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Electronic excitation and ionization temperature measurements in a high frequency inductively coupled argon plasma source and the influence of water vapour on plasma parameters
Specimchhica Acta, Vol. 35B, pp. 163 to 175 Pergamon Press Ltd., 1980. F’rinted in Great Britain
Electronic excitation and ionization temperature measurements in a high frequency inductively coupled argon plasma source and the influence of water vapour on plasma parameters J. F.
ALDER,
R.
M.
BOMESELKA*
and G. F.
KIRKBRIGHT
ChemistryDepartment, Imperial College, London SW7, U.K. (Receiued 25 April 1979; in revised form 16 October 1979) Abstiet-Excitation temperatures measured from Fe I lines with a wide range of upper energy levels have shown that the environment of the central channel of an argon ICP is not in a state of local thermal equilibrium, as predicted from Griem’s criterion. Deviations from a Boltzmann distribution have been observed in the population of the excited states of Fe I. The significance of water in providing extra electrons by ionization in the ICP is demonstrated and discussed.
1.
INTRODUCTION
ELECTRON density and temperature measurements in the type of high frequency inductively coupled argon plasma (ICP) used for analytical atomic emission spectroscopy have been made by numerous workers over a range of operating and observation conditions. The methods used for these measurements have been discussed recently by KORNBLLJM and DE GALAN [l]. If local thermal equilibrium (LTE) prevails in the source, a commonly employed convenient method of temperature determination is the “two-line” method [2] which, with the addition of a suitable thermometric species, yields the electronic excitation temperature, T,, which can then be equated to the electron temperature, T,. The ionization temperature, Ti,,may be determined from measurements of the relative intensities of an atom and ion line of a suitable species and knowledge of the electron density, n,, via the Saha equation. Under LTE conditions the value obtained is again equivalent to the electron temperature, T,. Two methods are commonly employed for the determination of the electron density in ICP sources; these are the HP line-broadening method and via the measurement of the absolute intensity of the argon recombination continuum at a suitable wavelength. These methods are fundamentally quite accurate, theoretical considerations giving rise to less than 10% uncertainty in the values obtained; greater errors may be incurred in the experimental observations and the application of the Abel inversion technique to the data obtained. One of the most useful features of both of these methods, however, is that the values of n, obtained may be employed directly to evaluate the extent to which LTE prevails in the system without the need to resort to rotational, vibrational and electronic excitation temperature measurements for a wide range of species. Experimental results available from measurements with the argon ICP indicate that over a range of operating conditions and heights of observation above the core in this source, LTE does not prevail [3-51. This has been shown, for example, from determination of the translational gas temperature, Tg, by measurement of the Doppler width of emission lines from analyte species [6] and comparison with other excitation and
* Present address: Institut Fiir Spektrochemie und Angewandte Spektroskopie, Bunsen-Kirchhoffstr. 4600 Dortmund, West Germany.
11,
G. R. KOFWELUM and L. DE GALAN, Specfmchim. Acta 32B, 71 (1977). [2] P. W. J. M. BOUMANS, Theory of Spectrochemical Excitation. Adam Hilger, London (1966). [3] J. M. MERMET, Spectrochim. Acta 3OB, 383, (1975). [4] J. M. ~~ERMET, J. JAROSZ and J. P. ROBIN, Preprints 17th Coil. Spectrosc. Int., Florence 1973, Vol. 1, p. 101 (1973). [5] J. M. MERMET, Thesis. University of Lyon (1974). [6] H. G. C. HUMAN and R. H. SCOTT, Spectrochim. Acta, 3lB, 459, (1976).
[l]
SA(B I--r\
163
164
J. F. ALDER,
R. M. BOMBELKA and G. F. KIRKBRIGHT
ionization temperature values obtained by other workers [7-lo]. As observed by KORNBLUMand DE GALAN [l], the temperatures obtained differ widely. For laboratory plasmas, the parameter which determines whether LTE exists in the system is the magnitude of n,. One criterion for LTE has been proposed by GRIEM[ll] who considered the various processes concerned in the population and depopulation of a given energy level in a plasma. The criterion adopted was that the collisional depopulation rate should be 10 times greater than the radiative depopulation rate for less than 10% deviation from LTE. This consideration leads to a requirement for LTE that n,(mp3) 2 9.2 x 10z3 (Z,,
ye>‘,
(1)
where EH is the ionization energy of hydrogen (in eV), k is Boltzmann’s constant, and (E,-El) represents the largest energy level difference in the term system (in eV). For T, = 7000 K (0.603 eV), E2-E1 = 11.548 eV, and EH = 13.597 eV, this yields for argon a value of n, 2 1.19 x 1O23m-3. Griem also concluded that if the resonance lines are self-absorbed, the above criterion could be relaxed by up to an order of magnitude. If (i) we adopt the criterion of the optical depth (7) being ~1 at the line centre for predominantly Doppler broadened lines under conditions of appreciable selfabsorption (Stark broadening of the argon resonance lines would be expected even at high electron densities to be relatively small), (ii) use the formula given by CILLIERS,HAY and RASH [12], viz.
(2) where r, = classical electron radius (=2.818x lo-l5 m), A = wavelength (m); d = thickness of the emitting layer (m); fi2 = absorption oscillator strength; N; = ground state population (m-“); c = velocity of light (m s-l) and M = the mass of the radiator (kg); and (iii) take oscillator strength data from WIJZSEet al. [13], we find the values of 7 with an argon ground state atom density of 1.22 x 1O24rne3, T = 6000 K and d = 10e2 m, to be 75200 for the 106.67 nm line and 308,000 for the 104.82 nm line. Using the Holstein approximation g(T) = [~(r In 7)1’2]-1,
(3)
the escape factors g(T) are thus found to be 2.2~ 1O-6 and 5.2X lo-’ respectively [14] and thus the Ar resonance lines are almost totally self-absorbed. Hence, for the argon ICP, LTE can be expected to prevail only at electron densities greater than ca. 1 x 1O22rnp3. This value agrees well with measurements made by BACRI et al. [15] in an argon arc. These authors found departures from LTE at electron densities lower than ca. 2 x 1O22rnp3. The argon 4s resonance and metastable levels were found to be overpopulated at lower electron densities. The overpopulation of the 4 s levels was the consequence of radiative transitions from upper energy levels which were not balanced by the inverse absorption transitions. Electron densities measured from non-inverted 43 profiles yield values of about an order of magnitude lower in the “optimum viewing [71 J. F. ALDER and J. M. MERMET, Spectrochim. Acta 28B, 421, (1973). @I J. JAROSZ, J. M. MEEWET and J. P. ROBIN, C. R. Acad. Sci. Paris, 278B, 885 (1974). r91 G. R. KORNBLUMand L. DE GALAN, Spectrochim. Acta 29B, 55, (1978). r101 J. JAROSZ, J. M. MERMET and J. C. ROBIN, Spectrochim. Acta 33B, 55 (1978). [Ill H. R. GRIEM, Plasma Spectroscopy. McGraw-Hill, New York, (1964). Cl21W. A. CELIERS, J. D. HEY and J. P. S. RASH, J. Quant. Spectrosc. Radiat. Transfer 15,963,(1975). 1131W. L. WESE, M. W. SMITH and B. M. Mnxs, Atomic Transition Probabilities, Vol II, NSRDS-NBS 22 (1969). ZWICKER, Plasma Diagnostics, (edited by W. LOCHTE-HOLTGRE~), Chapter Amsterdam (1968). Cl51J. BACRI, A. M. GOME~ and S. BENZAID, J. Phys. D. Appl. Phys. 9, 1943, (1976).
[141 H.
4. North Holland,
Electronic excitation and ionizationtemperature measurements
165
zone” in the ICP [16]. Since BACRI et al. [15] also worked under optically thick conditions for the resonance lines, it seems justifiable to assume the non-existence of LTE for argon in that part of the tail-flame of the ICP used for simultaneous multielement analysis (SMEA). The non-existence of LTE in the low-power argon ICP is manifested in the spectral characteristics of the source. In this paper we show that there is no unique excitation temperature for Fe I in the argon ICP, as the excitation temperature is dependent upon the energy levels of the selected thermometric species when LTE does not prevail. The excitation temperatures obtained are then compared with ionization temperatures obtained from a number of elements with widely different ionization potentials. Finally, the significance of water vapour in providing a substantial increase in the electron density and excitation and ionization temperatures in the central channel is also discussed. 2. THEORY 2.1 Iron electronic excitation temperature measurements The intensity I,, of a spectral line is given by the equation: = 1he2NAgpf&E~‘kT
I gP
2A~,Whm,
’
(4)
where 1 is the length of the emitting region, h = Planck’s constant, e = the electron charge, Q, = the permittivity of free space, m, the mass of the electron, NA the total number of atoms of species A, g, the statistical weight of the lower state p and U(T) the partition function of species A. A graph of log &,h3/g&,) vs E,, the energy of the upper level q, should, under LTE conditions, yield a straight line with a slope of (-2.303 kT)-‘, from which T,, can be determined provided that errors in the intensity measurements, inconsistency in the transition probabilities and inhomogeneous emitting layers are absent. Any deviation from a straight line will represent a deviation from a thermal (LTE) distribution which can then be interpreted to correspond to an overor under-population of various energy levels. In this study, Fe I was chosen and a set of twenty lines was selected from the data of BRIDGES and KOFWBLITH [17] (Table 1). These authors measured absolute transition probabilities for 534 Fe I lines and performed a thorough analysis of all previous work on Fe I, stating that their values were believed to be consistently more accurate; no data from other authors were therefore used. The range of upper energy levels covered was 26,875-55,754cm-l and all oscillator strengths were estimated to be accurate to f 10%. BRIDGET and KOFWBLITH [17] used a well-stabilized argon arc and found no change in the measured Fe I oscillator strengths when the electron density was varied from 2 x 10” to 5 x 10” rnp3, although inconsistencies appeared for lower electron densities. These deviations are thus a direct indicator of departures from a thermal population of the Fe I levels and from the foregoing we see that such deviations are to be expected for values of n, below cu. 1022m-3. 2.2 Ionization temperature measurements The Saha equation:
can be used to relate total densities,
spectral lines originating
n, of species of charge z and (z + 1). For two from levels q(ion) and j(atom), the Saha and Boltzmann
1161 D. J. KALNICKY, V. A. FASSELand R. N. KNSELEY, Appl. Specrrosc.31, 137 (1977). [17] J. M. BRIDGES and R. L. KORNBLI-IH, AstrophysJ. 192,793, (1974).
J. F. ALDER,
166
R. M. BOMB=
and G. F. KIRKBRIG~
Table 1. Energy levels and oscillator strengths of Fe I emission lines employed
Wavelength
1020A3
Ek
A(nm)
(cm-‘)*
368.222 368.411 370.108 370.446 371.993 372.438 372.762 373.239 373.486 373.713 373.830 374.826 374.948 375.361 375.823 376.005 376.053 376.379 376.554 376.719
55754 49135 51192 48703 26875 45221 34547 44512 33695 27167 53094 27560 34040 44184 34329 45978 44512 34547 52655 34692
2.6202 9.5279 5.1878 18.8875 13.8549 25.8918 17.1512 16.4422 2.5517 19.3916 4.6561 53.8876 3.5638 41.0532 5.3082 26.0364 82.3658 8.2580 1.5757 11.6964
* Ek = excitation energy of the emitting level. Oscillator strength data from BFUDGES and KORNBL~~H[17]. Table 2. Emission line data employed for evaluation of TIoN via the Saha equation A
(cm-‘)
[600uo K]
Mg II
285.213 279.553
35051 35761
1.05 2.00
14.85 [13] 10.72 [13]
14.85 10.72
CaI Ca II
422.673 396.847
23652 25192
1.42 2.40
6.54 [13] 2.92 [13]
6.54 2.92
Cd1 Cd II
228.802 226.502
43692 44136
1.006 2.000
14.28 4.16
[18], 15.79 [19] [18], 5.88[19]
15.035 5.02
ZnI Zn II
213.856 206.191
46745 48481
1.004 2.000
17.13 6.56
[18], 20.69 [19] [18], 6.59[19]
18.91 6.575
Fe I Fe II
252.285 258.588
39626 38660
33.1 [20], 25.15 [21] 5.087 [22], 10.93 [23]
29.125 8.009
BaI Ba II
553.548 493.409
18060 20262
Ti I Ti II
451.274 450.127
28896 3 1,207
WI
181 191 201 211 221 231 241
Mean Ag, (10%‘)
Ek
(nm)
Species
31.6 47.6 3.85 4.82 36.0 61.2
3.45 [24] 1.91 [24]
3.45 1.91
0.950 [25] 1.393 [26]
0.950 1.393
S. R. BAUMANNand W. H. SMITH, J. Opt. Sot. Am. 60, 345, (1970). T. ANDERSEN and G. SORENSEN,J. Quant. Spectrosc. Radiat. Transfer 13, 369 (1973). C. H. CORUSS and J. L. TECH, NBS Monograph 108 (1968). F. P. BANFIELDand M. C. E. HUBER, Astrophys. J. 186, 335, (1973). G. E. Assous~ and W. H. Sm, Astrophys. J. 176, 259 (1972). M. C. E. HUBER, Astrophys. .Z. 190, 237, (1974). B. M. MILI?S and W. L. WIESE, Critically evaluated transition probabilities for BaZ and Ball, Technical Note 474 (1969).
NBS
Electronic
excitation
and ionization
temperature
measurements
167
equations can be combined to yield the well-known relation in LTE:
I,‘, _ 2g’A’Aii (2Tr$T)3’z exp [-(Ei
XL- negjAj&
- Ej + Ei,-
AEi,)/kT] ,
(6)
where A is the Einstein spontaneous emission transition probability and hEion is the lowering of the ionization energy. Seven elements with lines whose transition probabilities are known with moderate accuracy from recent literature were selected, viz. Mg, Ca, Zn, Cd, Ti, Ba and Fe. The wavelengths employed and additional data for these lines are given in Table 2. The line pairs were selected on the grounds of similarity of excitation energy and closeness of wavelength. Transition probabilities obtained by CORLISSand BOZMAN[27] were not considered sufficiently accurate to allow choice of additional elements [28]. Partition function data were extrapolated from DRAWINand FELENBOK [29] at T= 6000 K and the lowering of the ionization energy was calculated from the Debye formula [30]: (7) with n, = 1021 me3 and T, = 6000 K. The value derived from this equation, 0.012 eV or 100 cm-l, was used in the subsequent calculations. The UnsGld formula for the lowering of the ionization energy used by KALNICKY er al.[16] is considered incorrect [31] yielding a value high by a factor of ca. 4. 3. INSTRUMENTAL The instrumental system, consisting of a 2 kW high frequency plasma generator (International Plasma Corporation, Model 120-27) and monochromator (Rank Hilger Ltd., Monospek 1000) fitted with an EM1 6256B photomultipher tube and a 1200 grooves mm-l diffraction grating blazed at 330 nm, has already been described elsewhere [32]. The viewing height was selected by racking the complete coupling-box and torch assembly vertically. A 2 mm high viewing zone through the centre of the plasma was always chosen, and the entrance and exit slits of the monochromator were set at 15 pm corresponding to a spectral half band pass of ca. 0.019nm. The photomultiplier output was fed directly to a chart recorder (Servoscribe Model RE 541.20) across a 22 KQ input resistor. Measurements were performed with a demountable silica torch which terminated 2 mm above the top of the load coil. An all-glass concentric nebulizer (Meinhard Associates, Model T-230-A*) was used (uptake rate 0.93 ml min-’ at an argon flow rate of 0.9 1. min-‘, efficiency 2.2%; uptake rate of 1.20 ml min-* at an argon flow rate of 1.31 min-‘, efficiency 2.7%). The internal diameter of the injector tube orifice, governing the injector gas velocity and thus a primary factor for comparison of torch performance between different authors, was 1.60* 0.02 mm.
[25] S. J. WOLNIK and R. 0. BERTHEL, Astrophys. J. 179, 665, (1973). [26] J. R. ROBERTS, T. ANDERSEN and G. SORENSEN, Astrophys. J. 181,567,(1973). [27]C. H. CORLISS and W. R. Bow, Experimental Transition Probabilities for Spectral Lines of Sever@ Elements, NBS Monograph 53. U.S. Government Printing Office, Washington 25, D.C. (1962). [28] B. WARNER and C. R. COWLEY, J. Quant. Spectrosc. Radiat. Transfer 7, 751, (1967). [29] H. W. DRAWIN and P. FELENBOK, Data for Plasmas in Local Thermal Equilibrium. Gauthier-Villars Paris (1965). [30] A. P. T&o-, Spectrophysics. Chapman and Hall London (1974). [31] H. N. OLSEN, Proc. V Bienn. Gas Dynamics Symp., Evanston 1963 Ch. 3,Northwestern (1963). [32] J. F. ALDER, A. M. GUNN and G. F. KIRKBRIG~, Anal. Chim. Acta 92, 43, (1977).
Univ. Press
168
J. F. ALDER, R. M. BOMESELKA and G. F. KIRKBRIGHT
4.
EXPERIMENTAL
4.1 Excitation and ionization temperature measurements
and the determination of the
electron density
Relative line intensities for the set of twenty Fe I lines and for the ion and atom lines of the selected species were determined at various viewing heights. A standard tungsten ribbon lamp was used to calibrate the response of the detection system at different wavelengths and all measured intensities were corrected for all lines with A greater than 290 nm. Regression lines were calculated to evaluate T,, for the Fe I levels in four different ways: (i) T,, calculated from all Fe I levels; (ii) T,, calculated from Fe I levels between 26,875-34,692 cm-‘; (iii) T,, calculated from Fe I levels between 33,695-45,978 cm-l; (iv) T,, calculated from Fe I levels between 44,184-55,754 cm-l. In addition, excitation temperatures from the Fe 1 373.486/373.713 nm line pair are also presented for comparison with later work. A 100 pg ml-’ Fe solution was used to examine the lower energy lines (up to 34692 cm-‘) in order to minimize self-absorption and a 1000 pg ml-’ Fe solution was used for the remainder. For the ionization temperature measurements, all solutions were 20 pgml-’ except for Fe and Ti (100 pg ml-l). The half-width of HP at 486.1 nm was used to determine n,, the hydrogen arising from the decomposition of the aqueous solutions nebulized. GRIEM [ll] has tabulated values of c(T, n,) for the expression: n, = c(T, n,) A:;:, where c(T, n,) is a weak function of both parameters. We have interpolated his values at ~(6000 K, n,) in order to calculate n, from the measured half-widths. The instrumental half-width for a 15 pm slit was 0.019 nm in the first order. The Doppler half-width AA& of Ha at 6000 K, 0.027 nm, was convoluted with instrumental half-width AA:,, (assumed Gaussian) to give a combined Gaussian contribution, AAl,*, to the half-width of 0.033 nm, from the expression: (Ah1,2)2= (Ah$)‘+ (AAi,)2. This Gaussian contribution was then removed from the measured half-width of I-& with the aid of tables of half-widths of the Voigt function [33]. The resulting values of AA,,,(0.04-0.5 nm) were then used to evaluate n,(l X 1020- 4 x 1021 mp3) from Griem’s expression and the values obtained at high electron densities should be reasonably accurate as then the contribution from the Doppler and instrumental broadening is very small. The background under the line was taken as that value at 484.0 nm. 4.2 Variation of the injector flow rate and aerosol desolvation Electron densities and cadmium ionization temperatures were measured at three different viewing heights in the central channel, whilst the argon carrier gas flow-rate was varied in two different ways: (i) by increasing the input pressure to the nebulizer in steps of 69 kPa over the range 172-517 kPa; (ii) by adding dry argon via a flow-meter to the aerosol leaving the spray chamber. At a constant flow-rate, the latter procedure would be expected to yield a lower constant water transport rate into the plasma. A small U-tube filled with up to 7 g of coarse silica gel was used to remove various amounts of water from the aerosol, the quantity of water removed being a function of the weight of the silica gel. Taps were arranged such that the aerosol leaving the spray chamber could either enter the plasma directly or first pass through the U-tube; no significant reduction in the argon flow-rate through the U-tube was observed. The electron density was determined from the half-width of HP and correlated with the [33]
W. L. WISE, Plasma Diagnostic Techniques Chapter 6. Academic Press, New York (1965).
(Mited
by R. H. HUDDLEST~NE
and S. LEONARD),
169
Electronic excitation and ionization temperature measurements
continuum intensity variation at 484.0 nm. The cadmium ionization and iron excitation temperatures from the Fe I 373.486/373.713 nm line pair were also determined. 5. RJSJLTSAND DISCUSSION 5.1 Excitation and ionization temperature measurements The excitation and ionization temperatures obtained under different operating conditions are presented in Table 3. Both BOUMANS and DE BOER [34] and MEFCMET[35] have suggested that sample excitation and ionization by interaction with metastable species in the argon ICP can explain why ion lines are more strongly excited than expected for a plasma with a relatively low (ca. 6000 K) excitation temperature. BOUMANSand DE BOER [34] have published values of (IJIa)_/(Ii/IJI where these Table 3. Excitation and ionization temperatures obtained experimentally*? Viewing height (mm)
10
20
30
10
20
30
5990
*270 4600 *200
6140 *250 5160 *270
*170 4480 *200
20 Fe I levels
7490
6910
9 Fe I levels from 26875 to 34692 cm-’
*190 6920 l440
*290 5990
*I330
5370 *210 4660 +250
11 Fe I levels from 33695 to 45978 cm-’
7190 *290
7000 *t810
5230 *290
6130 =t280
5890 *240
4920 *180
11 Fe I levels from 44184 to 55754 cm-i
8510 *390
7720 *1280
6220 *810
7040 *610
7620 *590
6010 *530
Fe I 373.491373.71 nm
7620
6510
4860
4690
5380
4700
Mg II 279.5/ Mg I 285.2 nm
7950
7650
6860
7140
6680
5930
Ca II 396.8/ Ca I 422.7 nm
8100
8110
6320
6480
6800
5610
Cd II 226.5/ Cd I 228.8 nm
7940
7700
7130
7390
6850
6260
Zn II 206.1/ Zn I 213.9 nm
7720
7530
7040
7140
6600
6110
Fe II 258.6/ Fe I 252.3 nm
8260
7850
6960
7510
6890
6060
Ba II 493.4/ Ba I 553.5 nm
8440
7650
6470
6040
7060
5580
5940
6150
5250
Ti II 450.1/ TiI 451.2nm Mean T from II/I ratio Electron density n, (m-Y Carrier gas flow-rate (I mini)
5090
8070 rt260 3.34 x 102’
7750 *210 2.27 x 10”
6800 *330 7.44 x loa0
6810 k.650 2.23 x lO*i
6720 *290 6.38 x 1020
5830 l360 1.77 x 1020
0.90
0.90
0.90
1.30
1.30
1.30
* Power 1200 W; plasma gas flow rate = 12.8 1.mm’; auxillary gas = 0 1. mm’. t The errors in the excitation temperatures were evaluated using a program written for the HewlettPackard HP97 calculator to obtain the error in the slope of the regression line and represent 20 uncertainties in the excitation temperature, i.e. the excitation temperatures lie with 95% probability within the stated limits. [34] P. W. 3. M. BOUMANSand F. J. DE BOER,Spectrochim. Acta 32B, 365 (1977). [35] J. M. h&m, C. R. Acad. Sci. Paris 28lB, 273, (1975).
170
J. F.
ALDER,
R. M. BOMBELKA and
G. F. KIRK~RIGHT
ratios ranged from one to three orders of magnitude. Similarly, KAJ_NICKY et al. [16] using space resolved radial values of (Ii/I,),, have calculated the electron density, n,, via equation (6) and found values of n, from 30-50 times lower than the measured Abel inverted values derived from H,. We believe that the values of n, derived from H, are essentially correct and that there are no additional electric forces acting in the plasma which might further broaden Ha, which are not accounted for in the present line-broadening theories. The ionization temperatures derived from our measurements on seven elements with a wide range of ionization potentials, under various conditions and using measured averaged values of n,, are found to be similar to the values derived from the Fe I levels of high excitation energy. It is therefore possible that the true value of T, is closer to the value derived from high Fe I levels rather than the value derived from the low energy Fe I levels. This interpretation follows from the fact that, for a non-thermal level distribution, the thermal limit, defined as the level above which all higher levels have a true Boltzmann population at T,, decreases as n, increases and the plasma becomes more collision dominated until, at a sufficiently high electron density, LTE is achieved down to the ground level. Our Fe I excitation temperature values at 10 mm using the Meinhard nebulizer at an argon injector flow rate of 0.9 1.mine1 demonstrate that under these conditions the Boltzmann plot is tending towards a straight line, indicating an approach to LTE conditions. All of our Fe I excitation temperatures indicate an increasing trend with upper energy level. However, from Fig. 1, it is apparent that either the high levels are overpopulated with respect to the low levels at a low electron temperature, or that the low levels are overpopulated with respect to the high levels at a higher electron temperature. Although this effect could be due to a non-homogeneous emitting region, it is more likely that any effects due to inhomogeneity are small and that the lower Fe I levels are over-populated at a higher electron temperature due to radiative decay of upper levels down to these levels which is not balanced by the inverse absorption processes. Under all conditions studied, no unique excitation temperature for Fe I was found and thus the non-LTE state of this plasma has been demonstrated for our experimental conditions. The ionization temperatures derived from the seven species are in all cases in reasonable agreement. This result is perhaps surprising, as non-thermal level distributions are even more probable for ionic species under the conditions employed in this work due to their higher ionization potentials, but in all cases atomic and ionic lines with similar excitation
Fig. 1. Boltzmann plot for Fe1 levels at a viewing height of 20 mm. Power = 1200 W; plasma gas flow-rate = 12.8 Lrnin-‘; carrier gas flow rate = 1.30 l.min-‘. The derived excitation temperatures are shown in column 5 of Table 3.
Electronic excitation and ionization temperature measurements
171
energy were chosen so that they should be similarly affected. The excitation temperatures measured from HP, I-I, and El, in the ICP [36] also reflect the fact that non-thermal level populations must be expected for species at the conditions employed in the ICP. In view of this fact, two-line temperature measurements using low energy lines with accurately known transition probabilities cannot be relied upon to yield the correct electron temperature in the ICP and the excitation temperatures obtained should be interpreted with caution. 5.2 The influence of the carrier gas flow rate and water on plasma parameters The variation in the cadmium ionization temperatures determined from the Cd II 226.5 nm/Cd I 228.8 nm line pair and the electron density at three different viewing heights when the argon injector flow rate was varied is shown in Figs. 2 and 3. The effect of the variation in the water injection rate is masked by the large dependence of these parameters on the injector flow rate. In order to study the effect of water, all gas flows were kept constant and some water was removed using the U-tube containing silica gel. The reduction in electron density, using H,, and cadmium ionization temperatures are depicted in Figs. 4 and 5, and a significant reduction in the electron density was observed when water was removed. The absolute recombination continuum intensity is proportional to ~zZ/T,~‘~and also showed a decrease which is correlated with the reduction in the electron density in Fig. 6. In order to examine this effect further, measurements of the electron density and cadmium ionization and iron excitation temperatures at different viewing heights were performed, and the results are presented in Figs. 7-9. It can be seen that the presence of water in the aerosol is noticeable at all viewing heights and significant at viewing heights above 15 mm, thus increasing the electron density and ionization and excitation temperatures in the ICP. BOUMANS and DE BOER [37] estimated that the water entering the central channel would consume about 20% of the total power input to the carrier gas itself, implying a decrease in temperature with water present. For a nebulizer working at 3% efficiency and a carrier gas flow rate of 1.01. min-‘, the mole ratios of argon, hydrogen and oxygen atoms in the plasma are in the ratio Ar : H : 0 as 89.9 : 6.7 : 3.4, as at 6000 K the concentrations of other species such as O,, H2, OH, H+, 0’ and 02+ are relatively low [38]. As the ionization energies of hydrogen and oxygen are about 2 eV below that
”
6000 -
‘*A
I 14
I IO
d6 Injector
argon flow
rate,
I 18
Cmin.’
Fig. 2. Cadmium ionization temperatures under the following conditions: 0 nebulizer pressure increased to increase the argon flow rate, x nebulizer pressure constant at 140 kPa, and dry air added to the gas stream. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘. [36] K. VISSER, F. M. HAMM and P. B. ZEEMAN, Appt. Spectrosc. 30, 34 (1976). [37] P. W. J. M. BOUMANSand F. J. DE BOER, Spectrochim. Acta 31B, 355 (1976). [38] F. BURHORN and R. WIENECKE,Z. Phys. Chem. 215, 285 (1960).
172
J. F.
ALDER,
”
R. M. BOMBELKA and G. F. KIRKBRIGHT
I 0.6
I IO Injector
I IB
I 1.4
cfgon
flow
1min.’
rate,
Fig. 3. Electron densities under the following conditions: 0 nebulizer pressure increased to increase the argon flow rate, x nebulizer pressure constant at 140 kPa, and dry Ar added to gas stream. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘.
of argon, their degrees of ionization at 6000 K would be expected to be about five times higher than that of argon and hence an increase in the electron density would be expected when water is present. We have observed a 30°k reduction in the electron density when the aerosol was dried substantially. The presence of water vapour and its significance in increasing the electron density can partially explain why the measured electron densities with water present are higher than the LTE values for a pure argon plasma, i.e. 2.94 x 1019(6000 K) to 6.64 x 1020(7500 K) [31]. 5.3 Analyte excitation mechanisms in the argon ICP BOLJMANSand DE BOER [34] have recently proposed a mechanism for the ICP operating in argon under analytical conditions in which metastable argon play a central role both as an easily ionizable constituent in the plasma and as a reactant with the analyte species, through Penning ionization. ~~ERMET [35] has already earlier postulated
“I2 Massof
0.1 water
02 collected,
g
Fig. 4. Variation in the electron density (observed from Ho) on removal of some water vapour from a 10.0 ml sample solution after nebulization. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘; carrier gas flow-rate = 0.88 1. min-*; viewing height = 20 mm; nebulizer efficiency 2.16%.
Electronic
excitation
and ionization
I
temperature
I
Mass
measurements
173
I
ai
0.2
ofwater collected,
g
Fig. 5. Variation in the cadmium ionization temperature on removal of some water vapour from a 10.0 ml sample solution after nebulization. Power = 1200 W; plasma gas flow-rate= 12.8 1. min-I; carrier gas flow-rate = 0.88 1. min-‘; viewing height = 20 mm; nebulizer efficiency = 2.16%.
similar Penning ionization mechanisms and also proposed excitation mechanisms with ionized argon metastable ions. There are presently two competing theories for analyte excitation processes in the argon ICP: direct electron collisional excitation and excitation by metastable argon atoms. Argon has two metastable energy levels, 4s3Pz, ‘e; the other two 4s levels, ‘e and le, are radiatively linked to the 3P6 ‘S, ground level. Although the lifetimes of the argon metastable species in vacuum are greater than 1.3s [39] their lifetimes in the ICP are shorter by at least nine orders of magnitude due to the high electron collisional transition rates between all four 4s Ar I levels [40]. Nevertheless, because of the almost total self-absorption of the argon resonance lines, all four 4s levels behave as “pseudometastables” showing a rather long collective efficient lifetime [39], although, as stressed, the actual mean lifetime of an excited argon atom in any one of the 4s states in the ICP is extremely short. This does
Electron density from li@mtio
Fig. 6. Correlation between continuum intensity and electron density from HP as increasing quantities of water vapour were removed from a lO.Ornl sample solution after nebulization. The slope of the line is 0.93 and “ratio” represents the value of the measured parameter with some water removed to the value with no water removed; the continuum intensity was measured at 484.0 nm and all parameters are the same as in Fig. 4.
I
WI J. L. DELCROIX, C. M. FERREIRA and A. RICARD, Principles of Larer Plasmas (edited by G. BE-), Chapter 5. J. Wiley, New York (1976). 1401 S. VAC~UIJS, J. P. DINGUIRARD, H. KAFROUNI and I. PAGES, J. Quunt. Spectrosc. Radiat. Transfer 17, 755 (1977).
174
J. F. ALDER, R. M. BOMJSJSLKAand
G. F. KIRKEWGHT
IO Viewing
Fig. 7. Electron
density
20 height,
mm
(from Ho) in the central channel with water water removed x Parameters as in Fig. 4.
0 and with most of the
not, however, rule out the possibility of the excited argon atoms playing a role in analyte excitation processes. MCGREGOR [41] in 1963 working with an argon plasma jet suggested that the relatively strong nitrogen second positive band system [N2(C311,,) --, N,(B311,)] observed was the direct result of energy transfer between a metastable argon atom and a ground state nitrogen molecule. The N2(C311,) state is similar in energy to the argon 4s levels and therefore an energy transfer is feasible. This same band system is also easily observed when air is entrained into the conventional short torch in the ICP and can be effectively excluded when an extended torch, whose outer silica tube has been lengthened by 30 mm, is used with observation through the silica wall. The extended torch also reduces the observed intensity of other band systems like NO, CN, N2+and NH, and we have found it very useful for examination of the variation of the true argon recombination continuum with wavelength. Penning ionization is an electron exchange process involving an intimate, short duration collision. In this process, the atom M is ionized by donating an electron to the (metastable) argon atom, which in turn ejects an electron, collapsing to the 3p6 ‘S, ground state. As the central channel in the ICP has been shown not to be in LTE, the electron temperature T, in that zone must necessarily be higher then the heavy particle temperature T,, resulting in a two-temperature plasma. A difference between T, and ‘I,
I
Y
!*Ooo\ .$ .P
7500
-
‘x.X
E -A -2 B ”
X
\ x, a
I
I IO Vv?wing
Fig. 8. Cadmium
I 20 helgM,
mm
ionization temperature in the central channel with water the water removed X. Parameters as in Fig. 4.
[41] W. K. MCGREGOR, Pm. University Press (1963).
0 and with most of
V Bienn. Gas Dynamics Symp., Evanston 1963, Chapter
8. Northwestern
Electronic
excitation
and ionization
temperature
Viewmg hight ,
measurements
175
mm
Fig. 9. Fe I excitation temperatures from the 373.49 nm/373.71 nm lines at different viewing heights with water 0 and with most of the water removed X. Parameters as in Fig. 4.
is a necessary consequence of a departure from a thermal level population and is due to the electron-atom collision frequency being too low. We expect T, to be closer to the value derived from the Fe I levels of high excitation potential rather than the value from the commonly used lower levels. A high electron temperature would considerably reduce the importance of analyte ionization via Penning ionization with excited argon atoms (not only metastable atoms [34]) as the dominant ionization mechanism and we expect that analyte excitation and ionization in the ICP are primarily dominated by electrons, with radiative deexcitation influencing the analyte level populations and giving rise to the observable nonthermal effects.
Acknowledgement-We to one of us (R.M.B.).
wish to thank the Science Research
Council for the award of a research studentship
Electronic excitation and ionization temperature measurements in a high frequency inductively coupled argon plasma source and the influence of water vapour on plasma parameters J. F.
ALDER,
R.
M.
BOMESELKA*
and G. F.
KIRKBRIGHT
ChemistryDepartment, Imperial College, London SW7, U.K. (Receiued 25 April 1979; in revised form 16 October 1979) Abstiet-Excitation temperatures measured from Fe I lines with a wide range of upper energy levels have shown that the environment of the central channel of an argon ICP is not in a state of local thermal equilibrium, as predicted from Griem’s criterion. Deviations from a Boltzmann distribution have been observed in the population of the excited states of Fe I. The significance of water in providing extra electrons by ionization in the ICP is demonstrated and discussed.
1.
INTRODUCTION
ELECTRON density and temperature measurements in the type of high frequency inductively coupled argon plasma (ICP) used for analytical atomic emission spectroscopy have been made by numerous workers over a range of operating and observation conditions. The methods used for these measurements have been discussed recently by KORNBLLJM and DE GALAN [l]. If local thermal equilibrium (LTE) prevails in the source, a commonly employed convenient method of temperature determination is the “two-line” method [2] which, with the addition of a suitable thermometric species, yields the electronic excitation temperature, T,, which can then be equated to the electron temperature, T,. The ionization temperature, Ti,,may be determined from measurements of the relative intensities of an atom and ion line of a suitable species and knowledge of the electron density, n,, via the Saha equation. Under LTE conditions the value obtained is again equivalent to the electron temperature, T,. Two methods are commonly employed for the determination of the electron density in ICP sources; these are the HP line-broadening method and via the measurement of the absolute intensity of the argon recombination continuum at a suitable wavelength. These methods are fundamentally quite accurate, theoretical considerations giving rise to less than 10% uncertainty in the values obtained; greater errors may be incurred in the experimental observations and the application of the Abel inversion technique to the data obtained. One of the most useful features of both of these methods, however, is that the values of n, obtained may be employed directly to evaluate the extent to which LTE prevails in the system without the need to resort to rotational, vibrational and electronic excitation temperature measurements for a wide range of species. Experimental results available from measurements with the argon ICP indicate that over a range of operating conditions and heights of observation above the core in this source, LTE does not prevail [3-51. This has been shown, for example, from determination of the translational gas temperature, Tg, by measurement of the Doppler width of emission lines from analyte species [6] and comparison with other excitation and
* Present address: Institut Fiir Spektrochemie und Angewandte Spektroskopie, Bunsen-Kirchhoffstr. 4600 Dortmund, West Germany.
11,
G. R. KOFWELUM and L. DE GALAN, Specfmchim. Acta 32B, 71 (1977). [2] P. W. J. M. BOUMANS, Theory of Spectrochemical Excitation. Adam Hilger, London (1966). [3] J. M. MERMET, Spectrochim. Acta 3OB, 383, (1975). [4] J. M. ~~ERMET, J. JAROSZ and J. P. ROBIN, Preprints 17th Coil. Spectrosc. Int., Florence 1973, Vol. 1, p. 101 (1973). [5] J. M. MERMET, Thesis. University of Lyon (1974). [6] H. G. C. HUMAN and R. H. SCOTT, Spectrochim. Acta, 3lB, 459, (1976).
[l]
SA(B I--r\
163
164
J. F. ALDER,
R. M. BOMBELKA and G. F. KIRKBRIGHT
ionization temperature values obtained by other workers [7-lo]. As observed by KORNBLUMand DE GALAN [l], the temperatures obtained differ widely. For laboratory plasmas, the parameter which determines whether LTE exists in the system is the magnitude of n,. One criterion for LTE has been proposed by GRIEM[ll] who considered the various processes concerned in the population and depopulation of a given energy level in a plasma. The criterion adopted was that the collisional depopulation rate should be 10 times greater than the radiative depopulation rate for less than 10% deviation from LTE. This consideration leads to a requirement for LTE that n,(mp3) 2 9.2 x 10z3 (Z,,
ye>‘,
(1)
where EH is the ionization energy of hydrogen (in eV), k is Boltzmann’s constant, and (E,-El) represents the largest energy level difference in the term system (in eV). For T, = 7000 K (0.603 eV), E2-E1 = 11.548 eV, and EH = 13.597 eV, this yields for argon a value of n, 2 1.19 x 1O23m-3. Griem also concluded that if the resonance lines are self-absorbed, the above criterion could be relaxed by up to an order of magnitude. If (i) we adopt the criterion of the optical depth (7) being ~1 at the line centre for predominantly Doppler broadened lines under conditions of appreciable selfabsorption (Stark broadening of the argon resonance lines would be expected even at high electron densities to be relatively small), (ii) use the formula given by CILLIERS,HAY and RASH [12], viz.
(2) where r, = classical electron radius (=2.818x lo-l5 m), A = wavelength (m); d = thickness of the emitting layer (m); fi2 = absorption oscillator strength; N; = ground state population (m-“); c = velocity of light (m s-l) and M = the mass of the radiator (kg); and (iii) take oscillator strength data from WIJZSEet al. [13], we find the values of 7 with an argon ground state atom density of 1.22 x 1O24rne3, T = 6000 K and d = 10e2 m, to be 75200 for the 106.67 nm line and 308,000 for the 104.82 nm line. Using the Holstein approximation g(T) = [~(r In 7)1’2]-1,
(3)
the escape factors g(T) are thus found to be 2.2~ 1O-6 and 5.2X lo-’ respectively [14] and thus the Ar resonance lines are almost totally self-absorbed. Hence, for the argon ICP, LTE can be expected to prevail only at electron densities greater than ca. 1 x 1O22rnp3. This value agrees well with measurements made by BACRI et al. [15] in an argon arc. These authors found departures from LTE at electron densities lower than ca. 2 x 1O22rnp3. The argon 4s resonance and metastable levels were found to be overpopulated at lower electron densities. The overpopulation of the 4 s levels was the consequence of radiative transitions from upper energy levels which were not balanced by the inverse absorption transitions. Electron densities measured from non-inverted 43 profiles yield values of about an order of magnitude lower in the “optimum viewing [71 J. F. ALDER and J. M. MERMET, Spectrochim. Acta 28B, 421, (1973). @I J. JAROSZ, J. M. MEEWET and J. P. ROBIN, C. R. Acad. Sci. Paris, 278B, 885 (1974). r91 G. R. KORNBLUMand L. DE GALAN, Spectrochim. Acta 29B, 55, (1978). r101 J. JAROSZ, J. M. MERMET and J. C. ROBIN, Spectrochim. Acta 33B, 55 (1978). [Ill H. R. GRIEM, Plasma Spectroscopy. McGraw-Hill, New York, (1964). Cl21W. A. CELIERS, J. D. HEY and J. P. S. RASH, J. Quant. Spectrosc. Radiat. Transfer 15,963,(1975). 1131W. L. WESE, M. W. SMITH and B. M. Mnxs, Atomic Transition Probabilities, Vol II, NSRDS-NBS 22 (1969). ZWICKER, Plasma Diagnostics, (edited by W. LOCHTE-HOLTGRE~), Chapter Amsterdam (1968). Cl51J. BACRI, A. M. GOME~ and S. BENZAID, J. Phys. D. Appl. Phys. 9, 1943, (1976).
[141 H.
4. North Holland,
Electronic excitation and ionizationtemperature measurements
165
zone” in the ICP [16]. Since BACRI et al. [15] also worked under optically thick conditions for the resonance lines, it seems justifiable to assume the non-existence of LTE for argon in that part of the tail-flame of the ICP used for simultaneous multielement analysis (SMEA). The non-existence of LTE in the low-power argon ICP is manifested in the spectral characteristics of the source. In this paper we show that there is no unique excitation temperature for Fe I in the argon ICP, as the excitation temperature is dependent upon the energy levels of the selected thermometric species when LTE does not prevail. The excitation temperatures obtained are then compared with ionization temperatures obtained from a number of elements with widely different ionization potentials. Finally, the significance of water vapour in providing a substantial increase in the electron density and excitation and ionization temperatures in the central channel is also discussed. 2. THEORY 2.1 Iron electronic excitation temperature measurements The intensity I,, of a spectral line is given by the equation: = 1he2NAgpf&E~‘kT
I gP
2A~,Whm,
’
(4)
where 1 is the length of the emitting region, h = Planck’s constant, e = the electron charge, Q, = the permittivity of free space, m, the mass of the electron, NA the total number of atoms of species A, g, the statistical weight of the lower state p and U(T) the partition function of species A. A graph of log &,h3/g&,) vs E,, the energy of the upper level q, should, under LTE conditions, yield a straight line with a slope of (-2.303 kT)-‘, from which T,, can be determined provided that errors in the intensity measurements, inconsistency in the transition probabilities and inhomogeneous emitting layers are absent. Any deviation from a straight line will represent a deviation from a thermal (LTE) distribution which can then be interpreted to correspond to an overor under-population of various energy levels. In this study, Fe I was chosen and a set of twenty lines was selected from the data of BRIDGES and KOFWBLITH [17] (Table 1). These authors measured absolute transition probabilities for 534 Fe I lines and performed a thorough analysis of all previous work on Fe I, stating that their values were believed to be consistently more accurate; no data from other authors were therefore used. The range of upper energy levels covered was 26,875-55,754cm-l and all oscillator strengths were estimated to be accurate to f 10%. BRIDGET and KOFWBLITH [17] used a well-stabilized argon arc and found no change in the measured Fe I oscillator strengths when the electron density was varied from 2 x 10” to 5 x 10” rnp3, although inconsistencies appeared for lower electron densities. These deviations are thus a direct indicator of departures from a thermal population of the Fe I levels and from the foregoing we see that such deviations are to be expected for values of n, below cu. 1022m-3. 2.2 Ionization temperature measurements The Saha equation:
can be used to relate total densities,
spectral lines originating
n, of species of charge z and (z + 1). For two from levels q(ion) and j(atom), the Saha and Boltzmann
1161 D. J. KALNICKY, V. A. FASSELand R. N. KNSELEY, Appl. Specrrosc.31, 137 (1977). [17] J. M. BRIDGES and R. L. KORNBLI-IH, AstrophysJ. 192,793, (1974).
J. F. ALDER,
166
R. M. BOMB=
and G. F. KIRKBRIG~
Table 1. Energy levels and oscillator strengths of Fe I emission lines employed
Wavelength
1020A3
Ek
A(nm)
(cm-‘)*
368.222 368.411 370.108 370.446 371.993 372.438 372.762 373.239 373.486 373.713 373.830 374.826 374.948 375.361 375.823 376.005 376.053 376.379 376.554 376.719
55754 49135 51192 48703 26875 45221 34547 44512 33695 27167 53094 27560 34040 44184 34329 45978 44512 34547 52655 34692
2.6202 9.5279 5.1878 18.8875 13.8549 25.8918 17.1512 16.4422 2.5517 19.3916 4.6561 53.8876 3.5638 41.0532 5.3082 26.0364 82.3658 8.2580 1.5757 11.6964
* Ek = excitation energy of the emitting level. Oscillator strength data from BFUDGES and KORNBL~~H[17]. Table 2. Emission line data employed for evaluation of TIoN via the Saha equation A
(cm-‘)
[600uo K]
Mg II
285.213 279.553
35051 35761
1.05 2.00
14.85 [13] 10.72 [13]
14.85 10.72
CaI Ca II
422.673 396.847
23652 25192
1.42 2.40
6.54 [13] 2.92 [13]
6.54 2.92
Cd1 Cd II
228.802 226.502
43692 44136
1.006 2.000
14.28 4.16
[18], 15.79 [19] [18], 5.88[19]
15.035 5.02
ZnI Zn II
213.856 206.191
46745 48481
1.004 2.000
17.13 6.56
[18], 20.69 [19] [18], 6.59[19]
18.91 6.575
Fe I Fe II
252.285 258.588
39626 38660
33.1 [20], 25.15 [21] 5.087 [22], 10.93 [23]
29.125 8.009
BaI Ba II
553.548 493.409
18060 20262
Ti I Ti II
451.274 450.127
28896 3 1,207
WI
181 191 201 211 221 231 241
Mean Ag, (10%‘)
Ek
(nm)
Species
31.6 47.6 3.85 4.82 36.0 61.2
3.45 [24] 1.91 [24]
3.45 1.91
0.950 [25] 1.393 [26]
0.950 1.393
S. R. BAUMANNand W. H. SMITH, J. Opt. Sot. Am. 60, 345, (1970). T. ANDERSEN and G. SORENSEN,J. Quant. Spectrosc. Radiat. Transfer 13, 369 (1973). C. H. CORUSS and J. L. TECH, NBS Monograph 108 (1968). F. P. BANFIELDand M. C. E. HUBER, Astrophys. J. 186, 335, (1973). G. E. Assous~ and W. H. Sm, Astrophys. J. 176, 259 (1972). M. C. E. HUBER, Astrophys. .Z. 190, 237, (1974). B. M. MILI?S and W. L. WIESE, Critically evaluated transition probabilities for BaZ and Ball, Technical Note 474 (1969).
NBS
Electronic
excitation
and ionization
temperature
measurements
167
equations can be combined to yield the well-known relation in LTE:
I,‘, _ 2g’A’Aii (2Tr$T)3’z exp [-(Ei
XL- negjAj&
- Ej + Ei,-
AEi,)/kT] ,
(6)
where A is the Einstein spontaneous emission transition probability and hEion is the lowering of the ionization energy. Seven elements with lines whose transition probabilities are known with moderate accuracy from recent literature were selected, viz. Mg, Ca, Zn, Cd, Ti, Ba and Fe. The wavelengths employed and additional data for these lines are given in Table 2. The line pairs were selected on the grounds of similarity of excitation energy and closeness of wavelength. Transition probabilities obtained by CORLISSand BOZMAN[27] were not considered sufficiently accurate to allow choice of additional elements [28]. Partition function data were extrapolated from DRAWINand FELENBOK [29] at T= 6000 K and the lowering of the ionization energy was calculated from the Debye formula [30]: (7) with n, = 1021 me3 and T, = 6000 K. The value derived from this equation, 0.012 eV or 100 cm-l, was used in the subsequent calculations. The UnsGld formula for the lowering of the ionization energy used by KALNICKY er al.[16] is considered incorrect [31] yielding a value high by a factor of ca. 4. 3. INSTRUMENTAL The instrumental system, consisting of a 2 kW high frequency plasma generator (International Plasma Corporation, Model 120-27) and monochromator (Rank Hilger Ltd., Monospek 1000) fitted with an EM1 6256B photomultipher tube and a 1200 grooves mm-l diffraction grating blazed at 330 nm, has already been described elsewhere [32]. The viewing height was selected by racking the complete coupling-box and torch assembly vertically. A 2 mm high viewing zone through the centre of the plasma was always chosen, and the entrance and exit slits of the monochromator were set at 15 pm corresponding to a spectral half band pass of ca. 0.019nm. The photomultiplier output was fed directly to a chart recorder (Servoscribe Model RE 541.20) across a 22 KQ input resistor. Measurements were performed with a demountable silica torch which terminated 2 mm above the top of the load coil. An all-glass concentric nebulizer (Meinhard Associates, Model T-230-A*) was used (uptake rate 0.93 ml min-’ at an argon flow rate of 0.9 1. min-‘, efficiency 2.2%; uptake rate of 1.20 ml min-* at an argon flow rate of 1.31 min-‘, efficiency 2.7%). The internal diameter of the injector tube orifice, governing the injector gas velocity and thus a primary factor for comparison of torch performance between different authors, was 1.60* 0.02 mm.
[25] S. J. WOLNIK and R. 0. BERTHEL, Astrophys. J. 179, 665, (1973). [26] J. R. ROBERTS, T. ANDERSEN and G. SORENSEN, Astrophys. J. 181,567,(1973). [27]C. H. CORLISS and W. R. Bow, Experimental Transition Probabilities for Spectral Lines of Sever@ Elements, NBS Monograph 53. U.S. Government Printing Office, Washington 25, D.C. (1962). [28] B. WARNER and C. R. COWLEY, J. Quant. Spectrosc. Radiat. Transfer 7, 751, (1967). [29] H. W. DRAWIN and P. FELENBOK, Data for Plasmas in Local Thermal Equilibrium. Gauthier-Villars Paris (1965). [30] A. P. T&o-, Spectrophysics. Chapman and Hall London (1974). [31] H. N. OLSEN, Proc. V Bienn. Gas Dynamics Symp., Evanston 1963 Ch. 3,Northwestern (1963). [32] J. F. ALDER, A. M. GUNN and G. F. KIRKBRIG~, Anal. Chim. Acta 92, 43, (1977).
Univ. Press
168
J. F. ALDER, R. M. BOMESELKA and G. F. KIRKBRIGHT
4.
EXPERIMENTAL
4.1 Excitation and ionization temperature measurements
and the determination of the
electron density
Relative line intensities for the set of twenty Fe I lines and for the ion and atom lines of the selected species were determined at various viewing heights. A standard tungsten ribbon lamp was used to calibrate the response of the detection system at different wavelengths and all measured intensities were corrected for all lines with A greater than 290 nm. Regression lines were calculated to evaluate T,, for the Fe I levels in four different ways: (i) T,, calculated from all Fe I levels; (ii) T,, calculated from Fe I levels between 26,875-34,692 cm-‘; (iii) T,, calculated from Fe I levels between 33,695-45,978 cm-l; (iv) T,, calculated from Fe I levels between 44,184-55,754 cm-l. In addition, excitation temperatures from the Fe 1 373.486/373.713 nm line pair are also presented for comparison with later work. A 100 pg ml-’ Fe solution was used to examine the lower energy lines (up to 34692 cm-‘) in order to minimize self-absorption and a 1000 pg ml-’ Fe solution was used for the remainder. For the ionization temperature measurements, all solutions were 20 pgml-’ except for Fe and Ti (100 pg ml-l). The half-width of HP at 486.1 nm was used to determine n,, the hydrogen arising from the decomposition of the aqueous solutions nebulized. GRIEM [ll] has tabulated values of c(T, n,) for the expression: n, = c(T, n,) A:;:, where c(T, n,) is a weak function of both parameters. We have interpolated his values at ~(6000 K, n,) in order to calculate n, from the measured half-widths. The instrumental half-width for a 15 pm slit was 0.019 nm in the first order. The Doppler half-width AA& of Ha at 6000 K, 0.027 nm, was convoluted with instrumental half-width AA:,, (assumed Gaussian) to give a combined Gaussian contribution, AAl,*, to the half-width of 0.033 nm, from the expression: (Ah1,2)2= (Ah$)‘+ (AAi,)2. This Gaussian contribution was then removed from the measured half-width of I-& with the aid of tables of half-widths of the Voigt function [33]. The resulting values of AA,,,(0.04-0.5 nm) were then used to evaluate n,(l X 1020- 4 x 1021 mp3) from Griem’s expression and the values obtained at high electron densities should be reasonably accurate as then the contribution from the Doppler and instrumental broadening is very small. The background under the line was taken as that value at 484.0 nm. 4.2 Variation of the injector flow rate and aerosol desolvation Electron densities and cadmium ionization temperatures were measured at three different viewing heights in the central channel, whilst the argon carrier gas flow-rate was varied in two different ways: (i) by increasing the input pressure to the nebulizer in steps of 69 kPa over the range 172-517 kPa; (ii) by adding dry argon via a flow-meter to the aerosol leaving the spray chamber. At a constant flow-rate, the latter procedure would be expected to yield a lower constant water transport rate into the plasma. A small U-tube filled with up to 7 g of coarse silica gel was used to remove various amounts of water from the aerosol, the quantity of water removed being a function of the weight of the silica gel. Taps were arranged such that the aerosol leaving the spray chamber could either enter the plasma directly or first pass through the U-tube; no significant reduction in the argon flow-rate through the U-tube was observed. The electron density was determined from the half-width of HP and correlated with the [33]
W. L. WISE, Plasma Diagnostic Techniques Chapter 6. Academic Press, New York (1965).
(Mited
by R. H. HUDDLEST~NE
and S. LEONARD),
169
Electronic excitation and ionization temperature measurements
continuum intensity variation at 484.0 nm. The cadmium ionization and iron excitation temperatures from the Fe I 373.486/373.713 nm line pair were also determined. 5. RJSJLTSAND DISCUSSION 5.1 Excitation and ionization temperature measurements The excitation and ionization temperatures obtained under different operating conditions are presented in Table 3. Both BOUMANS and DE BOER [34] and MEFCMET[35] have suggested that sample excitation and ionization by interaction with metastable species in the argon ICP can explain why ion lines are more strongly excited than expected for a plasma with a relatively low (ca. 6000 K) excitation temperature. BOUMANSand DE BOER [34] have published values of (IJIa)_/(Ii/IJI where these Table 3. Excitation and ionization temperatures obtained experimentally*? Viewing height (mm)
10
20
30
10
20
30
5990
*270 4600 *200
6140 *250 5160 *270
*170 4480 *200
20 Fe I levels
7490
6910
9 Fe I levels from 26875 to 34692 cm-’
*190 6920 l440
*290 5990
*I330
5370 *210 4660 +250
11 Fe I levels from 33695 to 45978 cm-’
7190 *290
7000 *t810
5230 *290
6130 =t280
5890 *240
4920 *180
11 Fe I levels from 44184 to 55754 cm-i
8510 *390
7720 *1280
6220 *810
7040 *610
7620 *590
6010 *530
Fe I 373.491373.71 nm
7620
6510
4860
4690
5380
4700
Mg II 279.5/ Mg I 285.2 nm
7950
7650
6860
7140
6680
5930
Ca II 396.8/ Ca I 422.7 nm
8100
8110
6320
6480
6800
5610
Cd II 226.5/ Cd I 228.8 nm
7940
7700
7130
7390
6850
6260
Zn II 206.1/ Zn I 213.9 nm
7720
7530
7040
7140
6600
6110
Fe II 258.6/ Fe I 252.3 nm
8260
7850
6960
7510
6890
6060
Ba II 493.4/ Ba I 553.5 nm
8440
7650
6470
6040
7060
5580
5940
6150
5250
Ti II 450.1/ TiI 451.2nm Mean T from II/I ratio Electron density n, (m-Y Carrier gas flow-rate (I mini)
5090
8070 rt260 3.34 x 102’
7750 *210 2.27 x 10”
6800 *330 7.44 x loa0
6810 k.650 2.23 x lO*i
6720 *290 6.38 x 1020
5830 l360 1.77 x 1020
0.90
0.90
0.90
1.30
1.30
1.30
* Power 1200 W; plasma gas flow rate = 12.8 1.mm’; auxillary gas = 0 1. mm’. t The errors in the excitation temperatures were evaluated using a program written for the HewlettPackard HP97 calculator to obtain the error in the slope of the regression line and represent 20 uncertainties in the excitation temperature, i.e. the excitation temperatures lie with 95% probability within the stated limits. [34] P. W. 3. M. BOUMANSand F. J. DE BOER,Spectrochim. Acta 32B, 365 (1977). [35] J. M. h&m, C. R. Acad. Sci. Paris 28lB, 273, (1975).
170
J. F.
ALDER,
R. M. BOMBELKA and
G. F. KIRK~RIGHT
ratios ranged from one to three orders of magnitude. Similarly, KAJ_NICKY et al. [16] using space resolved radial values of (Ii/I,),, have calculated the electron density, n,, via equation (6) and found values of n, from 30-50 times lower than the measured Abel inverted values derived from H,. We believe that the values of n, derived from H, are essentially correct and that there are no additional electric forces acting in the plasma which might further broaden Ha, which are not accounted for in the present line-broadening theories. The ionization temperatures derived from our measurements on seven elements with a wide range of ionization potentials, under various conditions and using measured averaged values of n,, are found to be similar to the values derived from the Fe I levels of high excitation energy. It is therefore possible that the true value of T, is closer to the value derived from high Fe I levels rather than the value derived from the low energy Fe I levels. This interpretation follows from the fact that, for a non-thermal level distribution, the thermal limit, defined as the level above which all higher levels have a true Boltzmann population at T,, decreases as n, increases and the plasma becomes more collision dominated until, at a sufficiently high electron density, LTE is achieved down to the ground level. Our Fe I excitation temperature values at 10 mm using the Meinhard nebulizer at an argon injector flow rate of 0.9 1.mine1 demonstrate that under these conditions the Boltzmann plot is tending towards a straight line, indicating an approach to LTE conditions. All of our Fe I excitation temperatures indicate an increasing trend with upper energy level. However, from Fig. 1, it is apparent that either the high levels are overpopulated with respect to the low levels at a low electron temperature, or that the low levels are overpopulated with respect to the high levels at a higher electron temperature. Although this effect could be due to a non-homogeneous emitting region, it is more likely that any effects due to inhomogeneity are small and that the lower Fe I levels are over-populated at a higher electron temperature due to radiative decay of upper levels down to these levels which is not balanced by the inverse absorption processes. Under all conditions studied, no unique excitation temperature for Fe I was found and thus the non-LTE state of this plasma has been demonstrated for our experimental conditions. The ionization temperatures derived from the seven species are in all cases in reasonable agreement. This result is perhaps surprising, as non-thermal level distributions are even more probable for ionic species under the conditions employed in this work due to their higher ionization potentials, but in all cases atomic and ionic lines with similar excitation
Fig. 1. Boltzmann plot for Fe1 levels at a viewing height of 20 mm. Power = 1200 W; plasma gas flow-rate = 12.8 Lrnin-‘; carrier gas flow rate = 1.30 l.min-‘. The derived excitation temperatures are shown in column 5 of Table 3.
Electronic excitation and ionization temperature measurements
171
energy were chosen so that they should be similarly affected. The excitation temperatures measured from HP, I-I, and El, in the ICP [36] also reflect the fact that non-thermal level populations must be expected for species at the conditions employed in the ICP. In view of this fact, two-line temperature measurements using low energy lines with accurately known transition probabilities cannot be relied upon to yield the correct electron temperature in the ICP and the excitation temperatures obtained should be interpreted with caution. 5.2 The influence of the carrier gas flow rate and water on plasma parameters The variation in the cadmium ionization temperatures determined from the Cd II 226.5 nm/Cd I 228.8 nm line pair and the electron density at three different viewing heights when the argon injector flow rate was varied is shown in Figs. 2 and 3. The effect of the variation in the water injection rate is masked by the large dependence of these parameters on the injector flow rate. In order to study the effect of water, all gas flows were kept constant and some water was removed using the U-tube containing silica gel. The reduction in electron density, using H,, and cadmium ionization temperatures are depicted in Figs. 4 and 5, and a significant reduction in the electron density was observed when water was removed. The absolute recombination continuum intensity is proportional to ~zZ/T,~‘~and also showed a decrease which is correlated with the reduction in the electron density in Fig. 6. In order to examine this effect further, measurements of the electron density and cadmium ionization and iron excitation temperatures at different viewing heights were performed, and the results are presented in Figs. 7-9. It can be seen that the presence of water in the aerosol is noticeable at all viewing heights and significant at viewing heights above 15 mm, thus increasing the electron density and ionization and excitation temperatures in the ICP. BOUMANS and DE BOER [37] estimated that the water entering the central channel would consume about 20% of the total power input to the carrier gas itself, implying a decrease in temperature with water present. For a nebulizer working at 3% efficiency and a carrier gas flow rate of 1.01. min-‘, the mole ratios of argon, hydrogen and oxygen atoms in the plasma are in the ratio Ar : H : 0 as 89.9 : 6.7 : 3.4, as at 6000 K the concentrations of other species such as O,, H2, OH, H+, 0’ and 02+ are relatively low [38]. As the ionization energies of hydrogen and oxygen are about 2 eV below that
”
6000 -
‘*A
I 14
I IO
d6 Injector
argon flow
rate,
I 18
Cmin.’
Fig. 2. Cadmium ionization temperatures under the following conditions: 0 nebulizer pressure increased to increase the argon flow rate, x nebulizer pressure constant at 140 kPa, and dry air added to the gas stream. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘. [36] K. VISSER, F. M. HAMM and P. B. ZEEMAN, Appt. Spectrosc. 30, 34 (1976). [37] P. W. J. M. BOUMANSand F. J. DE BOER, Spectrochim. Acta 31B, 355 (1976). [38] F. BURHORN and R. WIENECKE,Z. Phys. Chem. 215, 285 (1960).
172
J. F.
ALDER,
”
R. M. BOMBELKA and G. F. KIRKBRIGHT
I 0.6
I IO Injector
I IB
I 1.4
cfgon
flow
1min.’
rate,
Fig. 3. Electron densities under the following conditions: 0 nebulizer pressure increased to increase the argon flow rate, x nebulizer pressure constant at 140 kPa, and dry Ar added to gas stream. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘.
of argon, their degrees of ionization at 6000 K would be expected to be about five times higher than that of argon and hence an increase in the electron density would be expected when water is present. We have observed a 30°k reduction in the electron density when the aerosol was dried substantially. The presence of water vapour and its significance in increasing the electron density can partially explain why the measured electron densities with water present are higher than the LTE values for a pure argon plasma, i.e. 2.94 x 1019(6000 K) to 6.64 x 1020(7500 K) [31]. 5.3 Analyte excitation mechanisms in the argon ICP BOLJMANSand DE BOER [34] have recently proposed a mechanism for the ICP operating in argon under analytical conditions in which metastable argon play a central role both as an easily ionizable constituent in the plasma and as a reactant with the analyte species, through Penning ionization. ~~ERMET [35] has already earlier postulated
“I2 Massof
0.1 water
02 collected,
g
Fig. 4. Variation in the electron density (observed from Ho) on removal of some water vapour from a 10.0 ml sample solution after nebulization. Power = 1200 W; plasma gas flow-rate = 12.8 1. min-‘; carrier gas flow-rate = 0.88 1. min-*; viewing height = 20 mm; nebulizer efficiency 2.16%.
Electronic
excitation
and ionization
I
temperature
I
Mass
measurements
173
I
ai
0.2
ofwater collected,
g
Fig. 5. Variation in the cadmium ionization temperature on removal of some water vapour from a 10.0 ml sample solution after nebulization. Power = 1200 W; plasma gas flow-rate= 12.8 1. min-I; carrier gas flow-rate = 0.88 1. min-‘; viewing height = 20 mm; nebulizer efficiency = 2.16%.
similar Penning ionization mechanisms and also proposed excitation mechanisms with ionized argon metastable ions. There are presently two competing theories for analyte excitation processes in the argon ICP: direct electron collisional excitation and excitation by metastable argon atoms. Argon has two metastable energy levels, 4s3Pz, ‘e; the other two 4s levels, ‘e and le, are radiatively linked to the 3P6 ‘S, ground level. Although the lifetimes of the argon metastable species in vacuum are greater than 1.3s [39] their lifetimes in the ICP are shorter by at least nine orders of magnitude due to the high electron collisional transition rates between all four 4s Ar I levels [40]. Nevertheless, because of the almost total self-absorption of the argon resonance lines, all four 4s levels behave as “pseudometastables” showing a rather long collective efficient lifetime [39], although, as stressed, the actual mean lifetime of an excited argon atom in any one of the 4s states in the ICP is extremely short. This does
Electron density from li@mtio
Fig. 6. Correlation between continuum intensity and electron density from HP as increasing quantities of water vapour were removed from a lO.Ornl sample solution after nebulization. The slope of the line is 0.93 and “ratio” represents the value of the measured parameter with some water removed to the value with no water removed; the continuum intensity was measured at 484.0 nm and all parameters are the same as in Fig. 4.
I
WI J. L. DELCROIX, C. M. FERREIRA and A. RICARD, Principles of Larer Plasmas (edited by G. BE-), Chapter 5. J. Wiley, New York (1976). 1401 S. VAC~UIJS, J. P. DINGUIRARD, H. KAFROUNI and I. PAGES, J. Quunt. Spectrosc. Radiat. Transfer 17, 755 (1977).
174
J. F. ALDER, R. M. BOMJSJSLKAand
G. F. KIRKEWGHT
IO Viewing
Fig. 7. Electron
density
20 height,
mm
(from Ho) in the central channel with water water removed x Parameters as in Fig. 4.
0 and with most of the
not, however, rule out the possibility of the excited argon atoms playing a role in analyte excitation processes. MCGREGOR [41] in 1963 working with an argon plasma jet suggested that the relatively strong nitrogen second positive band system [N2(C311,,) --, N,(B311,)] observed was the direct result of energy transfer between a metastable argon atom and a ground state nitrogen molecule. The N2(C311,) state is similar in energy to the argon 4s levels and therefore an energy transfer is feasible. This same band system is also easily observed when air is entrained into the conventional short torch in the ICP and can be effectively excluded when an extended torch, whose outer silica tube has been lengthened by 30 mm, is used with observation through the silica wall. The extended torch also reduces the observed intensity of other band systems like NO, CN, N2+and NH, and we have found it very useful for examination of the variation of the true argon recombination continuum with wavelength. Penning ionization is an electron exchange process involving an intimate, short duration collision. In this process, the atom M is ionized by donating an electron to the (metastable) argon atom, which in turn ejects an electron, collapsing to the 3p6 ‘S, ground state. As the central channel in the ICP has been shown not to be in LTE, the electron temperature T, in that zone must necessarily be higher then the heavy particle temperature T,, resulting in a two-temperature plasma. A difference between T, and ‘I,
I
Y
!*Ooo\ .$ .P
7500
-
‘x.X
E -A -2 B ”
X
\ x, a
I
I IO Vv?wing
Fig. 8. Cadmium
I 20 helgM,
mm
ionization temperature in the central channel with water the water removed X. Parameters as in Fig. 4.
[41] W. K. MCGREGOR, Pm. University Press (1963).
0 and with most of
V Bienn. Gas Dynamics Symp., Evanston 1963, Chapter
8. Northwestern
Electronic
excitation
and ionization
temperature
Viewmg hight ,
measurements
175
mm
Fig. 9. Fe I excitation temperatures from the 373.49 nm/373.71 nm lines at different viewing heights with water 0 and with most of the water removed X. Parameters as in Fig. 4.
is a necessary consequence of a departure from a thermal level population and is due to the electron-atom collision frequency being too low. We expect T, to be closer to the value derived from the Fe I levels of high excitation potential rather than the value from the commonly used lower levels. A high electron temperature would considerably reduce the importance of analyte ionization via Penning ionization with excited argon atoms (not only metastable atoms [34]) as the dominant ionization mechanism and we expect that analyte excitation and ionization in the ICP are primarily dominated by electrons, with radiative deexcitation influencing the analyte level populations and giving rise to the observable nonthermal effects.
Acknowledgement-We to one of us (R.M.B.).
wish to thank the Science Research
Council for the award of a research studentship