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Atomic Emission Spectroscopy: A Tutorial Review*Anthony F. Lagalante a

a Chemical Science and Technology Laboratory, Physical and Chemical Properties Division, NationalInstitute of Standards and Technology, Boulder, CO, U.S.A.

Online publication date: 27 September 2004

To cite this Article Lagalante, Anthony F.(2004) 'Atomic Emission Spectroscopy: A Tutorial Review*', AppliedSpectroscopy Reviews, 34: 3, 191 207

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APPLIED SPECTROSCOPY REVIEWS, 34(3), 191207 (1999)

Atomic Emission Spectroscopy:

A Tutorial Review*

ANTHONY F. LAGALANTEChemical Science and Technology Laboratory

Physical and Chemical Properties Division

National Institute of Standards and Technology

Boulder, CO 80303

I. INTRODUCTION ........................................................................... 192

II. GENERAL DESCRIPTION ........................................................... 192

III. SOURCES ....................................................................................... 193

A. Flame Sources ......................................................................... 193

B. Inductively Coupled Plasma Sources...................................... 194

C. Microwave Plasma Sources .................................................... 196

D. DC Plasma Sources ................................................................. 197

E. DC Arc Sources....................................................................... 198

F. AC Arc Sources....................................................................... 199

G. High Voltage Spark Sources................................................... 200

H. Laser Microprobe Sources ...................................................... 201

I. Reduced Pressure Discharge Sources ..................................... 201

*Contribution of the U.S. Government. Not subject to copyright in the United

States.

Certain commercial instruments, materials, or equipment are identified in this

entry in order to specify adequately the experimental procedure or discussion. Such

identification does not imply recommendation or endorsement by the National Insti-

tute of Standards and Technology, nor does it imply that the materials or equipment

identified are necessarily the best available for this purpose.

191

Published by Marcel Dekker, Inc. www.dekker.com


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ATOMIC EMISSION SPECTROSCOPY 193

radiation that is produced is measured at discrete wavelengths by a high-resolution spectrometer. The source used in AES is typically much higher in

temperature (5000K to 10000K) than the source used in atomic absorption

spectroscopy (see the complementary review on atomic absorption spectro-

scopy in this journal issue). Consequently, both atomic and ionic excited

states of an element are produced, in contrast to the largely ground-state

atomic species produced by AAS. Although the resultant emission spectrum

is complicated due to the multitude of excited-state species produced, it con-

tains a wealth of atomic and ionic lines that can be used for quantitation.

The instrumentation required for AES (Figure 1) is similar to that re-

quired by AAS. The main difference between the two instruments is that

the thermal energy of the source produces the excited-state species in AES,

eliminating the need for an atomic line source to generate the excited-state

species. In addition, the resolution in AAS is determined by the atomic line

source; thus a monochromator of lower resolution can be used in AAS in

comparison to that required for AES.

III. SOURCES

A wide variety of sources exist for production of excited-state atoms

or ions in AES. These sources include flames, plasmas, arcs, sparks, and

lasers. Ideally, a source should be hot enough to atomize and excite as many

elements as possible, operate in a chemically inert environment, be able to

analyze solid, liquid, or gaseous samples, possess a low background, produce

reproducible, accurate, and precise results, and be easy to operate and main-

tain.

A. Flame Sources

Historically, the flame was the first source to be used with AES.

Flames are capable of producing temperatures ranging from 1500K to 3000K,

which, for atomization purposes, is a relatively low operating range. Al-

though the lowtemperature flame is not an ideal source for AES, it is possible

to use a flame-AAS instrument operated in the AES mode by simply not

FIG. 1. The essential components of an atomic emission spectrophotometer.
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194 LAGALANTE

using the hollow cathode line source. Typical flame oxidant gases include air,oxygen, and nitrous oxide, while fuel gases include hydrogen and acetylene.

Consequently, the number of excited-state species generated in the flame is

low, producing low sensitivities. These temperature limitations make the

flame suitable for analysis of easily excitable elements, such as the alkali

metals.

Flame gases can be either premixed before the burner head or be mixed

in the burner head. Premixing the fuel and oxidant gases produces a laminar

flame. Burners, which operate by premixing the fuel and oxidant gases, in-

clude the Meker, Alkemade, single slot, and multi-slot burners. As in flame-

AAS, the sample to be combusted must first be dissolved in a solvent for

subsequent nebulization into these burners. The efficiency of the nebulizer

will affect the amount of sample available for combustion, and ultimately

the sensitivity of flame-AES. To circumvent the nebulizer efficiency factor,

the total-consumption burner can be used to deliver the entire sample to the

flame. The fuel and oxidant gases of the total-consumption burner are not

premixed and, consequentially, a turbulent flame is produced. The turbulent

flame, in addition to the excess of solvent accompanying 100% of the sample

delivery, produces a very inhomogeneous flame with a high noise level.

In addition to nebulization efficiency, many of the same considerations

of flame AAS must be taken into account when operating a flame-AES. The

background emission from a flame is high by AES standards. This back-

ground emission can be attributed to both the band spectra produced by vari-

ous molecular fragments and radicals, such as OH O2, CH, CO, CN, CH,

and NH, and the continuum spectra produced by blackbody radiators and

chemiluminescent reactions. Like flame-AAS, the highly reactive speciesgenerated in a flame can result in chemical interferences that lower the sensi-

tivity.

B. Inductively Coupled Plasma Sources

Of all the sources to be discussed, the one that comes closest to achiev-

ing all of the ideal source characteristics is the inductively coupled plasma

(ICP), and therefore it is the source most commonly used in AES today.

The ICP emission source was co-invented by Fassel (USA) and Greenfield

(England) during the 1960s. Due to the popularity of AAS at that time, its

potential in AES was not fully realized until the 1970s. The ICP (Figure 2)

uses a high-frequency generator that supplies 1 to 5 kW of power to an

induction coil that is wrapped about a 15 to 30 mm diameter quartz tube. Aspark from a Tesla coil provides seed electrons into the argon support gas (1

L min1

flow rate) to initiate the plasma. Under the influence of the magnetic

field produced by the induction coil, the seed electrons will collide with the
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FIG. 2. The inductively coupled plasma source.

argon gas to produce the hot, partially ionized argon gas plasma. The ICP

can achieve temperatures as high as 9000K to 10000 K, and consequentially

a second coolant gas stream of argon (10 L min1

flow) must be directed

around the quartz tube to prevent it from melting. As in flame-AES, the

sample to be analyzed by ICP-AES must first be dissolved in a solvent and

nebulized into the argon support gas. Typical nebulizers used include thepneumatic and cross-flow nebulizers.

The radio-frequency generator typically operates over a range of 3 to

100 MHz, and is usually set to 27 MHz of higher to take advantage of the
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196 LAGALANTE

skin effect of the plasma. Below an operating level of 27 MHz, eddy cur-rents reside in the center of the plasma and the nebulized solution preferen-

tially flows to the cooler outer edges of the plasma. This effectively dilutes

the analyte species in the observation zone of the plasma, decreasing the

analytical sensitivity. Furthermore, the cooler regions of the plasma are

higher in ground state species that are available for self-absorption. At and

above 27 MHz, these eddy currents move outward toward the skin or edges

of the plasma, creating a torroid that has a cooler center (7000 to 8000 K).

It is through this central cooler region that the nebulized solution preferen-

tially flows. Therefore, the skin effect effectively concentrates the sample

in the narrow central portion of the plasma, resulting in increased sensitivity.

This also results in a narrower emission profile that minimizes self-absorp-

tion and further increases the sensitivity.

The hottest portion of the plasma is the plasma core, which is located

inside the induction coil. Complete vaporization and atomization is achieved

in the 2 to 3 millisecond residence time of the sample in the plasma. The

hottest zone extends only a few millimeters above the top of the induction

coil. To achieve a compromise between the signal and background emission,

the observation zone of the ICP is placed at a height of 1 to 3 cm above the

induction coil. In the observation zone, the only background emission is due

to argon line emission and the OH band emission. The chemically inert envi-

ronment of the plasma results in a long lifetime for the excited state species

that are produced. Within the observation zone, there is a temperature gradi-

ent and consequently emission intensities for a particular element vary along

the observation zone. Therefore, the observation height for the element of

interest can be optimized. Above the observation zone of the plasma is thecooler, flame-like zone that is suitable as an observation zone for the easily

excited elements, such as the alkali metals. The high temperatures generated

by the plasma are capable of producing ionized excited-state species. Emis-

sion due to ionized species is typically more intense than ground state atomic

emission. Typically, parts-per-billion (where l part-per-billion is 107

percent)

detection limits with a linear range of five orders of magnitude are obtainable

using ICPAES. Although the ICP approaches an ideal source for AES, the

primary disadvantages of an ICP-AES are that it is expensive to purchase,

the plasma consumes large quantities of relatively expensive argon gas, and

that the instrument cannot directly analyze solid samples.

C. Microwave Plasma Sources

Another plasma source that has been used, as a source in AES, is the

microwave induced plasma (MIP). Similar to the ICP, the MIP is initiated

by seed electrons from a Tesla coil discharge. The seed electrons collide with
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the support gas in a microwave cavity that is commonly operated at 2450MHz. Nitrogen, argon, or helium have been used as support gases for the

plasma. Operating power for the MIP can range from 25 to 250 W.

Sample introduction into the MIP is again accomplished through nebu-

lization into the supply gas for the plasma. However, the low power levels

used in the MIP are not effective for solvent evaporation. Consequentially,

complicated sample introduction techniques that will desolvate and vaporize

the sample prior to introduction into the plasma must be performed. For the

plasma to exist, the coupling element of the microwave cavity must be tuned

to match the system impedance. The system impedance can be easily per-

turbed during sample introduction into the plasma. For example, large nebu-

lized volumes of sample and solvent readily disrupt the system impedance

and as a result the plasma may be extinguished.

Greater plasma stability is obtainable using a capacitively-coupled mi-

crowave plasma. However, the cost advantage of MIP is not realized due to

the higher power requirements (200 to 1000 W) of the capacitively-coupled

system. The temperatures generated in the MIP are lower than those of the

ICP and range between 4000K to 5000K. Microwave plasmas have low back-

ground emission due to the inert gases used, however, the lower temperature

of the plasma can make chemical interferences troublesome.

Sample introduction techniques remain the greatest hindrance to the

use of microwave plasmas in AES. However, the same sample introduction

limitations associated with the MIP as a source for AES, allow the MIP-AES

to function as an ideal element selective detector in gas chromatography.

In gas chromatography, the sample to be analyzed has already undergone

desolvation and vaporization and is ideally suited to undergo combustion inthe MIP. Selective detection of H. C, N. P. B. and O has been accomplished

by the interfacing of a MIP-AES to the exit of the chromatographic column.

D. DC Plasma Sources

The three-electrode dc plasma (Fig. 3) uses two carbon anode elec-

trodes and a tungsten cathode in an inverted Y configuration. The support

gas for the plasma is argon (8 L min1

flow rate). The plasma is initiated by

moving the electrodes together, and once ignited requires very low power

levels (


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198 LAGALANTE

FIG. 3. The do plasma source. (Reproduction courtesy of Beckman Instru-

ments)

The temperature in the excitation region of the plasma is approximately

6000K and offers detection limits similar to that of the ICP. Like the ICP,

the observation region and the main plasma are spatially separated. The dc

plasma suffers from similar disadvantages to those of other plasma sources.

The dc plasma can only be used to analyze samples that have been previouslydissolved in a solvent, and it is relatively expensive to operate due to its

consumption of large volumes of argon gas. A major disadvantage of the dc

plasma is that the high temperature of the plasma physically deforms the

electrodes. Therefore, it is necessary to reshape the electrodes after only ap-

proximately two hours of use.

E. DC Arc Sources

The various plasma sources discussed provide many advantages for the

analysis of liquid samples. However, it is the arc and spark sources that are

predominantly used for the analysis of solids. Since the 1920s, the dc arc has

been used extensively as a qualitative method of trace-elemental analysis for

metallurgical samples. A continuous high-current discharge (1 to 30 A) atlow voltage (10 to 25 V) is formed between the two metal or graphite elec-

trodes under atmospheric conditions. Graphite is the electrode material that

is most commonly used because of its inertness, high-purity, and refractory
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properties. Like the dc plasma, the arc can be initiated by physically movingthe electrode gap together, or by a low-current spark discharge. Following

initiation, the arc plasma is maintained by ionization of the material between

the gap.

In a dc arc source manufactured in the United States, the anode is used

to hold the sample and the cathode is the counter electrode, whereas in a

European instrument the cathode is the electrode that holds the sample while

the anode is the counter electrode. The solid sample to be analyzed can be

mixed with graphite and placed in the electrode cup in the case of powders,

or evaporated to dryness in the cup in the case of dissolved samples. In the

analysis of conductive solids, the electrode can be machined directly from

the sample to be analyzed. The resulting arc discharge generates temperatures

from 3000K to 8000K along the distance of the arc, with the highest tempera-

tures generated at the sample surface. The high temperatures at the electrode

surface vaporize the sample, which then subsequently undergoes collisional

excitation in the discharged plasma.

The background emission accompanying dc arcs is severe. Both band

and continuum emission from species in the atmospheric plasma is prevalent.

To eliminate the troublesome CN (cyanogen) band emission accompanying

atmospheric operation, the dc arc can be operated in an argon or helium

atmosphere. An additional source of background radiation is due to the

heated electrodes, which emit as blackbody radiators. Self-absorption is also

troublesome in dc arc sources due to the ground state species residing in the

outer, cooler portions of the arc. Since the dc arc source slowly wanders

about the electrode surface, only a small portion of the entire sample is vapor-

ized at a single instant. This slow random sampling of different areas of thesample may produce errors in the results, often because the analyte concen-

tration at the surface of a solid sample is not homogeneous. However, the

high-detection limits of the dc arc have popularized the technique as a quali-

tative method of analysis. Today, the dc arc is used to qualitatively identify

approximately 70 elements in solid samples ranging from geological ores to

biological solids.

F. AC Arc Sources

Two types of ac arcs can be used as a source for AES. The high-

voltage ac arc operates from 2000 to 4000V, while the low-voltage ac arc

operates from 100 to 400V. Both of the ac arcs self-extinguish at every half-

cycle due to the change in the voltage polarity. The high-voltage ac arc self-ignites itself every half-cycle when the applied voltage exceeds the dielectric

breakdown voltage of the gas between the two electrodes. The low-voltage

ac arc is re-ignited by a low-current spark every half-cycle. An advantage of
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200 LAGALANTE

the voltage polarity change at every half-cycle of the ac arc is that bothanodic and cathodic sampling is possible. Using a 60 Hz source, the ac arc

re-ignites 120 times per second, allowing the ac arc to rapidly sample differ-

ent portions of the sample surface. This higher sampling rate gives the ac arc

better precision than the slow wandering sampling of the dc arc. However,

the self-extinguishing and re-ignition lowers the arc temperature, making the

ac arc less sensitive than the dc arc source. The lower temperatures generated,

however, do extend the lifetime of the electrode material.

One solution to the compromises evident between ac and dc arcs is to

use an intermittent ac arc. The intermittent ac arc does not change voltage

polarity and samples the anode on both half-cycles of the voltage wave. The

intermittent ac arc produces the sensitivity advantages of the dc arc, with the

increased precision advantages of the ac arc.

G. High Voltage Spark Sources

Similar to the ac arc source, the high-voltage ac spark source randomly

samples the electrode surface, resulting in higher precision. As in the other

electrical discharge sources, electrodes can be machined from conducting

samples, or mixed with graphite and pressed into pellets for powdered sam-

ples. The firing of the spark is regulated either by a capacitively-controlled

discharge (voltage-thresholded spark) or by a rotary gap (time-thresholded

spark). The gap breakdown occurs at a rate of hundreds of times per second.

The high-current levels of the discharge generate high temperatures from a

spark source. The high temperatures allow metals to undergo single and mul-tiple ionization in spark sources to produce an emission spectrum that is quite

complex. This high extent of ionization decreases the sensitivity in compari-

son to a single atomic emission line and reduces the overall sensitivity of the

measurement for a particular element. Time-resolved emission spectroscopy

is possible with the spark source because the emission due to atomic and

ionic species will vary over the duration of the spark. By maximizing the

signal-to-background emission intensity, in time one can alleviate some of

the background problems that plague electrical-discharge sources.

Additionally, both arc and spark sources, or electrothermal atomizers

(see the review on AAS), can be used as a sample introduction technique for

ICP-AES. Since ICP is not suited for analysis of solids, arc and spark sources

can be used for the sole purpose of vaporizing a solid into the support gas

stream of an ICP source. The spark source has also been used as a gas chro-matography detector to provide element selective detection of C, B. N. O.

and S.
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H. Laser Microprobe Sources

A high-powered pulsed laser such as the ruby or Nd:YAG laser can be

used as a source for the volatilization and excitation of samples for AES.

The laser is focused on a spot 5 to 50 m in diameter on the sample. A vaporof atoms, ions, and molecular species is produced when the laser discharges.

As the vapor passes through the electrode gap of two graphite electrodes, a

spark will ignite. The energy from the spark provides the energy to excite

the atoms and ions of the sample. The laser can be focused on select portions

of a sample for spatial analysis, and the region of the sample that is vaporized

is so infinitesimally small that the technique can be considered non-destruc-

tive.

I. Reduced Pressure Discharge Sources

The hollow cathode lamp (HCL), which is used primarily as a line

source in AAS, also can be used as an AES source. The element selectivity

of the HCL is achieved by fabricating the cathode, or coating the interior of

the cathode, with the element of interest. The removable cathode HCL design

is most suited to function as a source in AES. The cathode is machined from

conductive solid samples, or can be pressed into a pellet in the case of pow-

dered samples. As in the HCL, the Pyrex cylinder containing the anode and

cathode is back-filled with an inert fill gas (Ar, Ne) at reduced pressures. At

low currents (10 to 30 mA), the fill gas is ionized and accelerated toward the

cathode surface, where upon contact with the cathode, the accelerated gas

atoms sputter atoms off the cathode into the vapor phase. These atoms un-dergo further collisions with other accelerated fill-gas ions, and are elevated

to their excited-state configurations. Upon relaxation back to the ground

state, the sample atoms emit radiation at their characteristic atomic lines. The

HCL is operated at low temperatures, and therefore narrow atomic lines are

produced. The electrical discharge source is primarily suited for elements

with high vapor pressures in matrices of low vapor pressure.

IV. INSTRUMENTATION

The key to the selectivity of AES is to have a monochromator of suffi-

cient resolution to separate closely spaced elemental emission lines and the

ability of the monochromator to separate these emission lines from the back-ground emission. The entrance optics of the monochromator selects a solid

angle element from the emission source and focuses a maximum amount of
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(Figure 4). In the Czerny-Turner configuration a single concave mirror func-tions as both the collimating and final imaging mirror, whereas in the Ebert

mounting, two independent mirrors are used. Both configurations utilize pla-

nar ruled or replicated gratings that are commonly rotated under computer

control. Additionally, the spectral lines that are dispersed are true images of

the slit (stigmatic); therefore, the intensities of these lines are independent of

FIG. 4. The (a) Czerny-Turner and (b) Ebert mounting plane grating configu-

rations.
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204 LAGALANTE

the slit height. In these configurations the gratings are typically used in first-order dispersion to avoid the large rotation angles of the grating. The design

of the monochromator makes the spectrometer compact due to the position-

ing of the slits and detector placement on the same side of the instrument.

Overall resolution of the instrument depends on several instrumental vari-

ables. Both configurations are capable of providing the high-resolution (


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FIG. 5. The Paschen-Runge mounting.

C. Echelle Gratings

The Echelle grating configuration is accomplished by placing a prism

before an Echelle grating using the Czerny-Turner mounting. The Echelle

grating configuration is capable of maximizing the intensity of the separated

incident radiation at the optimum order. This is accomplished by using a

prism to order separate the radiation and the Echelle grating to wavelength

separate the radiation. The result is a compact two-dimensional array on

which exit slits can be positioned to measure the emitted radiation. The sensi-

tivity of the instrument is increased because the blaze angle for each elementis maximized in a shorter focal length. When using an Echelle grating, the

slit height must be kept small (1 mm) to minimize the overlap of radiation

of different orders, which reduces the sensitivity some. The Echelle spec-
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206 LAGALANTE

trometer eliminates spectral interferences due to line overlap by non-analyteatoms through order separation.

D. Detectors

The first detectors in AES were those using photographic film. Follow-

ing dispersion of the emitted radiation from the source, the spectral lines

were recorded on photographic film. The photograph was typically compared

to a standard photograph for a given element to qualitatively ascertain the

existence of a particular element. Although semi-quantitative analysis is pos-

sible using photographic film, through line-to-background ratios using a den-

sitometer, the procedure is tedious compared to other quantitative techniques.

Photographic detection was used, however, in qualitative analysis such as

when samples were analyzed using arc and spark sources.Photomultiplier tubes have been commonly used as detectors for AES.

The two main sources of noise in AES arise from the source flicker and the

dark current from the photomultiplier tube. Generally, the entrance slit is

narrowed until the noise from the photomultiplier tube is the dominant source

of noise. The signal-to-noise ratio of the element of interest can be enhanced

by modulating the source and through signal averaging of the response from

the photomultiplier tube.

Many modern instruments now feature array-based detector systems.

In most cases these minimize or eliminate the use of scanning devices or even

the use of conventional-style monochromators. The use of arrays provides a

means to record the entire spectrum in a very short time, and it provides the

means to produce a complete output of an entire spectral region. In such

arrangements it is possible to have an accurate accounting of the backgroundeffects, although the impact of stray light and its rejection can be an issue.

Several types of array devices are available that are suitable for emission

spectrometry, these include linear photodiode arrays, charge-coupled device

(CCD) detectors and charge injected device (CID) detectors. All have been

used in various commercial instruments. At issue is often how to provide

adequate spectral resolution for separation of the atomic lines while provid-

ing an adequate spectral range. Sometimes compromises have to be made

relative to range and/or spectral resolution. Various designs of instruments

have been designed that include either limited scanning (for range selection),

custom-designed detectors or the use of special grating designs that provide

two dimensional imaging on a two dimensional CCD array.

V. PERFORMANCE

In the case of the quantitative analysis of solutions, standardization of

an unknown sample by AES is accomplished by creating a calibration curve.
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The Beer-Lambert law is used to directly relate emission intensity to theconcentration of an element in a standard solution. The measured emission

intensity is dependent on atomizer efficiencies, instrumental variables, and

the source excitation temperature. In flame-AES, the calibration curve spans

2 to 3 orders of magnitude in sensitivity, whereas in plasma-AES the calibra-

tion curve spans 4 to 5 orders of magnitude in sensitivity. Care must be

taken to ensure that the unknown solution lies within the linear region of the

calibration curve. Atomic emission calibration curves are non-linear at higher

concentrations due to self-absorption by atoms in cooler parts of the source.

If one is working in a non-linear portion of the calibration curve, the solution

may be diluted until it is within the linear region. In the case of solids analy-

sis by AES, standardization may be accomplished by the use of standard

reference materials that are available from the National Institute of Standards

and Technology. As in AAS, care must be taken to ensure that vaporization

and excitation conditions are similar between unknowns and standards and

that the impact of interferences is minimized.

BIBLIOGRAPHY

Ingle, J. D., Crouch, S. R. Spectrochemical Analysis, Prentice Hall, New Jersey, 1988.

Mika, J., Torok, T. Analytical Emission Spectroscopy, Crane, Russak and Company,

New York, 1974.

Robinson, J. W. Atomic Spectroscopy, Marcel Dekker, New York, 1996.

Zander, A. T., Atomic Emission Sources for Solution Spectrochemistry, Anal.

Chem. 1986, 58, 1139A.

Varnes, A.W., Inductively Coupled Plasma Atomic Emission Spectroscopy, Hand-book of Instrumental Techniques for Analytical Chemistry, ed. Frank Settle,

pp. 395418, Prentice Hall PTR, NJ, 1997.

Dean, J.A., Atomic Spectroscopy, Analytical Chemistry Handbook, McGraw Hill,

NY, 1995.

Kellner, R.A., Mermet, J.-M., Otto, M., and Widmer, H.M. Eds., Elemental Analy-

sis: Atomic Emission Spectrometry, Analytical Chemistry, pp. 435452, Wi-

ley-VCH Verlag GmbH, Weinheim, 1998.

Hareland, W.A., Atomic Emission Spectroscopy, Analytical Instrumentation Hand-

book, 2nd

Edition, Galen Wood Ewing Ed., pp. 221256, Marcel Dekker, NY,

1997.
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Artigo 2_ Emissão atômica

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