Anal. Chem. 1985, 57, 131-136
131
Modification in the Surface Composition of Sparked Electrodes and Its Relation to Relative Sensitivity Factors in Spark Source Mass Spectrometry Jozef A. A. Verlinden, Karin M. E. Swenters, and Renaat H. H. Gijbels* Department of Chemistry, University of Antwerp (U.I.A.),B-2610 Antwerp- Wilrijk, Belgium
The In-depth dlstrlbutlons of a number of elements In the sparked surfaces of some metals (Iron, alumlnum, and copper) were measured by uslng secondary Ion mass spectrometry. The surface composltlon of the sparked electrodes was found to dlffer from that of the bulk. .The ratlo of the secondary Ion lntensltles In the sparked surface and In the bulk of the electrodes, both normallred to the correspondlng slgnal of Fe’, for Instance, agreed remarkably well with relative sensltlvlty factors (vs. Fe as the Internal standard) which were experlmentally determlned for spark source mas8 spectrometry uslng homogeneous standard reference materials. Thls observatlon would suggest that differences In relatlve sensltlvlty factors are, at least to some extent, caused by varlous processes whlch occur In the electrode surface whlle sparklng.
In mass spectrometric analysis, just as in any other instrumental method of analysis, the concentration of impurities is found from indirect measurements. A mass spectrometer is a comparator, it can be used to measure the ratio of the ion currents I x / I y corresponding to elements of interest X and
Y. Because of the various processes which occur in a vacuum discharge and because of possible discrimination effects in the mass spectrometer, the ratio of the number of detected ions corresponding to X and Y usually differs from the true concentration ratio of these elements in the sample, Cx/Cy. Consequently, in order to calculate the concentration of impurities from mass spectrometric results, correction factors are introduced which relate these two ratios I x / l y = (Cx/Cy).RSF(X/Y)z (1) The proportionality factor RSF(X/Y)z is called the relative sensitivity factor when X is determined relative to Y (internal standard) in a matrix Z. Its value characterizes the dependence of the sensitivity of the spark source mass spectrometric method on the type of elements and on the sample matrix. In practice the RSF is experimentally determined by analyzing standard samples. The values of the RSF in spark source mass spectrometry (SSMS) are usually within the range 0.1-10. This means that this technique exhibits a reasonably uniform sensitivity for various elements. It is very tempting to try calculating the RSF theoretically; this would, indeed, allow use of SSMS for analyses without the need to resort to standard reference materials. Relations between experimental RSF’s and physical and chemical properties such as melting point ( I ) , boiling point (2-4), heat of sublimation (5-9), ionization potential (4-7, l o ) ,covalent radius ( 6 , 9 ) ,ionization cross-section ( 7 , 8 ) ,and temperature for constant vapor pressure (11)have been studied and empirical relationships for calculating RSF’s have been proposed. However, most of these relationships rely upon a single matrix only; in fact, there are very few reports dealing with the influence of matrices (12).
On the other hand, many reports deal with the influence of experimental conditions on the value of an RSF. The factors were shown to be influenced by the shape of the sample electrodes (13),spark-gap width (14,15),spark voltage, repetition frequency, and accelerating voltage (16),temperature of the sample electrodes (I7),etc. The above considerations lead to the conclusion that it is inadequate to express the RSF only as a ratio of some physical property of a measured element to that of an internal standard element and that the equation for calculating the RSF should contain a factor determined by the measurement conditions ( I 7 ) . It would be very interesting to know the effect of the experimental factors on different stages of the spark process and the recording of ions. The accelerating voltage affects the RSF because of the nonuniform energy distributions of different ions. The degree to which the RSF is influenced by this parameter is very dependent on the energy-defining slit width (18). The effect of other parameters such as spark voltage, which is interrelated with the gap width (19),the repetition frequency, and pulse width is complex and affects, among others, the ion abundances (20), the temperature of the electrodes (17,21),the material consumed per pulse (19),and thus the crater diameter and depth (22). Another series of papers has been dealing with the surface investigation of sparked alloy electrodes by various techniques such as electron microprobe and Auger electron spectroscopy (AES) combined with sputter removal (23-28). It was shown that the composition of the sample surface may change with repeated sparking. Atmospheric pressure spark emission spectroscopy work points to the interpretation that “matrix effects” which have plagued alloy analysis are in fact reflections of microscopic chemistry or alterations of the sample surface by the analytical instrument (29, 30). In addition it was demonstrated by optical emission spectroscopy that the vapor composition may differ from that of the solid sample (23,25, 31,32). It is therefore probable that relative sensitivity factors in spark source mass spectrometry are to some extent determined by the surface composition while sparking, since the atomized material originates from the surface. Also this composition may be influenced by spark parameters. Observations on sparked metal electrode surfaces by scanning electron microscopy revealed molten and resolidified structures (19,20,33)which means that during sparking thin liquid layers are formed. From these observations and with the results from material consumption studies (19) in SSMS we assume that for metals, under the given experimental conditions, material is transported into the spark gap as a liquid particle, according to a mechanism such as described by Davies and Biondi (34). It should be mentioned however that under milder sparking conditions electrode surfaces could be observed which rather resemble sputtered surfaces (35). Differences in the composition of sampled material as compared to the bulk were reported to be due to selective vaporization (32),segregation (26,28),oxide layer formation (23,24,28),etc. Also, condensation effects may play a role. In this respect Franzen and Schuy (13)reported the possibility
0003-2700/85/0357-0131$01.50/0 0 1984 American Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
Table I. Composition of the Samples (wt W ) element Be Mg A1 Si Ti V Cr Mn Fe
co Ni cu Zn
A1 8306
CuBe 1121
Cu 1101
1.89
0.0005
0.07 0.11
0.0006 0.005
steel 662
8.35 0.10 0.18 1.25 0.14 1.26 0.78
0.002 0.004 0.085 0.30 0.012 97.49 0.01
0.005 0.037 0.013 69.6 30.3 0.009
Zr 0.27
0.34
0.01
0.002
0.016 0.012
matrix
AI
0.58
As Nb Mo Sn Sb Ta W Pb
Table 11. Spark Parameters
0.084 0.041 0.30 1.04 95.3 0.30 0.59 0.50 0.092 0.19 0.29 0.068 0.016 0.012 0.20 0.21
0.05
that during each discharge evaporated material of one electrode recondenses on the other electrode surface in proportion to the surface areas presented by the electrodes and fractional condensation and enrichment effects could occur during recrystallization. In this work we studied the effect of sparking on the surface composition of metals by secondary ion mass spectrometry (SIMS),which is a much more sensitive technique than the methods referred to above. A comparison is made between relative surface enrichment or depletion factors and RSF’s, used in SSMS, corrected for discrimination effects occurring after ion production. The matrices used were homogeneous reference materials of aluminum, copper, and steel.
EXPERIMENTAL SECTION The samples investigated were NBS low alloy steel 662, NBS copper SRM 1101, NBS beryllium-copper alloy 1121and Pkhiney aluminum standard 8306. The composition of the samples is given in Table I. The microlevel homogeneity of the standard reference materials was studied by Van Craen et al. (36) and the suitability of the samples for microprobe analysis demonstrated. The spark source mass spectrometer used was a JEOL JMS01-BM-2 double focusing instrument with electrical detection equipment. The electrical detection system of the apparatus was used in the magnetic peak-switching mode with a Hall probe magnetic field monitor, thus allowing coverage of the entire mass range at a constant accelerating voltage. The spark-gap width was kept constant with an automatic spark-gap controller. The vacuum in the analyzer was maintained at about torr and the source pressure just before sparking was usually about torr. The experimental RSF’s used for comparison in this work were determined by Van Hoye (37-39) on the same standard samples and with the same instrument. Our experimental SSMS conditions were kept as close as possible to those used by Van Hoye and are given in Table 11. All metallic samples were cut in the form of bars (1.5 X 1.5 mm2 section) and the top was subsequently wet-polished with silicon carbide abrasive paper and fine-polished with 25,15, 10, 3, and 1pm metallurgical diamond paste. The polished surfaces of a pair of identical electrodes were then sparked for at least 15 min in a plane-to-plane geometry. In-depth analysis was performed with a CAMECA IMS-300 instrument with electrostatic sector attachment. The sparked samples were mounted in the SIMS sample holder and actual primary ion beam. analyses were carried out with a 6.0 keV 1602+ The primary ion current was approximately 0.5 pA while the bombarded area was measuring ca. 600 pm on side (scanning). Oxygen gas was flooded over the sample surface through a bleed-in
spark-gap voltage, kV repetition frequency, kHz pulse width, ~ 1 s
30 1 20
60 3 20
cu
Fe
45 1 20
60 3 20
system in the immersion lens until a partial pressure of about 2X torr was maintained. A 400-pm diaphragm localized in the immersion lens restricted the area analyzed. When necessary, mass spectral interferences from molecular ions were eliminated by energy selection up to 120 eV by lowering the sample voltage. A detailed description of the data acquisition and reduction system used is given by Van Espen et al. (40).AU secondary ion intensities measured in the sparked surfaces were normalized to the signal of Fe+, measured in the same spectrum. Where possible, an environment sensitive matrix ion species ratio M2+/M+(41)was measured in order to correct intensity data where necessary.
RESULTS AND DISCUSSION Aluminum Electrodes. In-depth analysis in the unsparked polished electrode surfaces by secondary ion mass spectrometry did not reveal any depletion or enrichment of the elements of interest at the surface with respect to the bulk. After the same sample was sparked against an identical electrode in a plane-to-plane configuration under vacuum, the in-depth distributions of the elements were measured again by SIMS. In Figure 1the obtained profiles of some elements in the aluminum standard reference material are shown. It is clear that the surface concentration now differs from that of the bulk. The estimated erosion rate in our SIMS measurements is 0.5-1 nm/s. We preferred to plot relative secondary ion intensities (Ixt/IF,+)as the ratio allows a direct comparison with the RSF’s used in SSMS with iron as the internal standard element (see further). From our measurements it follows that most elements are surface enriched with respect to iron, whereas Ni and Cu are sometimes slightly depleted. In order to quantitatively describe the presence of impurities in the surface layer, a relative surface enrichment factor Fx is introduced for each element X, as the ratio of the secondary ion intensity measured for this element in the surface and the mean secondary ion intensity measured for the same element in the bulk, both relative to the respective intensities of Fe+. Thus
Fx =
(IXt / IFe’) surface
(IX+/IFe’)
(2)
bulk
We assume that the concentration of the elements in the surface, which was temporarily molten during sparking, corresponds to that of the sampled liquid leaving the surface. If this assumption is correct, a SIMS analysis of the electrode surface yields information on the composition of the material sampled during the SSMS analysis (under the chosen experimental conditions). In Table I11 the experimental surface enrichment factors for the aluminum samples sparked at 30 and 60 kV spark voltage, respectively, are compared with the corresponding RSF’s for SSMS (Fe = 1) as obtained by Van Hoye (39) with the same instrument and the same experimental parameters. A remarkable agreement is found between our SIMS values of Fx and the RSF’s. The RSF’s were measured by electrical detection in the peak switching mode and were corrected for discrimination effects which may occur after ion production. In order to arrive a t a correct comparison with the SIMS results, however, it is necessary to take into account possible discrimination effects occurring in the ionization and recombination processes in the spark; in particular it should be mentioned that SSMS analyses are usually based on the 1+
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
133
\
\
Sn
0
200
100
400
300
SPUTTER TIME
(5)
Figure 1. In-depth distribution of some elements at the sparked surface of an aluminum electrode.
Table 111. Values of F x , R x , and RSF for Aluminum Matrix Sparked at 60 kV, 3 kHz and 30 kV, 1 kHz 30 k V and 1k H z
60 k V and 3 k H z
element
FX
Mg AI Si Ti Cr Mn Fe Ni
5.9 f 1.7 1.4 f 0.2 1.1 f 0.4 1.5 f 0.3 1.8 f 0.6 1.7 f 0.4 1 1.1f 0.2 1.1f 0.4 2.1 f 0.8 2.7 f 1.0 4.0 f 1.6
cu Zn
Sn Pb
RX
2.9 1.3 1.2 1.4 1.7 1.3 1 1.1 1.1 2.0 2.8 4.4
RSF 7.0 1.3 0.9 1.5 1.7 1.9 1 0.7 0.7 3.4 2.9 4.1
no. measd
Fx
RX
RSF
no. measd
f 1.0 f 0.1 f 0.1 f 0.2 f 0.2 f 0.2
5 5 5 5 7 5 5 7 3 4 3
1.5 1.1 1.0 1.2 1.6 1.0 1 1.0 1.0 2.1 3.1 2.8
3.0 f 0.4 1.1f 0.1 0.8 f 0.1 1.6 f 0.2 1.6 f 0.1 1.5 f 0.1 1 0.7 f 0.1 0.7 f 0.1 1.8 f 0.2 2.2 f 0.3 2.8 f 0.4
4 3 3 4 4 3
i 0.1 f 0.1 f 0.5 f 0.4 f 0.8
3.0 f 1.1 1.1 f 0.2 1.0 f 0.2 1.3 & 0.1 1.2 f 0.4 1.3 f 0.2 1 1.0 f 0.1 1.0 f 0.4 2.2 1.0 3.0 & 1.0 2.5 & 0.7
ions, whereas in the spark a variety of multiply charged ions are produced. According to Chupakhin et al. (42) all elements in a spark discharge have an equiprobable ionization. Following calculations by Ramendik et al. (43),the relative content of singly charged ions of different elements after recombination is constant. This implies in our case, that no correction on the given RSF’s is necessary and that a direct comparison with Fx is justified. Owens and Giardino (44), however, noted that the ratio of singly to multiply charged ions may differ from element to element resulting in a bias in the calculated concentrations if the rather abundant doubly charged ions are ignored. The total number of ions of each element, however, is assumed to be proportional to ita concentration. The result is that relative sensitivities are affected to some extent by differences in the relative abundance of the analytically important 1+ ions. In order to correct for this possible effect of discrimination, the abundances of the ions (1+/2+/3+/4+) were measured for a number of elements (pure matrices and alloys) with electrical detection in the peak-switching mode, under the following experimental conditions: spark voltage, 60 kV; repetition frequency, 3 kHz; pulse width, 20 ~ s a; and /3 slit
*
5 4 3 3 3
widths, 2 mm; main slit width, 100 Km; collector slit width, 1200 wm. The results are given in Table IV, together with the normalized abundances Ax+/AFe+. Corrections for this possible source of discrimination in SSMS (when basing on the 1+ ions) were made by multiplying our values of Fx obtained with SIMS by the normalized abundance to give a value Rx
R x = Fx
AX+ -
AFe+
(3)
The Rx values are also given in Table 111. The relative standard deviation on Rx is assumed to be about equal to that of Fx, since the uncertainty on Ax+/AFe+is probably less than 10-20%. The agreement between Rx and RSF is satisfactory but not better than that between the Fx values and the RSF. It is worth mentioning here that discrimination effects in our SIMS measurements (Le., ion-optical, energy, and mass discrimination) are ruled out by measuring relative ratios Fx (eq 2). The agreement between the RSF and the composition of the sparked surface (relative to the bulk) would suggest that differences in SSMS relative sensitivities are, at least, to some extent, caused by a different composition of sampled surface layer. Studies by a number of authors (45,46) indicate that
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
51
V
52
Cr
90
Zr
60
Ni
l Z OSll 75
AS
123
Sb
0
1
I
I
I
0
200
400
600
I
I
1000
800
SPUTTER
L
TIME
(S)
Figure 2. In-depth distribution of some elements at the sparked surface of a steel electrode.
Table IV. Abundance6 of Multiply Charged Ions as Measured by SSMS in the Peak-Switching Mode (60 kV,3 kHz, 20 /AS) element
1+
2+
3+
Be
64.1 41.3 87.6 87.1 78.2 78.7 80.9 65.5 83.8 86.4 86.9 82.7 80.0 76.7 82.8 53.6 86.2 87.8 47.1 46.5 82.4 92.1
35.9 52.8 9.9 9.5 17.8 18.7 14.6 30.0 13.7
20.1 5.4 2.5 2.5 3.6 2.4 4.0 4.1
11.2 11.1
2.0 1.7 2.5 0.8 5.0 3.4 8.5 1.8 1.9 10.6 9.5 3.2 0.9
Mg
A1 Si Ti
v
Cr
Mn Fe
co Ni cu
Zn Zr Nb Mo Sn Sb Ta W AU Pb Nan C aa Ka As" a
14.4 19.1 16.9 12.9 35.8 11.2
9.7 39.8 40.8 13.0 6.6
61.3
30.0
74.8 87.7 64.9
23.0 9.8 19.3
2.1
6.8 2.1
5.0 14.8
4+ 0.4 0.02 0.9 0.4 0.3
A(l+)/A(Fet) 0.76 0.49 1.05 1.04 0.93 0.94 0.97 0.78
0.5 0.9 0.4 0.5 0.4 0.3 0.1 1.4 0.9 2.2 0.9 0.6 2.4 3.2 1.4 0.3
1.03 1.04 0.99 0.95 0.92 0.99 0.64 1.03 1.05 0.56 0.55 0.98 1.10
2.1
0.73
0.6 0.3 0.9
0.89 1.05 0.77
1
Photoplate detection.
bulk diffusion of major constituents through the solid electrode
to the sampled surface layer occurs only to a negligible extent during spark discharges. Secondary electron images of sparked aluminum electrodes showed molten and resolidified surfaces. A typical diffusion coefficient in the liquid phase is cmz/s which means that for times of s the diffusion length of an element in the liquid surface layer may be of the order of 50 nm. No numerical data are however available to reliably
estimate the thickness of this molten layer or the time it stays molten. The process of selective extraction from the solid bulk and transport of impurities may be of the same kind as in zone refining as the molten zone moves to deeper layers in the electrode. According to Ramendik et ai. (42) the high RSF of volatile elements may be attributed to a nonequiprobable atomization of material due to evaporation of atoms from the heated region surrounding the crater walls. In our study the elements Mg and Zn have a much larger vapor pressure than the other elements studied in aluminum. When comparing Fx,Rx, and RSF, however, relatively small deviations are found for Mg and Zn, even a t 60-kV spark voltage. From the results in Table I11 it follows that changing the spark parameters may result in the formation of surface layers of slightly different composition. The consequence is that also the RSF measured in SSMS will alter. Steel Electrodes. Figure 2 shows the SIMS in-depth distributions of some elements in a sparked steel surface. Again it can be seen that most elements are surface enriched with respect to iron, whereas Ni is depleted. No significant difference was observed between the composition of steel electrode surfaces sparked for 15 and 30 min, respectively, indicating that after 15 min of sparking a steady state is reached. No surface enrichment or depletion was observed in the unsparked metal. The Fe2+/Fe+ratio was used as the environment sensitive ratio (40). The values of Fx, Rx, and RSF are compared in Table V. A somewhat better agreement is found between Rx and RSF than between Fx and RSF; the correction factor Ax+/AF,+ significantly improves the agreement for the elements Mo, T a and W. It is remarkable that the agreement Rx-RSF or Fx-RSF is even satisfactory for the volatile elements, such as As. It is of interest to mention a study made by Van Oostrom et al. (28) who investigated the surface composition of stainless steel electrodes, after sparking under vacuum, by combined scanning Auger microscopy and in-depth profiling by sputter removal of electrode material. Differences in surface and bulk composition were explained by oxide layer formation, surface
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
135
6 10
lo5 ?
4
U
Y
>
-
+ v
,
5c
4
10
E 3
10
2
10
.-
I 0
200
400
600
800
1000
1200
SPUTTER TIME
1400
(5)
Figure 3. Indepth distribution of some elements at the sparked surface of sparked steel electrodes. 8
1\
6
a J 4
U
-d \
-X 2
0 0
200
400
1
,
I
600
800
1000
SPUTTER TIME
c
(s)
Flgure 4. In-depth distribution of some elements at the surface of sparked Cu and CuBe electrodes.
segregation, and evaporation. The degree of surface segregation (increased surface concentration) was calculated. The predicted magnitude of this effect in steel decreased in the order K > Na > Ca > Mg >> Cr > Ni > Fe. Especially for K, Na, Ca, and Mg segregation phenomena are to be expected. For K the surface segregation was explicitly demonstrated by Auger imaging (28). In the present work, some attention was paid to all the above mentioned elements. Figure 3 shows in-depth profiles measured in steel, in absolute secondary ion currents. The results qualitatively agree with the predicted magnitude of surface segregation, namely, a strong enrichment of K, Na, Mg, and Ca, in that order. From these results the
following Fx values can be calculated Na, 73; Mg, 59; K, 160; Ca, 54. If Fx is a measure for the RSF, unusually high values would be found. Unfortunately, no certified values are available for the coqcentration of these elements in steel standard reference materials, so that no comparison between Fx and RSF values can be made. Copper and Copper-Beryllium Electrodes. For copper and copper-beryllium electrodes the same procedure was followed as for aluminum and steel. Typical in-depth profiles are shown in Figure 4. A number of elements now appear to be depleted with respect to iron. This is in agreement with what can be expected from the RSF's. In Table VI the ex-
136
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1,lJANUARY 1985
T a b l e V. V a l u e s of 60 kV, 3 k H z , 20 ps
F x , R x , and RSF for S t e e l S p a r k e d a t
element
FX
RX
Ti
1.5 f 0.2 1.5 f 0.15 1.5 f 0.17
1.4 1.5 1.5
1 1.1 f 0.2
1 1.1
0.95 f 0.3 1.7 f 0.2 2.9 2.1 f 0.4 2.4 f 0.1 2.3 f 0.4 5.5 4.0 2.3 1.4
0.99 1.7 2.2 2.0 2.4 1.5 5.7 4.2 1.3 0.8
v
Cr Fe co Ni
cu As Zr
Nb Mo Sn Sb Ta
w
RSF 2.0 f 0.2 1.6 f 0.2 1.7 f 0.1
no. measd
3 3 3
1
0.86 f 0.09 0.84 f 0.09 1.6 f 0.2 3.0 f 0.4 1.9 f 0.2 1.6 f 0.2 1.2 f 0.1 4.1 f 0.8 3.6 f 0.6 0.9 f 0.1 0.6 f 0.1
3 3 3 1
3 2
3 1 1 1 1
T a b l e VI. V a l u e s of F x , R x , and RSF M e a s u r e d for E l e m e n t s in a S p a r k e d Cu a n d C u B e Matrix at 40 kV, 1 k H z , 20 ps
element
FX
RX
RSF
Be
0.31 f 0.17 1.2 f 0.6 0.9 f 0.1 1.3 f 0.4 1.4 f 0.6
0.24
0.63 f 0.08 1.3 f 0.2 0.98 f 0.08 1.7 f 0.2 1.5 f 0.1
A1 Si Cr Mn Fe
co Ni cu Sn Sb
1
0.88 f 0.34 0.76 f 0.25 1.1 f 0.6 3.5 2.2
1.1
0.9 1.3 1.1 1
0.91 0.79 1.1
3.6 2.3
no. measd 2
4 2
4 3
1
0.86 f 0.06 0.72 f 0.07 0.66 f 0.05 2.6 f 0.5 3.1 f 0.7
4 3 4 1 1
perimental values of Fx,Rx,and the RSF's are tabulated. Again the agreement with RSF is reasonable and better for Fx than for Rx. The matrix element Cu was sometimes enriched and sometimes depleted as appears from the fluctuations of the Fx values (Table VI). This could suggest that heterogeneous structures may occur at the surface of sparked copper. Figure 4 shows an in-depth profile in which depletion was observed.
CONCLUSION The above considerations may lead to the conclusion that a thin surface layer of the specimen controls actions in the spark gap. The composition of this layer depends on the discharge parameters, type of electrodes, and discharge atmosphere. The phenomena responsible for the differences in surface and bulk composition may be oxide layer formation, segregation, vaporization, fractional condensation, selective sputtering, zone refining, and others. The relative importance of each factor can differ markedly depending upon conditions. Differences in relative sensitivities in SSMS appear to be rather due to differences between sampled material and the bulk than to differences in atomic quantities (such as ionization potential, heat of sublimation, etc.) proposed in earlier empirical approaches to rationalize relative sensitivity coefficients. In other words SSMS reveals the real composition of the sparked surface.
ACKNOWLEDGMENT The authors thank J. Van Puymbroeck for many useful discussions. Registry No. Mg, 7439-95-4; Si, 7440-21-3;Ti, 7440-32-6; Cr, 7440-47-3; Mn, 7439-96-5; Fe, 7439-89-6; Ni, 7440-02-0; Zn,
7440-66-6; Sn, 7440-31-5;Pb, 7439-92-1;V, 7440-62-2; Co, 744048-4; As, 7440-38-2; Zr, 7440-67-7; Nb, 7440-03-1; Mo, 7439-98-7; Sb, 7440-36-0; Ta, 7440-25-7; W, 7440-33-7; Be, 7440-41-7; AI, 7429-90-5;Cu, 7440-50-8;beryllium-copper alloy, 11133-98-5;steel, 12597-69-2.
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RECEIVED for review August 6, 1984. Accepted October 10, 1984. This paper was presented a t SAC 83, an international conference organized by the Analytical Division of the Royal Society of Chemistry, University of Edinburgh, July 17-23, 1983. This work was carried out under Research Grant 8085/ 10 from the Interministerial Commission for Science Policy, Belgium. K.S. is indebted to the "Instituut tot Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw (IWONL)" for financial support.
