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Anal. Chem. 1983,55, 1934-1939
Microprocessor-Controlled Instrument for the Simultaneous Generation of Square Wave, Alternating Current, Direct Current, and Pulse Polarograms J. E.Anderson’ and A. M. Bond* Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds, Victoria 321 7, Australia
single mercury drops (9, 11-13, 19, 22). This approach, however, can be applied to almost all modern polarographic techniques (21) including differential pulse polarography (23, 24). The main disadvantage reported is the complex and more expensive instrumentation necessary to perform SWV (9). This is probably true with respect to analog instrumentation requiring sophisticated timing and gating circuitry, but with the advent of computer-based systems where essentially the only difference between differential pulse polarography and square wave polarography is the software, this hardly can be considered as a disadvantage (3,11). Certainly, hardware cost is not a discrimination factor as instrumentation employing microprocessor-based systems will be less expensive than its analog counterpart. In microprocessor-based square voltammetry more than one cycle of the square wave can be advantageously applied on each ramp step for signal averaging purposes and the square waves need not be applied during the entire ramp step. Since The electrochemical technique of square wave polarography, this is obviously a very similar experiment to AC polarography SWP, was first reported by Barker and co-workers (1,2) a t in which a sine wave is superimposed onto a potential ramp, a dropping mercury electrode, DME. The technique was it is not surprising that AC polarograms should be extractable reviewed in 1975 by Sturrock and Carter (3) who noted that from the square wave experiment. Indeed, examination of applications had been limited by the lack of good commercial theory demonstrates that all the transient techniques are instrumentation a t reasonable cost. However, even by 1975, closely related and can be interconverted by mathematical developments in theory and instrumentation in a limited transformations (25-27), so it may be considered to be a number of laboratories, coupled with detailed data presenting conservative approach to regard techniques of square wave, analytical applications, indicated that square wave polarogalternating current, pulse polarography, etc. as inherently raphy should be an electroanalytical method of great advandifferent from each other. tage. Ramaley and Krause presented the theory for reversible From the practical viewpoint the transformations can be systems (4) and examined the analytical usefulness of square undertaken if the instrumentation has the capability of colwave voltammetry, SWV, at a hanging mercury drop electrode lecting sufficient data over a wide enough time (or frequency) (HMDE) (5). The technique was then reexamined by Christie domain from a single experiment. In the present work we have et al. who provided a more comprehensive theory for reversible chosen to demonstrate that application of a series of square systems including its application a t the DME (6, 7). The only waves on to the potential ramp can provide data leading to distinction made between SWP and SWV above is that the generation of not only square wave polarograms but also, via former is restricted to use of the DME, the latter being the very simple software manipulation, the DC, AC, and pulse more general term. polarograms. An approach of this kind makes it practical for In 1979, Boudrea and Perone stated that SWV may replace the analytical chemist to conveniently decide on the relative conventional differential pulse polarography, DPP, for merits of a particular technique since a single experiment has quantitative determinations of electroactive species in solution (8). Recent editorial comment in the journal, ANALYTICAL only to be performed to generate the required data. The same approach obviously offers equal advantage in studies of CHEMISTRY, focused attention on this prospect (9). These electrode mechanisms. The fast fourier transform of Smith editorial remarks now have catalyzed new activity in SWV (26) could obviously be used to achieve this objective, as could as in the last year or so a great resurge in interest has emerged. the Hadamard function applied by de Levie (27). Our apThus, new theory, applications, instrumental designs, and the proach is mathematically simpler than either of these techappearance of modern commercially available instrumentation niques and ideally suited to microprocessor-based instruhave been forthcoming in a very short period of time as perusal mentation not having access to “number crunching” capaof ref 10 to 20 will indicate. bilities. The sensitivity of both square wave and differential pulse With the instrumentation described in this work, experivoltammetric techniques is excellent with detection limits in ments may be performed as in polarography at a DME when the to M concentration range (1-21). The main a new drop is grown at each ramp potential or as in fast scan advantage claimed for SWV over DPP appears to be the voltammetry when the staircase ramp steps are very short and possibility of using high scan rates which leads to a reduction the experiment is performed on a single drop at a DME or in analysis time by allowing experiments to be performed on a t a stationary electrode. The data collected during an experiment may be treated as a “Barker” type SWP experiment Present address: Department of Chemistry, Murray State or as an AC experiment. Additionally, the ability to perform University, Murray, KY 42071. With a square wave superlmposed onto a staircase potential ramp the technlque of square wave polarography (voltammetry) is shown to be capable of produclng data whlch enables square wave, DC, AC, pulse, and other polarograms (voltammograms) to be obtained from a single experlment. The method of data acqulsitlon and large number of methods of data presentation possible hlghllght the close slmllarltles of many polarographlc techniques rather than dlfferences more normally discussed in the literature. The ablllty to obtain the response for a range of polarographlc technlques from a single experlment with mlcroprocessor-basedInstrumentation leads to a hlghly efflclent approach to analytlcal and klnetlc lnvestigatlons employing polarographlc and voltammetrlc methods.
0003-2700/83/0355-1934$01.50/00 1983 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
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Figure 1. Comparison of wave forms superimposed onto DC ramp
voltage in square wave and sinusoidal AC polarography: square wave (-) sine wave (-). DC polarography and DPlP in the same experiment also exists. These forms of polarography originate from data collected prior to application of the square wave and from data obtained during the first half of the square wave minus data obtained prior to application of the square wave, respectively. Our instrumental approach is most closely related to that of Buchanan and Sheleski (12). These authors also applied a multiplicity of square waves to each step of the staircase ramp with a microcomputer-controlled square wave polarographic instrument but used the data to only generate square wave polarograms. EXPERIMENTAL SECTION Instrumentation. The microprocessor-based wave form generator and data acquisitiion system used in this work have been described in part elsewhere (30, 31) as has the homemade potentiostat (31). The software used to generate the square waves was essentially identical with that described previously for generation of digital AC sine waves (32,33)with the exception that a square wave “look-up” table was used instead of a sine wave “look-up” table. Software to generate AC polarograms from square-wave data is described in ref 33 and based on simulation of a lock-in-amplifier. The experiments were performed with w static mercury drop electrode (SMDE) from EG&G Princeton Applied Research. The SMDE was used with a Pt auxiliary electrode and a Ag/AgCl (saturated KCl) reference electrode. The theoretical work was performed on a DEC System 20 computer with a program written in Fortran. This theory is the square-wave version of that described iin detail for digital AC polarography (33). Reagents and Procedures. Analytical reagent grade chemicals were used throughout. All solutions were degassed with high purity nitrogen prior to the experiments. Experiments were performed at 21 A 1 “C.
RESULTS AND DISCUSSION (a) Techniques Based on Square Wave Polarography. In digital AC polarography (32, 33) a digital sine wave is superimposed onla a staircase ramp potential rather than the analog sine wave used in conventional AC polarography. A “look-up” table wm used to generate the digital sine wave. The sine wave consists of 36 potential steps and the current was measured during each step. Under these circumstances, current measurements can be considered to be made once every 10” with respect to the applied sine wave. Upon completion of a given experiment, any of the 35 total current polarograms generated in the experiment may be examined after the direct current has been filtered out digitally. Alternatively, with software to simulate a lock-in amplifier, the phase sensitive fundamental, second harmonic, etc. AC POlarograms may be obtained. The digital AC technique therefore has many of the characteristics of fast Fourier transform or Hadamard based AC polarography (25, 26,28, 29). The wave form emplo,yed in this study is a square wave. Figure 1 shows the close relationship between square wave and sinusoidal Ab=polarography. The Fourier transform of a square wave indicates that a signal E = E d , + IPE (sin w t + y3 sin w t I- ‘/5 sin w t ...) (1) is being applied instead of E = E d , + A E sin w t (2)
Figure 2. Square wave polarograms at 35 points in time with software M Cd(I1) in 1 correction of DC component for reduction of 5 X M KCI: ten cycles of ,-6.24 mV peak to peak amplitude (40 Hz) averaged; staircase ramp = -5 mV per step; potential range = -0.4 V to -0.8 V vs. SCE; electrode area = 0.0156 cm2;drop time = 0.5 s; temperature = 22 O C .
In eq 1and 2 hE is amplitude (half peak to peak), w is the angular frequency, t is time, E is potential, and E d c is DC potential. Although a “look-up” table is not required to generate a trivial wave form such as a square wave, the approach is advantageous in generating timing sequences and measuring phase relationships. Furthermore, it means that software for square wave and digital AC polarography differs only by the nature of the “look-up” table. With a “look-up” table consisting of 36 data points per square wave, current data can be conveniently taken 36 times per cycle on the square wave or 18 times (equally spaced) o n the positive half and 18 times on the negative half of each cycle. The square wave is superimposed onto a staircase ramp and each mercury drop from the SMDE is grown at the next ramp potential. A time delay occurs before the applicatioln of the square wave. This time delay is usually substantiallly longer than the period during which the square waves are applied and essentially determines the drop time. As in digital AC polarography (32,33)current data from multiple cycles were averaged to ohtain 35 current measurements representative of the square wave at a given DC potential. Unless otherwise stated data from the first 1.5 cycles (54 points) were discarded prior to the averaging process to allow for the decay of any transient signals. A high pass filter was also simulated via the software associated with the data treatment process, in that prior to averaging the cycles, the data from each cycle were averaged to obtain an estimate of the direct current flowing during each cycle. This direct current can then be subtracted from each current measurement in that particular cycle to eliminate the DC component. If the DC correction is undertaken as above, the resulting polarograms are closely related to those obtained in differential pulse polarography. In DDP, the DC term is measured prior to the pulse and then subtracted from a pulse superimposed onto the ramp. That is, the experiment uses data from the first half of the square wave. Figure 2 shows 35 differentid pulse type polarograms which may be generated in a single experiment from the square wave experiment and by using the procedure for correcting for the direct current described above. Each of these differential pulse type polarograms is generated from data taken at successive times in the squaipe wave cycle. Note however that in this case the average DC component is actually obtained at the same time and potential as the pulse data.
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Figure 3. Alternate alternating current display method of the data in Figure 2. Curves (a) are phase sensitive fundamental harmonic polarograms from 0 ' to 190' using the simulated lock-In-amplifler. Curves (b) and (c) are, respectively, the third and second harmonic polarograms of the same data starting at ' 0 relative to the applled square wave.
The current-time profile of the peaks in Figure 2 is characteric of a chronoamperometric t-1/2response. In the particular case of Figure 2 a square wave of frequency 40 Hz was used, so the data were taken at 694-ysintervals, and chronoamperometric data could be displayed in other formats. The data could also have been displayed in the Barker ( I , 2) format as the difference in current sampled during the positive and negative halves of the square wave. Since the Barker square wave technique is so well-known, examples of this readout format will await a later part of the manuscript on SWV. Similarly the DC polarograms could have been displayed because the average DC current has been evaluated by software. Alternatively, the software used could be modified to include a measurement of the dc current prior to the application of the square wave. By use of the nomenclature common to AC polarography, the curves in Figure 2 represent total current AC polarograms at 35 phase angles loo apart. However, the phase angles are not exactly identical with those in AC polarography when a digital sine wave is used since the current at exactly Oo and 180' is not represented. That is, since current measurements cannot be made on the edges of the square wave, an offset is introduced. For convenience of presentation and describing the relationshiR with AC techniques the first data point taken after the negative edge of the square wave will be considered to be taken at Oo or in phase with the applied signal. This is allowed for in computations on the theory of digital AC technique which predict that the exact phase angle of measurement depends on the time a t which the current was measured during each step (33). The close similarity of square wave, differential pulse and AC techniques is revealed by the above discussion. An alternate method of displaying the data via the aid of the simulated lock-in-amplifier produces phase-selective fundamental and higher harmonic polarograms. This procedure has been described with a hardware lock-in-amplifier previously (34). To achieve conversion of data from the time to frequency domain, the data from each cycle are multiplied by a reference square wave with an amplitude of *l. The resulting 36 points are then averaged to provide the phasesensitive current at the given potential. The complete phase-sensitive AC polarogram may then be generated, by combining data from all the DC potentials used. By change of the phase angle of the reference with respect to the original applied potential, phase sensitive detection is possible. By change of the reference frequency to integer multiples of the
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Flgure 4. Comparison of different harmonics for square wave polarograms displayed in alternating current format. Curve (a) is the fundamental harmonlc at,'O curve (b) is the third harmonic at Oo, and curve (c) is the second harmonic at 80' after multiplication by 2. Experimental parameters are given in Figure 2 except potential range was -0.4 to -1.0 V vs. SCE.
applied square wave frequency, the first, second, and third harmonics may be obtained. Figure 3 shows the first, second and third harmonic AC polarograms obtained from the data in Figure 2. The phase-sensitive harmonic polarograms from 0 to 190' are shown at IOo intervals. As expected for a square wave, the third harmonics are substantially larger than the second harmonic. This is explicible from the Fourier series of a square wave given in eq 1. The shape of the peak height vs. phase angle curve is different from that obtained with digital sinusoidal AC polarography. However, when the theory for digital sinusoidal ac polarography (reversible) (33) was modified to comply with an applied square wave, the shape of the theoretical peak height w. phase plot was in qualitative agreement with the experimental data. Individual fundamental, second and third harmonic polarograms at a given phase angle may also be displayed as shown in Figure 4. These polarograms and many others which could have been displayed in the same expanded scale format are closely related to the kind of display that would be obtained and used by the analytical chemist using conventional AC instrumentation. The difference is that many different experiments and probably different instruments would be required with the analog approach. The substantial degree of digital filtering, discrimination against charging current via a variety of approaches, and ability to simultaneously generate data in different time and/or frequency domains from a single experiment leads to high quality data, great flexibility, the ability to characterize the electrode processes from a kinetic viewpoint, and, thereby, the use of kinetic information to detect possible interferences in analytical applications of polarography. The charging current discrimination can be obtained by using data a t the end of the square wave, phase-angle detection with the lock-in-amplifier and by use of second-order techniques such as the second harmonic response, etc. all from the one experiment. From the analytical
ANALYTICAL CHEMISTRY, VOL. 55, NO. 12, OCTOBER 1983
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E (VI Figure 5. Efficiency of the software high pass filter for DC component correction. Experimental conditions are given in Figure 4. Curves (a) are the total current square 'wave polarograms in the presence and absence (base line) of the square wave. Curves (b) are the equivalent fundamental harmonic (0') alternating current polarograms. point of view the multitechnique approach appears to be optimal in most directions. The efficiency of the riimulated high pass filter used to measure and correct for the DC component in the differential pulse and AC type polarograms was examined by performing the square-wave based expieriments described above both with and without the square wave potential superimposed on the ramp potential. When the experiment is performed without an applied square wave, the polarogram obtained should be identical with the base line if the DC or ramp currents are effectively filtered out. It was found by inspection of the resulting polarograms that more effective DC filtering could be obtained by averaging the first 31 data points per cycle rather than 36. This is apparently due to the nature of decay of the DC component (approximately t-lIz) as a function of time and necessitates weighting of currents toward those obtained early in the square wave cycle. Figure 5 shows the total current, and fundamental and higher harmonic AC polarograms obtained at Oo for the reduction of cadmium with and without the square wave applied. It is apparent that ad this drop time (0.5 s) the DC current is effectively removed. To test the effectiveness of this filtering method under conditions where the DC) component may be of more substantial amplitude, the square wave was superimposed onto the pulse used in normal pulse polarography instead of onto the staircase ramp used in DC polarography. This wave form naturally led to generation of a wide range of polarographic techniques related to differential normal pulse voltammetry and normal pulse AC polarography etc. (21 26,3639). Figure 6 shows examples of phase selective fundamental harmonic polarograms at 0" with aind without the square wave superimposed onto the pulse. In both cases two cycles of the square wave were averaged for noise reduction. In Figure 6a the square wave was applied 100 ms after application of the pulse, while in Figure 6b it was applied 40 ms after the pulse. It is apparent that the combination of the high pass filter and simulated lock-in amplifier are remarkably effective in eliminating the DC component in square wave normal pulse polarography particularly when applied 100 ms after the pulse. It is obvious that the techniques described above represent only a fraction of the polarographic responses possible from
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Figure 6. Test of efficiency of the digital high pass filter under normal pulse ramp conditions. Experimental parameters are the same as those given in Figure 4 except for use of pulsed rather than staircase ramp: curve a pulse wa!3 appiled 100 ms and curve b applied 40 ms before the square wavo. Base line represents data obtained for square-wave of zero amplitude. a single experiment. Current measurements undertaken just prior to the application of the square wave (ramp current,) provide the user witlh the DC polarographic data directly rather than by the computation described above. Chronoamperometric data are automatically incorporated into this experiment as well as in the framework of data obtained after addition of a square wave. If one or more data points frorn the first half of the firwt cycle of the square wave are retained, the conventional differential pulse polarogram(s) can be obitained by subtraction of the DC component obtained prim to the first half cycle of the square wave. Clearly, conventional square wave polarograms could be obtained by subtraction of data points from the first half of each square wave frorn data at equivalent points of time on the second half. Finally, the three-dimensional pseudoderivative DC data (35) could have been collected prior to the square wave data or from the time domain response from the square wave. This threedimensional approach could also be used if the square wave had been superimposed onto the normal pulse wave form to produce the equivalent three-dimensional normal pulse polarograms (35). It is clear that an overwhelming amount of data or numerous polarographic techniques can be incorporated into a single experiment. It, is our goal in the future, using microcomputer instrumentation, to provide the analyst with these data in a useful format. (b)Techniques Based on Square Wave Voltammetry. The main advantage of SWV as promoted in the literature is the ability to perform the experiment on a single drop. The SMDE is ideally suited for this purpose since effects of drop growth during the scan, which arise when a dropping mercury electrode is used (3),are absent. The square wave techniques described in detail in the section on polarography can be easily performed on a single drop by decreasing the ramp step time, thereby enabling faster scan rates to be used without loss of resolution. In addition, the number of cycles of the square wave that are averaged may be reduced. In the results shown here only two cycles of the square were averaged instead of 16 as in the square wave polarography described above. Square voltammograms may be obtained by subtracting any of the 18 points from the first half of the first (or any other) cycle from any of the' 18 points from the second half of the same cycle. For simplicity the only data presented in this report are obtained for the case in which the last point from
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Figure 7. Comparison of S W V data by using high pass filter for correction of DC ramp (experiments a, b, and c) and conventional square wave voltammetry as developed by Barker (experiments d and cadmium in 1 M KCI. e). System studied is for reduction of 5 X In both classes of experiment the scans were performed on a single Hg drop of area = 0.0156 cm2. Experimental parameters and their description are described in detail in the text. the first half of the cycle is subtracted from the last point on the second half of the cycle. However, note that while the square wave is always symmetrical, the subtraction process need not be. That is any of the datum taken on the second half of the cycle could be subtracted from any of the datum on the first half. To realistically compare square wave voltammetry described in this work with conventional SWV as performed by Christie et al. (6, 7), the amplitude of pulses and the potential of the ramp steps need to be set equal in the two methods. In addition, the times of the ramp and pulse steps were set equal to those in the conventional SWV method, and the sum of these steps was set equal to the cycle time in the square wave technique we have developed. The results of two conventional SWV experiments were averaged to achieve equivalent signal to noise ratios, as obtained by averaging two cycles per ramp step as in our experimental approach. The curves in Figure 7a-c show the ways in which the data obtained from square wave voltammetry with DC component correction may be displayed, while those in Figure 7d,e arise from conventional square wave voltammetry. The conditions for the Figure 7a-c sequence were a 25-mV peak-to-peak square wave (25 Hz) superimposed on a staircase ramp (-5 mV per ramp step). The square wave was applied immediately after the ramp step and the first 1.5 cycles were discarded as described previously with the averaging of the remaining two cycles. The conventional SWV data are the average of two scans in which the ramp step size was -5 mV, the square-wave ampltidue was -25 mV peak-to-peak, and the DC ramp and positive and negative edges of the square wave were 20 ms in width. In both sets of experiments the potential was scanned from -0.4 V to -0.6 V vs. Ag/AgCl and the twoelectron reduction of cadmium ions to cadmium amalgam in 1 M KC1 is considered. Figure 7a contains the two square wave components which are obtained without prior elimination of the DC or ramp component (i.e., without software high pass filter). Figure 7b contains the two components corrected for the DC component which are subsequently subtracted from each other to yield the curve in Figure 7c. Figure 7d contains the two components which are subtracted to obtain curve e, the curve usually presented in SWV. The final responses from the two approaches to SWV (curves 7c and 7e) are similar for the reversible reduction of cadmium as would be expected. The differences in curves a and d arise from the differences in the DC time domain from which the data are obtained. The experiment time for each ramp affects the concentrations at the electrode surface and needs to be considered, particularly for electrode process other than reversible. However, in essence, the major difference of analytical significance between the conventional method and
ours is that averaging to improve the signal to noise ratio is undertaken in the same experiment rather than from consecutive experiments. With respect to actual data acquisition time required to perform the experiments described in Figure 7, the technique utilizing DC correction requires approximately 1 s longer than the two conventional SWV experiments. It needs to be noted that when using a "look-up" table to generate the square-wave, 17 symmetrical difference curves beside that displayed in Figure 7e could have been presented in addition to the asymmetrical difference curves. Obviously from the discussion in the polarography section, the alternating current voltammograms could have been generated and displayed via the lock-in-amplifier approach as could have staircase voltammograms and chronoamperometric curves, etc. as described in the polarographic section.
CONCLUSIONS The alternate method of performing square wave voltammetry described in this paper illustrates the ability of computer-based instrumentation to effectively perform several polarographic (or voltammetric) experiments simultaneously. The need to consider DC, AC, pulse, and other techniques as individual experiments requiring separate instrumentation may not be required in the future. Work in these laboratories is now aimed at exploiting the substantial scope for improving the efficiency of polarographic methods of analysis when using microprocessor-based instrumentation. Studies of electrode kinetics (mechanisms) should also be facilitated by this approach. This aspect will also form part of our future research objectives. Registry No. Mercury, 7439-97-6. LITERATURE CITED Barker, G. C.; Jenkins, 1. L. Anak.9 (London) 1952, 77, 685-696, and references cited thereln. Barker, G. C. Anal. Chlm. Acta 1958, 18, 118-131, and references cited therein. Sturrock, P. E.; Carter, R. J. CRC Crit. Rev. Anal. Chem. 1975, 5 , 201-223. Ramaiey, L.; Krause, M. S.Anal. Chem. 1989, 4 1 , 1362-1365. Ramaley, L.; Krause, M. S.Anal. Chem. 1989, 4 1 , 1365-1369. Christie, J. H.; Turner, J. A.; Osteryoung, R. A. Anal. Chem. 1977, 47, 1899-1903. Turner, J. A.; Chrlstie, J. H.; Vukovic, M.; Osteryoung, R. A. Anal. Chem. 1977, 4 9 , 1904-1908. Boudrea, P. A.; Perone, S.P. Anal. Chem. 1979, 5 1 , 811-817. Anal. Chem. 1980, 5 2 , 229A-230A. Yarnitzky, C.; Osteryoung, R . A.; Osteryoung, J. Anal. Chem. 1980, 5 2 , 1174-1178. Buchanan, E. G., Jr.; Sheleski, W. J. Talanta 1980, 2 7 , 955-961. Feder, A. L.; Yarnitzky, C.; O'Dea, J. J.; Osteryoung, J. Anal. Chem. 1981, 5 3 , 1383-1386. O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1981, 5 3 , 695-701. Qiang-skeng, F.; Sin-rhu, L. Anal. Chem. 1981, 5 3 , 1006-1011. Sin-rhu, L.; Qiang-Sheng, F. Anal. Chem. 1982, 5 2 , 1362-1367. Borus-BoszormOnyl, N.; Schoket, B. Nahrung , 1979, 2 3 , 537-547. PorubskB, M. Tenslde Deferg. 1978, 15, 241-243. Malihski, T.; ZagBrski, 2. P. H S I , Hung. Sci. Insfrum. 1980, 47, 13-17. Stojek, 2.; Osteryoung, J. Anal. Chem. 1981, 5 3 , 847-851. He, P.; Avery, J. P.; Fauikner, L. R. Anal. Chem. 1982, 5 4 , 1313A1326A. Bond, A. M. "Modern Polarographlc Methods in Analytical Chemistry"; Marcel Dekker: New York, 1980. Christie, J. H.; Osteryoung, R. A. J . flectroanal. Chem. 1974, 4 9 , 391-31 1. Blutstein, H.; Bond, A. M. Anal. Chem. 1976, 48, 248-252. Bond, A. M.;Grabaric, B. S. Anal. Chem. 1979, 5 1 , 337-341. Smith, D. E. I n "Eiectroanalytlcal Chemistry"; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol. 1, Chapter 1. Smith, D. E. I n "Fourier, Hadamard and Hilbert Transforms in Chemistry"; Marshall, A. G., Ed.; Plenum: New York, 1982; pp 453-525, and references cited therein. Ruzic, I . J . Electroanal. Chem. 1972, 39, 111-122, Seelig, P. F.; de Levie, R. Anal. Chem. 1980, 5 2 , 1506-1511. de Levie, R. Anal. Chem. 1980, 5 2 , 1535-1537. Anderson, J. E.; Bond, A. M. Anal. Chem. 1981, 5 3 , 504-508. Anderson, J. E.; Bagchi, R. N.; Bond, A. M.; Greenhill, H. B.; Henderson, T. L. E.; Walter, F. L. A m . Lab. (Fairfield, Conn.) 1981, 13 (2), 21-32. Anderson, J. E.; Bond, A. M. Anal. Chem. 1981, 5 3 , 1394-1398. Anderson, J. E.; Bond, A. M. Anal. Chem. l982, 5 4 , 1575-1576.
Anal. Chem. 1983, 55,1939-1942 (34) Bond, A. M.; Flego, U. S.Anal. Chern. 1975,4 7 , 2321-2324. (35) Anderson, J. E.; Bond, A. M., J . Elecfroanal. Chem. 1983, 145, 21-34. (36) Bond, A. M.; Grabarlc, B. S. J . Necfroanal. Chem. 1978, 8 7 , 251-260. (37) Hayes, J. W.; Smith, D. E. J . Elecfroanal. Chem. 198% 114 I 283-292. (36) Hayes, J. W.; Smith, D. E. J . E/ectroana/. Chern. 1980, 1 1 4 , 293-297.
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RECEIVED for review December 28, 1982. Accepted May 2, 1983. The financial contribution of the Australian Research Grants S&eme in support ofthe work described in this Paper is gratefully acknowledged.
On-Line Supercritical Fluid Chromatography/Fourier Transform Infrared Spectrometry Kenneth H. Shafer* Battelle, 505 King Avenue, Columbus, Ohio 43201
Peter R. Griffiths Department of Chemistry, University of California, Riverside, California 92521
The interface between a wpercriticai fluid chromatograph and a Fourier transform infrared spectrometer (SFC/FTIR) has been demonstrated for the first time. Separations are performed on a wide-bore fused silica capillary column. An ultraviolet detector, FTIR flow cell, and flame ionlzation detector are integrated in series, demonstrating the use of both GC and HPLC detection with SFC and Identlfication by on-line FTIR spectrometry. The I R transparency of CO,, Just above the critical pressure makes it a nearly ideal solvent, butt some absorptlon bands intensify dramatlcaliy as the pressure Is Increased. Even so, a I-cm pathlength can be employed for supercritical COP,which i ! ~ 1 to 2 orders of magnitude greater than Is commonly used in flow cell HPLC/FTIR. Resuits show that easlly identifiable spectra may be obtalned for 3 p$ each of anisole, acetophenone, and nitrobenzene Injected Into the chromatograph.
A supercritical fluid is a substance raised above its critical temperature and pressure, such that it takes on solubility properties common to liquids. In supercritical fluid chromatography (SFC), separations are performed by using a supercritical fluid as the mobile phase with either u highperformance liquid Chromatography (HPLC) packed column (I) or a gas chromatography (GC) wall-coated open-tubular (WCOT) capillary (2) column. The diffusion coefficient and viscosity of supercritical fluids are intermediate between those of a gas and a liquid, which allows the use of HPLC or GC columns. The selectivity of SFC can resemble that of gas or liquid chromatography depending on the volatility and solubility of the compounds to be separated and the operational temperature and pressure of the supercritical fluid. A recent overview of the methods and principles of supercritical fluid chromatography has been reported by Peaden and Ltee ( 3 ) . SFC may be used to separate high molecular weight, multifunctional compounds which neither GC nor HPLC can separate as well as many compounds separable by GC and/or HPLC. In particular since many nonvolatile or thermally labile compounds can be !separated by SFC, many separations previously effected by HPLC should be able to be accomplished more rapidly by SFC using the same columns. Several
detectors commonly used for either GC or HPLC can be used equally well for SFC, but a need for rapidly identifying each eluting peak still exists. In view of the success of FTIR spectrometry for the on-line identification of GC and HPLlC peaks, it can be expected that it may also serve the samle purpose for SFC. This paper describes the first results from an SFC/FTIR interface. We initially investigated the use of a flow cell for SFC/ FTIR, even though for HPLC/FTIR flow cells sacrifice sensitivity and spectral information because of limitations imposed by interfering solvent absorption bands (4-6). Few infrared spectra of supercritical fluids have been reported in the past, so that the IR transparency of mobile phases used in SFC/FTIR is not well-known. If a fluid having favorable infrared transparenc:y can be selected, SFC/FTIR measurlementa should have greater sensitivity and spectral information than the corresponding HPLC/FTIR measurement. Furthermore, unlike HPLC, the selectivity of SFC can be varied by pressure programming; at higher pressures the density of the supercritical fluid, and therefore the solubility of samples in the mobile phase, it3 increased. Thus pressure programmirrg in SFC can have the same effect as temperature programming in GC or gradient elut,ion in HPLC. A single supercritical fluid may often be used throughout the analysis, thereby reducing, but not entirely eliminating, the constraints imposed for spectral subtraction of the mobile phase in gradient elution HPLC/FTIR measurements using a flow cell interface. It will be seen later that the absorptivities of bands in the spectrum of supercritical COz iincrease with pressure, with some barid intensities being more sensitive to pressure than others. Thus even though the problems of solvent compensation which are encountered in gradient elution HPLC/FTIR are not as severe for SFC/FTIR, a single reference spectrum cannot be used throughout the analysis. It may also be noted that SFC/M[S is being developed as an alternative to HPLC/MS to minimize solvent interferences of the type found in the latter technique (7).No doubt the complementary nature of MS and FTIR which leads to improved identification capability of GC peaks (8-10) will likewise lbe beneficial to SFC. In this paper we have described the first practical demonstration of SFC/FTIR measurements. The performance of a wide-bore fused sillica capillary column with supercritical
0003-27C10/83/0355-1939$01.50/00 1983 Amerlcan Chemlcal Society
