A low-power oxygen inductively coupled plasma for spectrochemicaln analysis—III. Excitation mechanism

Specrrochamtca Acre Vol. 45B, Nos Printed III Great Bntam

05~8547/90 53 oo+ 00 Pergamon Press plc

l/Z, pp. 157-165, 1990

A low-power oxygen inductively coupled plasma for spectrochemical analysis-III. Excitation mechanism PENGYUAN YANG

and RAMON M. BARNES*

Department of Chemistry, University of Massachusetts, GRC Towers, Amherst, MA 01003-0035, U.S.A. (Receioed 15 November 1988; in revised form 15 June 1989)

Abstract-The excitation mechanism in an analytical oxygen inductively coupled plasma (ICP) is discussed. In addition to the local thermal equilibrium mechanism, charge transfer from oxygen ions to the analytes in the analytical zone also plays an important role in excitation. A comparison of the excitation mechanism between the oxygen ICP and an argon ICP discharge is presented.

1. INTRODUCTION inductively coupled plasma (ICP) has been generated and investigated previously [l-6]. For spectrochemical analysis, the choice of analytical wavelengths in the oxygen ICP is important and requires an understanding of the excitation mechanism. Excitation mechanisms in the argon [7-153, air [16], and oxygen ICPs [4, 121 have been proposed. Our recent studies on a low-power (2 kW) oxygen ICP discharge include computer simulation [ 171 and plasma diagnostics [ 181. In general, the oxygen ICP has been found to be near local thermal equilibrium (LTE). Some minor departures from LTE were observed in the analytical zone [S, 183. For example, the intensity ratio of a Ca II to a Ca I line was 3 times that of the LTE value. This deviation is smaller than those reported in an argon ICP. A number of mixed-gas ICPs have been described with nitrogen, air, oxygen, hydrogen, or helium replacing the outer, intermediate and/or central gas flows [ 19-271. Appropriate mixing of another gas with argon can improve the detection limits for many elements [21,22]. However, for most elements [2,28,29] a pure molecular gas ICP results in 10 to 100 times poorer detection limits than with an argon ICP. The addition of a small amount of a molecular gas to argon normally increases the thermal conductivity of the mixture compared to argon, but it does not change the excitation mechanism. As the amount of doped gas increases, energetic species derived from the molecular gas, such as O+, H+, and N+, might become dominant in the excitation process [9, 14, 303. In this paper a number of elements and wavelengths are monitored to investigate the charge transfer process in the oxygen ICP discharge. The data obtained, along with previous results, are used to verify proposed excitation mechanisms. A PURE OXYGEN

2. EXPERIMENTAL 2.1. Instrumentation

The instrumentation used in this study has been described previously [18, 281. Briefly, the 2 kW oxygen ICP was generated at 40.68 MHz in a conventional l&mm i.d. quartz torch. Typical oxygen flow rates were 15, 0.5, and 0.8I/min for the outer, intermediate, and central streams, respectively. Emission spectra were recorded with a lP28A photomultiplier (RCA) after dispersion by a 0.75-m Czerny-Turner monochromator (SPEX 1700-H). 2.2. Line intensity measurement Both blank and sample solutions were measured by scanning a wavelength window across an appropriate line. Solutions containing tested elements with concentrations from 10 to 1000 pg/ml were used. For convenience, the intensities in the oxygen ICP are normalized in such a way that for each element the most sensitive line is assigned a value of 100. *Author to whom the correspondence should be sent. 157

P. YANG and R. M. BARNES

158

3. RESULTSANDDISCUSSION 3.1. Assumption of excitation mechanism Several proposals have described as to how the excited and charged species are formed in an argon ICP [31, 321. These processes include Penning ionization with metastable atoms [7,8], charge transfer argon ions [9, lo], enhanced ionization through ambipolar diffusion [ll, 331, radiation trapping with population of the resonant level of argon [12, 133, and direct electron impact ionization and excitation [14, 151. In a molecular-gas ICP thermal excitation has often been considered to be dominant providing near-LTE conditions [3, 5, 16, 241. The selection of an appropriate analytical wavelength in the oxygen ICP depends on its excitation mechanism. If the oxygen ICP were in LTE, it should behave more like an equilibrium discharge than an argon ICP. For this reason, MEYERsuggested that wavelength selection for an air ICP could refer to the highest intensities found in the table of persistent arc lines of the elements [34,35]. As will be demonstrated later, this selection criterion is not always effective. Some elements exhibit similar emission behavior in either argon or oxygen ICP discharges [S, 181. If the oxygen and argon ICP discharges were alike, strong analyte lines observed in the argon ICP might also be expected to be strong in the oxygen discharge. In fact, many lines that are sensitive in the argon ICP are extremely insensitive in the oxygen ICP [3]. On the other hand, in the air ICP as well as in the oxygen ICP [S, 281, the alkali metals generally exhibit similar detection limits to those obtained in the argon ICP. As in the argon ICP, the alkaline-earth metals in the oxygen ICP are characterized by their ionic lines more than their atomic lines [28]. These similarities and differences between the argon and molecular gas ICPs lead to a consideration of the charge transfer process and gas properties. The gas properties play a major role in defining the excitation process. First, the high thermal conductivity and gas enthalpy of oxygen favors efficient heat transfer. The power density in the oxygen ICP in the induction coil is 5 times as large as in the argon ICP [ 173. Second, the electron number density of the oxygen ICP is about 4 x 1016/cm3 in the plasma in the induction coil, which is about one order of magnitude higher than that in an argon ICP [36]. Third, O+ species, like Ar+ species, are the particles with a considerable amount of internal energy, and have almost the same density as the electrons. Upon comparing the density of O+ with other energetic species such as metastable O”, and O-, the latter could be neglected because of the fact that they are 1000 times less populated than O+ species [37]. Therefore, the following mechanisms should be of principal concern: (1) electron collisional excitation and de-excitation, (2) charge transfer from oxygen ions, and (3) direct electron impact ionization and excitation. The third mechanism requires high-energy (e.g. > 5 eV) electrons with a considerable population. In the central channel of the oxygen ICP discharge the average electron energy is less than 1 eV, and the high-energy electron population is expected to be small compared to the total electron number density. As a consequence, the contribution of direct electron impact ionization and excitation in the oxygen ICP should be small. Although the O+ potential energy (13.61eV) is less than that of Ar+ (15.77eV), it is sufficient to provide energy for ionization and excitation. Therefore an assumption has been made that in addition to the electron collisional ionization and excitation, charge transfer from oxygen ions can also be an important process in analyte ionization and excitation: O++X=X+*+O+AE

(1)

O++X=X++O+AE

(2)

in which AE is the energy defect. 3.2. Charge transfer from oxygen ions Recently, GOLDWASSER and MERMET[lo] and TRASSY 1301 proposed that charge transfer from argon ion could play an important role in the analyte excitation mechanism. The high argon ionization potential favors the so-called “ionic line advantage”. VAN DER MULLENet al. [38] demonstrated that although charge transfer with Ar+ is an important ionization

Low-power

oxygen

ICP-III

159

pathway for analyte atoms, the reaction did not create large deviations from LTE. This approach might also be helpful for line selection in the oxygen ICP. The important conditions for a strong line are (a) high transition probability; (b) high upper-level density or (c) a combination of (a) and (b). The second condition can be fulfilled if a good match exists between the internal energy of Of and that of the upper level of the analyte atom or ion. Those elements for which the sum of ionization and excitation energy is less than the oxygen ionization potential (13.61 eV), can be expected to have strong ionic lines in the oxygen ICP. The 2.3eV difference between the argon and oxygen ionization potentials might affect the excitation mechanism for some elements, for which the total energy involved is greater than 13.61 eV but smaller than 15.77 (or 15.94)eV. As an example, a partial energy level diagram for the Ni II 22 1.6 nm transition is shown in Fig. la. The Ni II 221.6nm is the strongest Ni line observed in the argon ICP. Since the upper level of the transition lies above the sum energy of O+, the ionic line advantage for Ni II 221.6 nm could be lost in the oxygen ICP. The oxygen ionization potential cannot provide sufficient energy to excite the Ni II 221.6nm line, which requires a total energy of 14.2eV (7.63 eV for ionization and 6.63 eV for excitation). Another example is the Mg II line at 279.6 nm. Its partial energy diagram and the possible transition level are drawn in Fig. 1b. In this example, both ICP discharges can produce the same result, because the total energy involved is only 12.1 eV. The Mg II 279.6 nm line can be strong in the oxygen ICP. From these hypotheses, the periodic table generally can be divided into a few groups describing possible behavior in both argon and oxygen ICP discharges.

(1) The first group encompasses those elements for which the sum of ionization potential and excitation energy is higher than the argon ionization potential. These elements can be expected to have similar characteristics in both ICP discharges. The atomic line is generally strong and the ionic line is relatively weak. This group includes all alkali metals and the copper and zinc family. 20

hr

-

AT+ +

c

15

o--o’+

> m

14.26

c

-1

221.6 nm (6.63 rV>

10 I

7. 63

20 r

b 01

Fig. 1. Energy

diagram

and transition

levels for (a) Ni II 221.6nm

and (b) Mg II 279.6nm.

160

P. YANG and R. M. BARNES

(2) The second group of elements, in contrast, will exhibit an ionic line advantage in both ICP discharges, since the energy sum involved is less than the oxygen ionization potential. This group contains all alkaline-earth metals and most transition elements. (3) The third group comprises those elements with an energy sum between 13.6 and 16eV. These elements would exhibit strong ionic lines in an argon ICP, but only strong atomic lines in the oxygen ICP. These elements include Cd, Sn, Pb, Ni, Ru, Rh, OS, and Pt. (4) The last group of elements includes Hg, Ga, and Al [lo]. They show apparently abnormal behavior when viewed by a conventional spectrometer. A vacuum spectrometer is required to record ultraviolet emissions from these elements. 3.3. Comparison of line intensities in the oxygen ICP Intensities of more than 200 lines for 38 elements of interest are summarized and discussed in this section. To prove the assumption made in the previous section, these lines are selected in such a way that they are either sensitive in the argon ICP or sensitive in the arc and spark [38]. Some results for typical elements are listed in Tables 1 through 4. The fourth group of elements was not examined in this experiment. The results for some lines are not listed because of their relatively small intensities. When the results obtained in this study are compared with those reported by GOLDWASSER and MERMET[lo], the elements in Table 1 were found to exhibit different behaviors in argon and in oxygen ICP discharges. They belong to the third group of elements which have strong ionic lines in the argon ICP and strong atomic lines in the oxygen ICP. A particular example is nickel (Fig. la). The strongest atomic line in the oxygen ICP is almost 10 times as intense as the Ni ionic lines. The strongest ionic line in the argon ICP has an energy sum greater than the ionization potential of oxygen. Therefore, no ionic line enhancement advantage could be observed in the oxygen ICP. The elements in Table 2 exhibit more or less similar relative intensities in both ICP discharges, although in the oxygen ICP poorer sensitivities are observed than in the argon ICP. These elements are from the first group for which atomic lines are stronger than the ionic lines. The second group of elements can be divided into two subgroups according to whether or not their sensitive lines in an oxygen ICP are the same as in the argon ICP. Those elements for which the strong lines are essentially the same as in the argon ICP are listed in Table 3. A subgroup of elements is given in Table 4 for which the sensitive lines in the oxygen ICP differ from the ones in the argon ICP. In this case the energy sum of strong lines in the argon ICP is above 13.61 eV but in the oxygen ICP below 13.61 eV. For some elements in Tables 3 and 4 the atomic line is as sensitive as the ionic line. The ionic line enhancements are not as strong in the oxygen ICP as in the argon ICP, because of dominant thermal excitation in the former. The results listed in Table 4 are revealing. An example is cobalt. The second most sensitive line (Co II 228.62nm) in the argon ICP requires a total energy more than 13.61 eV (ionization potential 7.86 eV and excitation energy 5.84 eV). Therefore, another Co II line at 237.8nm becomes relatively stronger than 228.62nm in the oxygen ICP. The excitation energy for the Co II 237.8 nm line is 5.62 eV, so that the energy sum (13.48 eV) is less than the oxygen ionization potential. This result confirms that the sum of ionization potential and excitation energy must be less than oxygen ionization potential for an ionic line enhancement. 3.4. Selection of analytical line in oxygen ICP The above results are rearranged and plotted in Fig. 2 in the same way as illustrated by GOLDWASSER and MERMET[lo] and TRASSYand MERMET[30, 391. For comparison, the results obtained in the argon ICP [lo] also are included. The explanation of how this plot is made can be found in Ref. [lo]. In Fig. 2 the energy diagrams of the elements of the periodic table are arranged as a function of the Mendeleev families. A clear picture, showing how to choose a sensitive line in different ICP gas discharges, emerges from Fig. 2. Generally, the ionization potential represents the clue to determine whether an ionic line or an atomic line should be selected. For the oxygen ICP analytical

161

Low-power oxygen ICP-III Table 1. Relative intensities for the third group of elements in the oxygen ICP

Element Pt

State

Wavelength (nm)

E (ev)

9.0

I

265.945 217.467 210.333 264.689 262.803 248.717

4.6 5.99 4.68 4.81 4.98 5.08 4.23 6.05 4.05 4.4

100 90 38 42 32 29

55 70

203.646

6.68 5.78 6.37 6.68

352.454 341.476 351.505 361.939 232.003

3.54 3.65 3.63 3.85 5.34

100 58 50 34 13

299.797 204.937 306.471 283.030 Pt

Relative intensity

IP(eV)

18.56

II

224.552 214.423

29 25 22 19

12

Ni

7.63

Ni

18.15

II

221.647 230.330 231.604 216.556

6.63 6.54 6.39 6.76

9 9 9 7

Cd

8.99

I

228.802 346.620 361.000

5.41 7.37 7.37

100 39 23

Cd

16.90

214.438 226.502

5.78 5.47

23 70

OS

8.5

326.229 315.625 290.906 222.798

4.32 4.57 4.2 -

100 65 16 20

OS

17.0

233.66 253.80 248.624 236.735 228.226 219.439

5.75 4.89 5.35 5.43 6.09

53 35 31 26 18 16

286.333 270.651 283.999 242.949

4.32 4.78 4.78 5.51

100 67 52 67

189.98

7.37

50

405.783 283.307 368.348 239.379 216.999

4.38 4.4 4.34 6.50 5.67

100 40 77 27 14

220.35

7.25

60

II

II

Sn

7.34

I

Sn

14.63

II

Pb

7.41

I

Pb

15.03

II

lines can be selected based on the four groups as mentioned. For the first group of elements, comprising elements such as Li, Na, Zn, P and As, the selection of atomic lines is preferred as analytical wavelengths. Ionic lines, in contrast, are preferred for the second group of

162

P. YANG and R. M. BARNES Table 2. Relative intensities

Element

IP (eV)

for the first group

State

elements

Wavelength

(nm)

in the oxygen

E (eV)

ICP Relative intensity

Zn

9.39

I

213.856 636.235

5.80 7.74

100 2

Zn

17.96

II

202.548 206.20

6.12 6.11

58 39

cu

7.72

I

324.754 321.396 221.458 261.837

3.82 3.78 6.98 6.12

100 22 80 44

II

213.598

5.80

48

I

588.995 589.592

2.11 2.10

100 51

20.9

CU

5.14

Na

Table 3. Relative intensities

Element

IP (eV)

for subgroup

State

1 of elements

in the second group

Wavelength

(nm)

in the oxygen

E (eV)

ICP

Relative intensity

279.553 280.270 279.079

4.43 4.42 8.86

100 34 86

7.64

I

285.213 383.826

4.34 5.94

13 8

Sr

11.03

II

407.771 421.552

3.04 2.94

100 35

Sr

5.69

I

460.333

2.69

1

Ca

11.87

II

393.367 396.847

3.15 3.12

100 35

Ca

6.11

I

422.673

2.93

1

Ba

10.0

II

455.403 649.7

2.72 2.51

100 20

In

18.86

II

230.606

5.31

100

In

5.78

I

303.969 325.609 451.5

4.1 4.1 3.02

80 58 29

Mn

15.64

II

257.610 259.373 260.569 293.920

4.81 4.77 4.75

100 60 45 24

Mn

7.42

I

403.076 403.307 279.482

3.08 3.08 4.44

58 26 45

II

257.590 223.948 260.349 238.706 233.198 226.23 248.870

Ta

Ta

16.2

7.88

I

289.184 296.332 285.015

5.86 5.53 5.74

6.22 5.52 6.65 4.53 4.43 5.04

100 100 75 60 60 60 60

48 36 30

Low-power Table 4. Relative

Element Be

intensities

for subgroup

oxygen

ICP-III

2 of elements

163

in the second group

Wavelength

(nm)

in the oxygen

ICP

Relative intensity

IP (ev)

State

18.2

II

313.042

3.95

100

E (eV

Be

9.32

I

234.861 332.109

5.28 6.45

80 28

co

17.05

II

237.862 238.636 236.379 23 1.405 230.786

5.62 5.74 5 74 5.97 5.87 7.60 5.84

100 98 69 83 80

340.512 345.350 347.412 232.609 248.923 245.145 222.589 239.709

4.07 4.03 4.12 6.09 5.56 5.24 5.57 5.56

90 85 28 90 100 51 4; 37

228.616 243.739

co W

7.86

17.7

I

II

43 45

W

7.98

I

400.875 255.509 257.144

3.45 5.24 5.21

17 15 15

Cr

16.49

II

283.563 286.511 205.552 206.149 286.257 285.568 276.654

5.93 5.84 6.03 6.02 5.86 5.84 6.03

100 67 55 36 45 36 42

Cr

6.76

I

425.438 427.480 428.973

2.91 2.90 2.89

90 70 35

II

221.426 227.525 197.317

5.60 5.44 6.28

100 47 16

Re

16.6

Re

7.87

I

346.047 346.473

3.58 3.57

42 6

Fe

16.18

II

259.940 239.562 238.204

4.71 5.26 5.20

100 23 17

Fe

7.87

I

247.29 1 253.56 294.443

5.06 5.01 4.17

83 80 26

elements. Some of these elements, such as Mg, Ca, Sr, Ba and Mn, will have the same sensitive line as those in an argon ICP, but others, such as Be, MO, and Co, will have ionic lines, of different sensitivity, since the sum of their ionization potential and excitation energy is greater than 13.61 eV. The third group of elements, such as Sn, Pb, Ni, OS and Pt, will exhibit completely different behavior in the oxygen ICP. Their ionic lines are sensitive in the argon ICP but not in the oxygen ICP discharge. Although the signal-to-background ratio and the signal-to-noise ratio are not discussed here, they will affect line selection and should also be considered in any practical application. Finally, weak F emission would be expected to occur in the oxygen ICP, because the lowest energy sum for F I is greater than the oxygen ionization potential.

(4

(4

3(

e\

31

e\

_ Lo

Na

K

Ab

Cs Cu

Ag Au

Be Mg Ca

Sr

Ba

1

\

Zn Cd

I - I

[

tip B

A,

Ga

In

TI

SC

Y

La

lo-

71

Fig. 2. Energy diagram of elements of the periodic table arranged according to “Mendeleev family” [lo]. The various ranges of excitation and ionization energies for most elements are indicated. The atomic state is the lowest and is indicated between resonance level and ionization energy. The ionic state is similarly indicated, including the ionization energy between states I and II. The filled diamond (lozenge) indicates the Iocatlon of the most sensitive line in an argon ICP [lo]; the square indicates the location of most sensitive line in an oxygen ICP 1s the same as in the argon ICP; the triangle indicates the location of most sensitive ionic line in an oxygen ICP that differs from an argon ICP, the circle indicates the location of most sensitive atomic line in an oxygen ICP whereas its ionic line is sensitive in the argon ICP, the open lozenge represents an abnormal behavior of the elements. (a) elements from Li to La, (b) elements from C to W, and (c)elements from F to U (see text for details).

(b)

30

,aV

Low-power

oxygen

ICP-III

165

4. CONCLUSION

In addition to the LTE properties of the oxygen ICP discharge, charge transfer from oxygen ions is an important process in the excitation of analyte elements. The results for an oxygen ICP, therefore, provide a verification of the general hypothesis proposed by GOLDWASSER and MERMET [lo]. The conclusion obtained in this study can serve as a guide for selecting appropriate analyte lines for an oxygen ICP or other molecular gas ICP discharges for spectrochemical analysis. Acknowledgement-Research

sponsored

by the ICP Information

Newsletter.

REFERENCED [l] G. A. Meyer, Ph.D. dissertation, University of Massachusetts, Amherst, MA (1982). [2] N. Kovacic and R. M. Barnes, Unpublished paper (1984). [3] G. A. Meyer and M. D. Thompson, Spectrochim. Acta 4OB, 195 (1985). [4] K. Liu, N. Kovacic, I. Bletsos and R. M. Barnes, 29th Annual Conference of the Spectroscopy Society of Canada, Paper 11, 1982. [S] P. Y. Yang and R. M. Barnes, 1986 Winter Conference on Plasma Spectrochemical Analysis, Paper 125, Kailua-Kona, Hawaii, 1986. [6] P. Y. Yang, Ph.D. dissertation, University of Massachusetts, Amherst. MA (1987). [7] P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta 32B, 71 (1977). [S] J. M. Mermet and C. Trassy, Reo. Phys. Appl. 12, 1219 (1977). [9] J. M. Mermet, 3rd Canadian Chemical Conference, Ottawa, Paper AN-9 (1980). [lo] A. Goldwasser and J. M. Mermet, Spectrochim. Acta 41B, 725 (1986). [11] H. V. Eckert, Deoelopments in Atomic Plasma Spectrochemical Analysis, Ed. R. M. Barnes, p. 35, Heyden, London (1981). [12] M. W. Blades and G. M. Hieftje, Spectrochim. Acta 378, 191 (1982). [13] J. W. Mills and G. M. Hieftje, Spectrochim. Acta 39B, 859 (1984). [14] J. F. Alder, R. M. Bombelka and G. F. Hirkbright, Spectrochim. Acta 35B, 163 (1980). [15] G. M. Hieftje, G. D. Rayson and J. W. Olesik, Spectrochim. Acta 4OB, 167 (1985). [16] N. Kovacic, G. A. Meyer, K. Liu and R. M. Barnes, Spectrochim. Acta 4OB, 943 (1985). [17] P. Yang and R. M. Barnes, Spectrochim. Acta 44B, 657 (1989). [18] P. Yang and R. M. Barnes, Spectrochim. Acta 44B, 1093 (1989). [19] S. Greenfield and D. J. Smith, Anal. Chim. Acta 59, 341 (1972). [20] S. Greenfield, ICP Information Newslett. 2, 167 (1976). [21] A. Montaser, V. A. Fassel and J. Zalewski, Appl. Spectrosc. 35, 292 (1981). [22] E. H. Choot and G. Horlick, Spectrochim. Acta 41B, 925 (1986). [23] E. H. Choot and G. Horlick, Spectrochim. Acta 41B, 889 (1986). [24] E. H. Choot and G. Horlick, Spectrochim. Acta 41B, 907 (1986). [25] A. Montaser and R. L. Van Hoven, CRC Crit. Rev. Anal. Chem. l&45 (1987). [26] K. D. Ohls, S. W. Golightly and A. Montaser, Mixed-gas, molecular-gas, and helium inductively coupled plasmas operated at atmospheric and reduced pressures, in: Inductioely Coupled Plasma in Analytical Atomic Spectrometry, A. Montaser and S. W. Golightly Eds, Ch. 15, p. 563. VCH Publishers, New York (1987). [27] P. E. Walters and C. A. Barnard& Spectrochim. Acta 43B, 325 (1988). [28] R. M. Barnes and G. A. Meyer, Spectrochim. Acta 4OB, 893 (1985). [29] P. Yang and R. M. Barnes, Spectrochim. Acta 45B. 167 (1990). [30] C. Trassy, Rev. Int. Hautes. Temp. Refract. 26, 192 (1986). [31] M. W. Blades. B. L. Caughlin, Z. H. Walker and L. L. Burton, Prog. Anal. Spectrosc. 10, 57 (1987). [32] W. M. Blades, Excitation mechanisms and discharge characteristics-Recent developments, in: Inductiuely Coupled Plasma Emission Spectroscopy, P. W. J. M. Boumans Ed. Part 2, Ch. 11. John Wiley, New York (1987). 1331 F. Aeschbach, Spectrochlm. Acca 378, 987 (1982). [34] Handbooh of Chemistry and Physics, 57th edn, p. E213, Ed. R. C. Weast, Chemical Rubber Publishing, Cleveland. OH (1961). [35] G. A. Meyer, Spectrochim. Acta 42B. 201 (1987). [36] P. W. J. M. Boumans and F. J. De Boer, Spectrochim. Acta 32B, 365 (1977). 1373 S. V. Dresvin, Physics and Technology of Low Temperature Plasma, H. V. Eckert Ed. Iowa State University Press, Ames, IA (1977). [38] J A. M van der Mullen, I. J. M. M. Raaijmakers, A. C. A. P. van Lameren, D. C. Schram, B. van der Sijde and H. J. W. Schenkelaars, Spectrochim. Acta 42B, 1039 (1987). [39] C. Trassy and J. M. Mermet, Analytical Application of High Frequency Plasma, Technique and Documentatlon. Lavolsier, Paris (1984).