Anal. Chem. 1980, (12) Figura, P.; McDuffie, B. Anal. Chern. 1979, 57, 120. (13) Chau, Y . K.:Gachter, R.; Lum-Shue-Chan, K. J . Fish. Res. Board Can. 1974, 3 1 , 1515. (14) Batley, G . E.; Florence, T. M. Anal. Lett. 1976, 9 , 379. (15) Florence, T. M. Water Res. 1977, 1 1 , 681. (16) Shuman, M. S.;Shain, I. Anal. Chem. 1969, 47, 1818. (17) Figura. P.; McDuffie. B. Anal. Chem. 1977, 49, 1950. (18) Matson, W. R. Ph.D. Thesis, Massachusetts Institute of Technology, Cambridge, Mass., 1964. (19) American Public Health Association, "Standard Methods for the Examination of Water and Wastewater", 14th ed.; Washington: D.C., 1976. (20) Davison, W. J . Electroanai. Chem. 1978, 8 7 , 395. (21) Brezonik, P. L.; Brauner, P. A.; Stumm, W. Water Res. 1976, 70,605. (22) Raspor, B.; Branica, M. J . Electroanai. Chern. 1975, 59, 99. (23) Shuman, M. S.:Crorner, J. L. Environ. Sci. Technoi. 1979, 73, 543. (24) Frost, A. A.; Pearson, R. G. "Kinetics and Mechanism", 2nd ed.; John
52, 1439-1445
1439
Wiiey and Sons: New York, 1961; p 193. Stolzberg, R. J.; Rosin, D. Anal. Chern. 1977, 49, 226. Reid, J.; McDuffie. B., unpublished work, 1979. Gardiner, J. Water Res. 1973, 8 , 23. Sylva, R . N. Water Res. 1976, IO, 789. Shurnan. M. S.;Michael, L. C. Environ. Sci. Technol. 1978, 72, 1069.
RECEIVED for review December 10, 1979. Accepted April 25, 1980. The support of Paul Figura by t.he U S . Office of Water Research and Technology under Grant No. B-070-NY is gratefully acknowledged. Presented in part at the 5th Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Boston, Mass., Nov. 2, 1978.
Measurement of Direct Currents and Pulse Components for Analytical Evaluation of Differential Pulse Polarography and Voltammetry J. E. Anderson and A. M. Bond" Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 32 17, Victoria, Australia
Simple and inexpensive modifications to differential pulse poiarographs enable the dc and pulse components of the experiment as well as their differences to be measured. Using examples of electrochemically reversible and irreversible electrode processes, as well as kinetically complicated ones, demonstrates that access to additional data usually ignored in differential pulse polarography and voltammetry can be a valuable aid in analytical evaluation of the techniques and in methodology development. Theoretical studies would also appear to be substantially aided by access to such measurements.
Differential pulse polarography has become one of the most commonly used methods of polarographic analysis. Originally developed by Barker ( I ) , the technique uses a waveform consisting of a fixed amplitude pulse superimposed onto a dc ramp. Currents measured in the absence of the pulse are subtracted from currents in the presence of the pulse producing the differential current readout in which hi is plotted against the dc potential to produce a differential pulse polarogram (2). Theoretical work in differential pulse polarography has lagged far behind the analytical applications and i t is only recently (3-8) t h a t papers have appeared dealing with anything other than a reversible electrode process with planar diffusion. Use of the reversible theory has led to suggestions for improved instrumental design of the technique ( S I 2 ) . The basis for most of these innovations has been to eliminate residual terms arising from incomplete subtraction of the dc component of the experiment (9). Unfortunately, many electrode processes are not simple reversible diffusion controlled electrode processes and the validity of many of these ideas to the irreversible electrode process, adsorption, maxima, or all other well known phenomena associated with polarography is far from clear. Experimentally, in almost all theory vs. experiment correlations, and indeed in analytical work, the differential pulse polarograms are all that are measured. In view of the fact that the technique measures the difference between the pulse and 0003-2700/80/0352-1439$01 .OO/O
dc polarogram, i t is difficult to understand why the individual components of the experiment are not routinely monitored and results used for optimization in analytical work and particularly in theory vs. experiment correlations. This need has obviously been previously recognized by workers having access to very expensive computerized systems. The almost complete lack of studies in this area can be attributed to a lack of suitable readily available inexpensive equipment. In the present paper, it is demonstrated how simple modifications to existing circuits associated with differential pulse polarographs can enable far more information to be obtained than is conventionally the case. A qualitative survey of the information that can be readily extracted is presented to demonstrate the general use of these data automatically discarded in almost all previous studies using differential pulse (dp) polarography.
EXPERIMENTAL Instrumentation. The polarographic experiments described here were performed using a PAR h'Iodel 174 Polarographic Analyzer equipped with a Model 174170 drop timer. The modifications made to the PAR 174 are described in the Results and Discussion section. Current-potential curves were recorded on a Houston Instruments Model 2000 X-Y recorder. A platinum auxiliary (counter) electrode and a AgiAgCl saturated KC1 reference electrode were used in conjunction with the dropping mercury electrode (DME) in the three-electrode configuration. In the anodic stripping voltammetry (ASV) experiments, the DME working electrode was replaced by a Beckman Rotating Electrode Model 1885 equipped with a glassy carbon electrode. Glass sample cells were used throughout. Reagents a n d Procedures. Analytical reagent grade chemicals were used throughout the experiments as was distilled water. Polarograms were recorded at ambient temperatures of 21 f 1 "C. All solutions were degassed for at least 5 min prior to the experiments with nitrogen. The nitrogen used was passed through the following bubblers to ensure the absence of oxygen and to ensure that the gas was saturated with HzO: (1) HzO, (2) V(I1) (13),(3) 1.OM NaOH, and (4) HzO. RESULTS AND DISCUSSION Two methods of extracting the pulse and dc components of a differential pulse polarogram were examined with the 1980 American Chemical Society
1440
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980 O P E R . MODE O I F F . PULSE PULSE S A W L E O oc DiC. I I
--i I I
I I
I
I
I I
I I
I
22K
I I
I I
3K
I
I
1 OK
UTE
SYITCH 1 BEFORE PULSE PULSE I I I
L - - '
I I
I
I I I
I
I I
I
I
f
I .
I I
I
I
I
2
FRon I / E CONVERT.
I
I I
I
Figure 1. Modifications made to the PAR 174 for extraction of pulse and dc components. Method I: Switch 2 in component position, switch 1 in pulse position
PAR 174 Polarographic Analyzer. The first method (method I), and by far the simplest, involved changing the data acquisition time of the sample-and-hold circuit of the 174. This was accomplished by gating the signal to the sample-and-hold circuit either a t the normal time (for 16.7 ms before the end of the pulse via gate signal 111) or at a time before the application of the pulse (for 16.7 ms before the pulse via gate signal I which is normally used to gate this signal to the difference sample and hold). This modification is illustrated
in Figure 1. Also shown in Figure 1 are further modifications which were necessary for collecting data before and during the potential pulse. These consisted of breaking the connection between the sample-and-hold circuit to the difference amplifier, eliminating the amplification of 10 which is used in the differential pulse mode, and connecting the output of the sample-and-hold circuit to the final amplifier instead of the difference amplifier (while in the differential pulse mode of operation). In this way, when switch 2 is in the normal
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
1441
1.0uA
I
i
Figure 3. Auxiliary sample-ancChoM circuits added to PAR 174 in mothod I1 of extracting the dc and pulse components
- 0.4I
I I
I
I
- 0.5
-0.6
I
I
-0.7
- 0.8
E vs
AglAgCl Figure 2. Components of dp polarogram of 4.44 X M Cd(I1) in 1.0 M KCI using a 1.0-sdrop time with -25-mV pulses at a 2 mV/s scan rate. Curve a, pulse component in the dp mode; curve b, dc component in dp mode; curve c, dc component 50 ms before the end of the drop in the sampled dc mode; and curve d, dc component at the end of the drop time in the sampled dc mode
position, the 174 functions as usual and when switch 2 is in the component position (asshown in Figure 1) either the pulse or the dc component of the polarogram may be monitored (depending on the position of switch 1). A most useful additional aspect of these modifications is t h a t when the 174 is operated in the current sampled dc mode, the current may be sampled either a t its normal time (for 16.7 ms a t the end of the drop) or for 16.7 ms, 50 ms before the end of the drop. Figure 2 shows the various components of a d p polarogram of 4.4 X M Cd(I1) in 1.0 M KC1 which may be extracted using method I. This is a reversible electrode process, Cd(I1) 2e- e Cd(Hg). The pulse and dc components extracted while in the d p mode are shown in curves a and b, respectively. Curves c and d were obtained while in the current sampled mode and correspond to the current a t the end of the drop (curve d) and the current 50 ms before the end of the drop (curve c). I t is apparent that the sampled dc component a t the end of the drop (curve d) is the component which should be subtracted from the pulse component t o obtain the ideal differential pulse polarogram without distortion due to the dc term. This component is obtained a t the same point in time as the pulse component, and therefore variation in dc current levels due to drop size variation occurring because of drop growth between sampling times is not present. One problem with method I for extracting the various components of a differential pulse polarogram is apparent from Figure 2 in that curves b and c should be identical. This is true since sampling of the current in these two curves occurs a t the same point in time (same drop size) and a t the same potential and therefore represents an identical experiment since no pulse perturbation has been applied a t this point in time even in the d p mode. The difference in these two curves is due to the large difference in the time constant of the sample-and-hold circuit between the two modes of operation (120 ms for d p and 15 ms for sampled dc). It is clear that the distortion of the data occurs only in the rising portion of the curves and that the curves merge in the diffusion-limiting region where large changes in current do not occur between samples. This artifact of the 174 has been discussed elsewhere (16) and the time constants of the sample and hold circuits have been modified in some instances (17, 18) so that experimental results could be accurately compared with theoretical results. I t is clear then that the time constants of the
+
circuit must be changed to obtain distortion-free results with this extraction method. If this is not undertaken, only results in the diffusion-limiting region of the polarogram may be used quantitatively and all other data may be used only for qualitative comparison purposes. For example, in Figure 2 it is evident that if either curve b or c is subtracted from the pulse component (curve a), the same theoretically predicted positive offset in the dc limiting current region will result in the d p polarogram. As may also be seen, the subtraction of the curve d from a (in the diffusion-limiting region) will yield no offset in the d p polarogram and therefore the offset under the conditions of this experiment is attributable to the change in drop size between measurements for the reversible reduction of Cd(I1) in 1.0 M KCI. The difference in charging current due to the difference in potential a t which the measurements were made is negligible at this concentration. Since, when the 174 is in actual use in its commercially available form, it is component b which is subtracted from the pulse component, curve c will not be shown in further illustrations when method I is used and only qualitative use of the data will be made. I t should be noted that if curve b is distorted owing to the long time constant of the d p mode, then curve LL is also distorted. Minimization of this problem will be seen with the second method of extracting these data. The second method (method 11) of extracting the various components of a d p polarogram involved the addition of two alternate sample-and-hold circuits t o the 174. These were added a t the outputs of the FET gates of the 174 sample-and hold-circuits. In addition, the connections between the FET gates to the sample-and-hold amplifiers internal t o the 174 were broken when the alternate amplifiers were in use. The circuits added are shown in Figure 3. As may be seen, the time constant of these sample-and-hold amplifiers may be modified independently of the 174 and therefore may be adjusted t o suit the particular experiment. I t should be mentioned that method I and method I1 were designed such that the 174 could be easily returned to normal operation for routine work. This is accomplished with two switches with method I and by unplugging and switching off the connection a t the rear of the instrument for method 11. Using the 174 in the d p mode of operation, a negative shift in peak potential and decrease in peak height is observed as the scan rate is increased. This again is simply an artifact of the 120-ms time constant of the standard circuitry. In its standard form, no comparison may be made between theory and experimental results with the 174 unless scan rates of less than 0.5 mV/s are used. By contrast, using the circuit in Figure 3, the peak potential and amplitude were found to be independent of scan rate up to 20 mV/s. Figure 4 shows the 5 response curves which may be monitored using method 11. Curve a is the pulse component of a d p polarogram of 4.44 x IO4 M Cd(1I) in 1.0 M KC1. Curve b is the corresponding dc component while in the d p mode and curve c is the dc component a t the end of the drop obtained while in the sampled dc mode. I t is once again clear
1442
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
l.OuA
I
L
1
- 0.4
I
- 0.5
I I
- 0.6
I
-
0.1
- 0.8
E v s AglAgCl Figure 4. Components of dp polarogram of 4.44 X M Cd(I1) in 1.0 M KCI with 2 mV/s scan rate using -25-mV pulses and a 1-s drop time. Curve a, pulse component; curve b, dc component; curve c, dc component at end of drop. Curve d is t h e difference between curves a and b. Curve e is t h e difference between curves c and b
that it is this component (curve c) which should be subtracted from curve a if the change in drop size and the change in potential (due to t h e potential ramp) are to be taken into account. This subtraction would in fact be essentially the same as alternate drop differential pulse polarography (14, 15). Examination of the dc component 50 ms before the end of the drop (while in the sampled dc mode) yielded a curve identical to curve b as expected. Curve d is the difference response of curves a and b and is simply the d p polarogram. Curve e is the difference between curves c and d obtained while in the sampled dc mode. It should be noted that curve e then is the difference between the current flowing a t the end of the drop and the current 50 ms before the end of the drop. Therefore, curve e could be considered the error in the d p polarogram due to the difference in drop size and change in potential due to the potential ramp caused by the difference in the time in the measurements. Since the subtraction of curve e from curve d yields a zero positive offset (return to base line) in the diffusion-limiting region of the polarogram, it is clear that the effect of the potential pulse (-25 mV) under these conditions and for this electrode process is negligible in this region of the curve for the reduction of cadmium. However, further experiments indicate that this is not true when the potential pulse is in excess of -50 mV even for a reversible system such as Cd(I1) and that the presence of even small amplitude pulses is quite important for irreversible and quasi-reversible systems. T h e magnitude of the positive offset measured corresponded with a 1.9 and a 3.1% increase in the diffusion-limiting current between the two measurements for drop times of 2 and 1 s, respectively. This compares favorably with the calculated increases in diffusion current of 1.7 and 3.5% based on drop size increase. The measured increase in diffusion
current of 7.3% for 0.5-s drop times was somewhat in error compared with the calculated increase in current of 5.4%. Considerable differences in the theoretical offset and the experimental offset measured have been reported for short drop times (19). Our data indicate the origin of this problem arises solely from the dc terms. A number of nonreversible electrode reactions have been examined to evaluate the general usefulness of observing the components which make up the d p polarogram. Since this survey of electrochemical processes has been purely qualitative to date, only a few systems are described here in an attempt to illustrate the possible advantages of this approach. The advantages of the ability to be able to correlate the various components with their theoretical counterparts should be apparent. Figure 5 shows the pulse and dc components under a range of conditions which yield the differential pulse polarogram for Zn(I1) reduction in 1.0 M KC1 plus 0.01 M HCl. Curves a are with a 2-s drop time and with various pulse amplitudes. Curves b and c are with a 1-s and 0.5-s drop time, respectively, with 25 mV/pulse. T h e reduction of Zn(I1) a t a DME has always been a difficult system to deal with theoretically since a wide range of rate constants and transfer coefficients has been reported from experimental results (20,21). This appears to be due to the fact that the electrode process is more complex than simple slow electron transfer. Multistep charge transfer with various intermediates, second-order chemical reactions, and adsorption have been proposed to explain the results observed. When Figure 5 (curves b) is compared with the curves obtained for Cd(I1) (Figures 2 and 4), it is obvious that the peak height of the pulse component for unit concentration is greatly diminished. This is predicted by theory for systems with slow electron transfer (19). However, the distortion (marked by X in Figure 5) with large pulse amplitudes of the pulse component a t potentials more negative than the peak of the pulse component is not predicted theoretically and gives insight into the discrepancies which have been seen in comparisons between theory and experiment in this potential region of the d p and ac polarograms (22). The hump or distortion in the pulse component was a t first suspected to be due to possible contamination. However, separately prepared solutions a t various concentrations yield the same relative distortion. As mentioned previously and as may be seen, the presence of the pulse may not be assumed to play a negligible role in the resulting pulse component at potentials near the diffusion limiting region for the nonreversible reduction of zinc as is the case with the reversible Cd(I1) system. Clearly a rigorous theory should be able to account for the anomalous region of Figure 5 under all conditions, and mechanistic nuances will need to be added to d p theory for the reduction of zinc. It is interesting to note that in studies of Cd(I1) reduction in the presence of n-butanol, similar anomalous behavior is observed suggesting similar mechanisms in electron transfer. Stepwise electron transfer has been proposed for both of these processes (24, 25). Another system for which data are presented is the reversible reduction of Pb(I1) a t a DME under conditions in which maxima were present. Figure 6 shows the various components of d p polarograms of Pb(I1) in 0.02 M KCl using method I to measure the responses. Curves a, b, and c were obtained using pulse amplitudes of -10, -25, and -50 mV, respectively. It is interesting that the maximum currents of the pulse and dc components (curves 1 and 2 at each pulse amplitude) are of approximately the same amplitude. Furthermore, the difference in potential between the peak currents of the maxima of the dc and pulse components is simply the pulse amplitude. These observations
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
t
I
1
- 0.9
-08
-
I
1.0
I
I
1
I
- 1.1
I
I
I
loOrnv+
E vs AglAgCl
I I
I
I
1443
;-I
Figure 5. Components of dp pohrograms of 5 X l o 4 M Zn(I1) in 1.0 M KCI plus 0.01 M HCI. All curves obtained at a 2 mV/s scan rate. Curves a, 2-s drop time: (1) pulse component, -50-mV pulse; (2) pulse component, -25-mV pulse; (3) pulse component, 10 mV/pulse; (4) dc component, sampled dc; (5) dc component, dp mode. Curves b, 1-s drop time: (1) pulse component, -25-mV pulse; (2) sampled dc component; (3) dc component, dp mode. Curves c, 0 5 s drop time: (1) pulse component, -25-mV pulse; (2) sampled dc component; (3) dc component, dp mode. Curves d difference curves from b: (1) difference between b l and b3, (2) difference between b2 and b3. Method I1 used. X marks the region of anomalous behavior. All scans started at -0.85 V vs. Ag/AgCI
I
-02
I I
-0.3
I I
- 0.4
1 I
I
1
1 I
1
I
I L
100rnV--’
I
I
I
E vs AglAgCl
Figure 6. Effect of pulse amplitude On the components of the Pb(I1) dp polarogram in the presence of maxima. 5 X lo-‘ M Pb(I1) in 0.2 M KCI, 2 mV/s scan rate with a 1.0-s drop time. Curves a, -10-mV pulse; curves b, -25-mV pulse; and curves c, -50-mV pulse. For all set of curves: (1) pulse component, (2) sampled dc, and (3) dc component in dp mode. Curve b4 is the resulting dp polarogram using a -25-mV pulse. Method I used. All scans started at -0.2 V vs. Ag/AgCI
appear to be characteristic of the mode of mass transfer under streaming conditions associated with maxima of the kind occurring with lead reduction. Curves 3 a t the various pulse amplitudes may be considered the sampled dc polarogram
obtained at a drop time 50 ms shorter than obtained in curves 2. The smaller amplitude of the maxima in curves 3 supports the findings of Cover and Connery (26, 27) that short drop times are advantageous in that the effects of maxima are
1444
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
t
07
I
- 08
I I
- 0.9
I I
- 1.0
,I , 1 I - 0.7 - 0.8 E vs AglAgCl
I
1
I
- 0.9
I I
-1.0
I
1
-11
i
- 1.2
Figure 7. Comparison of dp polarograms and the various components thereof for 1 X M Cr(1II) in 1.0 M NaCIO, plus -0.02 M HCIOI without Fe(II1) (curves a ) and with 2 X M Fe(II1) (curves b). Each scan started at -0.7 V vs. Ag/AgCI using a 2-mV s-’ scan rate with -25-mV pulses. In each set of curves, from left to right: pulse component, sampled dc, and dc component in dp mode. The corresponding dp polarograms are shown above the component curves. Method I1 used decreased. The resulting dp polarogram for the -25-mV pulse is also depicted in Figure 6 (curve b4) and it is clear from a phenomenological point of view as to how negative values of Ai may be encountered in the presence of maxima in d p polarography. The method of measuring various responses in d p polarography obviously provides substantial and significant information that is missed when only the differential response is recorded. The last system illustrated in this paper having kinetic complications is the reduction of Cr(II1) with and without the presence of Fe(II1). The reduction of Cr(II1) to Cr(I1) is an irreversible process. The effect of Fe(II1) on this reduction process appears to be a catalytic one (28) whereby Fe(II1) readily oxidizes Cr(I1) formed at the electrode surface as it diffuses into the bulk of the solution. This causes an increase in the differential pulse peak height which obviously presents problems in the determination of Cr(II1) in samples with various concentrations of Fe(II1). Fortunately this interference may be effectively eliminated using the standard addition method (29). When the various components of the d p polarogram were examined, it was found that while the d p peak height increased by 53% (56% reported in Ref. 29) for M Cr(II1) with the addition of 2 X M Fe(III), the dc limiting current increased by only about 20%. The reason for this may be seen from the shape of the pulse components in each experiment, Figure 7 shows the various components for the reduction of Cr(II1) with (b) and without (a) Fe(II1) in 1.0 M NaC10,. When Fe(II1) is added to the solution, the reduction of Cr(II1) becomes more reversible (apparently) as indicated by the pulse component which yields a much larger difference between the pulse and dc components in the presence of Fe(II1). This system is obviously much more complicated than a change in electron transfer rate in that the peak potential is also shifted negative. However, measurement of the d p and dc components would be a considerable aid to an analytical chemist. Reference to calibration curves in the absence of Fe(II1) would lead to apparently
different chromium concentrations being determined using the two components and thereby reveal any interference. Extrapolation of the above ideas to differential pulse anodic stripping or conventional voltammetry also reveals the usefulness of measuring the individual responses in addition to the differential readout. Clearly the present work shows that this approach can be implemented with far simpler instrumentation than the larger and expensive computerized system originally used to generate such data (30). CONCLUSIONS The above examples are believed to demonstrate the usefulness in measuring the various responses rather than only the differential readout when undertaking an analytical evaluation of some aspect of dp polarography. Instrumentally, modifications to convert commercial d p polarographs are simple and inexpensive so that the additional information is readily gained and may be routinely implemented in methodology development. In rigorous theoretical vs. experiment correlations, access to the additional measurements would seem highly desirable if not essential. LITERATURE CITED Barker, G. C.; Gardner, A. W. Atomic Energy Research Establishement (Great Britain), AERE Harwell C/R 2297, 1958. Flato. J. B. Anal. Chem. 1972, 44(11). 75A. Dillard, J. W.; Hanck, K. W. Anal. Chem. 1976, 48, 218. Christie, J. H.; Osteryoung, R. A. J. €kcfroanal. Chem. 1974, 49, 301. Keller, H. E.; Osteryoung, R. A. Anal. Chem. 1971, 4 3 , 342. Blutstein, H.; Bond, A. M. Anal. Chem. 1978, 48, 248. Dillard, J. W.; Turner, J. A,; Osteryoung, R . A. Anal. Chem. 1977, 4 9 , 1246. Birke, R . L. Anal. Chem. 1978, 50, 1489. Parry, E. P.; Osteryoung, R. A. Anal. Chem., 1965, 3 7 , 1634. Osteryoung. J. G.; Christie, J. H.; Osteryoung, R. A. Bull. SOC. Chim. Belge 1975, 84, 647. Rifkin, S.C.; Evans, D. H. Anal. Chem. 1976, 48, 2174. Klein, H.; Yarnitzky, C. H. J . Nectroanal. Chem. 1975, 67 1. Meites, L.; Meites, T. Anal. Chem. 1948, 20, 984. Christie, J. H.; Jackson, L. L.; Osteryoung, R. A. Anal. Chem. 1976, 48, 242.
Christie, J. H.; Jackson, L. L.; Osteryoung. R. A. Anal. Chem. 1976, 48, 561.
Anal. Chem. 1980, 5 2 , 1445-1451 Christie, J. H.; Osteryoung, J.; Osteryoung, R. A. AM/. Chem. 1973, 45, 210. Abel, R . H.; Christie, J. H.; Jackson, L. L.; Osteryoung, J. G.; Osteryoung, R. A. Chem. Instrum. 1976, 7 , 123. Blutstein, H.; Bond, A. M. Anal. Chem. 1976, 48, 248. Bond, A. M.; Grabaric, B. S.;Rumble, N. W. J . Electroanal. Chem., 1980, 106, 85. Tamamushi, R. "Kinetic Parameters of Electrode Reactions of Metallic ComDounds": Butterworth: London. 1975. Tanchka, N.; Tamamushi. R. Nectrochim. Acta 1974, 9 , 963. Bond, A. M.; Grabaric, B. S.;Jones, R. D.; Rumble, N. W. J . Electroanal. Chem. 1979, 100, 237. Schwall, R. J.; Bond, A. M.; Smith, D. E. Anal. Chem. 1977, 49 1805. Goledeinowsiki, M.; Lopokowski, J.; Galus, Z . Kinet. Procesow €lektrodwycn Mater. Symp. Sekc. Necktrochem. Pol. tow. chem.. 3rd. 1975(Pub. 1978). 143.
1445
(25) Kisova, L.; Goledeinowski, M.; Lipowski, J. J . Electroanal. Chem. 1979, 95, 29. (26) Cover, R. E.; Connery, J. G. Anal. Chem. 1968, 4 0 , 87. (27) Cover, R . E.; Connery, J. G. Anal. Chem. 1969, 41, 918 1191, 1797. (28) Yamaoka, H. J . Electroanal. Chem. 1972, 3 6 , 457. (29) Bond, A. M.; Kelly, B. W.; Maloney, G . J. Anal. Chlm. Acta 1976, 8 1 , 31, and references cited therein. (30) Drake, K. F.; Van Duyne, R. P.; Bond, A. M. J . Electroanal. Chem. 1978, 89, 231.
RECEIVED for November 7,1979. Accepted April 29,1980. The work described in this paper was financially supported by the Australian Research Grants Committee.
Use of the Oxygen KLL Auger Lines in Identification of Surface Chemical States by Electron Spectroscopy for Chemical Analysis C. D. Wagner" Surfex Company, 29 Starview Drive, Oakland, California 9 4 6 18
D. A. Zatko Department of Chemlstty, University of Alabama, Tuscaloosa, Alabama 35486
R. H . Raymond Shell Development Company, P.O. Box 1380, Houston, Texas 77001
The oxygen KLL Auger group is useful in the identification of surface chemical states by ESCA (Electron Spectroscopy for Chemical Analysis). The vacancies in the final state of the Auger transition are partly in the core-like L, shell and partly In valence levels. The most intense transltlons Involve the valence levels, and so the lntenslty distrlbutlon and the spacing of the Auger group are dependent upon chemlcal Structure. The llne positions of the most Intense Auger llne and the 0 1s photoelectron line can be used In two-dlmendonal chemical state plots. When thls Is done wlth data on some 130 oxygen-containing organic and Inorganic compounds, It Is seen that classes of compounds occupy dlstlnct posltlons on the plots.
Auger transitions with final states involving valence levels can be used to identify chemical states. This is possible because the unique set of molecular orbitals involved produces unique Auger line distributions. T h e Auger spectra of gaseous compounds show especially great variations in spectral patterns (1-5). However, the broadened Auger lines in solid phase also show clear effects of structure on the line distributions, shown first for carbon and fluorine (6) and then for other elements ( 5 , 7-10). Valence-type Auger transitions in general have initial states t h a t are lower in energy than those of core-type transitions. T h e valence-type are often observed in relatively greater sensitivity by electron excitation, because of the inverse relationship of cross section and ionization energy (11). With photon interaction, the reverse is true (12). While electron excitation is more favorable for generation of valence-type Auger lines, radiation damage by the electron beam often makes it difficult t o use this technique for chemical state identification. Nevertheless, with care it has been used in solid 0003-2700/80/0352-1445$01 .OO/O
state successfully with KVV lines of C, N, and 0, and LVV lines of Mg, Al, Si, P, S, and C1. Although intensities of photon-excited Auger lines are less favorable, they may be recorded with relatively little concern about radiation damage. With the oxygen KVV transition, the initial state is unusual in that it has relatively a large amount of energy-more than 500 eV, and the oxygen Auger line is observed with high efficiency. Some observations have been made of Auger spectra in the solid state with metal oxides (13),MgO (14), NazO and NaOH ( 1 5 ) ,and HzO, MeOH, and MezO ( 5 ) . T h e last authors (Rye, Madey, Houston, and Holloway) have pointed out the advantage of examining the photon-excited Auger distribution. I t is dependent upon the molecular orbital pattern, and the Auger spectrum is many times more intense than t h e valence band portion of t h e photoelectron spectrum. I n the course of gathering spectral d a t a on many pure materials, spectra of the oxygen K W Auger group were taken in the expectation that the data would be analytically useful. The following examines the energy relationships of t h e KVV and 0 Is lines, and the line distributions in the KLL group.
EXPERIMENTAL Spectra were taken on a Varian IEE spectrometer, fitted with a high intensity magnesium or aluminum X-ray anode. X-rays were obtained at 10 kV, and 100 mA. Scanning was done in 0.14.5 eV steps by varying the retarding voltage applied t o the sample and sample cage. The analyzing voltage was a constant 100 eV, giving a constant 1-eV instrumental line width. Data were collected for periods ordinarily 1-10 min, at which time the peaks were >10 000 counts above background. Samples were the purest obtainable from commercial suppliers. They were gently ground in an alumina mortar in a nitrogen glove box, dusted onto polymer f i i tape on an aluminum cylinder, and transferred to the instrument without access to air. Spectra of all of the important lines were taken, to determine whether the spectrum was consistent with the structure. On this basis, data C 1980 American Chemical Society
