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Anal. Ghem. 1082,54. 1231-1233
Flgure 2. Pulse count Auger spectra for Na,SO, as a function of 5 keV primary electron beam current: (A) 10 nA, (B) 200 nA.
regulation of source current. It was quickly decided that regulation should be achieved via manipulation of filament heating current. The alternative of control by variation of extraction voltage was rejected because of concern about possible effects of changes in intragun potentials upon beam focus. A feedback parameter for monitoring of filament temperature, total emission, or actual beam current remained to be selected. Direct measurement and control of filament temperature was strongly considered due to prior experience of one of the authors with this technique in field-desorption sources for mass spectrometry (6). Rejection of this method was based primarily upon the extent of modification to existing filament heating circuitry that it would have required. Monitoring of actual beam current was likewise rejected as an unfeasible technique. Measurement of the beam current would have required electrically floating the high voltage acceleration power supply in the gun control chassis. Achieving isolation of this complex, power-consuming circuitry adequate to permit the accurate measurement of beam currents as low as I1 nA seemed unacceptably difficult. Total emission, as approximated by the current drawn by the emission control cup, seemed the most readily available estimator of beam current and was, therefore, the feedback parameter chosen for regulator control. Isolation of the cup
for the purpose of current measurement was already available to that instrument modification in the area of source optics was unnecessary. The fact that cup current was about 30 times greater than beam current promised to simplify needed measurement circuitry (currents on the order of tens to thousands of nanoamperes rather than picoamperes were involved) and to reduce sensitivity to a host of such error sources as cable leakage. The utility of the beam current regulator for pulse-counting AES is demonstrated by the results for Na2S03 The low beam currents available with the current regulator permitted acquisition of the Na2S03spectrum (Figure 2A). Higher beam currents precluded pulse-counting experiments as the result of severe electrical charging effects (Figure 2B) which were also encountered using beam currents (1MA)sufficient for direct derivative mode data acquisition. The lowest electron beam doses required for pulse-counting AES (2 X C/cm2) did not produce any detectable beam damage to the Na2S03 surface. However, higher electron beam doses (>4 X C/cm2) resulted in the desorption of volatile surface hydrocarbons and increasing surface sulfate formation as observed in correlative XPS measurements (7). In summary, the primary electron beam current regulator permits pulse count Auger spectra to be acquired using beam currents down to the low nanoampere range, thus minimizing the electron beam dose and current density required for electron-excited AES. Improved beam stability also should enhance the precision of integral methods for Auger quantitation at low beam currents or conventional derivative methods when relatively high beam currents can be used.
LITERATURE CITED Paimberg, P. W. J. Vac. Sci. Technol. 1075, 12, 379-384. Pantono, C. G.; Madey, T. E. Appl. Surf. Sci. 1081, 7 , 115-141. Madden, H. ti. J. Vac. Sci. Technol. 1081, 18, 677-689. Sickafus, E. N. Surf. Sci. 1080, 100, 529-540. Turner, N. H.; Murday, J. S.; Ramaker, D. E. Anal. Chern. 1080, 5 2 , 84-92. (6) Fraley, D. F.; Woodward, W. S.; Bursey, M. M. Anal. Chern. 1080, 52, 2290-2293. (7) Griffis, D. P.; Llnton, R. W., unpublished results, University of North Carolina-Chapel tiiii, 1961.
RECEIVED for review November 18,1981. Accepted February 18,1982. The authors wish to thank the Hercules Chemical Co. (Wilmington, DE) for a grant-in-aid in support of this research. The XPS/AES facility was funded in part by the National Science Foundation (Chemical Instrumentation) and the North Carolina Board of Science and Technology.
Amperometric Titrations with Hydrodynamic Modulation for End Point Detection Joseph Wang” and Bassam A. Frelha Department of Chwnistry, New Mexico State University, Las Cruces, New Mexico 88003
One of the most extensive applications of electrochemistry has been for end point detection in titrations. Amperometry is particularly useful as a method for detection in titrations of species at the milli- and submillimolar concentration levels. In its most common form amperometric titration consists of adjusting the potential of a microelectrode, usually the dropping mercury electrode, to be in the limiting current region for the species being monitored (analyte, product, or titrant). Sophisticated and sensitive electroanalytical techniques, such as ac polarography (I), differential pulse polarography (21, or anodic stripping voltammetry (3), have been incorporated with mercury electrodes to lower the detectability 0003-2700/82/0354-123 1$01.25/0
of amperometric titrations to the submicromolar concentration level. However, as applied to analyses of many important oxidizable substances, which cannot be determined a t the mercury electrode (due to its limited anodic potential range), the titrations are performed with solid electrodes (usually platinum) operated in the dc mode, resulting with detectability of only around the 1.O4 M concentration level (4). The purpose of the following work is to examine the feasibility of combining amperometric titrations with solid electrode end point detection based on hydrodynamic modulation voltammetry (HMV). HMV is proving to be a versatile technique for trace analysis at solid electrodes (5, 6). The 0 1982 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
major advantage of this technique is that the measured current amplitude due to pulsation in the convection rate is free from most background interferences. As applied to amperometric titrations of substances at micro- and submicromolar concentration levels, the changes in concentration due to each addition of the titrant are at the nanomolar concentration level; only hydrodynamic modulation measurements provide the sensitivity to monitor such changes and isolate them from the drifted (transient) background current of solid electrodes (at most solid electrodes the background current drifts for a long time after applying the desired potential (6)). Hydrodynamic modulation measurements may be easily incorporated with amperometric titrations; at each titrant addition a pulsed-convection modulation is applied, and the measured current amplitude is displayed against the amount of titrant added. Such method is simple and amenable to automation. Besides providing lower detection limits than other amperometric titrations at solid electrodes, the method can broaden the scope of HMV for measurements of nonelectroactive substances, in cases where the titrant or one of the titration products is electroactive. It can also improve the selectivity of HMV in cases where two substances are having similar redox potentials, and one of them is titrated selectively. Finally, amperometric titrations with modulated convection at solid electrodes with wide potential range (e.g., carbon) can be applied for both reduction and oxidation end point reactions, and thus avoiding the hazard and inconvenience of the dropping mercury electrode. The titration chosen to evaluate the method was iodine titration of ascorbic acid 12
+ CdHeO*(OH)C=COH
+
21-
+ C4H604C(=O)C=O
(1) This titration was chosen because of the sensitivity and selectivity problems associated with the determination of ascorbic acid a t solid electrodes (due to its irreversible oxidation and other species with similar redox potentials, respectively). The hydrodynamic modulation approaches used to evaluate the method were stopped-rotation and stoppedstirring amperometry.
EXPERIMENTAL SECTION Apparatus and Reagents. All measurements were conducted with a Model 364 polarographic analyzer (Princeton Applied Research) and a Houston Omniscribe strip-chart recorder. A three-electrode cell configuration was employed, using a 0.75-cm diameter glassy carbon disk working electrode (Model DDI, Pine Instruments Co.), a Ag/AgCl reference electrode, and a graphite rod counterelectrode. The disk electrode was attached to a rotating disk assembly (Model PIR, Pine Instruments Co.) used in the stopped-rotation experiments. For stopped-stirring experiments (using the same electode system) a 3.5 cm long stirring bar was placed in the center of the cell bottom, and a magnetic stirrer (Sargent-Welch) was employed, as was described elsewhere (7). Chemicals and reagents used have been described previously (7) except as noted. A 1.00 X 10" M stock solution of iodine was prepared daily by weighing out 25.4 mg of iodine and transfering it to a 100-mL volumetric flask containing 40 mg of potassium iodide dissolved in 25 mL of water. The solution was stirred with an ultrasonic stirrer until all the iodide was dissolved and then was diluted to volume. High potency vitamin and iron tablets were obtained from commercial sources. Procedure. A 200-mL aliquot of the buffer was deaerated for 8 min. During deaeration the working electrode was pretreated (without rotation or stirring) by cycling the applied potential between +0.9 V and -0.9 V, allowing 2 min at each potential. Following this, the working potential (+0.35 V) was applied, and transient currents were allowed to decay for about 3 min (+0.35 V is on the plateau region for ascorbic acid (8), while the oxidation of iodide, one of the titration products (eq l), starts at potentials more positive than +0.4 V). The blank stopped-convection current amplitude was recorded by switching the rotation or the stirring
0
0.2
0.4
I,
0.6
0.8
1.0
A D D E D ,PM
Figure 1. Typical HMV titration of 0.74 pM ascorbic acid with iodine; stopped rotatlon end point detection, 1600 rprn (on) for 15 s, 0 rprn (off) for 30 s.
on and off for 15 s (on) and 30-60 s (off). An aliquot of ascorbic acid was then added to the supporting electrolyte solution and the solution was deaerated for 30 s. The pulsed-convection current amplitude of ascorbic acid was recorded, before adding aliquots of the iodine. Following each addition of the titrant, nitrogen was bubbled and the solution was stirred for 30 s to ensure adequate mixing and deaeration; the current amplitude was measured as before. When the end point was reached (i.e., no further decrease in the current amplitude) the process was repeated an additional three to five times. The procedure for measuring vitamin C in commercial tablets is described in the following section.
RESULTS AND DISCUSSION Figure 1 shows an example of an amperometric titration with stopped-rotation end point detection of ascorbic acid at the submicromolar (7.4 X lo-' M) concentration level. It shows the actual current amplitudes-time traces of the titration, along with the resulting titration curve. The stopped-rotation current amplitudes are similar in shape to those reported earlier (8)) with response times of 3 s ("on") and 25 s ("off') (as determined by the rapid and slow adjustments, respectively, of the concentration profile to step changes in rotation rate). The titration curve consists of two straight (linear) line portions, as expected. No distinct curvature is observed even at this low concentration level (in direct current amperometric detection some rounding usually occurs especially with dilute solutions (9)). Each titrant addition accompanies an approximatley 100 nM decrease in the ascorbic acid concentration; such small changes are easily observed using the stopped-rotation monitoring. A detection limit around 20 nM ascorbic acid is expected, based on the signal-to-noise characteristics of the system. Although these titrations were performed by using 200 mL, it is possible to obtain a similar response a t the rotating disk electrode with only 10 mL. If this were the case, the limit of detection corresponds to about 34 ng of ascorbic acid. Such low detectability has not been reported yet for amperometric titrations at solid electrodes and is attributed to the ability of HMV procedures to isolate the convective-dependent response (which is only due to ascorbic acid in our example). The end point location in Figure 1corresponds to 6.9 X lo-' M ascorbic acid or to a titration error of -6.7 % . Such accuracy is relatively good when considering the very low concentration of the analyte. The deviation from the true value is attributed mainly to the instability (i.e., oxidation) of submicromolar ascorbic acid solutions (IO). The slow surface transient currents which require a long waiting period before performing direct current measurements of low analyte concentrations (under conditions of continuous rotation) are not affecting the pulsed-rotation
ANALYTICAL CHEMISTRY, VOL. 54, NO. 7, JUNE 1982
current amplitude (which can be measured accurately on top of this drifted background), resulting with reduced error and time. One of the major problems of voltammetric analysis at solid electrodes is the resolution of overlapping waves. This problem limits the applicability of voltammetric analysis when measuring multicomponent real samples. For example, a major problem in analyses of brain tissues is the presence of species, such as ascorbic acid and dopamine, with similar oxidation potentials. A direct determination of these species utilizing conventional HMV or other voltammetric techniques would be impossible. Ry combining a selective titration of one of these species with a sensitive hydrodynamic modulation procedure both r~peciescan be measured at the micromolar concentration level. In order to demonstrate such measurement, 5 pM ascorbic acid was titrated with iodine in the presence of 5 pM dopamine (conditions: as in Figure 1,except that the rotation “off“ time was 45 8). At the working potential employed (+0.35 V) both ascorbic acid and dopamine yielded a limiting current stopped-rotation response. The ascorbic acid was titrated, selectively, by the iodine, with a decreasing modulated response (part of which is a stable dopamine response) that reflects the titration progress. The resulting titration curve (not shown) had two linear lines with easily located end point at 5.4 pM. A stable current amplitude was obtained beyondl the end point (in constrast to the zero amplitude obtained1 in Figure 1)due to the dopamine left; the dopamine was quantified by using a standard addition procedure (the size of the amplitude beyond the end point is similar to that obtained for dopamine before adding ascorbic acid). The applicability of this approach to other pairs of species with similar redox potentials is obvious. In a mixture containing more than two components, having similar redox potentials, only the one being titrated would be measured. The utility of the method in the analysis of real samples was illustrated by the determination of vitamin C in high potency iron and vitamin tablets from commercial sources. The tablet was ground and 1% (in weight) of its powder was dissolved directly in the phosphate buffer present in the cell (after recording the blank modulated response). The resulting curve obtained for titration of this sample with iodine (not shown) was similar in shape to the one shown in Figure 1 (same conditionn, except that the “off“ time was 45 s). The end point was located at a value of 19.8 pM which corresponds to 390 pmol, or ’77.2 mg of vitamin C (as sodium ascorbate) per tablet. This value is in good agreement with a value of 75 mg specified Iby the manufacturer. The zero current amplitudes obtained beyond the end point indicate that none of the tablet components yields a detectable stopped-rotation response at the working potential employed. A series of seven titrations of 7.4 pM ascorbic acid solutions was used to estimiate the precision and accuracy of the method (stopped-rotation end point detection, 1600 rpm (on)). The mean concentration found was 7.1 pM with a range of 6.8-7.6 pM. The relative standard deviation over the complete series was 4%, and the mean titration error was -5.2%. Similar precision and accuracy have been reported for amperometric titrations of coppier employing differential pulse polarography at the dropping mercury electrode (2). The titration error is attributed, mainly, to the instability of ascorbic acid (as was
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discussed earlier), rather than to the inherent accuracy of the end point detection method itself. Slightly different slopes were observed for the linear line portion before the end point, even in cases where the same end point was located; this may be attributed to the aging effect of the carbon electrode (11) during the week period over which this series of titrations was performed. For more simple tiitrations with HMV end point detection and for laboratories where a rotating disk assembly is not available, the method can be performed employing stoppedstirring amperometry rather than the stopped-rotation end point detection. Stopped-stirring voltammetry is a new HMV approach, designed mainly to simplify the achievement of pulsation in the convective transport (7); this procedure can be easily incorporated for end point location purposes in amperometric titrations. A typical titration of 23.5 pM ascorbic acid employing stopped-stirring amperometry yielded a plot (not shown) with two linear regions, as in Figure 1 (conditions: stirring on (about 520 rpm) for 15 s, stirring off for 60 s). The end point was located at 22.2 pM ascorbic acid, which corresponds to a titration error of -5.5%. The noise level (when the stirring was on) was larger than that observed with stopped-rotation detection (Figure 1);this level decreased as the titration progressed (from 130 to 20 nA for 0 to 20 pM added iodine, respectively). This decreased noise may be due to small fluctuationci in the stirring speed which give rise to fluctuations in the concentration-dependent convective current. A detection limit around 0.7 pM ascorbic acid was calculated based on the signal-to-noise characteristics of the stopped-stirring data. In all of the titrations described above there was no need for blank titration (on the supporting electrolyte solution) because of the zero modulated response obtained for that solution. The titration time can be shortened by employing pulsed-convection approaches (e.g., pulsed-rotation mode) instead of the stopped-convection modes (used in the present study) which are characterized by their longer cycle; this will result with a slight loss in sensitivity (i.e., less than 100% modulation) and will require cumbersome instrumentation. The technique can be easily applied to many other chemical species. Future studies will include titrations of nonelectroactive species based on monitoring the HMV response of the titrant or one of‘the titration products.
LITERATURE CITED Tanaka, N.; Ogino, H. J . Electroanal. Chem. 1964, 7 , 332. Myers, D. J.; Bsteryoung, J. Anal. Chem. 1974, 4 6 , 356. Skobets, E. M.: Skobets, V. D.;Poplavskaya, N. A. Vkr. Khlm. Z h . 1971, 3 7 , 204. Laltlnen, H. A.; Burden, L. W. Anal. Chem. 1951, 23, 1268. Miller, B.; Bruckensteln, S. Anal. Chem. 1974, 4 6 , 2026. Wang, J. Talnnta 1981, 28, 369. Wang, J. Anal. Chim. Acta 1981, 129, 253. Wang, J. Anal. Chem. 1981, 5 3 , 1528. Christian F. D. “Analytical Chemistry”, 3rd ed.; Wlley: New York, 1980: p 357. Strohl, A. N.; Curran, D.J. Anal. Chem. 1979, 51, 353. Blaedel, W. J.; Wang, J. Anal. Chem. 1980, 52,76.
RECEIVED for review December 14,1981. Accepted March 15, 1982. This work was supported by the Society for Analytical Chemists of Pittsburgh and by the Research Center of the College of Arts and Sciences, New Mexico State University.
