equidensitometry—a method for plasma diagnostics—xiii

Spcnochmica Aera, Vol 399, Nos 9-11, pp 1225-1237. 1984. 0584-8547/84 so3.00 + .oo Printed in Great Entam. 0 1984. Perg~~on Press Ltd.

Equidensitometry-A method for plasma diagnostics-XIII. Investigations of the spatial emission distribution of different spectral

lines in an ICAP

K. NIEBERGALL, H. BRAUER and K. DITTRICH*

Karl-Marx-University, Department of Chemistry, Analytical Centre, Liebigstr. 18, 7010 Leipzig, German Democratic Republic

(Received 31 January 1984; in revised form 1 May 1984)

Abstract-Diagnostic plasma investigations of an inductively coupled plasma (ICP) in argon were carried out by photographing the plasma using a slitless spectrograph. The spectrograms so obtained were evaluated using the technique of equidensitometry. A background correction procedure has been developed so that net blackenings or net intensities can be obtained from the “equidensitograms”. This technique makes it possible to obtain spatial emission distributions of different spectral lines using a single exposure lasting only one second. The possible applications of the method have been demonstrated by (i) comparing the emission distributions of Ar I, Cu I and N: lines, which, after Abel inversion of the Nf and Ar I distributions, makes it possible to propose a model for the entrainment of air in the ICP, (ii) determining the variations in the position and volume of the plasma torus and the corresponding variations in the LTE and non-LTE zones with changes in power, and (iii) the influence of power and gas flow rates on the emission profiles of a number of spectral lines from different elements, where it is shown that the effects are dependent on the excitation energies of the spectral lines.

1. INTRODUCTION

TODAY the inductively coupled plasma (ICP) in argon has, after a relatively short period of development, become an important analytical method with a wide range of application. During this period many studies of the fundamental processes occurring in the plasma have been carried out and will no doubt continue in the future. In particular the processes in the plasma and their influence on the spatial emission profiles of analyte substances are being investigated as they are as yet not fully understood.

During the last 7 years many papers have been published on the excitation mechanisms, interferences and matrix effects in the ICP. In the fundamental paper of BOUMANS and DE BOER [l] it was shown that the favourable analytical properties of this source could only be explained by the absence of local thermal equilibrium, LTE. In addition in the earlier years spatial distributions of temperature and electron number densities [2, 43 and emission intensities [3] were determined by many authors most of them using direct photoelectric measurements although self-scanning photodiode arrays have been used [3, 61.

Other authors have investigated the influence of matrix substances on the radial distribution of light intensities [S-8]. In particular UCHIDA et al. [9] have measured the spatial distribution of argon atoms in metastable states using absorption techniques as well as by measuring temperature and electron number density distributions. Vertical intensity distributions were measured to explain matrix effects [lo] and to determine gas temperatures [ 111.

*For correspondence.

[l] P. W. J. M. BOUMANS and F. J. DE BOER, Spectrochim. Acta 32B, 365 (1977). [2] D. J. KALNICKI, V. A. FASSEL and R. N. KNISELEY, Appl. Spectrosc. 31, 137 (1977). [3] T. E. EDMONDS and G. HORLICK, Appl. Spectrosc. 31, 536 (1977). [4] G. R. KORNBLUM and L. DE GALAN, Spectrochim. Acta 32B, 71 (1977).

[5] G. R. KORNBLUM and L. DE GALAN, Spectrochim. Acta 32B, 455 (1977). [6] H. KAWAGUCHI, T. ITO, K. OTA and A. MIZUIKE, Spectrochim. Acta 35B, 199 (1980). [7] N. W. BLADES and G. HORLICK, Spectrochim. Acta 36B, 861 (1981). [8] N. W. BLADES and G. HORLICK, Spectrochim. Acta 36B, 881 (1981). [9] H. UCHIDA, K. TANABE, Y. NOJIRI, H. HARAGUCHI and K. Fuw~, Spectrochim. Acta 36B, 711 (1981).

[lo] S. R. KOIRTYOHANN, J. S. JONES, C. P. JESTER and D. A. YATES, Spectrochim. Acta 36B, 49 (1981). [l l] H. KAWAGUCHI, T. ITO and A. MIZUIKE, Spectrochim. Acta 36B, 615 (1981).

1225

1226 K. NIEBERGALL et al.

In a fundamental paper by FURUTA and HORLICK [ 123 results of vertical, lateral and radial distribution measurements of analyte emission were given. These authors proposed a particular structure for the ICP identifying zones of LTE and non-LTE.

ROEDERER et al. [ 131 measured the molecular emission distributions and showed that these were dominated by air entrainment in the plasma. RYBARCZYK et al. [14] investigated the effect of the alkali metals in the lower regions of the plasma where interaction of the sample with the plasma begins. A comparison of the spatial distributions of Ar and Ca atoms lead NOJIRI et al. [15] to the conclusion that to obtain an accurate model of the ICP spatial number densities are necessary.

This large number of papers shows the usefulness of the measurement of spatial distributions in explaining the structure and excitation mechanisms in an ICP. A disadvantage of all the measurements is that one must measure the line intensities of several atoms and ions one by one. The possibility of measuring all the distributions from one exposure is given when using a slitless spectrograph and measuring the plate using the technique of equidensitometry.

The combination of these two methods was developed earlier by the authors [ 163 and has been used to explain atom distributions in arc plasmas [ 17-251 and in other discharges [26]. The technique has also recently been used by HORLICK and FURUTA [27] for ICP diagnostic measurements although the spectrograms were not measured.

The advantages of this method are: (i) A picture of the whole plasma-atom, ion and molecular emissions+n be obtained

with a single exposure of about one second. (ii) The spectrogram can be evaluated using different techniques such as computerised

microdensitometry and by equidensitometry. The latter is to be recommended if very high blackenings, very small plasma images or many equidensity contours are required, even though the method is much more time consuming.

In this paper the usefulness of the method in investigating the structure of and element distribution in an ICP is demonstrated. In particular the emission distributions of Ar, analyte atoms and Nz molecules as well as the influence of some parameters on the Ar and analyte emission distributions were measured.

2. EXPERIMENTAL

2.1. ICP-Spectrometer

An ICP attached to an Ebert-type grating spectrograph was developed for trace analysis. The only commercial components were the r.f. generator and the spectrograph. All other components, e.g. impedance matching networks, coil, nebuliser, etc., were built in the laboratory [28]. The working coil

[12] N. FURUTA and G. HORLICK, Spectrochim. Acta 37B, 53 (lY82). [13] J. E. ROEDERER, G. J. BASTIAANS, M. A. FERNANDEZ and R. J. FREDEEN, Appl. Spectrosc. 36,383 (1982). [14] J. P. RYBARCZYK, C. P. JESTER, D. A. YATES and S. R. KOIRTIOHANN, Anal. Chem. 54, 2162 (1982). [ 151 Y. NOJIRI, K. TANABE, H. UCHIDA, H. HARAGLJCHI, K. FUWA and J. D. WINEFORDNER, Spectrochim. Acta 38B,

61 (1983). [16] K. DITTRICH, K. NIEBERGALL and H. R~SSLER, Spectrochim. Acta 31B, 331 (1976). [ 173 K. DI~RICH, H. R&SLER and K. NIEBERGALL, Acta Chim. Ad. Sci. Hung. 89, 347 (1976). [18] K. DITTRICH, K. NIEBERGALL and H. R~SSLER, Acta Chim. Acad. Sci. Hung. 89, 365 (1976). [19] A. V. KARJAKIN, K. DITTRICH, L. I. PAVLENKO, G. G. BAB~SCHEWA and R. ZSCHOCKE, Zh. Anal. Khim. 34,98

(1979). [20] K. DITTRICH, K. NIEBERGALL and R. ZSCHOCKE, Spectrochim. Acta 36B, 35 (1981). [21] I. R. SCHELPAKOVA, K. DITTRICH, I. G. JUDELEWITSCH, R. WENNRICH and 0. I. SCHTSCHERBAKOVA, Zh. Prikl.

Spektrosk. 34, 604 (1981). [22] K. DI~RICH, A. SULEVA and K. NIEBERGALL, Spectrochim. Acta 36B, 505 (1981). [23] K. DITTRICH, T. ORE~CHKOW and A. PETRAKIEV, Spectrochim. Acta, 36B, 515 (1981). [24] K. D~RICH, K. NIEBERGALL, R. WENNRICH, A. V. KARJAKIN, L. I. PAVLENKO and I. A. POPOVA, Spectrochim.

Acta 37B, 199 (1982). [25] K. DIITRICH, K. NIEBERGALL, N. KRASNOBA~VA and N. NEDJALKOVA, Spectrochim. Acta 38B, 461 (1983). [26] A. PETRAKIEV, I. KOLEVA, S. WUDIMIROV and K. DITIXICH, Acta Chim. Ad. Sci. Hung. 104, 1 (1980). [27] G. HORLICK and N. FURIJTA, Spectrochim. Acta 37B, 999 (1982). [28] K. NIEBERGALL, K. DFTTRICH, U. PFEIFFER and H. BRAUER, Exptl. Method. und wissensch. Geriitebau 82/8,1

(1982).

Equidensitometry 1227

and torch are exchangeable making it possible to use powers of up to 10 kW when N,/Ar is used. Details are given in Table 1.

2.2. Production of spectrograms The slit of the spectrograph was removed. Very small photographic images were obtained by

focussing the ICP with a 20-fold reduction in size onto the plane where the slit of the spectrograph would be normally. The gamma of the plate was obtained from an exposure made with a 6-step transmission filter and a Xe arc lamp using an exposure time equal to that used to obtain the spectrogram thus avoiding the Schwarzschild effect.

2.3. Production of equidensitograms Using an automated contact copying procedure the ICP spectrogram was converted photographi-

cally into well defined equidensity contours representing blackenings which increase in steps of S = 0.1 [16]. The broadening of the contours caused by the photographic emulsion and the procedure is very small.

Figure la shows a print of an enlarged ICP spectrogram taken from the original photographic plate. Three spectral lines of different wavelengths (Ar I 404 nm and Pb I 406 nm-the middle line is not

Pb 1405.8

(a)

Fig. 1. (a) A plasma spectrogram of the ICP between wavelengths of 404 and 406 nm. (b) A single equidensity contour, S = 1.8, from the spectrogram.

Table 1. Instrumentation and operating conditions

Rf generator

Rf power Coil

Torch Outer gas flow Intermediate gas flow Carrier gas flow

Nebuliser Nebuliser efficiency Solution uptake rate

Spectrograph

LSV 10 (VEB Funkwerk K(ipenick, G.D.R.) 27.12 MHz, crystal controlled 0.7; 1.1;; 1.4 kW 3.5 turns copper, silver plated

Fassel type; injector tube has i.d. of 1.5 mm. 13’ and 16 Imin-’ 0.5* and 2.0 1 min- 1 0.7* and 0.85 1 mitt-’

Meinhard coaxial glass nebuliser 3% 1.1 mlmin-‘, controlled by pump

PGS 2 (VEB Carl Zeiss Jena, G.D.R.) Stigmatic Ebert mounting

Optics One quartz lens, f = 40 mm, focussing an image of the source on the spectrograph slit plane with a 20-fold reduction in size.

Grating 1300 lines mm- i, blazed at 300 nm first order, dispersion of 0.363 nm nun i.

Photographic plates ORWO UV 1. Exposure 1.0s

*Conditions normally used in analysis.

1228 K. NIEBERGALL~~ al.

identified) on this print demonstrate that the resolution is good enough to separate spectral lines with wavelength differences of less than 0.5 nm. The broad black band under the peaks represents the continuous background radiation partially masked by the three turns of the load coil.

Figure lb shows one equidensity contour with S = 1.8 revealing the structure of the ICP in more detail, e.g. the difference in shape of the Ar and Pb profiles. The upper horizontal line represents the blackening of the continuous spectrum at the position indicated in the Fig. la showing that this technique also gives details of structure inside the continuum. An equidensity contour for a lower blackening value would give information about the outer regions of the plasma.

2.4. Background correction All the equidensity contours obtained for a given spectral line were combined by magnifying the

contours 1%fold and drawing them in by hand. To ensure that the contours were correctly positioned relative to each other small marks, made with Chinese ink, were put on the original plate. These could be located on the individual equidensity contour photographs and used to ensure correct alignment.

Background corrections were made by converting the blackening values into relative intensities, subtracting the background intensity from the line intensity and converting this corrected intensity back into a blackening value. (The emulsion was calibrated using the spectrum obtained with the 6-step filter and the Xe arc lamp.)

Finally the positions of the contours of blackening values equal to 0.1,0.2,0.3, etc., are located and drawn to give the “equidensitogram” for the spectral line. Thus in the diagrams the outer contour represents a blackening of S = 0.1, the next S = 0.2 and so on. The inner contour has a number assigned to it equal to ten times its blackening value. As an example Fig. 2a shows the equidensity contours for the Zn II 255.8 nm line before background correction and Fig. 2b after background correction, the horizontal lines representing the background. (Since the plasma is symmetrical only half of the distribution is drawn.) In addition the position of the top of the torch and of the top of the coil are included in the figure.

Thus the “equidensitograms” show the two-dimensional emission structure of a given spectral line without Abel inversion.

2.5. Analyte solutions For this investigation solutions of Ba, Cu, Hg, Pb, Mn, Y and Zn were prepared with concentrations

in the pgg/ml range, the actual concentration being chosen to give spectrograms with suitable blackenings after an exposure of one second. The elements were chosen so as to give a wide range of excitation energies and norm temperatures as shown in Table 2. The choice of the spectral line is limited by the technique and to obtain spectral lines free from interference a high resolution is required. Thus a single exposure covered a wavelength range of only 80 nm on a 24 cm plate.

Distance from center line, mm

Burner

Fig. 2. Equidensity contours of the Zn II 255.8 nm line, (a) without background correction and (b) with background correction.

Equidensitometry

Table 2. Parameters of the transitions used

1229

Spectral line

(nm)

Cu I 324.754 Pb I 283.307 Mn I 279.827 Zn I 307.590 Hg I 253.652 Y II 324.228 Ba II 264.729 Zn I 328.233 Mn II 257.610 Zn II 255.796 Ar I 360.652 N: 391.4

First and second ionization Excitation Norm

Potentials Vi Potential VP temperature TN*

0 0 vi+ vq (K)

7.1 3.8 11.5 6000 7.4 4.4 11.8 6000 7.4 4.4 11.8 6000 9.4 4.0 13.4 8000

10.4 4.9 15.3 llooo 6.5 12.2 4.0 16.2 12000 5.2 10.0 7.2 17.2 12 500 9.4 7.8 17.2 12 500 7.4 15.6 4.8 20.4 14000 9.4 18.0 11.0 29.0 18000

15.7 15.0 30.7 20000 15.6+ 3.2

*Rough estimates based on ne = 10’6cm-3 (cf. Refs [l, 291. tThe dissociation energy of N, is 9.8 eV, that of N: 8.7 eV.

3. RESULTS AND DISCUSSION

3.1. Emission distributions of different species The emission distributions of an analyte atom (Cu), Ar I from the plasma gas and N:

molecules from the air would be expected to be different. Spatial distributions of Ar species have been measured ip some instances [9, 151 but the authors have not found any similar determinations of Nz, although BARN= and NIKDEL [31] used gas chromatography to measure the N2 distriblltion in an ICP and ZEEMAN et al. [32] determined temperatures of a 9.2 MHz N,-ICP using N: bands.

Figures 3a and 3b illustrate the measured emission distributions for the Ar I 360.6 nm line and for the Cu I 324.7 nm line respectively, both distributions corrected for background. The

I ,

(a) (b) (c) Dietance from center he, mm

Fig. 3. Equidensity contours of different spectral lines after background correction. (a) Ar I 360.6, (b) Cu I 324.7, (c) N: 391.4 (band head).

[29] P. W.J. M. BOIJMANS and M. CH. bJX-!bi3N~R,~~ctm&?i. Acta 37B, 97 (1982). [30] K. DITTRICH, K. NIEBERGALL, H. R~SSLER and F. H~PPNER, Acta Chim. Ad. Sci. Hung. 95,117 (1977). [31] R. M. BARNES and S. NIKDEL, Appl. Spectrosc. 29,477 (1975). [32] P. B. ZEEMAN, S. P. TERRLANCHE, K. VINIER and F. H. HAMM, Appl. Specrrosc. 32, 572 (1978).

1230 K. NIEBERGALL et al.

distributions are as expected: the Cu emission occurring in the plasma channel while the Ar emission is primarily outside the channel. Similar results have been obtained for Ca and for metastable Ar atoms [15].

Figure 3c shows the emission distribution of the N: bandhead at 391.4 nm corresponding to the O-O transition in the BZZC+-X%+ system. The equidensity contours indicate that this molecular emission from the plasma mantle arises from excitation of the surrounding atmospheric nitrogen.

Figure 4 shows the three distributions superimposed on each other, the contours representing a blackening value of 0.1. This figure gives a clear impression of the relative distributions of the individual species in the plasma. The conical shape of the Ar emission is a result of the gas in the plasma torus being blown upwards from the centre of the coil. The argon gas which does not become part of the torus is not excited. The horizontal direction of the inner equidensity contours in Fig. 3c indicate the strong radial dependence of the N: emission at an observation height of between 5 and 10 mm giving the bell-shaped distribution seen in Fig. 4.

After Abel inversion of the N: intensity profile (compare with Ref. [30]), the true radial emission profiles show a maximum at distances between 9 and 12 mm from the centre of the plasma as shown in Fig. 5. The bell-shaped emission distribution results from the gas flow dynamics of a free burning ICP in which turbulence brings the surrounding atmosphere into the plasma mantle. Figure 6 shows schematically this turbulence.

Note: It is clear that diffusion of Nz into the plasma is minimal as a result of the high vertical velocity of the plasma gas. Such diffusion would result in an emission distribution of N: similar to that of Ar I. The entrained atmospheric gases are directed inwards and upwards through the action of the forced aerosol flow.

3.2. Influence of rf power on the argon emission The plasma forming processes in an ICP (Ar --, Ar’” and Ar + Ar + + e-) and the rotational

symmetry of the plasma are of critical importance when considering analyte emission


Fig. 4. Equidensity contours with S = 0.1 from Fig. 3 superimposed on each other.

1231

r,mm

Fig. 5. The radial intensity distribution of N: at different heights above the coil.

/ ‘Y/ Cd

40-

z 30-

* s al 5 20-

8

E .P 2 io- Max.Ni

emlsslon

Air

9 15 15

Distance from center he, mm

Fig. 6. The emission structure and gas flow in an ICP burning freely in air.

structures. For this reason the influence of different power levels on the Ar emission structure was studied.

Figure 7 shows the equidensity contours of the Ar I 360.6 nm line and the associated background at 3 different powers. (Since the gas flows were kept constant the powers that

1232 K. NIEBERGALL~~~.

P-07 P=l.l


P-l 4 (kW)

Fig. 7. Equidensity contours for Ar I 360.6, corrected for background, at different powers.

could be used without changing the torch were restricted to between 0.7 and 1.4 kW.) This figure shows that the Ar and the background intensities increase as the power increases, as is well known. It also shows that as the power increases the radiating Ar plasma torus, and thus the region of high temperature, moves towards the outer quartz walls of the torch.

Abel inversion of the data in Fig. 7 at a height of 10 mm above the coil gives the curves shown in Fig. 8. It is clear that as the power increases the position of maximum emission of the Ar line moves towards the centre of the plasma while at the same time the radius where the relative intensity is zero increases. (Similar results are obtained at heights of 7 and 15 mm.) It appears, therefore, that the volume of the plasma torus increases as the power increases. Consequently the plasma channel as a region of relatively low temperature and assumed LTE is increasingly confined as the power increases. If one accepts the proposal of FURUTA and HORLICK about the spatial distribution of regions in the plasma where substantial departures from LTE occur [12] it can be concluded that the regions of LTE and non-LTE change with changing power.

Figure 9 illustrates the movement of the Ar emission maximum at different heights in the plasma as the power increases. The movement is very marked at a height of 7 mm but negligible at 20 mm above the coil, where the maximum lies on the channel axis. These results are clearly related to the plasma torus which at heights greater than 15 mm is weak or non- existent. Lower in the plasma, however, the changes can be unambiguously related to changes

I P=l4kW

r, mm

Fig. 8. The radial dependence of the Ar I 360.6 emission on the power at an observation height of 10 mm. (i, = intensity after Abel inversion.)


--__ 20 mm

I I.1

P, kW

I 1.4

Fig. 9. The dependence of the radius at which the maximum intensity of the Ar I line occurs in dependence on the power at different observation heights.

in the position and volume of the torus. Thus it is possible to predict that in the upper regions of the plasma the excitation mechanisms will be homogeneous but lower in the plasma a heterogeneous structure exists with both LTE and non-LTE conditions as described in Ref. [12].

In summary, the Ar I equidensitograms have shown that increasing the rf power supplied to the plasma causes the torus region to expand and the higher temperature and non-thermal excitation associated with it to move nearer the channel axis. At the same time there are corresponding changes in the plasma structure in the vertical direction. These changes clearly will result in changes in the intensity and emission profiles of analyte atoms and ions as demonstrated below.

3.3. E$ect of di$erent parameters on analyte emission FURUTA and HORLICK [12] have published reliable radial and vertical distributions of

analyte emission intensities. Other authors have also considered distributions of the analyte when investigating matrix effects [7,8, 10,13,14]. The effects of rf power and gas flow rates on the emission profiles was studied and the results are given below.

3.3.1. The influence of rf power. The influence of rf power on spectral line and background intensities is well known [ 1,291. If the concept of torus growth as described above is accepted then it can be expected that changes in the analyte emission distribution will differ depending on the specific spectral transition. The influence of the power at constant gas flow rates on the line intensities and emission structure was studied for a number of spectral lines-Ba II 260.7 nm, Mn 1279.8 nm, Zn I 307.6 and 328.2 nm, and Zn II 255.8 nm-each with different norm temperatures as given in Table 2.

The results obtained are shown in Fig. 10, from which the following can be derived. (i) The heights of the radiating zones for both spectral line and background increases with

power in all cases. (ii) The intensity of the central emission zone increases with power but the relationship

between intensity and power is not the same for all of the spectral lines. (iii) The intensities of spectral lines with low excitation energies (Ba II 260.7, Mn I 279.8

and Zn I 307.6) increase markedly with increasing power in the region just above the coil or inside the burner. At heights of 10 mm or more above the coil the intensities are scarcely

1234 K. NIEBERGALL~~~~.

b C d

I I

e


Fig. 10. Equidensity contours of some analyte lines at different powers. (a) Ba II 264.7, (b) Mn I 279.8, (c) Zn 1307.6, (d) Zn I 328.2, (e) Zn II 255.8.

affected by variations in the power. On the other hand spectral lines with higher excitation energies (Zn I 328.2 and Zn II 255.8) show marked increases in intensity with increasing power higher up in the plasma. A direct correlation with the increase in intensity and the norm temperature was not observed, e.g. Zn I 328.2 and Ba II 264.7 have the same norm temperature but show different plasma emission profiles.

(iv) Changes in power result in changes in the spatial emission profiles. These changes are shown by the increasing contraction of the emission zones for the spectral lines with low excitation energies at low observation heights as the power increases. This results in the region of optimum excitation moving downwards towards the coil space. Spectral lines with higher excitation energy, however, are only marginally affected in the same way or in some instances may even be shifted upwards.

These results support the concept of a torus which changes with changing power thus giving rise to the variations in the spatial emission structure of the plasma.

It is possible to calculate from the equidensity contours in Fig. 10 the dependence of the net intensity on the power at different heights in the plasma. This was done for a height of 5 mm above the coil: the difference in emission structures is greatest at this height. The results are not presented here but were similar to those given in Refs [ 1 and 291, namely: transitions with high norm temperatures show a positive net intensity vs power correlation while transitions with low norm temperatures show a maximum as described in Ref. [ll].

The results show that the power influences emission intensities and distributions in different ways depending on the excitation energy of the spectral lines. This has the consequence that the compromise conditions used in analysis affect the accuracy and reproducibility differently.


3.3.2. Influence of gasf?ow rates. The outer and intermediate gas flows, which maintain the discharge, and the aerosol carrier gas flow, which introduces the analyte into the plasma, contribute to the emission intensity distributions to different extents. These gas flow effects have been fully investigated by many authors using spectrometric measurements [ 1,3,5,6, 29, 33-371. The effects of changing the outer and intermediate gas flow rates on the Mn II 257.6 line at a constant power of 1.1 kW when measured using equidensitometry are shown in Fig. 11.

Fig. 1 la shows the Mn line and the background under conditions normally used in this laboratory for trace analysis (see Table 1). Increasing the plasma gas flow rate from 13 to 16 lmin-’ results, as expected, in little change in the distribution or intensity as shown in Fig. 1 lb.

Increasing the intermediate gas flow from 0.5 to 2 1 min- 1 lifts the whole plasma upwards, as shown in Fig. llc and at the same time the emission structure is changed and the blackening from the central emission zone is reduced from 1.3 to 1.2.

3.3.3. InJuence of aerosol carrier gasflow rate. The aerosol gas flow rate is known to have a large influence on line intensities and line-to-background ratios. The role of this parameter in affecting line intensities and spatial emission structures needs to be investigated to a greater extent than described in the literature. In this study the aerosol gas flow rate was raised from 0.70 to 0.85 1 min- ’ , i.e. by 21%. It follows that the gas velocity at the orifice also increases by 21%. The other parameters were kept constant. Figure 12 shows the equidensity contours obtained with both flow rates for the spectral lines (a) Cu I 324.7, (b) Pb I 283.3, (c) Zn I 307.6, (d) Hg I 253.6, (e) Y II 324.2, (f) Mn II 257.6 and (g) Zn II 255.8, arranged in order of the norm temperatures.

A comparison of the two sets shows: (i) That the height of the central emission zone increases with higher gas flow rates for all

of the lines with the exception of Zn II 255.8 which is not easy to excite. The easily excited or “soft” lines (a-c), whose maximum intensities are initially at heights of less than 5 mm from


Fig. 11. Equidensity contours, corrected for background, of the Mn II 257.6 line at different gas flow rates at a power of 1.1 kW. Gas flows:

outer gas

13 lmin-’ I; 16 (c) 13

intermediate gas carrier gas

0.5 lmin-’ 0.7 lmin-’ 0.5 0.7 2.0 0.7.

[33] R. M. BARNES and R. G. SZHLEICHER, Spectrochim. Acta 36B, 81 (1981). [34] R. M. BARNES and J. L. GENNA, Spectrochim. Acta 36B, 299 (1981). [35] M. I. BOIJLOS and G. DUE, Can. J. Spectrosc. 22, 68 (1977). [36] G. HORLICK and M. W. BLADES, Appl. Spectrosc. 34, 229 (1980). [37] M. FRANKLIN, C. BADER and S. R. KOIRTYOHANN, Spectrochim. Acta 31B, 589 (1976).

1236 KNIEBERGALL~~ al.


the top of the coil, show the greatest increase in height. These lines also show a club-shaped profile which becomes more elongated as the flow rate increases, the effect being particularly noticeable with the Cu and Pb lines. It can also be seen that the width of the emission zones near the top of the burner depends strongly on the excitation potentials and the norm temperatures of the spectral lines. That this effect is not due merely to the magnitude of the blackening can be seen in Fig. llg which shows a broad distribution and a relatively low intensity. These results confirm the zone model of FURUTA and HORLICK [12].

(ii) Considering the intensity changes two opposite trends can be seen: for the Cu, Pb and Zn I lines there is an increase in intensity with increasing gas flow rate and for the other lines the intensity decreases. These two trends correspond to the classification made by BOUMANS of “soft” and “hard” lines. The decrease in intensity is related to the difficulty with which the line is excited. Thus at an observation height of 15 mm the decrease in the intensity of the Hg line in the centre of the channel is approximately 20 % while for the Zn II line the decrease is 70 %.

(iii) The background intensities show a relatively small decrease as the gas flow rate increases although their vertical heights decrease significantly. This may be due to the plasma channel expanding causing the surrounding argon plasma to become thinner resulting in a decrease in the background emission.

Figure 12 shows the complex relationship which exists between the outer and inner structures of the emission zones and how changes in the aerosol gas flow rates can influence both the structure and the intensity of spectral lines as well as the background in the observation zone. One can therefore conclude that small deviations from the optimum gas flow rate caused by a fluctuating or drifting gas supply, partial blocking of the nebuliser, etc., lead to a corresponding change in the gas velocity. Another source of error is the possibility of a change in the diameter of the orifice of the injector tube. It is easily shown that a 10% reduction in the diameter of the orifice, e.g. from 1.5 to 1.35 mm, results in a 23 y0 increase in the gas velocity even though the gas flow rate is constant. This increase is approximately the same as would result in increasing the gas flow rate from 0.70 to 0.85 1 mini. Reduction in the orifice diameter could result from the deposition of material in the orifice, melting and by roughening of the glass surface.

These results show the importance of the velocity of the aerosol gas as it leaves the injector tube, a parameter which is not easily monitored, and demonstrate that changes in this velocity can cause both systematic and random errors. It is unfortunately not possible to reduce such disturbances by increasing the inner diameter of the orifice since if it is too wide the aerosol carrier gas does not penetrate the plasma and form a channel but only acts as the plasma gas [38].

4. CONCLUSIONS

The study has led to the following results and conclusions: (i) The technique of equidensitometry is a useful method for ICP investigations. It is

possible to get qualitative, semi-quantitative and quantitative results concerning the spatial emission distribution of all species using a single exposure.

(ii) A model of the entrainment of the atmosphere surrounding the ICP into the plasma has been suggested based on the emission of Ni.

(iii) Changes in parameters such as power and gas flow rates lead not only to changes in intensity but also to structure changes. The volumes of the LTE and non-LTE zones of the ICAP are affected strongly by these parameter changes resulting in the different responses of the “hard” and “soft” lines.

[38] H. BRAIJER, K. NIEBERGALL and K. DITTRICH, 2nd ICP-Meeting Chem. Soo. G.D.R., Leipzig 1981.

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