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Studies of the cathode layer of a d.c. arc
SpectrochimicaActa,Vol. ZEB,pp.339to 358.Pergamon Press 1873. Printed in Northern Ireland
Studies of the cathode layer of a d.c.
arc*
R. J. DECKER University of Rhodesia, P.O. Box MP 167, Sal&bury, Rhodesia (Received1 November1972; revtiion received24 April 1973) Abstract-Some factors which affect the cathode la.yyer of a d.c. arc were investigated. It is shown that: (i) varying concentrationsof elements with low ionisstion potentials in the sample oan markedly affect the distributionof atoms in the cathode layer and hence the detection limits of the technique; (ii) variations in arc current and arc gap have only a small effect on the cathode layer provided that these factors are maintained at values above certain minimum levels; (iii) the use of artifical atmospheres and inhomogeneous magnetic fields generally increase detection limits; (iv) detection limits obtained with samples containingrelatively high concentrationsof elements with low ionisation potentials can be two or three times better than those obtained with conventional anode evaporation techniques provided wandering of the cathode spot is kept to a minimum, a condition which often cannot be realised in practice. 1. INTRODUCTION
little has been written about the conventional use of the cathode d.c. arc as a light source in spectrogmphic analysis as initially described by ~WANNKOPPF and PETERS [l], although Atn~ and BOUKOBZA [2,3] have used this region with the sample being evaporated from the anode. In this case, the reason for using the cathode layer with the arc in this configuration is different from that underlying MANNKOPF and PETERS’ experiments [a]. It has been claimed that the use of the cathode layer technique has the following advantages : (i) lower detection limits for a large number of elements, particularly those with low ionisation potentials, (ii) insensitivity to small external changes such as variations in arc current and arc gap. The reasons for the above are, perhaps, not fully understood, but it is generally accepted that the cathode layer effect may be explained by the presence of the steep potential gradient in the close vicinity of the cathode. If only axial transport mechanisms are considered we can distinguish between two transport velocities [5, 61:
IN RECENT layer in a
YEARS
(i) convection velocity v,, which represents the rate at which the v&pour of an element is borne upward from the cathode by convection processes, and * This investigation was carried out at the Institut fiir Spektrochemie und Angewandte Spektroskopie, Dortmund, Germany, and was supported by a generous scholarship from the ALEXANDER VON HUMBOLDT Foundation, Bonn Bad Codesberg. [I] [2] [3] [4] [5]
R. R. R. R. P.
1MANNgo~rrand C. PETERS,2. Phy&. 70, 444 (1931). AVNI and A. BOUKOBZA,Appl. Spectrosc. 28, 483 (1969). AVNI and A. BOUKOBZA,Appl. Spectroac. %4,406 (1970). Aarr and Z. GOLDBART, Spectrochim. Acta 28B, 241 (1973). W. J. M. BOGS, Proc. 6th Colloq. Spectrosc. Intern., Amsterdam 1966, Spectrochim. Acta 11,146 (1967). [S] P. W. J. M. BOWS, Excitation of Spectra, Chapter 6, p. 50 in Analytical Emiaion Spectroecopy, (Ed. E. L. Grove) Vol. 1, Part II. Marcel Dekker, New York (1972). 339
340
R. J. DE-
(ii) ionic velooity, -v,, which represents the rate at which the ionio component of the element vapour would move towards the cathode (hence the negative sign) in the absence of convection. The resulting velocity of the ions is thus = o, - ui. If the degree of ionisation of the element is cc then the resulting velocity, V, of the element vapour away from the cathode is given by, V = u,(l - CC)+ a (u, - u‘), or V = v, - at),, vf cm be replaced by pE, where 1 is the mobility of the ion and E the electric field strength in the plasma giving, V=u,apE. Clearly then the upward velocity of an element, under constant conditions of temperature and electron pressure-which affect both the convection velocity and ionisation of the element-is determined by the degree of ionisation and hence by the ionisation potential of the element and also by the magnitude of the electric field strength. In the cathode layer the field strength is so large that V ia negative and the element v&pour, as a whole, migrates back towards the cathode forming a region of high concentration directly in front of the cathode, this effect clearly being more marked with elements with low ion&&ion potentials. This high concentration of atoms results in the emission of more intense atomic spectral radiation from the cathode region than from the main body of the arc column. Since transference of energy from the arc column to the cathode is small, the cathode temperature is relatively low. This may be beneficial in two respects: (i) more controlled evaporation of the sample, and (ii) the possibility of using thinner electrode cavity walls when evaporating the sample from the anode giving a higher ratio of sample mass to electrode msss in the aro column. Both SCOTTand M~CHELL 17-111 developed the use of the cathode layer as an emission source for the analysis of rocks, soils and plant material. The methods were suocessful and the technique became fairly widely used. On the debit side it is recognised that significant concentrations (about 6 per cent) of the elements with low ionisation potentials, such as the alkali metals, can diminish the potential gradient in the cathode region, and it is generally accepted that relatively high concentrations of such elements in the plasma can markedly reduce or even destroy the cathode layer effect. Because of this Scorr and MITCHELL developed separation techniques [ 121which removed the alkali metals, alkaline earth metals and [7] [S] [9] [lo] [ll]
R. 0. SCOTT, J. Sot. Ckm. Itd 64, 189 (1945). R. 0. SCOTT, J. Sot. Chem. Id 66, 291 (1946). R. L. MITCEELL, J. Sot. Chem. Id. 69, 210 (1940). A. M. M. DAVIDSON and R. L. MITCHELL, J. Sot. Chem. Ind. SO,21 3 (1940). R. L. M.ITCEELL, Tech. Commun. No. 44 of the Commonweelth Bureau of Soil Soience, Harpenden, England (1948). [12] R. 0. SCOTT and R. L. MITCHELL, J. Sot. Chm. Ind. 63,4 (1943).
Studies of the cathode layer of a d.c.
an:
341
phosphates from the sampIe substances which SCOTThas shown to adversely affect the measured line intensity [13]. Of significance, perhaps, is the observation of MANXKOPPF and PETERSthat the detection limits obtained using norm& twc techniques were of the same order as those obtained using a spark soure%,an observation in conflict with the experience of modern analysts. It is possible that the arc techniques were then in an undeveloped state and that with improvements in the method, little is to be gained in using the cathode layer. Indeed, AXRENS and LIW_IIZ:NBERG [l&j, and PIERG~C, TORRESand &Cnsrr~,,~ El&] have found little diflSren%ein detection limits when using either the cathode layer technique or conventional methods (referred to in this paper as anode barques), although the absolute de&&ion limits of the former method> expressed in terms of the minimum mass of an analyta that is detectable, were undoubtedly superior iu as much as the sample required for analysis was very much less. The work described below was carried out to investigate the cathode layer more fully and to see whether its appar%nt advantages could be extended.
(i) J&w$wM~~. The apparatus used is detailed in Table 1. (ii) Ibctrodee. The electrodes used are detailed in Table 2. (iii) ~~~~1~. In most of this work a synthetic sample, referred to in this report as the standard mix, was used hawing the ~rn~i~on given in Table 3. The elements were ohos%n on the bases of their ion&&ion bunts and the simplicity of their spectra. TabIe 1. Apparatus used in this study Spectrograph Grating Nit width Photographiaplates Development PoweFsource ~c~pho~rne~r
A mod&d 3.4 m JACO grating spectrograph. 600 lines per mm used in t&6 first order. Grating width 10 cm. 20 pm. Kodak S.A. 1. 3 min in Agfa Radix101diluted I:26 at 20% Current stabilised d.c. source, Zeiss ~c~6~photom3~~.
(iv) CaIcuZat&ns.The arc temperatures, Tin degrees Kelvin, welp8calculated from the int%nsity ratios of the zinc lines 3676 and 3232. The electron pressures, % in Newtons/ma, were calculated from the arc temperstur%s and the vanity ratios of the m&~esium lin%s23S2 and 2796. The relevant equations for oalculating T and pe were given by BOUMAXS1171.
R. J. DECKER
342
Table 2. Electrodes used in this study
crater Wall thickness Electrode
Dia (mm)
Depth (mm)
A
080
10.0
B
1.68
476
0.74
C
2.44
6.35
0.37
D
2.0
Counter
Reference
(-) 1.0
According to MITCHELL [ 1I] RingsdorfI Cat. No. RW0033. Ringsdorff Cat. No. RWO034.
I.0 tapering to O-1 taper angle 7” I Blunted carbon rod 6 mm dia 8.0
DECKERand EVE [16]
Table 3. Composition of standard mix and the spectral lines used in this study
Compound
(A)
Cone. of element in graphite (%)
Li,CO,
3232 3082 3011 2796, 2852 3039 3076, 3282
0.0008 0.001 0.002 Electrode impurity 0.0002 0.04
Line Element Li Al Zr Mg Ge Zn
AU& ZrOCl, GeO, ZnO
Ion Pot.
5.39 5.98
(W
6.84 7.64 8.13 9.39
All the calculations, including the emulsion calibrations, were carried out on an I.B.M. 1130 computer using a modified version of the program previously described by DECKER and EVE [ 181. (v) Detection limits. In emission spectrography it is common practice to define the detection limit of a method as being that concentration of analyte which gives a line intensity equal to three times the standard deviation of the background intensity at the wavelength of the analytical line [19]. The mean intensity and standard deviation of the background are commonly calculated from background intensities obtained from 15 or 20 exposures of blank samples. The detection limit is then obtained by assuming that the working curve is linear near the detection limit and extrapolating the curve to the intensity representing the detection limit. A more rapidly determined measure of the detection limit, useful for comparing methods on a given instrument, can be obtained by measuring the line intensity emitted by a given concentration of analyte relative to the background intensity. This method is only valid, however, if the background intensity does not vary too drastically between the different methods. (It has been shown that the standard [18] R. J. DECKERand D. J. EVE, Spectrochim.Acta SSB, 479 (1970). [lQ] H. RAISER,Spectrochim.Acta 3, 41 (1947).
Studies of the
cathodelayer of a d.c. arc
343
deviation of the background on a photographic plate is related to the density of the background but over a fairly wide range of densities can be considered to be approximately constant [IS]). Since the background intensities did not vary considerably in this investigation, line/background ratios were used to compare detection limits obtained under the different conditions employed in this investigation. For a more detailed discussion of the determination of detection limits the reader is referred to the publications dealing specifically with this topic [19-231. 3. FACTORS AFFECTING THE AXIAL DISTRIBUTION OF SPECTRAL RADIATION IN THE CATHODE LAYER
3.1 Presence of elements with low ionisation potentials 3.1.1 Experimental
conditions
(i) Optics. The arc was focussed on the slit of the spectrographwith a B-fold magnification using front surface mirrors. Only radiation from a layer I.6 mm in height directly above the cathode was allowed to enter the slit of the spectrograph. During the exposure the image of the leading edge of the cathode was kept, by manual adjustment, at the edge of the fishtail diaphragm over the slit. The maximum movement of the cathode from this position during the exposures did not exceed a distance equivalent to f0.1 mm in the arc. (ii) Measurement. Measurementswere taken along the spectral lines at distances equivalent to 0.05 mm in the arc. The length of the microphotometerslit was equivalent to 0.02 mm in the arc. (iii) Samples. The standard mix was diluted with NasCO, and KsCO, to form two series of mixtures containing 0 per cent; 0.01 per cent; 0.1 per cent; 1 per cent; and 10 per cent of the compounds respectively. (iv) Sample electrode. electrode C (see Table 2). (v) Arc conditions. Current 8A; gap 9 mm; exposure time 2 min.
3.1.2 RESULTS Figure 1 shows the distributions of temperature, electron pressure and line intensity obtained with the samples containing 0 per cent; O-1 per cent and 10 per cent K&O,. (The curves for 0.01 per cent and 1 per cent K&O, are omitted for clarity but their positions may be readily predicted from the trends shown in each diagram). Identical results were obtained with Na,CO,. The curves show: (i) The temperature and electron pressure decrease slightly away from the cathode. As would be expected, a large increase in the concentration of the alkali metal in the sample causes a reduction in the arc temperature. (ii) When the concentration of alkali metals is low a steep increase in intensity is observed in the vicinity of the cathode for elements with low ionisation potentials. The curves shown in Fig. 2 are the line intensity curves obtained for the undiluted sample corrected to constant temperature (5000°K) and constant electron pressure ( 1O-2Nm-a), th e values being normalised to an initial intensity of 100, and, therefore, [20] [21] [22] [23]
H. KAISER, P. W. J. M. P. W. J. M. P. W. J. M.
Opt% 91, 309 (1964). BOUMANSand F. J. DE BOER, Spectrochim. Acta 27B, 391 (1972). BOUMAXSand F. J. M. J. MAESSEN,2. Anal. C&m. 220, 241 (1966). BOUMANSand F. J. M. J. MAESSEN,2. Anal. Chem. 235, 98 (1967).
344
0.5
I.0
I.5
r 2 82 0
f
--. _.-.
-.
-k*., ‘\
-iFI
\\
B o
~
‘i
Mg2652
I
. 0
0.5
I.0
Distance from cathodqmm
I.5
0.5
FO
I.5
Distance from cathodqmm
03
to
I.5
Distance from cathode,mm
Fig. 1. The effect of inch inundation of K&O, on the distribution of &PC temperature, de&on pressure, and line intensity emitted by elements with different ionisation potentials in the vicinity of the cathode. ape--0 % K&O,,- - -o0.1% K&OS, -*-~--a10% K&O,,
represent the relative distribution of the atoms in this region [24]. It is clear that the ‘con~ntr~tion gradient’ in the cathode layer is directly related to the ion&ration potential of the element. (iii) Under oonditiona in which the oathode layer is marked, %n‘intensity plateau’ follows the ‘en~~hment zone’, after which the intensity of the spectral line decreases markedly. The region of the ‘cut-off’ is the same for all elements affected and lies at 1.1 f 0.1 mm from the cathode. (iv) The l~e~b~~k~o~d curves, shown in Fig. 3, generally follow the shape of the line intensity curves in Fig. 1, although they tend to be closer together. They show that for some elements s, little potassium in the arc can be beneficial. Perhaps most interesting is that with 10 per cent buffer the ~e~b&ckg~~d curves go through a maximum away from the cathode. The position of this maximum, [24]
R. J. DEUJIEBand D. J. Em, .A&
&u&mm. !#I, 497
(1969).
Studies of the oathads layer of a
d.c. &ro
345
Mg (7.65&l Zr (6.95eV) Al t5.99eV) ti (5.39
f
I
055
I I.0
eV1
t I+
Distance from cathode,
mm
Fig. 2. The relative distribution of atoms in the vicinity of the cathode C&XIlated from the data given in Fig. 1 for the undiluted samples.
OCGe
0-s I.0 t.5 Distance from cofhodff.mm Disfancefrom cafho*mffl
Fig. 3. The &Tact of increwing ~n~ntr~t~o~ of K&O, on the ~ue~~~o~d ratio, and hence on the detection limita, in the vicinity of the cathode. 0 % K&O,, - - - - O-1% K&O,, -.-a-. 10 % K,CQS.
representing the region where detection limits are optimum, appears to be related to the ion&&ion potential of the element as shown in Fig. 4, each point on the curve being a mean of the vahes obtained from the samples diluted to contain 10 per cent K&O, and Na&09 respeotively. (Magnesium does not fall on this curve since the element is supplied from the electrode walls and not from the sample and hence does not enter the cathode layer directly.) The presenoe of the maxima indicator that even with 10 per oent alkali metal compound the detection limits are somewhat better in
R. J. DECKER
346
lonisation Fig.
potential,
eV
4. The relationship between the ionisation potential of an element and the
distance from the cathode of the maximum of the line/backgroundratio curves in arcs containing 10 % of either Ne,COs or K&O,.
$-i-; 15, [z$ 0.1
0.2
0.3
Distonce.
0.4
05
mm
0.1
0.2
0.3
Distance.
0.4
0.5
mm
0.1
0.2
o-3
Distance.
0.4
0.5
mm
Fig. 6. The effect of increasing concentration of silicon on the line intensity emitted by elements with different ionisation potentials in the cathode layer. 0 % SiO,, - - - - 1% SiO,, -.-e-* 10% SiO,, -*+-*.30 % SiO,.
the cathode region than in the main body of the arc column, although the maxima for elements with low ionisation potentials fall outside the region normally accepted as the cathode layer. (v) Figure 5 shows that increasing concentrations of silicon-an element with a relatively high ionisation potential of 8.12 eV-affects the cathode layer enrichment of the elements to a lesser degree than the alkali metals. Increasing the concentration of the silicon also appears to reduce the overall line intensity in the cathode layer.
studies
of the
cathode layer of 8 d.c. aFc
347
Most of these results are readily predictable if one applies the theoretical considerations given in the introduction. The ‘intensity plateau’ is more difficult to explain, but appears to be a region in which the forces carrying the sample away from the electrod~p~ma~ly convection and radial di&sion-and the forces attracting the sample to the e~e~trod~le~trostat~~ are in dynamic eq~b~um~ and is probably the region of the transition of the cathode fall to the uniform Geld in the arc column. The cut-off in the line intensity indicates that there is a sudden, marked dilution of the sample vapour by the plasma vapour as it leaves the influence of the potential gradient and enters the main body of the arc column. Subsequent experiment has shown that the position of the cut-off is unaltered by varying the current between 6-12 A. 3.2 Effect of arc current 3.2.1 Experimental conditions (i) Opt&x. Results from the previous section indicate that the cathode k+y@r ‘enrichment zone” does not extend beyond about 04 mm from the cathode. !L’hisportion of the arc was thus imaged on the slit of the spectrograph so that w&h a lo-fold ma~~catio~ a 4 mm image of the region w-as obtained. (ii) Snw@e. Standard mix, undiluted. (iii) Sample electrode. &&rode C (see Table 2). (iv) Arc canditiolzs. current 4, 6, 8, 10 and 12 A; gap
9 IWIL
The results obtained are represented by the curves shown in Fig. 6. (The curves for 6 A and 10 A are omitted for clarity, but their positions may be readily predicted from the trends shown in eaoh diagram.) The curves for line intensity and line/background ratio were very similar and show: (i) The deletion Emits improve with increasing current up to a maximum of 10 A. At higher currents the electrode walls burn away rapidly, resulting in a loss of sample and a corresponding decrease in fine intensity. An exception to the above trend is shown by germanium which gives higher line intensities at the lower currents. This can probably be explained on the basis of the properties of GeU,. This compound is appreciably volatile at temperatures above 1250% [25], whereas germanium itself is involatile (boihng point 2~30~~)* The increase in cathode temperature due to the increase in arc current would favour the reduction of the oxide to the metal thereby reducing the amount of germanium entering the plasma. (ii) The variation in line intensity (not shown in the figure) and detection limit with changing arc current above 6 A is small. At lower currents the cathode layer enri~~~~t effect weakens, the position of the rna~rn~m of the line intensity and line/background ratio curves of elements with higher ionisation potentials moving away from the cathode. (iii) The arc temperature distribution in this region undergoes a rather interesting change as the current varies. As it increases the position of the maximum temperature
R. J. DECKER
348
%
ii 0
::
T, /Do
h cr -Q-+.
;’
-I
0.1
]
~~~,
0.1
0.1
0.2
0.2
0.3
0.3
0.4
04
05
05
Distance, mm Fig.
‘b
0.2
0.3
0.8
0.4
,, 15,,p-;;\;, 0.1
oi
0.2
0.3
0.4
05
0.2
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04
05
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0.1
0.2
0.3
04
06
12730;6
,
,
,
0.1
0.3
0.4
0.3
0.2
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6. The effeot of ero ourrent on temperature, el&xon pressure, and line/ background ratio on different elementa in the oathode layer. w 4 A, - --o8 A, -.-.--_o12 A.
moves away from the cathode indicating that the transference of energy to the cathode by becomes increasingly less efficient. (Temperatures greatm than 6600’K-shown the curves for both 10 A and 12 A-are probably inaccurate due to the possibility of the optimum temperature of the Zn 3076 line being exceeded, or of the selfabsorption of this line becoming significant at the higher currents due to an increase in the distillation rate of the zinc caused by the higher electrode temperatures. Both of these effects would cause the calculated arc temperatures to be higher than the true temperatures. Since these temperatures may be inaccurate it follows that the corresponding electron pressures may also be high. Nevertheless, the trends shown by the graphs are still valid). 3.3 Effect of arc gap 3.3.1. Experivnental conditiona The conditions used were identical to thoee described in the previous section. The am wee burnt with a current of 8 A with arc gaps maintained at 4 to 12 mm in 2 mm steps. (Currenta greeter than 8 A could not be used since at the smaller am gape the higher temperetmw of the osthode causes the orater walls to burn away too rapidly).
3.3.2 Resulte In all instances the line intensities and detection limits were markedly inferior At greater arc gaps no significant trend could be observed, atanarcgapof4mm.
Studies of the oathode layer of a d.c. arc
349
perhaps due to the variation in the volatiliaation rates of the elements, caused by the changes in the electrode temperature a.t the diEerent arc gaps, having more effect on line intensity than the variation of the are gap it&f. In all ~8888the variation of line inteneity and detection limit is small with are gape greater than 6 mm. 4. THE RADTAL DISTRIBUTION OF INTENSITY IN TEE CATHODELAYER With the aid of a quartz Dove prism, the image of the 8rc was rotated through 90” and then focussed on the slit of the speotrograph (length 18 mm) with a (-fold magni&ation. The SpeCtr8 recorded were taken at 8 dist8nue corresponding to between 0.16 and 0.20 mm from the I I-
z
Zn 039eV Ge 8.13 MO 7.64
i-
Al 5.98
slrctrode
Li 5.39 I 0
I I
I 2
Dirtoncs from oxir, Fig.
‘7.
I 3
J 4
mm
The radial he intensity distribution obtained from elemente with different ionisation potentials in the oathode layer of an 8-A am.
cathode. The radial intensity distribution wae calculated from the recorded line profile using the numerical method of solving the Abel integral described by BIUCEWELL [26]. The results obtained with the standard mix burnt in an 8A arc are shown in Fig. 7, where the intensities heve been normal&d to give 8 central value of 10. It can be seen that 8s the ionisetion potential of the element increases, the distribution cnrve tends to broaden, indicating th8t the fectors causing the cathodelayer enriohment hinder not onIy the axial diffusion of stems, but also
radial diffusion. At 6 A the lithium ourve becomes narrower, the other onrvea remaining essentially unchanged. At 10 A and 12 A little variation was noticeable. An interesfing feature of the curves for zinc and germanium is the presence of a maximum awey from the centre of the arc column. This effect is more marked at 6 A and is absent at 12 A. The reasons for the ocourrence of the non-central maxima are uncertain but oould be due to either the self-absorption of the line or are wander, neither of which is taken into account in the Abel transformation. This effect has also been shown to occur to 8 greater extent in d.c. arcs using anode techniques 1271. [26] R. N. BRACEWELL, &&ml. J. phg8.9, IQ8 (1966). [273 R. J. DEand P. A. MCFADDEN, paper submitted to Spectrochim. Aots Part B.
R. J. DECKEB
350
5. EFFECT OF A MAGNETIC FIELD It has been reported by VUIUNO~IO and co-workers [28], 8nd by N1cx.x~ and co-workers [29, 301, thet the application of 8 st8tion8i-y inhomogeneousmagnetic field to 8 d.c. 81% can result in lower detection limits for many elements. Experiment showed, however, that the application of such 8 field caused 8 definite deterioretion in the detection limits when the cathode 18yer region is observed. This is probably due to the field ceusing the cathode spot to rotate &bout the w8lls of the electrode cavity rether then st8ying directly over the semple. As 8 result, the walls burn ewey preferentiallyend 8 significantproportion of the sample does not enter the cathode layer. 6. EFFECT OF ARTIFICUL ATMOSPHERES 6.1
Experimental conditions Two etmospheres were chosen for this p&r%of the investigation:
(i) Argon-oxygen mixture (70 per cent Ar and 30 per cent 0,) as it is often used to eliminrtte the interferenceof the cyenogen bend system. (The use of this mixture, in conjunction with the cathode layer, was recommended by MYERSand HENRY [31]). (ii) Helium as a gas with a high ionisation energy (24.48 eV), in en 8ttempt to enhence the c8thode layer effect. (The higher ionisetion potential of the atmosphere induces e greeter contraction of the arc column at the cathode resulting in a steeper potential gradient). The atmosphere around the 8rC was maintained using the Stallwood jet type of apparatus described by BOUM~S and MAESSEN[32] which ten be used to provide two concentric gas sheaths about the 8rc column, the outer she&h being used to shield the inner gas mixture from the 8tmosphere. To obtain conditions approximating that of e free burning erc three steps were taken: (i) the gas ~8s introduced via the outer, shieldinggas 8perture so th8t the upward stream of gas would have as small 8n effect on the erc column stability 8s possible. (ii) the arc w8s surroundedby a quartz dome with a window cut in it so that the recorded rcldietionfrom the 8rc was undisturbed by the dome itself, end (iii) the gas flow adjusted to the minimum to remove the CN bands from the recorded spectra, i.e. at e rate which maintained 8 slight positive pressurewithin the quartz dome. The erc gap ~8s 10 mm and the current 8 A. 6.2
Results The curves obtained
are represented
by those shown in Fig. 8, and show that:
(i) The use of an argon-oxygen atmosphere has an adverse effect on line intensity and detection limits. This is probably due to the fact that the introduction of appreciable amounts of argon to the arcing atmosphere causes a marked decrease in the voltage drop across the arc and thus in the power generated in the plasma. This is accompanied by a reduction in the arc column and cathode spot diameters, thereby encouraging wandering of the cathode spot about the electrode having, to some extent, the same effect as a magnetic field. 1281 V. Vux~~ov10. V. GEORGIJE&, D. Vux~~ov10 8nd 5. M. TODORO~IC, Spectrochim. Acta 24B, 555 (1909). [29] D. F. LEUSHAC~Eend H. NICIIEL,Spectrochim. Acta 26B, 391, 409 (1971). [30] D. LVMMERZHEIM and H. NICKEL,2. Anal. Chem. 246, 267 (1969). [31] W. C. MYERS end W. M. HENRY, Conference on the ~ltragnw@ation of Semiconductor Materials, Boston, Mass. 1961, p. 349. Macmillan Company, N.Y. (1962). 321 P. W. J. M. BLUMANSand F. J. M. J. MAESEEN,Spectrochim. Acta 24B, 585 (1969).
Studies of the cathode layer of
8
d.c.
361
are
fzf!y~,~~~, 0.1
0.2
0.3
0’4
0 I
0’5
0.2
0.3
04
0.5
~~~~~2, rf&; I’, 0.1
0.1
0.2
0.3
0.4
0.5
0.1
0.2
@3
0.4
0.5
0.1
I
I
I
I
1
0.2
0.3
0.4
0.5
0.1
02
0.3
0.4
0.5
0.1
02
0.3
0.4
0.5
Distance,
mm
Distance.
mm
0.2
0.3
0.4
0.5
Dlstance. mm
Fig. 8. The effect of different, atmospheres on the line/background -e air, - - - -o-Ar/O,, -.-.cathode 18yer.
ratios in the -n-He.
(ii) In the helium atmosphere the cathode leyer effect appears to be enhanced with the elements with low ionisation potentials in as much as the intensity or detection limit gradients are much steeper. Unfortunately, since the electrodes do not burn away during the arcing period, the amount of sample entering the column is relatively small, and the detection limits are inferior to those obtained using the other two atmospheres. Another interesting aspect is that a number of carbon ion lines (2572 A, 2836 A, 3509 A) was easily visible in the spectra obtained using helium as the arcing atmosphere. These lines, with excitation potentials of 16 eV end more, are generally only found in spark spectra. In one of the preliminary experiments aimed at determining the correct gas flow rate, the nitrogen O-O and O-l bands were also easily visible. These phenomena are not easily explained since the excitation mechanism in a He arc is not yet fully understood. 7. COMPARISON BETWEEN THE CATHODE LAYER TECHNIQUE AND ANODE TECHNIQUES In analytical techniques using the cathode layer it is generally recommended that the sample be mixed with carbon powder in a ratio of 1 :l. In spite of this dilution the prepared sample may still contain relatively high concentrations of elements with 3
352
R. J. DECKEB
low ionisation potentials which could seriously disturb the cathode layer enrichment thus reducing or even removing any benefit obtained in using the technique. 7.1 Distribution of line htenkty in the whole arc column To compare the effect of an excess of an alkali metal on the intensity distribution in the whole column, the arc was focuseed on the slit of the spectrograph (18 mm in length) with a e-fold magnification, and the standard mix burnt undiluted and also 2r
2
4
6
Distance from cathode,mm
Distance from cathodqmm
4 2 6 Distance from cathode.mm
Fig. 9. A oompariaon of line/background ratios obtained in the whole arc when buffered and unbuffered samples are eveporeted from the anode end oathode reepeotively. 0 % Ne,CO, ----2O%Na,CO, -**-‘-
0 % Ne,CO,
--.-.
20 % Na,C!O,
1
Cathode Evaporation,
Anode Eraporaticn.
with the addition of 20 per cent Na,CO, using the cathode (electrode C) and anode (electrode D) 8~ sample carriers respectively. The dietributions obtained are represented by the curve8 given in Fig. 9 which show: (i) Line intensities measured in the centre of the column are improved when the NqCO, ia present in the sample. This is probably due to the lower temperature of the arc favouring atomic emission since ionisation of the elementa is reduced. (ii) In all c&888the detection limits are very much better when using the cathode layer without the alkali metal. Interesting is the fact that when the cathode layer enrichment is marked, the element is not always generally detectable in the are column if the sample is evaporated from the cathode. (iii) A cathode layer enrichment is aleo present when the undiluted sample is evaporated from the anode. The intensity curvea obtained with the diluted sample
studies of the cathode 18y0r of 8 d.c. 8rc
363
also indicated the presence of slight enrichment in the cathode layer but the line/background ratios generally show little improvement. (iv) When an excess of alkali metal is present there is little difference in detection limits when using either the cathode layer or normal d.c. arc techniques. Samples containing high concentraClearly this latter point is of significance. tions of elements with low ionisation potentials must not be allowed to evaporate into the plasma at such a rate that the concentration of these elements is sufficient to destroy the enrichment effect. (MANNKOPPF and PETERS recommended that the sample should not distil into the plasma at a rate greater than 5 mg per min.) Two methods are immediately apparent; one can merely dilute the sample even further, or, one may use an electrode with a narrower cavity and thicker walls to reduce the rate of distillation of the sample into the plasma. The latter method was tested by comparing the results obtained in section 3.1 using electrode C, with results obtained using electrode B (see Table 2) under similar conditions. No significant difference, however, was found in the distributions or detection limits probably because the arc burns preferentially on those regions where the concentration of easily ionisable elements is greatest, i.e. usually on the sample and not on the electrode walls. (This effect has been used by MELLICH~MP to restrict wandering of the cathode spot in conventional d.c. arc methods [33].) 7.2 Comparison
of detection
7.2.1 Expwimentd
limits
cditionn
(i) Optica. The investigations 80 far described were carried out with the BTCfocueaed on the slit of the spectrogrsph. In norm81 an8lytic8l prsotice it i common to focus the 8ra on the collimator of the spectrograph in such 8 W8y thet the imege of the 8M: is as defocueeed aa possible at the alit, to reduce m eaaurement errors which m8y be C8used by dietribution effects in the plasma. Therefore, to comp8re the oathode layer technique with anode techniquea under conditions used in axmlysis, the 8rc was fooueeed onto 8 diaphragm using two, a& cylindrioel mirrors such thet the Bxial magnifkstion w8e IO-fold and the r8disl megniflcation, 2-fold. The diaphragm had 8 4 mm square aperture, thus light from the 6ret 04 mm from the cathode ~88 sllowed to enter the spectrograph. For the anode technique the cent4 portion of the 8ru column w8a focuseed on the diaphnxgm with no megnific8tion. Radiation pas&g through the diaphragm ~8s then focuesed onto the collimetor mirror. (ii) Sam@. The samples used were three earnplea containing N8,COs u88d in section 3.1, plus 8 sample contrrining SO per cant Ne&O,. (iii) Ekxtro&x~ Cathode layer technique, electrode C; Anode technique, electrode D. (iv) Excitation conditiona. current 8 A; g8p 8 mm; exposure time, burnt to completion.
7.2.2 Reeults Plots of line/background obtained using both techniques at the different Na,CO, concentrations are shown in Pig. 10. They show that as the concentration of the alkali metal increases the difference in the detection limits decreases, and that at concentrations of between 10 per cent and 60 per cent of the carbonate (6 per cent25 per cent of sodium) the conventional anode technique is, in most cases, slightly more favourable. [33] J. W. %kLLICHAMP, Appt. Spectrosc. 21, 23 (1967).
R. J. DECKEB
0
0.01 01
I
so
IO
NasCOs,
0
0.01
0 I
Na,CO,
%
,
IO
50
, %
2-
I
L
1/q-I
“t
.,2752,
,
00010~1
I
,
0
IO so
Na,Cs.
_ _ ‘Ge3039
0
%
O-01
I
04
Na,CO,.
IO
50
%
Fig. 10. A comparison of line/background ratios obtained with containing increasing concentrations of Na using the cathode layer __f_ cathode 18yer technique, -0--and anode technique. technique.
7.3
8 sample technique anode
The analysis of naturally occurring samples
The majority of naturally occurring samples generally contain signilkant quantities of elements with low ionisation potentials-commonly sodium and potassium and often aluminium. It was therefore decided to compare the two techniques using three geological samples, containing different, but significant, amounts of elements with low ionisation potentials. 7.3.1 Experimental
con&time
(i) Opt&x. The optic81 systems used were as described in section 7.2.1. (ii) Samples. The three samples used and the concentrations of the major elements in the samples are given in Table 4. (iii) Arc&g conditiona. For the cathode layer technique the samples were diluted with graphite powder to form two sets of samples containing 50 per cent and 75 per cent graphite respectively. The former set of samples were p8cked into electrodes A, B and C, and the latter Table
4. Composition
of the three ssmples used in Section Composition
Sample
SiO
C80
Limestone B8&t Granite
14.1 41.7 69.2
41.3 11.5 0.1
Fes0s 7.6 11.4 0.08
N8,0 0.4 2.9 2,8
7.3
( %) KsO 0.7 0.9 10.1
Al,0, 4.2 13.3 10.8
2.2 12.7 -
Studiesof the cathode layer of a d.e. arc
366
set into electrodesB and C. The samples were burnt in a 9-A are with an arc gap of 9 mm. The exposure time after a pre-burn of 16 set was 1 min. For the anode technique, the samples diluted to contain 50 per cent graphite powder were pseked into electrode D and burnt to completion using an arc current of 8 A and an arc gap of 6 mm. Under these conditions the btbckgroundintensities were, for a given geological sample, appro~ma~ly the same for the four arcing conditions.
7.3.2
Results
The mean of 3 line/background ratios obtained for eight trace elements found in the three samples under each set of conditions is given in Table 5. They show that very little, if anything, is to be gained by using the cathode layer technique with the basalt and limestone samples. The granite results show, however, that the use of the technique give more favourable detection limits for most elements particularly when using electrode A as recommended by M~TCRELL [111.The reason for this difference almost certainly lies in the burning qualities of the sample. The granite sample burnt well, the arc column rerna~~g steadily over the sample cavity with very little wander of the cathode spot. The other two samples, however, burnt with marked diffusion of the sample to and through the electrode walls causing severe wandering of the cathode spot about the crater edge and thus a substantial amount of the sample did not enter the cathode layer, 8. SAMPLE SIZE The three electrodes used in Section 7.3 in conjunction with the cathode layer technique burn at approximately the same rate under the given conditions and it is thus possible to obtain from Table 5 a measure of the amount of sample required to obtain a given line/background ratio. Under the conditions given in section 7.3, the amounts of sample leaving electrodes A, B, C and D are approximately in the ratio 1:4:9 :33 respectively. A study of the figures in Table 5 reveals that the amounts of sample required to obtain a given line/background ratio using the cathode layer and anode layer techniques is 1:3 respectively in the worst cast (zirconium in basalt) and 1:330in the best case (titanium in granite). It has been suggested by BOUMAN~ [34] that this very large increase in the Ti and Ni intensity in the granite samples may be in part due to blank enhancement. The Ti results in the other two samples are also high when using electrode A indicating that perhaps the electrode material was contaminated with Ti. Both electrode A and electrode D, however, were cut from the same box of electrode rods and, due to the larger dimensions of the electrodes more electrode material enters the arc when using the anode technique (electrode D burnt to completion) than in the cathode layer technique (electrode A burnt for 1 min). If one ignores the Ti results the best sample mass ratio drops to 1x50{zinc in limestone) which, perhaps, is more realistic. It is clear, therefore, that one obtains a marked decrease in the absolute detection limits of the method when the cathode layer is used in preference to the main body of the arc column. [34]P. W. J. M. BOWMANS,private communication.
R. J. DECKEB
366
Table 6. The mean value of three line/background ratios obtained using different techniques with the samples given in Table 4. The valuee in italics are the values lying within 10 per cent of the maximum values obtained for a particular element in a given sample and thus indicates the techniques which give the best detection limits for the element and sample conoemed Sample
BASALT
Graphite cont. Mnthod Electrode
A
60% Cathode Layer l3
C
13 0.8 10
11 @4 3.6
6.6 3.3 6.3 1.4
6.2 2.3 1.3 0.5
Element
Line A
Mn Pb Cd Ni
2784 2833 2981 3134
16 _+ 0.6 ;:;
Ti CU Zn ZI
3200 3274 3282 3438
;:; 0.9
50% Anode Tech. D
7
8
27
1;
0;
0.6
7.4
6.7
45
4.4 4.1 4.3 2.0
5.5 3.3 4.5 1.1
;:;
-
5.5
4.8
LIMESTONE
Sample Graphite conc. Method Electrode
A
Element
Lino A
Mn Pb cd Ni Ti CU Zn Zr
2794 2833 2981 3134 3200 3274 3282 3438
60% Cathode Layer B
16 -
21 0.5
;:; ;“6
2.0
C
26
76% Cathode Layer n C
60% Anode Teoh. D
16 0.5
22 0.7
15
0.9
0.1 4.9 1.4
0.1 63 1.1
0.1 0.9 4.5
0.1 0.8 6.1
77
1.0 0.6
0.4 1.4 0.4
0.5 1.4 0.9
0.7 1.1
;:4” 1.7 1.9
0.9
GRANITE
Sample Graphite cow. Method Electrode
A
Element
Line A
Mn Pb Cd Ni Ti CU Zn Zr
2794 2833 2981 3134 3200 3274 3282 3438
* -
76% Cathode Layer B C
7.4 1.0
;:;
60% Cathode Layer l3
3.8 2.6 I-2 1.6
C
12 3.9 1.1 1.9
76% Cathode Layer I3 C
3.7 2.8
0.8 1.2
50% Anode Tech. D
1.0 1.8
5.5 1.5 1.0 -
6.2
2.3
2.3
1.1
2.3
0.6
;I; 9.7
0.7 _ -
“” -
02 -
23 _
1.2 -
-
denotes element not detected.
9. Some of the more important
SUMMARY
points to emerge from this study are:
(i) Detection limits are not significantly affected by small changes in arc current and arc gap provided these factors are kept at values greater than 6 A and 6 mm respectively. The use of lower currents causes the cathode layer enrichment effect to weaken and the position of the maximum of the line/background ratio can move
Studies of the cathode layer of a d.c. arc
367
away from the cathode. Arc gaps smaller than 6 mm result in a marked deterioration of the detection limits. (ii) The application of a stationary inhomogeneous magnetic field causes the detection limits to deteriorate. (iii) The use of artificial atmospheres when using the cathode layer has no beneficial effect on detection limits, although the removal of the cyanogen band system is an advantage when determining elements which have their more sensitive lines in the wavelength region occupied by these bands. (iv) Increases in the concentration of elements with low ionisation potentials can markedly affect the distribution of atoms in the cathode layer, increasing the detection limits of the method. Detection limits, however, can be slightly better than those obtained with anode techniques, even with samples containing about 5 per cent alkali metal. Elements with high ionisation potentials appear to also affect the distribution of atoms in the cathode layer, but to a lesser extent. (v) The sample must evaporate directly into the cathode layer if any benefit is to be obtained by using the technique. Any movement of the cathode spot away from the sample can seriously affect the detection limits and this often happens with naturally occurring samples. (vi) At high concentrations of elements with low ionisation potentials the position for obtaining the optimum detection limit is at some distance away from the cathode, the distance being determined by the ionisation potential of the element and the degree to which the cathode layer enrichment has been suppressed. (vii) The amount of sample required to obtain a given line to background ratio is less when using the cathode layer technique than when using an anode technique. 10. CONCLUSIONAND DISCUSSION It is clear that the use of the cathode layer technique has some advantages in trace analysis. The detection limits obtained for samples with small concentrations of elements with low ionsiation potentials can be better than those obtained using anode techniques, even though in the latter case the sample is well buffered. In this investigation, using the synthetic samples which almost certainly give optimum results, the maximum improvement in detection limits was lo-fold, but was generally of the order of six times when the first O-4 mm of the arc was focussed on the collimator of the spectrograph (see Fig. 10). No doubt, by using a thinner layer closer to the electrode, detection limits could be improved even further. In view of this, one can forsee the technique’s successful application in many instances such as in the analysis of refractory material using carrier distillation methods (supplementary heating of the cathode may be necessary in some cases), in the analysis of semiconductor material, in the analysis of air-borne dust, etc. Its use in the analysis of carbon products is already well known. With samples containing high concentrations of elements with low ionisation potentials such as most naturally occurring material, detection limits can be two or three times better than when using anode techniques. With such samples, however, optimum detection limits may be obtained some distance away from the cathode.
368
R. J.
DECKER
Unfortunately, it appears that any movement of the cathode spot which enables the sample to distil from the electrode without entering the cathode layer can adversely affect the detection limits. It is, therefore, essential to use the technique under conditions which reduce wandering of the cathode spot to an absolute minimum. Two other disadvantages are apparent. Firstly, apart from the normal matrix effects which occur in d.c. arc. analysis, there is the additional effect of changes in the concentration of the major elements, particularly those with low ionisation potentials, affecting the atomic distribution in the cathode layer and hence the recorded line intensity. In choosing an internal standard, therefore, more consideration must be given to the ionisation potentials of the elements than is usual in the anode techniques. Secondly, the position of the cathode must be accurately maintained throughout the exposure, since a displacement of only O-1 mm can have a marked effect on recorded line intensity and detection limits. Because of this the technique is perhaps not particularly suitable for the routine analysis of a large number of samples unless precision requirements are fairly low. Perhaps the most promising field of application is in the analysis of very small samples (of the order of a mg). See, for example PREUSS [35] who used the technique successfully to analyse mineral grains in meteorites. In this respect the use of helium seems to warrant further study, particularly as it appears that some non-metals may be detected under these conditions. It should, however, be noted that although smaller samples may be used in this technique, the relative detection limits may not be significantly better than when using conventional anode techniques. It is apparent, therefore, that in some instances, particularly when analysing very small samples, the use of the cathode layer has much to commend it. It is recognised, however, that the successful application of the technique requires a certain degree of experimental skill. Finally, it is suggested that the cathode layer be defined as that region in the arc between the boundary at which the sample vapour leaves the influence of the cathode fall and enters the arc column, denoted by the sudden decrease in the intensity of spectral lines emitted by elements with low ionisation potentials (the ‘cut-off’ referred to in this paper) and the cathode, rather than vaguely referring to the cathode layer as the region close to the cathode. Acknowledgements-The author wishes to express his deep gratitude to the following: Prof. H. KAISER, Director of the Institut ftir Spektrochemie und Angewandte Spektroskopie, Dortmund, and to Dr. K. LAQUA and his colleagues in the ‘Emissionabteilung’ of the Institut for their assistance, both academic and personal, during his stay in Germany; to the Alexander von Humboldt Foundation for the award of a very generous scholarship, and to Dr. P. W. J. M. BOUNANS and Dr. D. J. EVE whose comments and suggestions have been most valuable. [35] E. PREUSS, Mikrochim.
Acta,
382 (1956).
Studies of the cathode layer of a d.c.
arc*
R. J. DECKER University of Rhodesia, P.O. Box MP 167, Sal&bury, Rhodesia (Received1 November1972; revtiion received24 April 1973) Abstract-Some factors which affect the cathode la.yyer of a d.c. arc were investigated. It is shown that: (i) varying concentrationsof elements with low ionisstion potentials in the sample oan markedly affect the distributionof atoms in the cathode layer and hence the detection limits of the technique; (ii) variations in arc current and arc gap have only a small effect on the cathode layer provided that these factors are maintained at values above certain minimum levels; (iii) the use of artifical atmospheres and inhomogeneous magnetic fields generally increase detection limits; (iv) detection limits obtained with samples containingrelatively high concentrationsof elements with low ionisation potentials can be two or three times better than those obtained with conventional anode evaporation techniques provided wandering of the cathode spot is kept to a minimum, a condition which often cannot be realised in practice. 1. INTRODUCTION
little has been written about the conventional use of the cathode d.c. arc as a light source in spectrogmphic analysis as initially described by ~WANNKOPPF and PETERS [l], although Atn~ and BOUKOBZA [2,3] have used this region with the sample being evaporated from the anode. In this case, the reason for using the cathode layer with the arc in this configuration is different from that underlying MANNKOPF and PETERS’ experiments [a]. It has been claimed that the use of the cathode layer technique has the following advantages : (i) lower detection limits for a large number of elements, particularly those with low ionisation potentials, (ii) insensitivity to small external changes such as variations in arc current and arc gap. The reasons for the above are, perhaps, not fully understood, but it is generally accepted that the cathode layer effect may be explained by the presence of the steep potential gradient in the close vicinity of the cathode. If only axial transport mechanisms are considered we can distinguish between two transport velocities [5, 61:
IN RECENT layer in a
YEARS
(i) convection velocity v,, which represents the rate at which the v&pour of an element is borne upward from the cathode by convection processes, and * This investigation was carried out at the Institut fiir Spektrochemie und Angewandte Spektroskopie, Dortmund, Germany, and was supported by a generous scholarship from the ALEXANDER VON HUMBOLDT Foundation, Bonn Bad Codesberg. [I] [2] [3] [4] [5]
R. R. R. R. P.
1MANNgo~rrand C. PETERS,2. Phy&. 70, 444 (1931). AVNI and A. BOUKOBZA,Appl. Spectrosc. 28, 483 (1969). AVNI and A. BOUKOBZA,Appl. Spectroac. %4,406 (1970). Aarr and Z. GOLDBART, Spectrochim. Acta 28B, 241 (1973). W. J. M. BOGS, Proc. 6th Colloq. Spectrosc. Intern., Amsterdam 1966, Spectrochim. Acta 11,146 (1967). [S] P. W. J. M. BOWS, Excitation of Spectra, Chapter 6, p. 50 in Analytical Emiaion Spectroecopy, (Ed. E. L. Grove) Vol. 1, Part II. Marcel Dekker, New York (1972). 339
340
R. J. DE-
(ii) ionic velooity, -v,, which represents the rate at which the ionio component of the element vapour would move towards the cathode (hence the negative sign) in the absence of convection. The resulting velocity of the ions is thus = o, - ui. If the degree of ionisation of the element is cc then the resulting velocity, V, of the element vapour away from the cathode is given by, V = u,(l - CC)+ a (u, - u‘), or V = v, - at),, vf cm be replaced by pE, where 1 is the mobility of the ion and E the electric field strength in the plasma giving, V=u,apE. Clearly then the upward velocity of an element, under constant conditions of temperature and electron pressure-which affect both the convection velocity and ionisation of the element-is determined by the degree of ionisation and hence by the ionisation potential of the element and also by the magnitude of the electric field strength. In the cathode layer the field strength is so large that V ia negative and the element v&pour, as a whole, migrates back towards the cathode forming a region of high concentration directly in front of the cathode, this effect clearly being more marked with elements with low ion&&ion potentials. This high concentration of atoms results in the emission of more intense atomic spectral radiation from the cathode region than from the main body of the arc column. Since transference of energy from the arc column to the cathode is small, the cathode temperature is relatively low. This may be beneficial in two respects: (i) more controlled evaporation of the sample, and (ii) the possibility of using thinner electrode cavity walls when evaporating the sample from the anode giving a higher ratio of sample mass to electrode msss in the aro column. Both SCOTTand M~CHELL 17-111 developed the use of the cathode layer as an emission source for the analysis of rocks, soils and plant material. The methods were suocessful and the technique became fairly widely used. On the debit side it is recognised that significant concentrations (about 6 per cent) of the elements with low ionisation potentials, such as the alkali metals, can diminish the potential gradient in the cathode region, and it is generally accepted that relatively high concentrations of such elements in the plasma can markedly reduce or even destroy the cathode layer effect. Because of this Scorr and MITCHELL developed separation techniques [ 121which removed the alkali metals, alkaline earth metals and [7] [S] [9] [lo] [ll]
R. 0. SCOTT, J. Sot. Ckm. Itd 64, 189 (1945). R. 0. SCOTT, J. Sot. Chem. Id 66, 291 (1946). R. L. MITCEELL, J. Sot. Chem. Id. 69, 210 (1940). A. M. M. DAVIDSON and R. L. MITCHELL, J. Sot. Chem. Ind. SO,21 3 (1940). R. L. M.ITCEELL, Tech. Commun. No. 44 of the Commonweelth Bureau of Soil Soience, Harpenden, England (1948). [12] R. 0. SCOTT and R. L. MITCHELL, J. Sot. Chm. Ind. 63,4 (1943).
Studies of the cathode layer of a d.c.
an:
341
phosphates from the sampIe substances which SCOTThas shown to adversely affect the measured line intensity [13]. Of significance, perhaps, is the observation of MANXKOPPF and PETERSthat the detection limits obtained using norm& twc techniques were of the same order as those obtained using a spark soure%,an observation in conflict with the experience of modern analysts. It is possible that the arc techniques were then in an undeveloped state and that with improvements in the method, little is to be gained in using the cathode layer. Indeed, AXRENS and LIW_IIZ:NBERG [l&j, and PIERG~C, TORRESand &Cnsrr~,,~ El&] have found little diflSren%ein detection limits when using either the cathode layer technique or conventional methods (referred to in this paper as anode barques), although the absolute de&&ion limits of the former method> expressed in terms of the minimum mass of an analyta that is detectable, were undoubtedly superior iu as much as the sample required for analysis was very much less. The work described below was carried out to investigate the cathode layer more fully and to see whether its appar%nt advantages could be extended.
(i) J&w$wM~~. The apparatus used is detailed in Table 1. (ii) Ibctrodee. The electrodes used are detailed in Table 2. (iii) ~~~~1~. In most of this work a synthetic sample, referred to in this report as the standard mix, was used hawing the ~rn~i~on given in Table 3. The elements were ohos%n on the bases of their ion&&ion bunts and the simplicity of their spectra. TabIe 1. Apparatus used in this study Spectrograph Grating Nit width Photographiaplates Development PoweFsource ~c~pho~rne~r
A mod&d 3.4 m JACO grating spectrograph. 600 lines per mm used in t&6 first order. Grating width 10 cm. 20 pm. Kodak S.A. 1. 3 min in Agfa Radix101diluted I:26 at 20% Current stabilised d.c. source, Zeiss ~c~6~photom3~~.
(iv) CaIcuZat&ns.The arc temperatures, Tin degrees Kelvin, welp8calculated from the int%nsity ratios of the zinc lines 3676 and 3232. The electron pressures, % in Newtons/ma, were calculated from the arc temperstur%s and the vanity ratios of the m&~esium lin%s23S2 and 2796. The relevant equations for oalculating T and pe were given by BOUMAXS1171.
R. J. DECKER
342
Table 2. Electrodes used in this study
crater Wall thickness Electrode
Dia (mm)
Depth (mm)
A
080
10.0
B
1.68
476
0.74
C
2.44
6.35
0.37
D
2.0
Counter
Reference
(-) 1.0
According to MITCHELL [ 1I] RingsdorfI Cat. No. RW0033. Ringsdorff Cat. No. RWO034.
I.0 tapering to O-1 taper angle 7” I Blunted carbon rod 6 mm dia 8.0
DECKERand EVE [16]
Table 3. Composition of standard mix and the spectral lines used in this study
Compound
(A)
Cone. of element in graphite (%)
Li,CO,
3232 3082 3011 2796, 2852 3039 3076, 3282
0.0008 0.001 0.002 Electrode impurity 0.0002 0.04
Line Element Li Al Zr Mg Ge Zn
AU& ZrOCl, GeO, ZnO
Ion Pot.
5.39 5.98
(W
6.84 7.64 8.13 9.39
All the calculations, including the emulsion calibrations, were carried out on an I.B.M. 1130 computer using a modified version of the program previously described by DECKER and EVE [ 181. (v) Detection limits. In emission spectrography it is common practice to define the detection limit of a method as being that concentration of analyte which gives a line intensity equal to three times the standard deviation of the background intensity at the wavelength of the analytical line [19]. The mean intensity and standard deviation of the background are commonly calculated from background intensities obtained from 15 or 20 exposures of blank samples. The detection limit is then obtained by assuming that the working curve is linear near the detection limit and extrapolating the curve to the intensity representing the detection limit. A more rapidly determined measure of the detection limit, useful for comparing methods on a given instrument, can be obtained by measuring the line intensity emitted by a given concentration of analyte relative to the background intensity. This method is only valid, however, if the background intensity does not vary too drastically between the different methods. (It has been shown that the standard [18] R. J. DECKERand D. J. EVE, Spectrochim.Acta SSB, 479 (1970). [lQ] H. RAISER,Spectrochim.Acta 3, 41 (1947).
Studies of the
cathodelayer of a d.c. arc
343
deviation of the background on a photographic plate is related to the density of the background but over a fairly wide range of densities can be considered to be approximately constant [IS]). Since the background intensities did not vary considerably in this investigation, line/background ratios were used to compare detection limits obtained under the different conditions employed in this investigation. For a more detailed discussion of the determination of detection limits the reader is referred to the publications dealing specifically with this topic [19-231. 3. FACTORS AFFECTING THE AXIAL DISTRIBUTION OF SPECTRAL RADIATION IN THE CATHODE LAYER
3.1 Presence of elements with low ionisation potentials 3.1.1 Experimental
conditions
(i) Optics. The arc was focussed on the slit of the spectrographwith a B-fold magnification using front surface mirrors. Only radiation from a layer I.6 mm in height directly above the cathode was allowed to enter the slit of the spectrograph. During the exposure the image of the leading edge of the cathode was kept, by manual adjustment, at the edge of the fishtail diaphragm over the slit. The maximum movement of the cathode from this position during the exposures did not exceed a distance equivalent to f0.1 mm in the arc. (ii) Measurement. Measurementswere taken along the spectral lines at distances equivalent to 0.05 mm in the arc. The length of the microphotometerslit was equivalent to 0.02 mm in the arc. (iii) Samples. The standard mix was diluted with NasCO, and KsCO, to form two series of mixtures containing 0 per cent; 0.01 per cent; 0.1 per cent; 1 per cent; and 10 per cent of the compounds respectively. (iv) Sample electrode. electrode C (see Table 2). (v) Arc conditions. Current 8A; gap 9 mm; exposure time 2 min.
3.1.2 RESULTS Figure 1 shows the distributions of temperature, electron pressure and line intensity obtained with the samples containing 0 per cent; O-1 per cent and 10 per cent K&O,. (The curves for 0.01 per cent and 1 per cent K&O, are omitted for clarity but their positions may be readily predicted from the trends shown in each diagram). Identical results were obtained with Na,CO,. The curves show: (i) The temperature and electron pressure decrease slightly away from the cathode. As would be expected, a large increase in the concentration of the alkali metal in the sample causes a reduction in the arc temperature. (ii) When the concentration of alkali metals is low a steep increase in intensity is observed in the vicinity of the cathode for elements with low ionisation potentials. The curves shown in Fig. 2 are the line intensity curves obtained for the undiluted sample corrected to constant temperature (5000°K) and constant electron pressure ( 1O-2Nm-a), th e values being normalised to an initial intensity of 100, and, therefore, [20] [21] [22] [23]
H. KAISER, P. W. J. M. P. W. J. M. P. W. J. M.
Opt% 91, 309 (1964). BOUMANSand F. J. DE BOER, Spectrochim. Acta 27B, 391 (1972). BOUMAXSand F. J. M. J. MAESSEN,2. Anal. C&m. 220, 241 (1966). BOUMANSand F. J. M. J. MAESSEN,2. Anal. Chem. 235, 98 (1967).
344
0.5
I.0
I.5
r 2 82 0
f
--. _.-.
-.
-k*., ‘\
-iFI
\\
B o
~
‘i
Mg2652
I
. 0
0.5
I.0
Distance from cathodqmm
I.5
0.5
FO
I.5
Distance from cathodqmm
03
to
I.5
Distance from cathode,mm
Fig. 1. The effect of inch inundation of K&O, on the distribution of &PC temperature, de&on pressure, and line intensity emitted by elements with different ionisation potentials in the vicinity of the cathode. ape--0 % K&O,,- - -o0.1% K&OS, -*-~--a10% K&O,,
represent the relative distribution of the atoms in this region [24]. It is clear that the ‘con~ntr~tion gradient’ in the cathode layer is directly related to the ion&ration potential of the element. (iii) Under oonditiona in which the oathode layer is marked, %n‘intensity plateau’ follows the ‘en~~hment zone’, after which the intensity of the spectral line decreases markedly. The region of the ‘cut-off’ is the same for all elements affected and lies at 1.1 f 0.1 mm from the cathode. (iv) The l~e~b~~k~o~d curves, shown in Fig. 3, generally follow the shape of the line intensity curves in Fig. 1, although they tend to be closer together. They show that for some elements s, little potassium in the arc can be beneficial. Perhaps most interesting is that with 10 per cent buffer the ~e~b&ckg~~d curves go through a maximum away from the cathode. The position of this maximum, [24]
R. J. DEUJIEBand D. J. Em, .A&
&u&mm. !#I, 497
(1969).
Studies of the oathads layer of a
d.c. &ro
345
Mg (7.65&l Zr (6.95eV) Al t5.99eV) ti (5.39
f
I
055
I I.0
eV1
t I+
Distance from cathode,
mm
Fig. 2. The relative distribution of atoms in the vicinity of the cathode C&XIlated from the data given in Fig. 1 for the undiluted samples.
OCGe
0-s I.0 t.5 Distance from cofhodff.mm Disfancefrom cafho*mffl
Fig. 3. The &Tact of increwing ~n~ntr~t~o~ of K&O, on the ~ue~~~o~d ratio, and hence on the detection limita, in the vicinity of the cathode. 0 % K&O,, - - - - O-1% K&O,, -.-a-. 10 % K,CQS.
representing the region where detection limits are optimum, appears to be related to the ion&&ion potential of the element as shown in Fig. 4, each point on the curve being a mean of the vahes obtained from the samples diluted to contain 10 per cent K&O, and Na&09 respeotively. (Magnesium does not fall on this curve since the element is supplied from the electrode walls and not from the sample and hence does not enter the cathode layer directly.) The presenoe of the maxima indicator that even with 10 per oent alkali metal compound the detection limits are somewhat better in
R. J. DECKER
346
lonisation Fig.
potential,
eV
4. The relationship between the ionisation potential of an element and the
distance from the cathode of the maximum of the line/backgroundratio curves in arcs containing 10 % of either Ne,COs or K&O,.
$-i-; 15, [z$ 0.1
0.2
0.3
Distonce.
0.4
05
mm
0.1
0.2
0.3
Distance.
0.4
0.5
mm
0.1
0.2
o-3
Distance.
0.4
0.5
mm
Fig. 6. The effect of increasing concentration of silicon on the line intensity emitted by elements with different ionisation potentials in the cathode layer. 0 % SiO,, - - - - 1% SiO,, -.-e-* 10% SiO,, -*+-*.30 % SiO,.
the cathode region than in the main body of the arc column, although the maxima for elements with low ionisation potentials fall outside the region normally accepted as the cathode layer. (v) Figure 5 shows that increasing concentrations of silicon-an element with a relatively high ionisation potential of 8.12 eV-affects the cathode layer enrichment of the elements to a lesser degree than the alkali metals. Increasing the concentration of the silicon also appears to reduce the overall line intensity in the cathode layer.
studies
of the
cathode layer of 8 d.c. aFc
347
Most of these results are readily predictable if one applies the theoretical considerations given in the introduction. The ‘intensity plateau’ is more difficult to explain, but appears to be a region in which the forces carrying the sample away from the electrod~p~ma~ly convection and radial di&sion-and the forces attracting the sample to the e~e~trod~le~trostat~~ are in dynamic eq~b~um~ and is probably the region of the transition of the cathode fall to the uniform Geld in the arc column. The cut-off in the line intensity indicates that there is a sudden, marked dilution of the sample vapour by the plasma vapour as it leaves the influence of the potential gradient and enters the main body of the arc column. Subsequent experiment has shown that the position of the cut-off is unaltered by varying the current between 6-12 A. 3.2 Effect of arc current 3.2.1 Experimental conditions (i) Opt&x. Results from the previous section indicate that the cathode k+y@r ‘enrichment zone” does not extend beyond about 04 mm from the cathode. !L’hisportion of the arc was thus imaged on the slit of the spectrograph so that w&h a lo-fold ma~~catio~ a 4 mm image of the region w-as obtained. (ii) Snw@e. Standard mix, undiluted. (iii) Sample electrode. &&rode C (see Table 2). (iv) Arc canditiolzs. current 4, 6, 8, 10 and 12 A; gap
9 IWIL
The results obtained are represented by the curves shown in Fig. 6. (The curves for 6 A and 10 A are omitted for clarity, but their positions may be readily predicted from the trends shown in eaoh diagram.) The curves for line intensity and line/background ratio were very similar and show: (i) The deletion Emits improve with increasing current up to a maximum of 10 A. At higher currents the electrode walls burn away rapidly, resulting in a loss of sample and a corresponding decrease in fine intensity. An exception to the above trend is shown by germanium which gives higher line intensities at the lower currents. This can probably be explained on the basis of the properties of GeU,. This compound is appreciably volatile at temperatures above 1250% [25], whereas germanium itself is involatile (boihng point 2~30~~)* The increase in cathode temperature due to the increase in arc current would favour the reduction of the oxide to the metal thereby reducing the amount of germanium entering the plasma. (ii) The variation in line intensity (not shown in the figure) and detection limit with changing arc current above 6 A is small. At lower currents the cathode layer enri~~~~t effect weakens, the position of the rna~rn~m of the line intensity and line/background ratio curves of elements with higher ionisation potentials moving away from the cathode. (iii) The arc temperature distribution in this region undergoes a rather interesting change as the current varies. As it increases the position of the maximum temperature
R. J. DECKER
348
%
ii 0
::
T, /Do
h cr -Q-+.
;’
-I
0.1
]
~~~,
0.1
0.1
0.2
0.2
0.3
0.3
0.4
04
05
05
Distance, mm Fig.
‘b
0.2
0.3
0.8
0.4
,, 15,,p-;;\;, 0.1
oi
0.2
0.3
0.4
05
0.2
0.3
04
05
Distance,
mm
0.1
0.2
0.3
04
06
12730;6
,
,
,
0.1
0.3
0.4
0.3
0.2
Dlstance. mm
6. The effeot of ero ourrent on temperature, el&xon pressure, and line/ background ratio on different elementa in the oathode layer. w 4 A, - --o8 A, -.-.--_o12 A.
moves away from the cathode indicating that the transference of energy to the cathode by becomes increasingly less efficient. (Temperatures greatm than 6600’K-shown the curves for both 10 A and 12 A-are probably inaccurate due to the possibility of the optimum temperature of the Zn 3076 line being exceeded, or of the selfabsorption of this line becoming significant at the higher currents due to an increase in the distillation rate of the zinc caused by the higher electrode temperatures. Both of these effects would cause the calculated arc temperatures to be higher than the true temperatures. Since these temperatures may be inaccurate it follows that the corresponding electron pressures may also be high. Nevertheless, the trends shown by the graphs are still valid). 3.3 Effect of arc gap 3.3.1. Experivnental conditiona The conditions used were identical to thoee described in the previous section. The am wee burnt with a current of 8 A with arc gaps maintained at 4 to 12 mm in 2 mm steps. (Currenta greeter than 8 A could not be used since at the smaller am gape the higher temperetmw of the osthode causes the orater walls to burn away too rapidly).
3.3.2 Resulte In all instances the line intensities and detection limits were markedly inferior At greater arc gaps no significant trend could be observed, atanarcgapof4mm.
Studies of the oathode layer of a d.c. arc
349
perhaps due to the variation in the volatiliaation rates of the elements, caused by the changes in the electrode temperature a.t the diEerent arc gaps, having more effect on line intensity than the variation of the are gap it&f. In all ~8888the variation of line inteneity and detection limit is small with are gape greater than 6 mm. 4. THE RADTAL DISTRIBUTION OF INTENSITY IN TEE CATHODELAYER With the aid of a quartz Dove prism, the image of the 8rc was rotated through 90” and then focussed on the slit of the speotrograph (length 18 mm) with a (-fold magni&ation. The SpeCtr8 recorded were taken at 8 dist8nue corresponding to between 0.16 and 0.20 mm from the I I-
z
Zn 039eV Ge 8.13 MO 7.64
i-
Al 5.98
slrctrode
Li 5.39 I 0
I I
I 2
Dirtoncs from oxir, Fig.
‘7.
I 3
J 4
mm
The radial he intensity distribution obtained from elemente with different ionisation potentials in the oathode layer of an 8-A am.
cathode. The radial intensity distribution wae calculated from the recorded line profile using the numerical method of solving the Abel integral described by BIUCEWELL [26]. The results obtained with the standard mix burnt in an 8A arc are shown in Fig. 7, where the intensities heve been normal&d to give 8 central value of 10. It can be seen that 8s the ionisetion potential of the element increases, the distribution cnrve tends to broaden, indicating th8t the fectors causing the cathodelayer enriohment hinder not onIy the axial diffusion of stems, but also
radial diffusion. At 6 A the lithium ourve becomes narrower, the other onrvea remaining essentially unchanged. At 10 A and 12 A little variation was noticeable. An interesfing feature of the curves for zinc and germanium is the presence of a maximum awey from the centre of the arc column. This effect is more marked at 6 A and is absent at 12 A. The reasons for the ocourrence of the non-central maxima are uncertain but oould be due to either the self-absorption of the line or are wander, neither of which is taken into account in the Abel transformation. This effect has also been shown to occur to 8 greater extent in d.c. arcs using anode techniques 1271. [26] R. N. BRACEWELL, &&ml. J. phg8.9, IQ8 (1966). [273 R. J. DEand P. A. MCFADDEN, paper submitted to Spectrochim. Aots Part B.
R. J. DECKEB
350
5. EFFECT OF A MAGNETIC FIELD It has been reported by VUIUNO~IO and co-workers [28], 8nd by N1cx.x~ and co-workers [29, 301, thet the application of 8 st8tion8i-y inhomogeneousmagnetic field to 8 d.c. 81% can result in lower detection limits for many elements. Experiment showed, however, that the application of such 8 field caused 8 definite deterioretion in the detection limits when the cathode 18yer region is observed. This is probably due to the field ceusing the cathode spot to rotate &bout the w8lls of the electrode cavity rether then st8ying directly over the semple. As 8 result, the walls burn ewey preferentiallyend 8 significantproportion of the sample does not enter the cathode layer. 6. EFFECT OF ARTIFICUL ATMOSPHERES 6.1
Experimental conditions Two etmospheres were chosen for this p&r%of the investigation:
(i) Argon-oxygen mixture (70 per cent Ar and 30 per cent 0,) as it is often used to eliminrtte the interferenceof the cyenogen bend system. (The use of this mixture, in conjunction with the cathode layer, was recommended by MYERSand HENRY [31]). (ii) Helium as a gas with a high ionisation energy (24.48 eV), in en 8ttempt to enhence the c8thode layer effect. (The higher ionisetion potential of the atmosphere induces e greeter contraction of the arc column at the cathode resulting in a steeper potential gradient). The atmosphere around the 8rC was maintained using the Stallwood jet type of apparatus described by BOUM~S and MAESSEN[32] which ten be used to provide two concentric gas sheaths about the 8rc column, the outer she&h being used to shield the inner gas mixture from the 8tmosphere. To obtain conditions approximating that of e free burning erc three steps were taken: (i) the gas ~8s introduced via the outer, shieldinggas 8perture so th8t the upward stream of gas would have as small 8n effect on the erc column stability 8s possible. (ii) the arc w8s surroundedby a quartz dome with a window cut in it so that the recorded rcldietionfrom the 8rc was undisturbed by the dome itself, end (iii) the gas flow adjusted to the minimum to remove the CN bands from the recorded spectra, i.e. at e rate which maintained 8 slight positive pressurewithin the quartz dome. The erc gap ~8s 10 mm and the current 8 A. 6.2
Results The curves obtained
are represented
by those shown in Fig. 8, and show that:
(i) The use of an argon-oxygen atmosphere has an adverse effect on line intensity and detection limits. This is probably due to the fact that the introduction of appreciable amounts of argon to the arcing atmosphere causes a marked decrease in the voltage drop across the arc and thus in the power generated in the plasma. This is accompanied by a reduction in the arc column and cathode spot diameters, thereby encouraging wandering of the cathode spot about the electrode having, to some extent, the same effect as a magnetic field. 1281 V. Vux~~ov10. V. GEORGIJE&, D. Vux~~ov10 8nd 5. M. TODORO~IC, Spectrochim. Acta 24B, 555 (1909). [29] D. F. LEUSHAC~Eend H. NICIIEL,Spectrochim. Acta 26B, 391, 409 (1971). [30] D. LVMMERZHEIM and H. NICKEL,2. Anal. Chem. 246, 267 (1969). [31] W. C. MYERS end W. M. HENRY, Conference on the ~ltragnw@ation of Semiconductor Materials, Boston, Mass. 1961, p. 349. Macmillan Company, N.Y. (1962). 321 P. W. J. M. BLUMANSand F. J. M. J. MAESEEN,Spectrochim. Acta 24B, 585 (1969).
Studies of the cathode layer of
8
d.c.
361
are
fzf!y~,~~~, 0.1
0.2
0.3
0’4
0 I
0’5
0.2
0.3
04
0.5
~~~~~2, rf&; I’, 0.1
0.1
0.2
0.3
0.4
0.5
0.1
0.2
@3
0.4
0.5
0.1
I
I
I
I
1
0.2
0.3
0.4
0.5
0.1
02
0.3
0.4
0.5
0.1
02
0.3
0.4
0.5
Distance,
mm
Distance.
mm
0.2
0.3
0.4
0.5
Dlstance. mm
Fig. 8. The effect of different, atmospheres on the line/background -e air, - - - -o-Ar/O,, -.-.cathode 18yer.
ratios in the -n-He.
(ii) In the helium atmosphere the cathode leyer effect appears to be enhanced with the elements with low ionisation potentials in as much as the intensity or detection limit gradients are much steeper. Unfortunately, since the electrodes do not burn away during the arcing period, the amount of sample entering the column is relatively small, and the detection limits are inferior to those obtained using the other two atmospheres. Another interesting aspect is that a number of carbon ion lines (2572 A, 2836 A, 3509 A) was easily visible in the spectra obtained using helium as the arcing atmosphere. These lines, with excitation potentials of 16 eV end more, are generally only found in spark spectra. In one of the preliminary experiments aimed at determining the correct gas flow rate, the nitrogen O-O and O-l bands were also easily visible. These phenomena are not easily explained since the excitation mechanism in a He arc is not yet fully understood. 7. COMPARISON BETWEEN THE CATHODE LAYER TECHNIQUE AND ANODE TECHNIQUES In analytical techniques using the cathode layer it is generally recommended that the sample be mixed with carbon powder in a ratio of 1 :l. In spite of this dilution the prepared sample may still contain relatively high concentrations of elements with 3
352
R. J. DECKEB
low ionisation potentials which could seriously disturb the cathode layer enrichment thus reducing or even removing any benefit obtained in using the technique. 7.1 Distribution of line htenkty in the whole arc column To compare the effect of an excess of an alkali metal on the intensity distribution in the whole column, the arc was focuseed on the slit of the spectrograph (18 mm in length) with a e-fold magnification, and the standard mix burnt undiluted and also 2r
2
4
6
Distance from cathode,mm
Distance from cathodqmm
4 2 6 Distance from cathode.mm
Fig. 9. A oompariaon of line/background ratios obtained in the whole arc when buffered and unbuffered samples are eveporeted from the anode end oathode reepeotively. 0 % Ne,CO, ----2O%Na,CO, -**-‘-
0 % Ne,CO,
--.-.
20 % Na,C!O,
1
Cathode Evaporation,
Anode Eraporaticn.
with the addition of 20 per cent Na,CO, using the cathode (electrode C) and anode (electrode D) 8~ sample carriers respectively. The dietributions obtained are represented by the curve8 given in Fig. 9 which show: (i) Line intensities measured in the centre of the column are improved when the NqCO, ia present in the sample. This is probably due to the lower temperature of the arc favouring atomic emission since ionisation of the elementa is reduced. (ii) In all c&888the detection limits are very much better when using the cathode layer without the alkali metal. Interesting is the fact that when the cathode layer enrichment is marked, the element is not always generally detectable in the are column if the sample is evaporated from the cathode. (iii) A cathode layer enrichment is aleo present when the undiluted sample is evaporated from the anode. The intensity curvea obtained with the diluted sample
studies of the cathode 18y0r of 8 d.c. 8rc
363
also indicated the presence of slight enrichment in the cathode layer but the line/background ratios generally show little improvement. (iv) When an excess of alkali metal is present there is little difference in detection limits when using either the cathode layer or normal d.c. arc techniques. Samples containing high concentraClearly this latter point is of significance. tions of elements with low ionisation potentials must not be allowed to evaporate into the plasma at such a rate that the concentration of these elements is sufficient to destroy the enrichment effect. (MANNKOPPF and PETERS recommended that the sample should not distil into the plasma at a rate greater than 5 mg per min.) Two methods are immediately apparent; one can merely dilute the sample even further, or, one may use an electrode with a narrower cavity and thicker walls to reduce the rate of distillation of the sample into the plasma. The latter method was tested by comparing the results obtained in section 3.1 using electrode C, with results obtained using electrode B (see Table 2) under similar conditions. No significant difference, however, was found in the distributions or detection limits probably because the arc burns preferentially on those regions where the concentration of easily ionisable elements is greatest, i.e. usually on the sample and not on the electrode walls. (This effect has been used by MELLICH~MP to restrict wandering of the cathode spot in conventional d.c. arc methods [33].) 7.2 Comparison
of detection
7.2.1 Expwimentd
limits
cditionn
(i) Optica. The investigations 80 far described were carried out with the BTCfocueaed on the slit of the spectrogrsph. In norm81 an8lytic8l prsotice it i common to focus the 8ra on the collimator of the spectrograph in such 8 W8y thet the imege of the 8M: is as defocueeed aa possible at the alit, to reduce m eaaurement errors which m8y be C8used by dietribution effects in the plasma. Therefore, to comp8re the oathode layer technique with anode techniquea under conditions used in axmlysis, the 8rc was fooueeed onto 8 diaphragm using two, a& cylindrioel mirrors such thet the Bxial magnifkstion w8e IO-fold and the r8disl megniflcation, 2-fold. The diaphragm had 8 4 mm square aperture, thus light from the 6ret 04 mm from the cathode ~88 sllowed to enter the spectrograph. For the anode technique the cent4 portion of the 8ru column w8a focuseed on the diaphnxgm with no megnific8tion. Radiation pas&g through the diaphragm ~8s then focuesed onto the collimetor mirror. (ii) Sam@. The samples used were three earnplea containing N8,COs u88d in section 3.1, plus 8 sample contrrining SO per cant Ne&O,. (iii) Ekxtro&x~ Cathode layer technique, electrode C; Anode technique, electrode D. (iv) Excitation conditiona. current 8 A; g8p 8 mm; exposure time, burnt to completion.
7.2.2 Reeults Plots of line/background obtained using both techniques at the different Na,CO, concentrations are shown in Pig. 10. They show that as the concentration of the alkali metal increases the difference in the detection limits decreases, and that at concentrations of between 10 per cent and 60 per cent of the carbonate (6 per cent25 per cent of sodium) the conventional anode technique is, in most cases, slightly more favourable. [33] J. W. %kLLICHAMP, Appt. Spectrosc. 21, 23 (1967).
R. J. DECKEB
0
0.01 01
I
so
IO
NasCOs,
0
0.01
0 I
Na,CO,
%
,
IO
50
, %
2-
I
L
1/q-I
“t
.,2752,
,
00010~1
I
,
0
IO so
Na,Cs.
_ _ ‘Ge3039
0
%
O-01
I
04
Na,CO,.
IO
50
%
Fig. 10. A comparison of line/background ratios obtained with containing increasing concentrations of Na using the cathode layer __f_ cathode 18yer technique, -0--and anode technique. technique.
7.3
8 sample technique anode
The analysis of naturally occurring samples
The majority of naturally occurring samples generally contain signilkant quantities of elements with low ionisation potentials-commonly sodium and potassium and often aluminium. It was therefore decided to compare the two techniques using three geological samples, containing different, but significant, amounts of elements with low ionisation potentials. 7.3.1 Experimental
con&time
(i) Opt&x. The optic81 systems used were as described in section 7.2.1. (ii) Samples. The three samples used and the concentrations of the major elements in the samples are given in Table 4. (iii) Arc&g conditiona. For the cathode layer technique the samples were diluted with graphite powder to form two sets of samples containing 50 per cent and 75 per cent graphite respectively. The former set of samples were p8cked into electrodes A, B and C, and the latter Table
4. Composition
of the three ssmples used in Section Composition
Sample
SiO
C80
Limestone B8&t Granite
14.1 41.7 69.2
41.3 11.5 0.1
Fes0s 7.6 11.4 0.08
N8,0 0.4 2.9 2,8
7.3
( %) KsO 0.7 0.9 10.1
Al,0, 4.2 13.3 10.8
2.2 12.7 -
Studiesof the cathode layer of a d.e. arc
366
set into electrodesB and C. The samples were burnt in a 9-A are with an arc gap of 9 mm. The exposure time after a pre-burn of 16 set was 1 min. For the anode technique, the samples diluted to contain 50 per cent graphite powder were pseked into electrode D and burnt to completion using an arc current of 8 A and an arc gap of 6 mm. Under these conditions the btbckgroundintensities were, for a given geological sample, appro~ma~ly the same for the four arcing conditions.
7.3.2
Results
The mean of 3 line/background ratios obtained for eight trace elements found in the three samples under each set of conditions is given in Table 5. They show that very little, if anything, is to be gained by using the cathode layer technique with the basalt and limestone samples. The granite results show, however, that the use of the technique give more favourable detection limits for most elements particularly when using electrode A as recommended by M~TCRELL [111.The reason for this difference almost certainly lies in the burning qualities of the sample. The granite sample burnt well, the arc column rerna~~g steadily over the sample cavity with very little wander of the cathode spot. The other two samples, however, burnt with marked diffusion of the sample to and through the electrode walls causing severe wandering of the cathode spot about the crater edge and thus a substantial amount of the sample did not enter the cathode layer, 8. SAMPLE SIZE The three electrodes used in Section 7.3 in conjunction with the cathode layer technique burn at approximately the same rate under the given conditions and it is thus possible to obtain from Table 5 a measure of the amount of sample required to obtain a given line/background ratio. Under the conditions given in section 7.3, the amounts of sample leaving electrodes A, B, C and D are approximately in the ratio 1:4:9 :33 respectively. A study of the figures in Table 5 reveals that the amounts of sample required to obtain a given line/background ratio using the cathode layer and anode layer techniques is 1:3 respectively in the worst cast (zirconium in basalt) and 1:330in the best case (titanium in granite). It has been suggested by BOUMAN~ [34] that this very large increase in the Ti and Ni intensity in the granite samples may be in part due to blank enhancement. The Ti results in the other two samples are also high when using electrode A indicating that perhaps the electrode material was contaminated with Ti. Both electrode A and electrode D, however, were cut from the same box of electrode rods and, due to the larger dimensions of the electrodes more electrode material enters the arc when using the anode technique (electrode D burnt to completion) than in the cathode layer technique (electrode A burnt for 1 min). If one ignores the Ti results the best sample mass ratio drops to 1x50{zinc in limestone) which, perhaps, is more realistic. It is clear, therefore, that one obtains a marked decrease in the absolute detection limits of the method when the cathode layer is used in preference to the main body of the arc column. [34]P. W. J. M. BOWMANS,private communication.
R. J. DECKEB
366
Table 6. The mean value of three line/background ratios obtained using different techniques with the samples given in Table 4. The valuee in italics are the values lying within 10 per cent of the maximum values obtained for a particular element in a given sample and thus indicates the techniques which give the best detection limits for the element and sample conoemed Sample
BASALT
Graphite cont. Mnthod Electrode
A
60% Cathode Layer l3
C
13 0.8 10
11 @4 3.6
6.6 3.3 6.3 1.4
6.2 2.3 1.3 0.5
Element
Line A
Mn Pb Cd Ni
2784 2833 2981 3134
16 _+ 0.6 ;:;
Ti CU Zn ZI
3200 3274 3282 3438
;:; 0.9
50% Anode Tech. D
7
8
27
1;
0;
0.6
7.4
6.7
45
4.4 4.1 4.3 2.0
5.5 3.3 4.5 1.1
;:;
-
5.5
4.8
LIMESTONE
Sample Graphite conc. Method Electrode
A
Element
Lino A
Mn Pb cd Ni Ti CU Zn Zr
2794 2833 2981 3134 3200 3274 3282 3438
60% Cathode Layer B
16 -
21 0.5
;:; ;“6
2.0
C
26
76% Cathode Layer n C
60% Anode Teoh. D
16 0.5
22 0.7
15
0.9
0.1 4.9 1.4
0.1 63 1.1
0.1 0.9 4.5
0.1 0.8 6.1
77
1.0 0.6
0.4 1.4 0.4
0.5 1.4 0.9
0.7 1.1
;:4” 1.7 1.9
0.9
GRANITE
Sample Graphite cow. Method Electrode
A
Element
Line A
Mn Pb Cd Ni Ti CU Zn Zr
2794 2833 2981 3134 3200 3274 3282 3438
* -
76% Cathode Layer B C
7.4 1.0
;:;
60% Cathode Layer l3
3.8 2.6 I-2 1.6
C
12 3.9 1.1 1.9
76% Cathode Layer I3 C
3.7 2.8
0.8 1.2
50% Anode Tech. D
1.0 1.8
5.5 1.5 1.0 -
6.2
2.3
2.3
1.1
2.3
0.6
;I; 9.7
0.7 _ -
“” -
02 -
23 _
1.2 -
-
denotes element not detected.
9. Some of the more important
SUMMARY
points to emerge from this study are:
(i) Detection limits are not significantly affected by small changes in arc current and arc gap provided these factors are kept at values greater than 6 A and 6 mm respectively. The use of lower currents causes the cathode layer enrichment effect to weaken and the position of the maximum of the line/background ratio can move
Studies of the cathode layer of a d.c. arc
367
away from the cathode. Arc gaps smaller than 6 mm result in a marked deterioration of the detection limits. (ii) The application of a stationary inhomogeneous magnetic field causes the detection limits to deteriorate. (iii) The use of artificial atmospheres when using the cathode layer has no beneficial effect on detection limits, although the removal of the cyanogen band system is an advantage when determining elements which have their more sensitive lines in the wavelength region occupied by these bands. (iv) Increases in the concentration of elements with low ionisation potentials can markedly affect the distribution of atoms in the cathode layer, increasing the detection limits of the method. Detection limits, however, can be slightly better than those obtained with anode techniques, even with samples containing about 5 per cent alkali metal. Elements with high ionisation potentials appear to also affect the distribution of atoms in the cathode layer, but to a lesser extent. (v) The sample must evaporate directly into the cathode layer if any benefit is to be obtained by using the technique. Any movement of the cathode spot away from the sample can seriously affect the detection limits and this often happens with naturally occurring samples. (vi) At high concentrations of elements with low ionisation potentials the position for obtaining the optimum detection limit is at some distance away from the cathode, the distance being determined by the ionisation potential of the element and the degree to which the cathode layer enrichment has been suppressed. (vii) The amount of sample required to obtain a given line to background ratio is less when using the cathode layer technique than when using an anode technique. 10. CONCLUSIONAND DISCUSSION It is clear that the use of the cathode layer technique has some advantages in trace analysis. The detection limits obtained for samples with small concentrations of elements with low ionsiation potentials can be better than those obtained using anode techniques, even though in the latter case the sample is well buffered. In this investigation, using the synthetic samples which almost certainly give optimum results, the maximum improvement in detection limits was lo-fold, but was generally of the order of six times when the first O-4 mm of the arc was focussed on the collimator of the spectrograph (see Fig. 10). No doubt, by using a thinner layer closer to the electrode, detection limits could be improved even further. In view of this, one can forsee the technique’s successful application in many instances such as in the analysis of refractory material using carrier distillation methods (supplementary heating of the cathode may be necessary in some cases), in the analysis of semiconductor material, in the analysis of air-borne dust, etc. Its use in the analysis of carbon products is already well known. With samples containing high concentrations of elements with low ionisation potentials such as most naturally occurring material, detection limits can be two or three times better than when using anode techniques. With such samples, however, optimum detection limits may be obtained some distance away from the cathode.
368
R. J.
DECKER
Unfortunately, it appears that any movement of the cathode spot which enables the sample to distil from the electrode without entering the cathode layer can adversely affect the detection limits. It is, therefore, essential to use the technique under conditions which reduce wandering of the cathode spot to an absolute minimum. Two other disadvantages are apparent. Firstly, apart from the normal matrix effects which occur in d.c. arc. analysis, there is the additional effect of changes in the concentration of the major elements, particularly those with low ionisation potentials, affecting the atomic distribution in the cathode layer and hence the recorded line intensity. In choosing an internal standard, therefore, more consideration must be given to the ionisation potentials of the elements than is usual in the anode techniques. Secondly, the position of the cathode must be accurately maintained throughout the exposure, since a displacement of only O-1 mm can have a marked effect on recorded line intensity and detection limits. Because of this the technique is perhaps not particularly suitable for the routine analysis of a large number of samples unless precision requirements are fairly low. Perhaps the most promising field of application is in the analysis of very small samples (of the order of a mg). See, for example PREUSS [35] who used the technique successfully to analyse mineral grains in meteorites. In this respect the use of helium seems to warrant further study, particularly as it appears that some non-metals may be detected under these conditions. It should, however, be noted that although smaller samples may be used in this technique, the relative detection limits may not be significantly better than when using conventional anode techniques. It is apparent, therefore, that in some instances, particularly when analysing very small samples, the use of the cathode layer has much to commend it. It is recognised, however, that the successful application of the technique requires a certain degree of experimental skill. Finally, it is suggested that the cathode layer be defined as that region in the arc between the boundary at which the sample vapour leaves the influence of the cathode fall and enters the arc column, denoted by the sudden decrease in the intensity of spectral lines emitted by elements with low ionisation potentials (the ‘cut-off’ referred to in this paper) and the cathode, rather than vaguely referring to the cathode layer as the region close to the cathode. Acknowledgements-The author wishes to express his deep gratitude to the following: Prof. H. KAISER, Director of the Institut ftir Spektrochemie und Angewandte Spektroskopie, Dortmund, and to Dr. K. LAQUA and his colleagues in the ‘Emissionabteilung’ of the Institut for their assistance, both academic and personal, during his stay in Germany; to the Alexander von Humboldt Foundation for the award of a very generous scholarship, and to Dr. P. W. J. M. BOUNANS and Dr. D. J. EVE whose comments and suggestions have been most valuable. [35] E. PREUSS, Mikrochim.
Acta,
382 (1956).