ASR-200054372

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This article was downloaded by: [Suleyman Demirel Universitesi] On: 24 November 2011, At: 02:09 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK Applied Spectroscopy Reviews Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/laps20 Recent Advances in Microwave Induced Plasma Atomic Emission Spectrometry with OkamotoCavity Kazuaki Wagatsuma a a Institute for Materials Research, Tohoku University, Sendai, Japan Available online: 19 Aug 2006 To cite this article: Kazuaki Wagatsuma (2005): Recent Advances in Microwave Induced Plasma Atomic Emission Spectrometry with OkamotoCavity, Applied Spectroscopy Reviews, 40:3, 229-243 To link to this article: http://dx.doi.org/10.1081/ASR-200054372 PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: http://www.tandfonline.com/page/terms- and-conditions This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae, and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand, or costs or damages

Transcript of ASR-200054372

Page 1: ASR-200054372

This article was downloaded by: [Suleyman Demirel Universitesi]On: 24 November 2011, At: 02:09Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,UK

Applied Spectroscopy ReviewsPublication details, including instructions forauthors and subscription information:http://www.tandfonline.com/loi/laps20

Recent Advances in MicrowaveInduced Plasma AtomicEmission Spectrometry withOkamoto‐CavityKazuaki Wagatsuma aa Institute for Materials Research, Tohoku University,Sendai, Japan

Available online: 19 Aug 2006

To cite this article: Kazuaki Wagatsuma (2005): Recent Advances in MicrowaveInduced Plasma Atomic Emission Spectrometry with Okamoto‐Cavity, AppliedSpectroscopy Reviews, 40:3, 229-243

To link to this article: http://dx.doi.org/10.1081/ASR-200054372

PLEASE SCROLL DOWN FOR ARTICLE

Full terms and conditions of use: http://www.tandfonline.com/page/terms-and-conditions

This article may be used for research, teaching, and private study purposes.Any substantial or systematic reproduction, redistribution, reselling, loan,sub-licensing, systematic supply, or distribution in any form to anyone isexpressly forbidden.

The publisher does not give any warranty express or implied or make anyrepresentation that the contents will be complete or accurate or up todate. The accuracy of any instructions, formulae, and drug doses should beindependently verified with primary sources. The publisher shall not be liablefor any loss, actions, claims, proceedings, demand, or costs or damages

Page 2: ASR-200054372

whatsoever or howsoever caused arising directly or indirectly in connectionwith or arising out of the use of this material.

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Recent Advances in Microwave InducedPlasma Atomic Emission Spectrometry

with Okamoto-Cavity

Kazuaki Wagatsuma

Institute for Materials Research, Tohoku University, Sendai, Japan

Abstract: Microwave induced plasmas with an Okamoto-cavity (Okamoto-cavity

MIP) are noted as a new excitation source in atomic emission spectrometry. The

Okamoto-cavity MIP can be sustained with various plasma gases, and can produce

each stable plasma with a high robustness against loading of various types of

samples. For example, the oxygen-containing MIP becomes an effective atomization

and excitation source for direct injection of organic solvents analysis because they

are completely burned in the plasma. In this review, the fundamental structure of the

microwave cavity, the spectrochemical characteristics, and the analytical applications

are summarized from reference papers.

Keywords: Microwave induced plasma, Okamoto cavity, atomic emission spec-

trometry, plasma gas, detection limit, excitation temperature, loading of organic

solvent

INTRODUCTION

Microwave induced plasmas (MIPs) have been known as an excitation source

in atomic emission spectrometry; however, the analytical application appears

only in particular fields such as a detector for gas chromatography (1). The

major reason for this is that conventional MIPs produced by using a

Beenaker cavity (2) or a Surfatron (3) have a limited tolerance to aqueous

aerosol introduction, because such MIPs should be maintained at relatively

Received 25 November 2004, Accepted 2 December 2004

Address correspondence to Kazuaki Wagatsuma, Institute for Materials Research,

Tohoku University, Katahira 2-1-1, Sendai 980-8577, Japan. E-mail: wagatuma@

imr.tohoku.ac.jp

Applied Spectoscopy Reviews, 40: 229–243, 2005

Copyright # Taylor & Francis, Inc.

ISSN 0570-4928 print/1520-569X online

DOI: 10.1081/ASR-200054372

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low powers (less than 500 W); otherwise, a serious damage of the torch

material may occur. In addition, the conventional MIPs cannot produce an

annular-shaped plasma like an inductively coupled plasma (ICP) because

they have the electric-field distribution yielding the maximum at the center

of their plasmas. These drawbacks have hindered the extensive applications

of MIPs in spite of their specific excitation ability as well as the low-

running cost.

In 1985, Jin et al. reported on a newly designed MIP which allows for

direct introduction of aqueous samples (4). His research group has made

apparatus comprising the MIP torch and a spectrometer for atomic emission

spectrometry (5, 6). In 1990, Okamoto developed a surface-wave-excited

non-resonant cavity for MIP (7). This cavity (Okamoto cavity) (7, 8) has

been expected as a new excitation source in MIP spectrometry because of

the unique feature beyond conventional MIPs. It produces a annular flame-

like plasma where the electric field can be concentrated at the edge of the

plasma, which enables sample aerosols to be easily introduced through the

center channel, as in the case of ICP (7, 8). Furthermore, the plasma can be

maintained at high powers up to 1.5 kW, making this plasma source highly

tolerant to wet aerosol loading. In addition, the Okamoto cavity can be

sustained with various gases, such as nitrogen, air, and helium, without any

modification of the torch design as well as the power supply system (9–13).

The use of nitrogen plasma gas enables the Okamoto cavity to be employed

as an ionization source in mass spectrometry due to its low mass interferences

(14, 15). The nitrogen and helium plasmas with the Okamoto cavity have been

studied also in atomic emission spectrometry. Several researchers have

reported on the excitation characteristics and the analytical performance of

these plasmas, indicating that MIPs with the Okamoto cavity have the

ability enough to be employed for practical applications (10–13,16–26).

These studies are introduced in this review.

PRINCIPLES OF OKAMOTO CAVITY

The apparatus and the basic principle of operation has been described in detail

by Okamoto (7, 8). A brief explanation is described and is cited from

Okamoto’s publications (8).

Figure 1 shows a schematic diagram of the Okamoto cavity comprising a

surface-wave exciter and a discharge tube. The surface-wave exciter consists

of a wave-guide and a mode transformer. The former part works also as an

impedance matching circuit between the microwave source and the

resulting impedance of the surface wave exciter, by reducing the area of the

wave-guide joining to the mode transformer. The latter part consists of an

inner conductor and an outer cylindrical conductor terminated by a front

plate. The surface wave is excited in the gap between the inner and the

outer conductors, where the electric field becomes the maximum and can be

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coupled with the plasma gas the most efficiently, which enables an annular-

shaped plasma like ICP to be produced. Microwave power is transferred

from a microwave generator running at a frequency of 2.45 GHz to the

surface wave exciter, through a uni-line, a directional coupler, and a three-

stub tuner. Matching in their impedances can be adjusted by using the

three-stub tuner.

The plasma torch comprises two concentric quartz tubes, and has two

paths of the gas flow: one is a tangentially-introduced gas flow which

sustains the plasma itself and the other is introduced through the inner tube

to carry the sample aerosol to the plasma. The detailed design of the torch

was described in Okamoto et al. (7). By selecting the flow rates and the

microwave power appropriately, the resulting plasma can be maintained

without any external cooling. The operating conditions for obtaining the

stable plasmas strongly depend on the kind of the plasma gas (9–13).

Nitrogen or air plasmas are produced by igniting the plasma with argon

under impedance matching conditions best suited to the nitrogen or the air

discharge. The argon plasma can be induced by sparks with a Tesla coil.

The argon flow is replaced by nitrogen or air immediately after the production

of the argon plasma, and then an annular-shaped stable plasma can be

produced at atmospheric-pressure without touching the wall of the outer

tube. Once the stable plasma is formed, the gas flow rates as well as the

microwave power can be varied over relatively wide ranges. In the case of

the nitrogen plasma, the flow rate of the plasma gas can be varied at the

plasma gas flow of 6–15 L/min when the microwave power ranges from

Figure 1. Cross section of the surface-wave exciter and the discharge tube of

Okamoto-cavity. See Okamoto (8).

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600 to 1000 W. It is possible to optimize the experimental conditions for each

sample as well as each analytical line among these discharge parameters.

CHARACTERISTICS OF OKAMOTO-CAVITY MIP

Excitation Temperature

Okamoto and his co-workers measured the excitation temperature, the rotation

temperature, and the electron number density in the nitrogen MIP (9). The

excitation temperature was estimated from the Boltzmann plot using

Fe I lines (370-nm lines), the rotation temperature was determined from the

band intensity of nitrogen molecule ion at 391.4 nm, and the electron

number density was from the intensity ratio of the Fe I line (252.3 nm) to

the Fe II line (258.6 nm) having a similar excitation energy. Figure 2 shows

variations in the excitation temperature as well as the electron number

density as a function of the microwave power supplied. Both of them

increase with increasing applied microwave power, if other operating con-

ditions are kept constant. At the power of 1.0 kW, the excitation temperature,

the rotation temperature, and the electron number density was estimated to be

5500 K, 5000 K, and 3 � 1013 cm23, respectively. The authors concluded that

the nitrogen MIP was in a local thermodynamic equilibrium (LTE), because

the excitation temperature is similar to the rotation temperature. They also

pointed out that conventional helium MIPs give a large difference between

the excitation and the rotation temperatures due to their non-LTE

Figure 2. Variations in the excitation temperature and the electron number density as

a function of the microwave power. Plasma gas: pure nitrogen; flow rate of the plasma

gas: 10.5 L/min; carrier gas flow rate: 0.62 L/min; observation height: 7.5 mm. See

Ogura et al. (9).

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characteristics. Okamoto et al. also reported on excitation temperatures of the

Okamoto-cavity MIP using helium and argon: 4900 K for helium and 5500 K

for argon measured under each optimum discharge condition (11).

Zhang and Wagatsuma (19) published on a comparison in the excitation

temperature between the pure nitrogen and the air Okamoto-cavity MIPs, indi-

cating that the excitation temperature of the air-MIP was in the range of 4150–

4750 K when the microwave power was varied from 0.8 to 1.3 kW. The values

were 300–400 K lower than those of the pure nitrogen MIP, which contributed

to the analytical performance of each plasma. Maeda and Wagatsuma (13)

reported on a variation in the excitation temperature when the oxygen

content was varied from 0 to 28 vol.% in nitrogen-oxygen Okamoto-cavity

MIP. As shown in Figure 3, the excitation temperature is reduced with increas-

ing oxygen content. This effect is probably due to the difference in the ioniz-

ation energy and the dissociation energy between nitrogen and oxygen

molecules.

Background Spectra

Okamoto (8) and Ohata (20) published background spectra of the nitrogen

Okamoto-cavity MIP, comprising N2 bands, N2þ bands, and NH band. It

should be noted that the background from these species appears over the wave-

length range of 300–420 nm; however, the OH bands (306 nm system) are very

weak compared to in argon-ICP. Although the nitrogen-oxygen MIP has more

intense backgrounds, such plasma is effective for removing bands originated

from carbon-containing species when introducing organic solvents (10).

Figure 3. Dependence of the excitation temperature on the oxygen content in the

nitrogen-oxygen mixed gas plasma at the microwave power of 1.0 kW. Flow rate of

the plasma gas: 14 L/min; carrier gas flow rate: 0.6 L/min; observation height:

7 mm. See Maeda and Wagatsuma (13).

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Interference with Easily-Ionizable Elements

Zhang and Wagatsuma (19) investigated the matrix effects of sodium,

calcium, and nitric acid on the emission intensities of a large number of

atomic and ionic lines over a wide rage of excitation energy in the nitrogen

Okamoto-cavity MIP. Figure 4 indicates the variations in the net intensities

of 20 spectral lines when adding 5 mg/mL sodium. Each emission intensity

without sodium is normalized as unity. Most of the atomic lines are

enhanced in the presence of sodium, whereas all of the ionic lines tested are

reduced. These effects decrease with increasing microwave power. The

authors also found that the degree of the matrix effect was strongly

dependent on the excitation energy of the atomic lines as well as the energy

sum (the excitation energyþ the ionization potential) of the ionic lines.

Larger intensity enhancement was observed in the atomic lines having

lower excitation energy, while larger intensity reduction was observed in

the ionic lines having larger energy sum. These phenomena can be

explained from the change in the ionization equilibrium in the MIP which

results from a dominant ionization of sodium. This shows similar behaviour

as the result obtained not in ICP but in flame emission spectrometry.

Figure 4. Effect of 5-mg/mL sodium on the relative emission intensities of various

atomic and ionic lines at three different microwave powers. The spectral lines are

arranged by their excitation energies. Concentration of the analytes: 0.005 mg/mL

for Li, Mg, Ca, Sr, and Ba, 0.1 mg/mL for Al, Cr, Cu, Fe and Mo, and 0.5 mg/mL

for Cd and Zn; plasma gas: pure nitrogen; flow rate of the plasma gas: 13 L/min; carrier

gas flow rate: 0.6 L/min; observation height: 12 mm. See Zhang and Wagatsuma (19).

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ANALYTICAL APPLICATIONS OF OKAMOTO-CAVITY MIP

Detection Limit

Ohata and Furuta (21) extensively investigated the detection limits of

55 emission lines for 21 elements excited by the nitrogen Okamoto-cavity

MIP, and compared those with the detection limits obtained by using an

argon ICP. The MIP and the ICP were operated under almost the same

plasma conditions and with the same measurement systems. Table 1 shows

Table 1. Comparison in the detection limits between nitrogen-MIP and

argon-ICP (21)

Wavelength

(nm)

Excitation

energy (eV)

Detection

limit N2-MIP

(ng/mL)

Detection

limit Ar-ICP

(ng/mL)

Na I 589.0 2.11 46 5.8

Al I 309.3 4.02 98 21

Al I 396.2 3.14 12 10

Ca I 422.7 2.93 3.7 1.1

Ca II 393.4 3.15 0.38 0.06

V I 437.9 3.13 27 20

V II 292.4 4.63 11 2.0

V II 309.3 4.40 8.8 1.3

Cr I 357.9 3.46 90 7.7

Cr I 429.0 2.89 24 18

Cr II 205.6 6.03 280 4.6

Cr II 267.7 6.18 25 3.3

Ti I 398.2 3.11 120 66

Ti II 334.9 3.74 4.3 0.9

Mo I 390.3 3.17 180 18

Mo II 202.0 6.13 340 6.0

Mn I 279.5 4.44 12 8.6

Mn I 403.1 3.08 18 9.7

Mn II 257.6 4.81 6.9 0.4

Ni I 232.0 5.34 54 14

Ni I 361.9 3.85 23 38

Ni II 221.6 6.63 580 6.8

Ni II 231.6 6.38 150 6.0

Mg I 285.2 4.34 2.2 1.3

Mg II 279.6 4.43 0.60 0.07

Cu I 324.7 3.82 2.3 2.0

Cu II 224.7 8.23 1800 6.9

Fe I 372.0 3.33 79 54

Fe II 259.9 4.77 13 1.6

Fe II 238.2 5.20 40 3.2

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the data extracted from their analytical results, indicating that the detection

limits obtained with the nitrogen MIP were one to two orders of magnitude

worse than those with the argon ICP.

Yamada and Okamoto (11) reported the determination of non-metal

elements when the helium Okamoto-cavity MIP was employed. It is interest-

ing to detect emission lines of halogen elements such as fluorine and chlorine

because their emission lines requiring high excitation energies cannot be suffi-

ciently excited in argon-ICP (11, 12). Figure 5 shows a typical spectrum in the

wavelength range of 683–692 nm emitted from the helium MIP (12), which

includes several fluorine atomic lines having the excitation energy of about

14.5 eV. The F I 685.6-nm line is the most intense and is thus recommended

as the analytical line for fluorine determination. The detection limit for this

line was estimated to be 150 ng/mL, yielding an improvement by a factor

of 25 over the literature value obtained with a helium Beenekker-cavity MIP.

Hydride Generation

Nakahara and his co-workers published a series of papers describing an

analytical application of the Okamoto-cavity MIP associated with hydride

Figure 5. Spectrum obtained by nebulizing an aqueous solution containing 5-mg/mL

NaF into the helium MIP. Microwave power: 600 W; plasma gas flow rate: 13.5 L/min;

carrier gas flow rate: 0.7 L/min: plasma gas: pure helium. See Okamoto et al. (12).

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generation technique (16–18, 22–26). The hydride generation technique is

known as an effective sampling method in atomic absorption spectrometry

and ICP spectrometry, enabling hydride-forming elements such as arsenic,

tellurium, antimony, and selenium to be detected more sensitively

compared to the conventional nebulization technique. Because the

Okamoto-cavity MIP has a high tolerance for loading of various forms of

samples, it can be considered that this is suitable for the atomization and exci-

tation source also in the case of hydride introduction. Figure 6 shows a

schematic diagram of their experimental apparatus. The sample introduction

system for hydride generation comprises a reaction path having a peristaltic

pump, a gas-liquid separator, a drying flask, and a nebulizer chamber. In com-

parison, a conventional spraying system with a concentric nebulizer is also

equipped with the nebulizer chamber. An acidified sample solution and an

alkaline tetrahydroborate solution are continuously introduced into the

hydride generator by running the peristaltic pump. After removing water

vapor produced during the hydride generation with the drying flask filled

with concentric sulfuric acid, the analyte elements changed into the hydride

vapor along with excess of hydrogen gas are fed into the MIP by nitrogen

carrier gas through the drain outlet of the nebulizer chamber.

Figure 6. Schematic diagram of the Okamoto-cavity MIP apparatus associated with a

hydride sampling system. A, microwave power supply; B, microwave generator; C1,

C2, tapered waveguide; D, uniline; E, direction coupler; F, three-stub tuner; I, mono-

chromator; J, photomultiplier; N, nitrogen tank; O, argon tank; P, gas controller; Q,

plasma gas; R, carrier gas; S, sample solution for nebulization; T, carrier gas and

sample aerosol or hydride; U, nebulizer; V, nebulizer chamber; X, three-way stop

cock; Y, sample solution for hydride generation; Z, NaBH4 solution; a, peristaltic

pump; b, mixing joint; c, gas-liquid separator; d, drying flask; e: Okamoto-cavity; g,

plasma torch. See Nakahara and Li (16).

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Under the optimized operation conditions, a comparison of the detection

limit for tellurium was performed between the hydride generation and the con-

ventional nebulization techniques. In case where the Te I 214.281 nm was

employed as the analytical line, the detection limit for the hydride generation

method was 15.0 ng/mL, while that for the solution nebulization method

740 ng/mL, resulting in the improvement in the detection sensitivity by a

factor of 50 (18). The detection limit for the As I 228.821 nm, the Se I

196.026 nm, and the Sb I 231.147 nm was 1.87, 2.99, and 0.86 ng/mL,

respectively, by the same hydride generation method.

More recently, Matsumoto, a member of Nakahara’s research group,

extensively investigated the determination of several elements in steel

samples by the Okamoto-cavity MIP associated with hydride-generation

technique (22–26). Table 2 summarizes the simultaneous determination of

bismuth and tellurium in three standard reference materials of steel.

The obtained analytical values were in good agreement with the certified

values (26).

Introduction of Organic Solvent

The direct aspiration of organic solvents into the plasma for emission analysis

can be useful for the rapid determination. The air-ICP (27–30), the air-MIP

(31–34), and the air capacitively coupled microwave plasma (35) have

been investigated for both aqueous and organic sample aerosol introduction.

Generally, the use of plasmas containing oxygen should be suitable to direct

organic solvent introduction due to the combustion. This promises a useful

application of this plasma source as a detector in liquid chromatography.

Further, the air-MIP may be used in the direct analysis of the organic

extract, in the case where solvent extraction is performed to separate minor

analyte elements from the matrix (27, 34).

Maeda and Wagatsuma (36) reported on the emission characteristics of

ethanol solution excited from the nitrogen-oxygen mixed gas MIPs.

Figure 7 indicates a variation in the excitation temperature as a function of

Table 2. Simultaneous determination of bismuth and tellurium in steel samples by

Okamoto-cavity nitrogen MIP with hydride generation (26)

Certified values (mg/g)a Obtained values (mg/g)

Sample Bismuth Tellurium Bismuth Tellurium

SS 192–1 0.098 0.031 0.0932 + 0.0007 0.0351 + 0.0004

ISS 193–1 0.034 0.026 0.0393 + 0.0004 0.0284 + 0.0005

ISS 195–1 0.032 0.042 0.0358 + 0.0003 0.0450 + 0.0008

aThe mean + standard deviation, based on five replicate determination.

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the ethanol content in the aspirated solution. The excitation temperatures were

estimated from Boltzmann plots using iron atomic lines ranging in 367–

368 nm. The excitation temperature is almost independent of the ethanol

content (about 4750 K) when nitrogen-28% oxygen is used as the plasma

gas. It is assumed that the excitation conditions are little changed even by

introducing 80 vol% ethanol into the plasma, probably because of complete

combustion of ethanol.

The same authors reported on direct analysis of 4-methyl-2-pentanone

(MIBK), which was frequently employed as an solvent extraction reagent to

remove minor elements from the iron matrix in steel samples, by using the

nitrogen-oxygen Okamoto-cavity MIP (37). Figure 8 shows the intensity

changes of the Swan band of C2 radical (516.5 nm, 0–0 band head) (38)

with increasing oxygen content in the nitrogen-oxygen mixed gas plasma,

when MIBK is introduced into the plasma. The band spectra originated from

C2 radical could not be observed if the oxygen content is more than 21 vol%.

It implies that carbon atoms from MIBK can be fully oxidized in

the oxygen-containing plasma and therefore few carbon radicals are in the

plasma. It is likely to say that the nitrogen-oxygen mixed gas MIP with the

Okamoto cavity has a high tolerance to loading of organic solvents. The

resultant plasma can be kept stable even when the MIBK solution is introduced.

SUMMARY

The Okamoto-cavity MIP has several noticeable features: a high robustness

against loading of various types of samples and a good matching for

Figure 7. Variation in the excitation temperature as a function of the ethanol content

when the nitrogen-28 vol.% oxygen mixed gas MIP is employed. Microwave power:

1.0 kW; total flow rate of the plasma gas: 14 L/min; carrier gas flow rate: 0.7 L/min;

observation height: 13 mm. See Maeda and Wagatsuma (36).

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various plasma gases. In the case of nebulization of aqueous samples, the

detection limits for the nitrogen Okamoto-cavity MIP are inferior to those

for a conventional argon ICP. However, wider and specified applications of

the Okamoto-cavity MIP can be expected; for example, direct analysis of

organic solvents in the oxygen-containing MIP or detection of non-metallic

elements in the helium MIP. This plasma would have a potential when

various kinds of samples must be analyzed in atomic emission spectrometry.

ACKNOWLEDGMENT

The author gratefully acknowledges Dr. Y. Okamoto for many useful sugges-

tions for the MIP instruments.

REFERENCES

1. Broekaert, J.A.C. (2001) Analytical Atomic Spectrometry with Flames andPlasmas; Wiley-VCH Verlag: Weinheim.

Figure 8. Variation in the intensities of the C2 Swan band (516.5 nm) as a function of

the oxygen content in the nitrogen-oxygen mixed gas MIP when MIBK is directly

introduced. Microwave power: 1.0 kW; total flow rate of the plasma gas: 14 L/min;

carrier gas flow rate: 0.7 L/min; observation height: 13 mm.

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