of nebulizer is the Nebulizer concentric-tube pneumatic type, shown in Figure 8- Liquid aerosol, 11a, in which the liquid sample is sucked through a mist capillary tube by a high-pressure stream of gas (霧化) flowing around the tip of the tube (the Bernoulli effect). This process of liquid transport is called aspiration. The high-velocity gas breaks the liquid up into fine droplets of various sizes, which are then carried into the atomizer. Also employed are cross-flow nebulizers, illustrated in Figure 8-11b, in which the high-pressure gas flows across a capillary tip at right angles. Often in this type of nebulizers, the liquid is pumped through the capillary. Figure 8-11c is a schematic of a fritted-disk nebulizer in which the sample solution is pumped onto a fritted surface through which a carrier gas flows. This type of nebulizer produces a much finer aerosol than do the first two. Figure 8-11d shows a Babington nebulizer, which consists of a hollow 43 sphere in which a high-pressure gas is pumped th h ll ifi i h ’ f FIGURE 8-11 Types of pneumatic nebulizers.

44 Ch8 An Introduction to Optical Atomic Spectrometry P.226 The most common kind of nebulizer is the concentric-tube pneumatic type, shown in Figure 8-11a, in which the liquid sample is sucked through a capillary tube by a high-pressure stream of gas flowing around the tip of the tube (the Bernoulli effect). This process of liquid transport is called aspiration. The high-velocity gas breaks the liquid up into fine droplets of various sizes, which are then carried into the atomizer.

FIGURE 8-11(a) Concentric. 45 Ch8 An Introduction to Optical Atomic Spectrometry P.226 Also employed are cross- flow nebulizers, illustrated in Figure 8-11b, in which the high-pressure gas flows across a capillary tip at right angles. Often in this type of nebulizers, the liquid is pumped through the capillary.

FIGURE 8-11(b) Cross-flow. 46 Ch8 An Introduction to Optical Atomic Spectrometry P.226 Figure 8-11c is a schematic of a fritted-disk nebulizer in which the sample solution is pumped onto a fritted (partially fused) surface through which a carrier gas flows. This type of nebulizer produces a much finer aerosol than do the first two.

FIGURE 8-11(c) Fritted disk. (partially fused)

47 Ch8 An Introduction to Optical Atomic Spectrometry P.226 Figure 8-11d shows a Babington nebulizer, which consists of a hollow sphere in which a high-pressure gas is pumped through a small orifice in sphere’s surface.

FIGURE 8-11(d) Babington. 48 Ch8 An Introduction to Optical Atomic Spectrometry P.226 8C-2 Introduction of solid samples p. 226 The introduction of solids in the form of powders, metals, or particulates into plasma and flame atomizers has the advantages of avoiding the often tedious and time-consuming step of sample decomposition and dissolution. Such procedures, however, suffer from severe difficulties in calibration, precision, and accuracy.

The techniques (intro. of solid samples) include: • 1). Direct manual insertion of solids into the atomization devices, such as between electric arc electrodes. • 2). Electrothermal vaporization of the samples and transfer of the vapor into atomization region. • 3). Arc, spark, or laser ablation. • 4). Slurry nebulization (aerosol consisting of suspension of fine solids). • 5). Sputtering in glow-discharge devices. (to be shown in Fig. 8- 12)

None of these procedure yields results as satisfactory as those obtained by introducing samples solutions by nebulization. 49 Cont’d

• 1. Direct Sample Insertion Page 227/textbook - In the direct sample-insertion technique, the sample is physically placed in the atomizer. For solids, the sample may be ground into a powder, which is then placed on or in a probe that is inserted directly into the atomizer. - With electric arc and spark atomizers, metal samples are frequently introduced as one or both electrodes that are used to form the arc or spark.

• 2. Electrothermal Vaporizers Electrothermal vaporizers, which were described briefly in the previous section, are also used for various types of solid samples. The sample is heated conductively on or in a graphite or tantalum (Ta, Z=73) rod or boat. The vaporized sample is then carried into the atomizer by an inert carrier gas.

50 Cont’d 3 Arc and Spark Ablation

• Electrical discharges of various types are often used to introduce solid samples into atomizers. The discharge interacts with the surface of a solid sample and creates a plume of a particulate and vaporized sample that is then transported into the atomizer by the flow of an inert gas. This process of sample introduction is called ablation.

• For arc or spark ablation to be successful, the sample must be electrically conducting or it must be mixed with a conductor. Ablation is normally carried out in an inert atmosphere such as an argon gas stream. Depending on the nature of the sample, the resulting analytical signal may be discrete or continuous. Several instrument manufacturers market accessories for electric arc and spark ablation.

• Note: • arcs and sparks also atomize samples and excite the resulting atoms to generate emission spectra that are useful for analysis. • A spark also produces a significant number of ions that can be separated and analyzed by mass spectrometry (see Section 11D). 51 5. The glow-discharge technique (p. 227) A glow-discharge (GD) device is a versatile source that performs both sample introduction and sample atomization simultaneously (see Fig. 8-12). A glow discharge takes place in a low-pressure atmosphere (1 to 10 torr) of argon gas between a pair of electrodes maintained at 250V to 1000 V.

The applied potential causes the argon gas to break down into positively charged argon ions and electrons. The electric field accelerates the argon ions to the cathode surface that contains the sample. Ejection of neutral sample atoms occurs by a process called sputtering. The rate of sputtering may be as high as FIGURE 8-12 100 µg/min. A glow-discharge atomizer. 52 End of Chap. 8 -Fundamental of Atomic spectroscopy

Next class: Chap. 9 - AA (absorption) and AF (fluorescence) spectrometry

- Instrumentation and applications of atomic spectroscopy methods

[Note: Atomic Emission (Chap. 10) is not covered.] 53