Anal. Chem. 1990, 62, 1615-1619
Henson, C. A.; Stone, J. M. J . Chmetogr. 1989, 469, 361. Armstrong, D. W.; Alak, A.; DeMond, W.; Hinze, W. L.; Riehl, T. E. J . Liq. Chromatogr. 1985, 8 , 261. Pirkle, W. H.; Pochapsky, T. C. J . Chrometogr. 1988, 369, 175. Llpkowitz, K.; Landwere, J. M.; Darden, T. Anal. Chem. 1986, 58, 1611
Toplol. S.; Sabio, M.; Moroz, J.; Caldwell, W. B. J . Am. Chem. SOC. im. 110.8367. BGhm. R.'E.; Martire, D. E.; Armstrong, D. W. Anal. Chem. 1988, 60, 522. Armstrong, D. W.; Ward, T J.; Armstrong, R. D.; Beesley, T. E. Science 1988, 232, 1132. Chang. C. A.; Wu, 0.; Armstrong, D. W. J . Chromafogr. 1986, 354, 454. Chang, C. A.; Wu, Q . ; Tan, L. J . Chromatogr. 1988, 367, 199. Armstrong, D. W.; Jin, H. L. J . Chromafogr. 1989, 462, 219. (13) Linder, K. R.; Mannschrek, A. J . Chromafogr. 1980. 793, 308. (14) Shlbata, T.; Morl, K.; Okamoto, Y. In Chiral SewraNOns by H R C ; Krstulovic, A. M., Ed.; John Wiiey 8 Sons: New York, 1989; pp 336-398.
1615
(15) Armstrong, D. W. US. Patent 4,539,399, 1985. (16) Berendsen, G.; Pikaart, K. A.; de Galan, L. J . Liq. Chromatogr. 1978, 3 , 561. (17) Pirkle, W. H.; Pochapsky, T. C. J . Am. Chem. SOC.1988, 708, 5627. (18) Seeman, J. I.; Secor, H. V.; Armstrong, D. W.; Timmons, K. D.; Ward, T. J. Anal. Chem. 1988, 60, 2120. (19) Armstrong, D. W.; Ward, T. J.; Czech, A.; Czech, B. P.; Bartsch, R. A. J . Org. Chem. 1985, 50, 5556. (20) Pirkle, W. H.; Pochapsky, T. C. J . Am. Chem. SOC.1988, 708, 352.
RECEIVED for review February 20,1990. Accepted May 3,1990. Support of this work by the Department of Energy, Office of Basic Sciences (DE FG02 88ER13819), and theSmokeless Tobacco Research council, lnc, ( G N ~~ 0, 1 5~~ - 0 ~~is) , gratefully acknowledged.
Transferability of Relative Sensitivity Factors in Secondary Ion Mass Spectrometry: An Evaluation of the Potential for Semiquantitative Ultratrace Analysis of Metals Gernot Friedbacher, Alois Virag, a n d Manfred Grasserbauer*
Institute o f Analvtical Chemistrv. Laboratorv for Phvsical Analysis, Technical University Vienna, Getreidemarkt 9, A-1060 W k n , Austria Secondary ion mass spectrometry (SIMS) has turned out to be a powerful tool for multleiementai ultratrace characterization of metals. I n many cases, a limiting factor for the evaluation of quantitative resuits is the avallablllty of reference materials. For that reason, It would be desirable to perform quantitative or at least semlquantltative analysis by means of a transfer of relative sensltivlty factors (RSFs) from one matrix to another and to calculate RSFs through interpolation using an experhnental regression functlon. The results shown In this paper suggest that a transfer of RSFs for semiquantltative multielemental ultratrace analysis with SIMS referrlng to the matrices W, Mo, Ta, Nb, WTIlO%, TaSI,, and SI Is possible whenever an error factor of 2 can be accepted.
INTRODUCTION According to the great number of different analytical questions arising from the technology of materials, it would be desirable to be able to perform quantitative trace analysis of any analyte-target combination under multielemental conditions. Secondary ion mass spectrometry (SIMS) in principle exhibits this capability. Due to the large variation of ionization probabilities for various elements and the dependence of these on sample composition (matrix effects), the use of matrix matched reference materials forms the most successful basis for quantitative analysis (1,2). The availability of reference materials is limited and their preparation rather tedious. A method for the preparation of multielement reference materials by homogeneous doping of a metal-here W and Mo-with trace elements has been described elsewhere ( 3 , 4 ) . The value of ion implantation for the preparation of reference materials for refractory metals is demonstrated in ref 5. In order to reduce the elaborate efforts for the preparation and the characterization of reference materials, it would be very useful to quantify SIMS data without matrix matched reference materials for a wide range of matrices and trace elements. For many cases, an accuracy within a factor of 2 would be sufficient.
Table I. Experimental Parameters instrument: Cameca ims 3f ion microanalyzer primary ions: 02+ primary ion energy: 5.5 keV primary ion beam size: about 100-pm diameter primary ion raster size: 250 X 250 pm2 for implantation standards 500 x 500 pm2 for powder standards primary ion current density: 10" A/cm2 mass resolution: 300 secondary ion polarity: positive secondary ion energy range: 0-70 eV for masses < 48 45-145 eV for masses 3 48 imaged field: 10-pm diameter for implantation standards 150-pm diameter for powder standards field aperture: 750 pm for implantation standards 1800 pm for powder standards contrast diaphragm aperture: 60 pm for implantation standards 400 pm for powder standards entrance slit: 700 pm exit slit: 700 pm detection system: electron multiplier residual sample pressure: approx Pa
The aim of this paper will be to show the practical value of empirical relations between relative sensitivity factor (RSF) data and the physical properties of elements for the transfer of RSFs. It is not intended to incorporate the theoretical approach in detail into the considerations, since, on the one hand, in most cases this is not possible for lack of known parameters and, on the other hand, it is not meaningful for many practical applications, because no significant improvement of accuracy can be achieved. Moreover, only relative ionization probabilities are used for evaluation. For that reason, theoretical approaches will only be involved as far as they provide plausible explanations for the patterns found. EXPERIMENTAL SECTION Instrumentation. All RSF data presented in this paper have been obtained with a Cameca ims 3f ion microanalyzer interfaced to a Hewlett-Packard 9825 A microcomputer. Instrumental parameters are listed in Table I.
0003-2700/90/0362-1615$02.50/00 1990 American Chemical Society
~
1616
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
Preparation of Powder Standards. The powder standards for tungsten and molybdenum were prepared by doping the corresponding oxides with aqueous solutions of nitrates of about 30 elements and by subsequent reduction under hydrogen. The resulting powders have been characterized in a round-robin exercise by atomic absorption spectroscopy and optical emission spectroscopy. For most dopants, the concentration is in the range from 10 to 100 pg/g. Detailed results for the tungsten powder can be found in refs 3 and 6 and for the molybdenum powder in refs 4 and 7 . The powders were pressed to pellets with a diameter of 1 cm and a thickness of about 1 mm. The samples can be considered to be homogeneous with respect to the analyzed area. Preparation of Implantation Standards. The silicon target was a commercially available wafer with a (100)surface. The TaSi, target was prepared by stoichiometric cosputtering of tantalum and silicon on a (100)silicon substrate, resulting in a TaSi, layer of 650-nm thickness. Some of the samples were annealed; that, however, did not influence the results. The targets for tungsten, molybdenum, tantalum, niobium, and WTilO% were prepared by physical vapor deposition of the corresponding elements on silicon wafers. For this process, diode sputtering using a Leybold Heraeus Z 400 instrument was applied. At a deposition rate of 0.5 pm/h and a pressure of 1 Pa, polycrystalline layers with a thickness between 0.5 and 2 pm were obtained. “i+ 23Na+,39K+, 24Mg+,and 40Ca+were implanted into these targets’at a polar angle of 7 O by using a High Voltage Engineering 350-kV instrument. The incident beam (2 mm in diameter) was rastered over an area of 1 X 1 cm2. The fluence was determined by current integration by means of a faraday cup. The fluences of the implants are in the range from 1014 to 1015 ern-,. The implantation energy was chosen between 40 and 350 keV so that the peak maxima are situated in a depth between 100 and 300 nm in all cases. Measurement Procedure. All SIMS measurements were carried out by means of cyclic peak switching using a somewhat modified depth profiling routine from Cameca. The masses were sequentially integrated for 1s, and up to several hundreds of data points were acquired. In the case of the powder standards, mean values of the acquired data points were calculated. Masses greater than or equal to 48 (Ti) were measured under energy discrimination in order to reduce interferences from molecular ions (see also Table I). The crater depths on the implantation targets were determined by means of mechanical stylus profilometry using a Sloan Dektak instrument.
RESULTS AND DISCUSSION Quantitative SIMS is usually performed by means of RSFs, which are defined here according to
RSF =
le1 fref cref
Table 11. Comparison of RSFs in Ta and TaSi, (Reference 181Ta+)U RSF element
Ta
TaSi,
RSFTB/RSFTdi,
Li
441 779 258
288 350 191
1.5 2.2 1.4
Na Ca
a All RSFs were determined from implantation reference mateprimary ions and the detection of positive rials by the use of 02+ secondary ions.
Table 111. Comparison of RSFs in Si and TaSi, (Reference 30Si+)n RSF element
Si
TaSi,
RSFsi/RSFTdiz
Li K
89 168 14 64
69 127 18 45
1.3 1.3 0.8 1.4
Mg
Ca
All RSFs were determined from implantation reference mateprimary ions and the detection of positive rials by the use of 02+ secondary ions. Table IV. Comparison of Calculated and Measured RSFs in WTilO%” element
RSFWTiloImeasdb
RSFWTilO% calcdc
P
Li K
1061 2486 52 320
770 2253 54 330
1.38 1.10 0.96 0.97
Mg
Ca
a All measured RSFs were obtained from implantation reference primary ions and the detection of posmaterials by the use of 02+ itive secondary ions. *Measured RSFs in WTilO% with ISOW+as the reference. Calculated RSFs in the same alloy with ISOW+ as the reference. The calculation was performed via RSFmilOSb(ref=w) = RSFmilOI(ref.~i) X 163 (RSF,(Ti) = 163). dRatio of measured to calculated RSFs.
should be constant over a large concentration range and the transfer from matrix A to matrix B should be simply possible by the use of (1)
(1)
(3)
where I is the ion intensity (counts/s); f is the isotopic abundance; c is the concentration (atom/atom); and el and ref are the analytical element and the reference element, respectively. At this point, it is worthwhile to note that for technological applications the concentration unit g/g is usually used. When RSFs are to be compared mutually, however, then of course the atomic concentration is of interest. In the case of reference materials prepared by ion implantation, eq 2 is used for the determination of RSFs. These values are comparable with those obtained by eq 1.
where the subscript specifies the matrix and the analyte is in parentheses. This relation has been tested for a number of systems. Table I1 shows a comparison of RSFs in pure tantalum and TaSi, (with lslTa+ as the reference). The mean scaling factor for the three elements is 1.7, which indicates that a transfer of RSFs for these elements from one matrix to the other is possible with reasonable errors. Furthermore, it is worthwhile to note that there is obviously only a relatively small influence of the matrix component Si on the relative ionization probabilities, which mainly determine the RSF. Table I11 shows the same type of comparison for four elements in the matrices Si and TaSi, (with 30Si+as the reference). The mean scaling factor for the RSFs is 1.2. This may serve as a further confirmation of the conclusion in the previous paragraph. Table IV shows a comparison of the calculated and measured RSFs for a WTilO% alloy. The second column shows the RSF values for the specified elements with ISOW+as the reference. These values were determined directly from measurements. The RSF values in the third column were obtained via eq 3, starting from the RSFs normalized to the
~
I r e f f e l ce1
where d is the crater depth (cm), QT is the implantation dose (atoms/cm2),N c is the number of measurement cycles, and cmfis the concentration of the reference element (atoms/cm3). Internal standardization by RSFs is performed in order to eliminate the influence of the measurement parameters and also of the matrix composition on the absolute ion intensities. If there were no mutual influences of elements, these RSFs
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
Table V. Calculated RSFs for the Matrices Mo, Nb, Ta, and Sia element Li Na
1817
61
calcd
measd RSFw
RSFM,
RSF,b
RSFT,
RSFsi
1061 1166
130 (1.9) 143 (1.2)
39 (0.8) 43 (0.5)
247 (0.6) 272 (0.4)
71 (0.7) 78
OThe values were obtained via eq 3, starting from RSFw(el)and RSFw(Mo) = 8.2, RSFw(Nb) = 27.2, RSFw(Ta) = 4.3, and RSFwprimary (Si) = 14.9 (measured values). All RSFs are valid for 02+ ions and positive secondary ions. The values in parentheses are the ratios between the calculated data shown and the corresponding measured values. reference 46Ti+and multiplying them by RSFw(Ti) (=163). The ratio between both values (fourth column) is around 1. This indicates that in this specific case only a slight matrix effect occurs and supports the transferability of RSFs from W to WTilO% for practical applications. One always has to keep in mind that these calculations also presume a constant value for RSFw(Ti) over 6 orders of magnitude (some micrograms per gram in the tungsten standard powder and 10% in the WTilO% alloy). Table V shows the results for analogous calculations for Li and Na in the matrices Mo, Nb, Ta, and Si. The RSFs in W were measured. The RSFs in the other matrices were calculated via eq 3 using RSFJMo), RSFw(Nb), RSFw(Ta), and RSFw(Si). The values in parentheses show the ratio between the calculated and measured RSFs. These values give further support to the transferability of RSFs between these matrices, if an accuracy in the range of a factor of 2 is acceptable. The results shown support the conclusion that for many practical SIMS applications in ultratrace analysis of metals matrix effects can be neglected in a first-order approximation and the transfer of RSFs from one matrix to another is possible in a very simple way with sufficient accuracy. A further approach of the transferability of RSFs is the use of experimental regression functions calculated for simple relations between the physical properties of elements and RSFs. Since it is not our intention to give a detailed theoretical description of the shown patterns, our attention should be mainly focused on the relation Pion+ = constant x e-Ei (4) where Pion+ is the ionization probability for positive ions and Ei is the ionization energy, which is the basic relation in SIMS theories describing the ionization process (local thermal equilibrium (LTE) (8),electron tunneling, bond breaking; for further references see ref 9). Of course, the ionization potential is not the only parameter that influences the ionization probability (and thus the RSF), but it is the most important one. Moreover, in most cases, it is not meaningful to introduce other parameters (e.g., work function), because they are not well-defined for technological samples and their influence on quantitative results can be eliminated or at least reduced to a great extent by normalization via RSFs. For that reason, it is worthwhile to investigate log RSF versus Ei patterns. Similar correlations have previously been published by some authors for other matrices (9-11). Figure 1 shows a log RSF versus E, plot for the matrices W, Mo, Nb, and WTilO7'. Similar patterns were also obtained for the matrices Ta, TaSi2, and Si. The relatively good linearity of the data suggests that new RSF values can be calculated by extrapolation or intrapolation using the regression line, allowing semiquantitative analysis. This approach is also supported by the comparison of the measured RSF values with those determined by the regression line, indicating good agreement within a factor of 2 (Table VI). Consideration of the error factors for the same type of comparison as shown
5
4
7
6
8 Ei (eV)
9
Figure 1. log RSF versus ionization energy for the matrices W, Nb, Mo, and WTilO%. For better presentation of the pattern, the data points are connected with straight lines. The regression lines are also plotted. All RSFs were obtained from implantation reference materials , ' primary ions and the detection of positive secondary by the use of 0 ions.
cow
-21
-
'Mo
-3.
-4 3
4
5
6
7
8
9
1
0
1
1
1
2
Ei (eV)
Figure 2. log RSF versus ionization energy for the matrices W and Mo. For better presentation of the pattern, the data points are connected with straight lines. The regression lines are also plotted. All RSFs were obtained from homogeneously doped powder reference materials by the use of 02+ primary ions and the detection of positive secondary ions.
Table VI. Comparison of Measured RSFs with the Values Obtained by Regression Calculation Using the log RSF versus Ei Pattern for the Matrix Tungsteno element
E,, eV
measd
K Na Li
4.341 5.139 5.392 6.113 7.646
2432 1140 1038 314 51
Ca Mg
RSF regression 2868 1096 808 339 53
dev, % 18 4 22 8 4
'The measured values were determined from implantation reference materials by the use of 02+ primary ions and the detection of positive secondary ions. (See also Figure 1.) in Table VI for Li, Na, K, Mg, and Ca in the matrices W, Mo, Ta, Nb, WTilO%, TaSi,, and Si reveals that 87% of all error factors are smaller than 1.5and 100% are smaller than 2. It has to be kept in mind, however, that only five elements with similar ionization probabilities are considered in this case. Figure 2 shows a log RSF versus E, plot for the matrices W and Mo. In this case, the RSFs were determined from refractory metal powders homogeneously doped with about 30 trace elements. For this set of elements, a larger scatter around the regression line occurs than that occurring in the previously shown plots. Nevertheless, it is worthwhile to note that the patterns are very similar for the two matrices. Table VI1 shows again a comparison between the measured RSFs
1618
ANALYTICAL CHEMISTRY, VOL. 62, NO. 15, AUGUST 1, 1990
Table VII. Comparison of Measured RSFs with the Values Obtained by Regression Calculation Using the log RSF versus Ei Pattern for the Matrix Tungsten" element K Na
Ba Sr A1 Ca
E'
v
Cr
Ti Zr Nb
Hf MO Sn
Mn Th Ni Mg
cu CO
Fe Ta
Si
B Sb As P
Ei,eV
measd
4.341 5.139 5.212 5.695 5.986 6.113 6.380 6.740 6.766 6.820 6.840 6.880 7.000 7.099 7.344 7.435 7.500 7.635 7.646 7.726 7.860 7.870 7.890 8.151 8.298 8.641 9.810 10.486
1283 1055 94 88 303 458 56 31 28 43 35 27 9.3 8.2 7.3 18 6.1 2.2 116 1.2 3 13 4.3 15 6 0.46 0.22 0.27
RSF regression 1312 387 346 166 106 87 58 33 32 30 29 27 23 19 13 12 10 8.5 8.4 7.4 6 5.9 5.8 3.9 3.1 1.8 0.31 0.11
The measured values were determined from homogeneously doped multielement powder reference materials by the use of 02+ primary ions and the detection of positive secondary ions. (See also Figure 2.)
Ca
-1
1 AH^
1
6 7.0
(Oxide) [kJ/moll
Flgure 3. log RSF versus heat of formation of the corresponding stoichiometric oxides (MeO) for the matrix tungsten. All RSFs were obtained from homogeneously doped powder reference materials by , ' primary ions and the detection of positive secondary the use of 0 ions.
and the regression values for the matrix tungsten; 54% of all error factors are smaller than 2 and 89% are smaller than 5. Only one value (Mg) deviates by 1 order of magnitude. For the matrix molybdenum, 50% of all error factors are smaller than 2 and 82% are smaller than 5. For most of the elements, the deviations are acceptable for many semiquantitative applications of SIMS. Besides the log RSF versus Ei patterns shown, the electron affinity (AE), the electronegativity ( X ) ,and the heat of formation of the oxides (A"f(oxide)) (see also ref 12) were also taken into consideration. Corresponding plots for AE and X indicated only a trend in the patterns with very large scattering. Relatively small scattering of RSFs around the regression line could be achieved in the case of log RSF versus
W
1
1
4'0 4 . 0 .. 3.0
Ti
tJ
I
2.0
E, rev1 Flgure 4. Ordinate section (d)of the log RSF versus E,regression lines (Figure 1) versus ionization energy of the matrix reference element (Ti was used for the matrix WTi10%). 0.8
0.6
f
0.5
1 Nbw T
0.7
Ti
\
0.4.
\
0.3.
'si
0.2.
0 i
i O
L
D
O
I
S
)
0
Ln
0
I\ m m (J1 E, [evl Flgure 5. Negative slopes ( 4 of ) the log RSF versus E, regression lines (Figue 1) versus Ionizationenergy of the matrix reference element (Ti was used for the matrix WTi10%). u)
u)
i\
AHf(oxide). Figure 3 shows the log RSF versus AHf(oxide) pattern for the matrix tungsten. The mf(oxide) values correspond to the formation of oxides with the stoichiometric composition MeO. There is good agreement between the measured values and the values obtained from the regression line; 86% of all error factors are smaller than 2 and 100% are smaller than 3. A similar pattern was also found for the matrix molybdenum, where 85% of all error factors are smaller than 2 and 100% are smaller than 3. The same procedure was also applied to some RSFs for TaSi, (normalized to Ta) recently published by Stevie et al. (13). A pattern similar to that shown in Figure 3 was obtained; 88% of the error factors are smaller than 2 and again 100% are smaller than 3 in this case. At this point, it should be mentioned that additional problems leading to poorer results could arise from the transfer of RSFs between different instruments. A detailed discussion of that problem and possible solutions can be found in ref 14. There is evidence, however, that the conclusions drawn in this paper are still valid if different instruments of the same type are considered and the alignment parameters are carefully documented. This is also confirmed by comparison of some RSFs from ref 15 with corresponding ratios obtained with our instrument. Since the log RSF versus Eipatterns have a similar shape for different matrices, we tried to find a relation between those patterns. Figure 4 shows the relation between the ordinate section ( d ) of the regression lines and the ionization potential of the corresponding matrix element used as the reference signal for the RSF calculation. There is a maximum for d somewhere in the middle range of the ionization potentials. One has to keep in mind that in this plot a linear scale has been used. The next step was to check whether or not the scatter of the d values might be caused by a limited precision of the measurements. Consequently, also the slopes ( k ) of the
Anal. Chem. 1890, 62, 1619-1623
14.0
2 12.0
\
y
10.0
is19
analysis of technological materials with SIMS. Investigation of additional systems will be needed to further confirm the relations found.
1
1
LITERATURE CITED (1) Newbury, D. E. Scanning 1080, 3 , 110-118. (2) Newbury, D. E. In QuanfnElthre Swface Analysis of Meted&, ASTM STP 643; McIntyre, N. S., Ed.; ASTM: Philadelphla, 1978; pp 127-149. (3) Wilhartitz, P.; Virag, A.; Friedbacher, G.; Grasserbauer, M.;Ortner, H. M. Fresenius’ 2.Anal. Chem. 1987, 329, 228-236. (4) Virag, A.; Friedbacher, G.; Grasserbauer, M.;Ortner, H. M.;Wilhartk, P. J . Mater. Res. 1988, 3 , 694-704. (5) Friedbacher,G.; Vkag, A.; Grasserbauer. M.;Bubert. H.; Palmetshofer, L.; Wilhartii. P.; Ortner, H. M.;RGdhammer, P.; Rltter, W. SIA, Surf. Interface Anal. 1988, 12, 165-167. (6) Scherer, V.; Hirschfeld, D. €rzmetalll98S, 39, 251-253. (7) Scherer, V.; Hirschfeld, D. €rzmetalll987, 40. 611-613. (8) Andersen, C. A.; Hinthorne, J. R. Anal. Chem. 1973, 45, 1421-1438. (9) Wilson, R. G. J . Appl. phvs. 1988. 6 3 , 5121-5125. (10) Lapides, L. E. SIA, Surf. Intefface Anal. 1985, 7 , 211-216. (11) Lapides, L. E.; Whlternan, G. L.; Wiison, R. 0 . Mater. Res. SOC. Symp. Roc. 1984. 25, 657-662. (12) Galuska. A. A.; Morrison, 0.H. Int. J . Mass Spectrom. Ion Processes 1984, 8 1 , 59-70. (13) Stevie, F. A.; Kahora, P. M.;Cochran, 0.W. J . Vac. Sci. Techno/. 1989, A7. 1539-1544. (14) RMenauer, F. G.; Steiger, W.; Rledel, M.;Beske, H. E.; Holzbrecher, H.; Diistemijft, H.; Gerlcke, M.; Richter, C. E.; Rieth, M.; Trapp. M.; Giber, J.; Solyom, A.; Mai, H.; Stingeder, G. Anal. Chem. 1985, 57, 1836-1643. (15) Ramseyer, G. 0.; Morrison, 0.H. Anal. Chem. 1983, 55, 1963-1970.
i
6 4.0 .0o I
l
D
n
l
o
D
l
h
n
h
o
l
m
n
m
o
m
E i [eVl
Flgure S. - d / k (from Figures 4 and 5) of the log RSF versus E , regression lines (Figure 1) versus ionization energy of the matrix reference element (Ti was used for the matrix WTi 10% ). The regression line is also plotted in the diagram.
regression lines were plotted versus the ionization potentials of the matrix elements. Figure 5 shows the corresponding pattern, which is rather similar to the d versus Ei pattern of Figure 4. The relatively good linearity of the -d/k versus Ei plot (Figure 6) supports the conclusion that the relations demonstrated in Figures 4 and 5 are not caused bv casual scattering of the measured values. We do not yet have an explanation for the relation found, but we think that it is worth further investigation, because it bears the potential for a more accurate transfer of RSF versus Ei patterns from one matrix to another, allowing semiquantitative analysis. This would be an important advancement in multielemental ultratrace
RECEIVEDfor review January 11, 1990. Accepted April 12, 1990. Support of this work by the Austrian Science Foundation, the Federal Ministry of Research, the Austrian National Bank, and the University Jubilee Fund of the City of Vienna is gratefully acknowledged.
Fabrication of Band Microelectrode Arrays from Metal Foil and Heat-Sealing Fluoropolymer Film David M. Ode11 and Walter J. Bowyer* Department of Chemistry, Hobart and William Smith Colleges, Geneva, New York 14456 We descrlbe a technique for easlly preparlng arrays of band microelectrodes by seallng metal foil and film of Tefrel fluoropolymer In “multldecker” sandwiches. Electrodes have been prepared from gold, platlnum, nickel, and sliver foils with thlcknesses ranglng from 4 to 100 pm. Double-layer capacitance measurements suggest that the seal between the film and the metal Is very good. Results of voltammetry experiments uslng arrays, single-band electrodes, and a conventional disk electrode are compared. Using anthraqulnone(O/-) In acetonltrlle as a reversible couple, we demonstrate three dlffuslon regimes ( h e a r diffusion to each dement, hemlcyllndrlcal ditfuslon, and h e a r ditfuslon to the array) at a slngle array. Results are compared to theoretlcal descrlp tlons. With the electrode arrays, relatlvely undlstorted cycllc voitammograms can be recorded at Scan rates up to 1000 V/s In aprotlc solvents wlth no compensation or correction.
INTRODUCTION There has been much interest in arrays of microelectrodes. Shortly after microelectrodes became popular, arrays were used to increase the current while maintaining microscopic *To whom correspondence should be directed. 0003-2700/90/0362-1619$02.50/0
dimensions of the electrode. Arrays have many advantages over both microelectrodes and macroelectrodes including improved signal-to-noise ratio and lower limits of detection
(1-13). Cylindrical, band, and dual-band microelectrodes also have been studied (14-17). Theoretical descriptions of the voltammetric response at microelectrode arrays have been well developed (18-20). Amatore et al. (18)described three regimes. At very short times (high scan rates), diffusion to each microelectrode element in the array is linear. At longer times, when diffusion is no longer linear, a plateau, rather than a peak, is recorded by cyclic voltammetry. Finally, a t very long times, voltammetric peak currents are the same as those that would be recorded at a macroelectrode, but the measured electrontransfer rate is predicted to decrease proportionally to the fractional area that is active microelectrode. Experimental work has been done to support the theoretical results (e.g. 3,5,18,19).However, the construction of arrays is difficult. Composite electrodes have complex and irregular geometries, and lithographic techniques are not available in most labs. Furthermore, lithographically prepared electrodes are not sufficiently robust to withstand polishing or rigorous electrochemical cleaning. Robust band electrode arrays of regular geometry have been described (12,13).Magee and Osteryoung (12)describe arrays 0 1990 American Chemical Society
