Radial particle distributions in a d.c. arc—maximum off-axis

SpectrochhlcaActa.Vol. 80B,pp.lto14. Pergamon Press.1075, Printed inNorthern Ireland

Radial particle distributions in a d.c. arc-Maximum offiaxh R. J. DECXER and P. A. MCFADDEN University of Rhodwia, Salisbury, Rhodesia (Received 6 September 1973. Rev&d

26 March 1974)

AbstrecGIt is shown that for some elementsevaporated from the anode of a d.c. arc the position of the maximum concentrationof atomic particles (i.e. atoms + ions) occurs not on the axis of the arc column but some distance away from the centre, this distance being largely determined by the inside diameter of the electrode crater. It is shown that the magnitude of the off-axis peak increases with increasing volatility of the element concerned, but decreases with increasing electrical power generated in the plasma as well as with decreasingionisation potential of the buffer metal. The proposed mechanism for this phenomenon is based on the fact that, due to thermal repulsion, cool vapours do not readily mix with hot gases. Thus volatile sample components evaporating from regions not immediately beneath the anode spot would tend to diffuse laterally around the am rather than vertically into the hot plasma. 1. INTR~IHJCTI~N

showed that in a d.c. arc easily ionisable elements tended to be concentrated in the outer regions of the plasma and that as the concentration of these elements was increased in the sample, the position of greatest concentration moved towards the centre of the arc. EBERHAQEN [2] investigated this effect with strontium in a gas-stabilised arc. He showed that the total concentration of atoms and ions of strontium was a maximum at some distance from the arc axis. He explained this effect as being due solely to radial ionic migration of the particles under the influence of the radial potential gradient. FRIE and MAECKER [3] showed mathematically that in a plasma subjected to a thermal gradient the heavier atoms in the plasma would normally migrate preferentially towards the cooler regions of the plasma. ROST [a], using the model proposed by FRIE and MAECKER, showed that if one assumed that sodium atoms were injected into the arc column from a point source then a radial distribution of sodium particles (atoms + ions) could be obtained which was similar to that obtained by Eberhagen with strontium. Vn~~ov16 [5] studying this effect in an arc with a 4 mm gap showed that in the vicinities of the anode and cathode the radial particle distribution showed a maximum away from the arc axis although in the middle of the arc the maximum occurred at the centre. In another report VUKANOVI~[6] showed that in the middle of the arc column and for a given element, the ratio of the particle density in the mantle of the arc to the particle density at the arc axis was directly related to the degree of ionisation of the element in the mantle, indicating that radial ionic IN 1903 LENARD [l]

[I] P. LENARD, Ann. P&&k. 11,636 (1903). [2] A. EBERHAQEN, 2. Physik. 142,312 (1955). [3] W. FRIE and H. MAJUXER, 2. Phy8ik. 162, 69 (1961). [4] L. ROST, Exp. Tech. Physik. 20, 337 (1972). [S] D. Vux~~ov16, Proceedings of the Seventh Internatimal Conference on Phenomena in Ionized &em, Vol. 1, p. 762 (Beograd 1966). [6] D. VUKANOVI~,Spectrochim. Aota 22, 815 (1966). 1 1

R. J.

2

DECXERand P. A. MOFADDEN

migration played a major part in this “separation effeot.” Her results also indicated that separation of the elements in the plasma due to mass differences as predicted by FRIE and IV~AECKER could play a significant, if minor, role. Other studies on radial distributions by AVNI and GOLDBART[7] and particularly by DE GALAN[8] showed that the radial distribution of particles in the arcs used by these authors had maxima on the arc axis, although in a more detailed report by DE GAIAN [9] tables of radial intensity emitted by some elements indicated the presence of off-axis maxima, although these maxima were sufficiently small to be considered as being within experimental error. The investigation in this laboratory using a d.c. arc with an 8 mm gap gave radial distributions with off-axis maxima for some elements which could not be explained in terms of ionic migration or mass separation effects and this is the subject of this report. 2. APPARATUSAND MEASTJRIN~ TECHNIQUE 2.1 Spectrograph A large Hilger spectrograph with a slit width of 16 pm and with quartz optics was used.

In this investigation a synthetic mixture of the composition given in Table 1 was used. The elements were chosen so that the sample contained elements with widely differing physical properties. This mixture was then diluted 1: 1 with one of either LiF, KF, CaF,, or BaF,, buffers which, when burnt in a d.c. arc, give a fairly wide range of excitation conditions. Table 1. The elements added as oxides to graphite to form the synthetio mixture used in this study and relevant data Concentration Element

(%)

Spectrel line (W

Zll Mg Pb Sn Al MO

3 0.001 o-01 0.01 0.001 0.01

307.6; 328.2 279.6; 286.2 283.3 326.2 308.2 317.0

Excitation potential (V)

Ionis&ion potential (V)

4.03; 7.78 4.43; 4.34 4.4 4.87 4.Q2 3.91

9.39 7.66 7.41 7.33 6.99 7.38

2.3 Electrode8 The anode was cut from 6 mm diameter graphite rod to give a sample crater 8 mm in depth, 2 mm in dirt and with a wall thickness of 1 mm tapering to 0.1 mm [7] R. Avru and Z. GOLDBART, Spedrochirn. Acta 28B, 189 (1973). [S] L. DE GALAN, J. Quant. Spectry Radiative Tramfer 5, 735 (1966). [9] L. DE GALAN, Ph.D. T?da. University of Amsterdam (1966).

Radial particle distributions in a d.c. mc4Gximum

off-h

3

at the top. This shape of eleatrode has been shown [lo] to give relatively oonstant electrode temperatures during the burn. The cathode was a blunted carbon rod 6 mm in dia with a rounded end. In all the work described the arc gap was maintained, manually, at 8 mm. 2.4 Source unit Since it was considered likely that volatilisation processes at the electrode could affect the particle distribution in the arc, it was thought advisable to ensure that when comparing the effects of different buffer compounds the temperatures of the sample electrodes (anodes) were approximately equal. Earlier work has shown that anode temperatures can be related empirioally to the electrical power generated in the arc column [l 11, and therefore in this work a d.c. source unit was used which provides the arc with a oonstant eleotrical power [12]. 2.6 Image

rotator

Since a quartz Dove prism was not readily available, a system using front surface mirrors to simulate a Dove prism was built [13]. The use of mirrors has

the advantage that the system is achromatic. 2.6 @ItiCS

A two lens system was used. The fist lens was used to pass parallel light from the arc through the “image rotator” and the second to focus this light on to the slit. No magnification of the image ooourred. The lenses were positioned so that radiation with a wavelength of 300 nm was focused on the slit of the speotrograph. 2.7 _Microphotometermeasurements

A Hilger non-recording miorophotometer with Galvoscale was used. The microphotometer slit was set at positions equivalent to a width of 10 pm and a height of O-6 mm on the photographic plate. Measurements were taken every 0.6 mm along the spectral line image. The deflection measurements were plotted and smoothed thus minimising random errors in measurement. 2.8 Calculation

of the radial atomic particle distribution

2.8.1 flalculation of radial intensity distribution. The intensity distribution obtained directly from the photographic image represents intensities integrated along the line of observation and is not a true radial distribution. This latter distribution can be obtained by solving the Abel Inversion Integral which can be represented by the equation

J(r) = _ 1 +@( I’(%) d;“l,ll (I) 77f --OD39-r) where J is the emission per unit volume per unit solid angle, I(x) the measured intensity at a distance x from the centre of the circle measured at right angles to the optio axis, I’(x) the first derivative of 1(z) with respect to 5, and r the radius [IO] [11] [12] [13]

R. R. R. C.

J. DECIKEIS and D. J. EVE, AppZ. Spectq f38,31 (1969). J. DECKER,Sptrmhim. Acta NIB, 137 (1971). J. DECKER, Spectrochim. Acta MB, 61 (1974). J. CRE~R.S and E. R. F. WINTEX, AppZ. Spectty 20,421 (1966).

R. J. DECKERand P. A. MCFADDEN

4

of the arc plasma. Fortunately this equation has a unique solution but since I(x) is obtained as a set of numerical data a numerical method of analysis is necessary. Three methods were considered: these being those of NESTOR and OLSEN [14],* PEARCE [ 1.51, and BRACEWELL [ 161. The first technique is based on line integration whereas the latter two techniques are based on strip integration identical results, the PEARUE method being somewhat

and give virtually

easier to apply.

KULAGIN, SOROKIN and DUBROVSKAYA [ 171 compared the methods of NESTOR and

was very much more accurate in the vicinity of the plasma axis. They also showed that the PEARUE method tended to introduce an off-axis maximum in the radial Both methods are inaccurate at some distance from the plasma axis. distribution. The calculated radial distributions obtained for a Zn 3076 line using both methods OLSEN with that of PEARCJEand showed that the NESTOR and OLSEN method

are compared in Fig. 1 as are the calculated relative atomic distributions

1.2 r

(see 28.2).

r

r,

mm

r,

mm

Fig. 1. Comparisonof the radial intensity distribution curves obtained by solving the Abel Inversion Integral by the NESTORand OLSENand the PEARCEmethods and the resultant relative radial distribution (n,,r) of atomic particles calculated using equation 2. -.-.-.Measured intensity distribution, NESTORand OLSENmethod, -----PEARCE method. * In the report by NESTORand OLE~EN equation 6a should be replaced by

fk: = -;

1

T,;%,nQ,+ Am--1QN [

[14] 0. H. NESTOE and H. N. OLSEN, Sot. Ind. App. Math. Rewiew 2,200 (1960). [IS] W. J. PEARaE,Conference cm Extremely High Temperatwm. John Wiley and Sons Inc., 1968, p. 123. [IS] R. N. BRACEWEU, Au&al. J. Bye. 9, 198 (1956). 1171 I. D. KULAQIN,L. M. SOROKINand E. A. DUBROVSKAYA, Opt. Speotyy 82,469 (1972).

Radial particledistributionsin a d.c. arc--Maximumoff-axis

6

It is apparent that the methods of NESTORand OLSEN and of PEARCEgive similar radial particle distributions although the radial intensity distributions are significantly different. In this study the method of NESTOR and OLSEN was used to calculate the radial distributions. 28.2 Determination of relative particle distribution. The number of particles, N, (atoms + ions) of a given element in a plasma, at a temperature of T emitting an atom line of frequency v with an intensity I is given by the equation IZ iv = (1 - a)dghv exp ( -E/kT)

(2)

where 2 is the partition function, a the degree of ionisation of the element, A the transition probability, h Plan&s constant, g and E the statistical weight and excitation energy of the upper level respectively and k Boltzmann’s constant. The value of a may be caloulrtted from the Seha equation [18, p. 1561 if the temperature and the electron pressure in the plasma are known. In this study the radial temperature distribution was calculated from the ratio of the radial intensities of the zino lines 3076 and 3282 [18, p. 111-J; the radial electron pressure distribution was calculated from the radial temperature distribution and the ratio of the intensities of the magnesium lines 2796 and 2852 [18, p. 1751. Using the calculated radial intensity, radial temperature and radial electron pressure distributions, the radial particle distribution for a given element was calculated from (2). Since the line intensities were measured on a relative scale the value of N cdculated in this work is purely relative. The symbol n,, is used here for this parameter. 2.9 Comment on the measuring technique Errors are introduced into the calculated radial distributions since, using the technique so far described, the intensities measured along the spectral line image are not necessarily average intensities, the actual value measured being determined by the rate of change of the density over the region of the spectral line superimposed over the microphotometer slit during measurement, and also by the slope of the emulsion calibration curve relating the density of the image to the intensity. A detailed investigation [19] of the error thereby introduced into the radial distribution has shown that it amounts to less than 3 per cent under the conditions in which the measurements were taken and in this study can, therefore, be taken as being negligible. Another source of error is arc wandering which cannot be entirely avoided. To reduce wandering to a minimum without introducing any external devioe which may affect the distribution, the arc was burnt at currents which ensured a fairly large anode spot thus reducing the degree of wandering [20]. 3. RADIAL TEMPERATUREAND ELECTRONPRESSURE DISTRIBUTIONS The radial variations in temperature and electron pressure in the plasma of an arc obtained by burning samples containing 50 per cent CaF, in the arc at different [IS] P. W. J. M. BOUMANS, Theory of Spectrochemical

Ex&ztion. Hilger (1966). [lS] P. A. MCFADDEN, M. Phil. T?mk. Universityof Rhodesia(1973). [20] R. J. DE~ICER and D. J. EVE, Spectrochim. Acta 25B, 411 (1970).

and Watts, London

R. J. DECKEB

and P. A. MOFADDEN

0123

0123

r.

mm

Fig. 2. Radial are temperature and electron pressuredistributions obtained from

samplesbufferedwith 60% CeF,atdiEerentarcpowere. Theeecurveearetypi4 ofthoseobtained throughout this investigation. arc powers are shown in Fig. 2. These curves are typical of those obtained throughout this work and are similar to those given by other authors [7, 81.

4. FACTORS AFFECTINQ THE ~UAQNITUDE AND OCCURRENCE

OF THE

OFF-AXIS PARTICLE &XIMA

This section describes some experiments designed to determine how the off-axis particle maxima reacts to changes in various parameters. Effect of the volatility of the elements in the sample The standard mixture, diluted with 50 per cent CaF,, was burnt in an arc at a constant power of 300 W ( ~10 A). The centre of the arc column, after rotation of the arc’s image through 90”, was focused on the spectrograph slit. The resulting relative atomic particle distributions, are shown in Fig. 3. The distributions have all been normalised such that at the centre of the arc column the particle density has a nominal value of 10. With the exception of aluminium it appears that as the volatility of the element decreases, as indicated by the order of appearance of the elements in the arc given by ZAIDEL, PROKOFJEV and RAISKI [21], the magnitude of the off-axis maximum also decreases. Of interest is the occurrence of a “secondary peak” in the distribution of MO. The curves obtained can be conveniently divided into three groups according to the magnitude of the off-axis maxima: (i) Large off-axis maxima Zn, Pb and Al. (ii) Average off-axis maxima Sn, Mg. (iii) Small or no off-axis maxima MO (often accompanied by a “secondary peak’). 4.1

[21]A. N. ZAIDEL, W. K. PROKOPJEV and 5. M. RAISIU,Tablesof &xchzZ Technik,Berlin (1966).

Liw.

Verlag

Radial particle distributionsin B da. arc-Maximum off-skis

I.6

I.6 r

r. mm

Fig. 3. The relative radial distribution of atomio particles (i.e. atoms + ions) in 8 da. are obtained by burning 8 aamplebuffered with 60% CaF, 8t 300 W.

Throughout this study the elements always fell into their respective categories and in the remainder of this report only curves for Zn, Sn and MO will be given as being representative of the above groups. 3.2 EfSect of sample matrix The effect of the four buffers LiF, KF, CaF, and BaF, was investigated by burning samples containing 50 per cent of a given buffer in the arc at 300 Wcorresponding to currents of approximately 9 and 12 A for the first two buffers mentioned respectively and 10 A for both the CaF, and BaF, buffers. The distributions obtained are represented by those in Fig. 4 and indicate that, in general, there is a decrease in the magnitude of the off-axis peak as the ionisation potential of the buffer metal decreases. 3.3 Effect of increasing the arc power The samples buffered as above, were burnt in arcs at 300 W, 400 W and 500 W and the distributions obtained were compared. Figure 5 shows the distributions obtained for samples buffered with LiF and CaF,. They show that as the power increases the magnitude of the off-axis peak decreases. In addition, MO and, at the higher powers, Sn, but not Mg, ahow a small secondary peak some distance away from the arc axis superimposed on a curve with the maxima lying on the arc axis. 3.4 Distance from the anode Figure 6 shows the distribution profiles at 1, 3, 5 and 7 mm from the anode obtained with the sample containing 50 per cent CaF, burnt in an arc at 350 W. These curves show that as the sample ascends, the off-axis maxima generally become less sharp and tend to move away from the arc axis. 3.5 Variation with anode crater diameter The samples were packed into electrodes with crater diameters varying from 1.2 to 4 mm. The samples were burnt in the arc with a power of 350 W. The results,

Buffer

Buffer

KF

Buffer

BaF,

Zn

Sn

Buffer

LiF

CaF,

W,-6.W)

iv, - 540V)

(V, = 52eV)

w,= 44eV)

3076

3262

I.0

0.4 0

0123

Fig.

1

kL 0

2'3

I2

3

4. The relative atomic particle distributions obtained with different buffer oompounds. All samples burnt at 300 W.

t$rk

II&

I:[;&

Sn

MO

3262

3170

Fig. 6. The effect of increasing the arc power on the relative atomio particle distributions. -----Arc bufferedwith 60% IS, ~ Arc buffered with 60% C&J?, 8

Radial particle distributionsin a d.c. arc----Maximum off-axis

Zn 3076

H”kwp

MO 3170

Sn 3262

5mm

3mm

IO

040 r.

Fig.

0.

mm

z I2

Imm

3

The relative atomio particle distribution in the arc at different distances from the anode. The sample was buffered with 50% Cal?, and burnt at 350 W.

Zn 3076 “4

I.4

0.6 mm

20mm

I.0

I.0

h!L

0.4 EL 0

I

2

o.4 0

3

I

2

3

MO 3170 I.4

I.4

0;6mm I.0

I.0

04

0

I23

0.4 t,

I.0

mm

I

2

I.0

‘\ 0

I

2

3

0.4 b

0

3

0

I

2

3

Fig. 7. Variation in the position of the off-axis maximum with changes in the anode crater diameter (indicatedby the broken line). Buffer, 50%; Power 350 W.

10

R. J. DECKER and P. A. MCFADDEN

rep~se~~d by the curves in Fig. 7, show that for wide craters, the off-axis peak generally occurs near the inside edge of the crater. As the crater diameter decreases, a point is reached when the off-axis maximum decreases in magnitude and finally cannot be detected.

The experiments described in Sections 3.1 through 3.6 give an indication of some of the oharacte~stios of the off-axis maxima. These are: (i) The magnitude of the off-axis maximum appears to be related to the volatility of the element being considered. Aluminium appears to be an exoeption to this trend but it is possible that in the presence of the fluoride buffers the aluminium is fluorinated to form AlF, with a boiling point of 1291°C a value which lies between the boiling points of Zn and Pb and is very close to that of PbF,. (ii) The off-axis maximum generally occurs on a line along the inside edge of the crater, although if the crater diameter is reduced a stage is reached when the magnitudes of the off-axis maxima begin to decrease and eventuaIly cannot be detected. (iii) The magnitude of the off-axis maximum appears to be inversely related to the ionisation potential of the buffer metal. (iv) For a given sample, as the power generated in the plasma (and hence ~u~ent) increases so the mag~tude of the ofF-axis maximum decreases. (v) Under certain circumstances, generally in the absence of an off-axis maximum, both Sn and MO show a small secondary peak superimposed on the distribution curve. Of the elements studied only these two elements form fluorides with relatively low volat~isation temperatures (SnF* sublimes at 7Ofi”C and &IoFB boils at 35OC). (vi) The off-axis maxima, when present, can be observed at all positions in the plasma, the peaks tending to move outwards as the vapour ascends the column. 4. A PROPOSED MECHANISM A mechanism which explains the occurrence of the off-axis maxima and the oharaote~sti~ given above in Section 3.6 is as follows: Consider the arc burning on the upper surface of the sample. The temperature at the anode spot is either equal to or slightly higher than the effective boiling point of the sample [22]. Volatilisation occurs of those sample components whose volatilisation temperat~es are equal to or lower than the burning spot temperature or which have significant vapour pressures under these oonditions. Elements and molecules immediately underneath the anode spot will evaporate and a significant proportion will enter the arc via the anode spot. The more volatile elements will also evaporate sig~fi~antly from regions outside the anode spot and, due to the thermal repulsion between the relatively cool vapour and the hot arc plasma [23], will tend to diffuse laterally either above or just below the sample surface rather than vertically into the plasma. The difficulty of introducing relatively cool fluids such as aerosols into hot plasmas has been known for some time and has been [22] 0. IiN.lQHl3, &xwtrooh~m Aota 4,237 (1960). 1231P. R~SENBLA~ and V. f(. LAMER, Phya. Rev. 70, 386 (1946).

Radial particle distributionsin a da. arc--BXasimum off-axis

11

discussed in detail by KRANZ[24, 251. Under the conditions in which the samples were burnt in this investigation the level of the sample was always below the top surface of the electrode walls and thus the lateral movement of the vapour is hindered by the electrode walls and perhaps slowed down sufficiently to be heated to a high enough temperature to enable it to enter the plasma directly, from the outside regions of the sample crater. This simple mechanism can be used to explain the characteristics given in Section 3.0 a5 follows: (i) l’olatility. The lower the volatilisation temperature of the component, the more pronounced is evaporation of the component from regions away from the anode spot. Thermal repulsion and thus lateral diffusion is significant under these conditions and thua the off-axis maximum is enhanced. (ii) Occuwence of the off-axis muxima at the inside edge of the electrode crater. This has already been explained above. The disappearance of the off-axis maxima at the smaller crater diameters is probably due to the fact that under these conditions the anode spot is as large as the electrode crater and the sample is forced to evaporate directly into the arc or diffuse away from the anode spot through the electrode walls. (iii) Ionisation potential of the buffer metal. As the ionisation potential of the buffer metal decreases so the diameter of the anode spot increases [20]. The heating effect of the are is thus spread over a wider area reducing the amount of sample evaporating from regions outside the anode spot, thereby reducing the magnitude of the off-axis maxima. (iv) Power. As the power used to burn a given sample is increased, the anode spot diameter also irmreases having the same effect as in (iii) immediately above. (v) Occurrence of the secondary peaks. It is clear that the entry of vapours into the plasma can be considered as two separate processes. (a) direct evaporation of the sample into the plasma via the anode spot giving a parabolic-type distribution, and (b) evaporation, particularly of the more volatile elements, from regions outside the anode spot, giving rise to a distribution with a maximum lying approximately above the inside edge of the sample crater. If an element evaporates from the electrode in one chemical form only or in a number of chemical forms with similar volatilisation temperatures, then one would expect a distribution which exhibits only one maximum, the height of any off-axis peak being determined by the relative magnitudes of the two processes. It is possible, however, for an element to evaporate in two or more chemical forms which have very different volatilisation temperatures. In such a case the more involatile component might evaporate in such a manner that the maximum lies at the centre of the plasma, while the volatile component might evaporate to give a marked off-axis maximum. Combining the two distributions could therefore result in a distribution with a maximum in the centre of the arc and a secondary peak lying approximately over the inside edge of the crater. In our investigation only Sn and MOgave distributions with a secondary peak and the fluorides of these twoelements, [.24] E. KRANZ, Proc. 15th C.S.I., Madrid 1969, Vol. 4, p. 96. Adam Hilger, London (1971). [25] E. KRANZ,S@rochim. AC&Z27B, 327 (1972).

12

R. J. DECKEB and P. A. MCFADDEN

in contrast to the other elements studied, have considerably lower boiling points than either the respective oxides or metals. It appears possible therefore that portions of the Sn and MO were fluorinated by the action of the fluoride buffers during the burn giving rise to the evaporation of these elements in two different chemical forms. Support for this theory is given by the fact that in a similar investigation in this laboratory using L&O as a buffer instead of LiF no secondary peak was detected in any of the Sn or MO distributions. (vi) Lateral movement of the off-axis maximum with increasing distance to anode. This is probably due to the plasma gas being deflected outwards as it passes around the cathode, a phenomenon mentioned by VTJKANO~I~! [6]. 5. POSSIBLECONTRIBUTORY FACTORS The following factors have been rejected as being significant in the formation of the off-axis maxima for the reasons given below. They may, however, play a small role in determining the magnitude of the phenomenon. 5.1 Errors in arc temperature and electron pressure Inaccurate determinations of temperature and electron pressure can clearly result in significant errors in the relative particle distribution curves. In particular if the measured temperature or electron pressure, or both, in the centre of the are are significantly higher than the true values, then in calculating the radial particle distribution using equation (2), the central values may be reduced to a much greater extent than is warranted thus creating the illusion of an off-axis maximum. This would be partioularly marked in the cases where the excitation energy of the spectral line concerned is relatively high or the ionisation potential is relatively low. A comparison of the relevant excitation and ionisation potentials (see Table I) with the relative magnitude of the off-axis peaks shows that no apparent correlation exists between these parameters. It is felt, in common with VUEANOVI~[6], that the errors involved are of the order of 10 % or less and that the distributions determined are a fair reflection of the true distributions. 5.2 Arc wandering Arc wandering would tend to flatten the recorded image of the line and if wandering was severe it is possible that a slight depression may occur in the distribution curve after solution of the Abel Inversion Integral. This depression, however, would occur in the distributions of all elements and to the same extent. 5.3 Ionisation Significant ionisation of the element in the centre of the column may result in a depression in the centre of the radial line intensity distribution of atom lines, but the off-axis maxima are also present when the line intensity is used to calculate the particle (i.e. atoms + ions) distribution across the arc. Ionic migration under the influence of the radial field may also contribute significantly to this phenomenon particularly with elements with low ionisation energies. Under the conditions used in this investigation this aspect can be considered to play only a minor part since Zn with a high ionisation potential

Radial particle distributionsin 8 d.c. em-Maximum

off-axis

13

exhibits this phenomenon to the 8ame degree a8 Al with a relatively low ionisation potential. 5.4 Self

absorpt&on

Radiation from the centre of the arc column can be absorbed by the vapours in the cooler outer layers of the arc. If self absorption is severe, it is possible for it to appear as if the central core is not radiating strongly. Tests in which the concentration of the Zn was increased stepwise to 15 %, the latter concentration being more than sufficient to induce noticeable self absorption, showed little difference in the relative magnitude of the off-axis peak, indicating that self absorption can only play a very minor role in determining the shape of the particle distribution curves under the conditions used in this investigation. 6.5 Di$czlsion through the electrode walls Diffusion of the elements through the electrode walls h&8, for some time been known to occur [26]. It seemed possible that the compound8 collecting on the outer surface of the electrode may evaporate vertically into the arc column forming a cylindrical sheath of sample v&pour about the arc axis giving rise to offaxis maxima. This possibility has been discounted since: (i) The off-axis peak occurs generally inside the diameter of the electrode crater, and (ii) When samples were burnt in crucibles of the same dimensions as the electrode craters but cut from vitreous carbon rods, a non-porous form of carbon, the off-axis maxima were not eliminated. 6.

CONCLUSION AND DISCUSSION

It ha8 been shown that the evaporation of samples into the plasma of a d.c. arc can play a significant role in determining the distribution of the particle8 (i.e. atom8 and ions) in the arc plasma. With sample components which are involatile or of medium volatility the effect of the volatilisation processes on the introduction of the sample into the plaema and thus on the detection limits of a particular technique is probably only of minor significance. Volatile substances, however, according to the mechanism proposed above, would tend to escape around the pla8ma rather than enter it, a tendency which would increase with volatility. This may explain, in part, why many of the very volatile elements such as As and Hg have poor detection limits in d.c. arc spectrographic analysis. It also explains why the formation of compound8 of these element8 which are relatively involatile can improve detection limits. For example LEUCHS[22] found that the deliberate formation of an Fe-As compound in the electrode significantly improved the line intensity emitted by a fixed weight of As. He attributed this to the lower volatility of the Fe-As compound when compared to that of A8 or As,O,. Understanding the mechanism of this phenomenon may be of some value in the selection of electrode and arcing conditions to be used in the analysis of volatile components. For example buffer compounds might be chosen such that they not [20] P. W. J. M. BOTJMANS, Spex Speaker 7, No. 4 (1962).

14

R. J. DECKERand P. A. MCFADDEN

only control the excitation conditions in the plasma but that they also give large and relatively cool anode spots, or chosen to affect the chemical nature of a particular element to improve its volatilisation properties. Aohowledgemlzte-The authors wish to recordtheir gratitude to the following people: Dr. D. J. EWE for his intereat in and edvice on this study; Dr. P. W. J. M. BOWMANS for many helpful suggestions, and to Dr. A. STRMEEIM,N. P. R. L., Republio of South Africa, for supplying us with vitreous carbon rods used in part of this inve&igation.