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Fast Sweep Differential Pulse Voltammetry at a Dropping Mercury Electrode with Computerized Instrumentation A. M. Bond"' and B. S. Grabaric2 Department of Inorganic Chemistry, University of Melbourne, Parkville, Victoria 3052, Australia
When used with computerized instrumentation enabling correction for background current and automation of readout, the technique of differential pulse voltammetry at a dropping mercury electrode combines the features of high sensitivity and reproducibility, rapid data acquisition, low noise level, simplicity of use, and other advantages. Sufficiently fast scan rates to obtain all the required data from a single drop of mercury can be obtained by employing much shorter periods between pulses than used in most previous work. Cadmium is determined down to the lo-' M concentration range by a pulse width of 40 ms and a duration between the commencement of each pulse of 80 ms.
Modern polarographic methods using ac, pulse or other waveforms have decreased t h e limit of detection of this M level for many species. method of analysis to below the Using stripping techniques, which concentrate materials on t h e electrode, levels of detection well below lo-' M are attainable ( I -3). However, operator skill and the length of time required to undertake measurements involving stripping voltammetry present difficulties with this method ( 2 ) . Clearly, if the simplicity of polarographic analysis can be coupled with very rapid data acquisition procedures and provide a sensitivity approaching t h a t of stripping methods, then such a n approach should provide close t o t h e optimum requirements in polarographic analysis in most directions. Differential pulse polarographic methods have been well established as being among t h e most sensitive available ( 4 , 5 ) . However, the necessity of recording only a single data point at each potential from a mercury drop necessarily means that t h e recording of a differential pulse polarogram is a slow procedure. Furthermore, t h e drop growth/drop fall pattern. as well as dc effects associated with this process (5-7), means t h a t t h e signal t o noise ratio and t h e limit of detection are not as likely to be as favorable as with stationary electrodes. T h e recording of dc voltammograms from a fast sweep of the d c potential on a single drop of mercury has long been established as a n attractive alternative t o d c polarography ( 8 ) and the same should be true with differential pulse techniques. However, the waveform associated with this technique requires t h e measurement of current values obtained in the presence and absence of t h e pulse. T h e assumed necessity for having a pulse width in t h e 50-ms region and a substantial duration between application of consecutive pulses means that, in order t o obtain adequate data resolution, a considerable reqtriction on scan rate has been indicated (9, 20). Thus, despite some considerable success with differential pulse voltammetry at a dropping mercury electrode (9),the need for slow scan rates a n d t h e concomitant problems with area growth terms have meant t h a t most work with this technique has used a staPresent address, Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 321 7, Victoria. Australia. *On leave from the Department of Inorganic Chemistry, Faculty of Technology, University of Zagreb, Zagreb, Yugoslavia. 1975 1977. 0003-2700/79/0351-0126$01.00/0
tionary electrode (10-24) and t h e many well recognized advantages of retaining t h e dropping mercury electrode have not been explored extensively. Contrary t o t h e above expectations, close examination of the theory for differential pulse voltammetry ( 1I , 12) and some of t h e d a t a obtained with computerized instrumentation (12-23) actually reveals that the duration between pulses may not need t o be very long t o provide high quality analytical data. Thus, if the duration between pulses can be made equal to, or less than, t h e pulse width, then clearly scan rates could in fact be made sufficiently fast so t h a t area growth at a dropping mercury electrode would not cause problems associated with t h e base line. Furthermore, t h e possibility of highly reproducible, simply obtained data with a very low noise level would be possible and, assuming t h e potential is set at an appropriate value prior to t h e commencement of t h e scan, differential pulse stripping voltammetry a t a dropping mercury electrode would be possible as has already been demonstrated with ac techniques (15). With this approach, data acquisition times a t least 100 times faster than with differential pulse polarography should be possible with an improved signal to noise ratio, higher reproducibility, and a lower limit of detection. Indeed, in almost every direction, optimization of results should be possible compared with polarography. In order to assess the feasibility of achieving all of the above goals, a fully computerized polarographic system was used. This system controlled t h e pre-sweep delay before commencing t h e scan and could be used for averaging of consecutive scans and correcting for background current as well as providing direct printout of peak height a n d position.
EXPERIMENTAL All chemicals used were of analytical grade purity. Solutions were thermostated at 22 & 1 "C and degassed with argon or nitrogen for a minimum of 15 min prior to recording a voltammogram. All voltammograms were obtained using a conventional three-electrode configuration. The reference electrode was Ag/AgCl (IM NaC1) and the auxiliary electrode was platinum wire. The electrochemical instrumentation consisted of a PAR Model 174 Polarographic Analyzer and PAR Model 1'74/61 linear sweep module (Princeton Applied Research Corporation, Princeton, N..J.) modified for differential pulse voltammetry (9) using a digital timer to control the period between pulses (16) and interfaced to a PDP 11/10 minicomputer (Digital Equipment Corporation, Marlhorough, Mass.) used in conjunction with a CAPS-11 operating system. The computer was equipped with a DR-11 general purpose interface which could he used t o provide the logic and buffer register necessary for program controlled parallel transfers of 16-hit data between the PDP-11 system and the Polarographic Analyzer. This interface also included status and control hits that crruld he controlled by either the program or t,he external device for command, monitoring, and interrupt functions. A TA-11 Cassette System Interface was used to load programs and for the input/output of data. Sampling of the current output from the Polarographic Analyzer waq performed using an AR 11 real time analogue subsystem which included a 16-channel,IO-hit AID converter with sample C 1978 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 5 1 , NO. 1, JANUARY 1979
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M Figure 1. Fast sweep differential pulse polarogram of 1 X cadmium in 1 M NaCI. Pre-sweep delay = 0.2 s, pulse width = 40 ms, duration between commencement of each pulse = 80 ms, pulse amplitude = -50 mV, scan rate = 50 mV/s. Initial potential = -0.45 V vs. Ag/AgCI, negative scan direction. (a) Differential pulse voltammogram uncorrected for background current. (b) Calculated background current. (c) As for (a) but corrected for background current
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Figure 2. Same as Figure 1 except that concentration of cadmium is 10.’ M
and hold, a programmable real time clock with one external input, and a display control with two 10-bit D/A converters. Sampled data as well as any transformed data were displayed on either a Tektronix D13 Storage Oscilloscope or on an X-Y recorder. A program operating the Polarographic Analyzer, acquiring data, evaluating data, correcting for background current and displaying data was written in BASIC language using system L assembly language. Some of these subroutines written in P ~ 11 features are described in more detail elsewhere ( 1 7 ) .
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RESULTS AND DISCUSSION Clearly, the major requirement in differential pulse voltammetry is t h a t the pulse be sufficiently wide to allow the charging current to decay to a very small value before sampling the current. We chose a value of not less than 40 ms for the pulse width. Assuming a duration between pulses should be as short as possible, but allowing for the need to measure the dc current just prior to application of the pulse and also with a low charging current, required that an equivalent time should be available between consecutive pulses. Thus a sequence of 40 ms between pulses and a 40-ms pulse width and similar combinations were examined for their analytical usefulness. The requirement of 80 ms for obtaining each data point meant t h a t with a scan rate of 100 m V / s a resolution of close to 8 mV could be obtained. At 50 mV/s, a resolution of about 4 mV was available with the same parameters so the above combination of pulse width and duration between pulses is compatible with a scan rate in the range of 50 to 100 m V / s while still retaining adequate discrimination against charging current and resolution. Parameters of these magnitudes require that 5 to 10 s be available to complete a scan encompassing 250 to 1000 mV so t h a t in our initial studies we chose t o use a capillary with a gravity controlled drop time in excess of 10 s and the pre-sweep delay before commencing the potential scan could be set a t any desired value. T h e above set of parameters is unusual with respect to standard practice in the sense t h a t the pulse duration and duration between pulses are close to 1:l but, as will be seen from subsequent data, this does not present any difficulty in the context of analytical chemistry and the ability to employ fast scan rates via this approach is essential. Figure l a shows a fast sweep differential pulse voltammogram of 1 X lo-’ M cadmium in 1 M KaC1. Figure l b shows the correction for background current including area growth terms and assuming a parabolic shape is valid for the nonfaradaic component. This method of background current correction is the same as t h a t described elsewhere for differential pulse polarography (17). Figure I C is the background
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corrected differential pulse voltammogram. Clearly, a t the M level, very high quality data are obtained even when commencing the scan early in the drop life and having a pulse-width equal to the delay between pulses. At this concentration level, a reproducibility of peak position of k0.5 mV peak height of &0.7% was obtained (10 scans). At lower concentrations, the background current becomes increasingly important and the conditions under which data are obtained are crucial in optimization of results. Figure 2 demonstrates that, if the same paraineters as in Figure 1 are M, then used, but with a cadmium concentration of 1 X the cadmium peak now resides on a severely sloping base line. However, using the computerized instrumentation, the background current can still be adequately corrected for. Under the conditions of Figure 2, a reproducibility of peak position of k2 mV and peak height of & 2 % was obtained (10 scans). Furthermore, the peak height is a linear function of M and the peak concentration over the range to position independent of concentration within the limit of experimental error with these parameters. The residual or background current includes terms which emanate from area growth which occurs during t h e scan of potential. Using a longer pre-sweep delay than in Figure 2 (or a faster scan rate) decreases the magnitude of the problem as shown in Figure 3. However, if the background pre-sweep delay is used as a period of controlled potential electrolysis (anodic stripping voltammetry), substantial enhancement of the faradaic component as well as decreased residual current terms arising from area is observed and the sensitivity is increased substantially as shown in Figure 4. Using a pre-sweep delay of 5 s and a scan rate of 50 m V / s M in 1 M enabled cadmium to be determined down to
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voltammetry a t a dropping mercury electrode would seem to be one of t h e most attractive electroanalytical methods available for routine analysis, particularly when coupled with computerized instrumentation t o correct for background current.
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is positive NaC1. Using a 10-s pre-sweep delay and the anodic stripping approach decreased t h e limit of detection to 2 X 1 0 M. In view of t h e high sensitivity, low noise level, simplicity of operation, and rapid d a t a acquisition available with this approach, t h e technique of fast sweep differential pulse
(1) E. Barendrecht in "Electroanalytical Chemistry", A J. Bard, Ed., Marcel Dekker, New York. 1967. Vol. 2, pp 53-109. (2) T. R. Copeland and R. K . Skogerboe. Anal Chem , 46, 1257A (1974). (3) A . M. Bond. in 'Modern Polarographic Methods in Analytical Chemistry", Marcel Dekker, New York, In press. (4) J. B. Flato, Anal. Chem.. 44(11), 75A (1972). ( 5 ) J G. Osteryoung. J. H. Christie, and R A. Osteryoung. SOC.Chim. Belge, 8 4 , 647 (1975). (6) J H. Christie and R A Osteryoung. J . Electroanal. Chem., 49, 301 (1974). (7) A M Bond and B S . Grabaric. Anal Chim. Acta. 8 8 , 227 (1977). (8) H. Schmidt and M. von Stackelberg, Modern Polarographic Methods", Academic Press, New York/London, 1963. (9) H. Blutstein and A. M. Bond, Anal. Chem.. 48. 248 (1976). (10) K C. Burrows, M. P. Brindle. and M. C. Hughes, Anal. Chem., 49, 1459 (1977) (11) S. C Rifkin and D. H. Evans, Anal. Chem.. 48, 248 (1976). (12) S. C. Rifkin and D. H. Evans, Anal. Chem., 48. 2174 (1976). (13) K . F. Drake, R. P. Van Duyne, and A. M. Bond, J . Electroanal Chem.. 89. 231 (1978). (14) H E. Kelier and R . A. Osteryoung, Anal. Chem., 43. 342 (1971). (15) N . G. Velghe and A. Ciaeys. J . Electroanal. Chem.. 35, 229 (1972) (16) A. M. Bond and R. J. O'Halloran. J . Nectroana/. Chem., 68, 257 (1976). (17) A. M. Bond and B. S. Grabaric. Anal. Chem., submitted for publication.
RECEIL-ED for review .July 12, 1978. Accepted October 10, 1978.
Fingerprinting and Partial Quantification of Complex Hydrocarbon Mixtures by Chemical Ionization Mass Spectrometry L. Wayne Sieck National Bureau of Standards, Washington.
D.C. 20234
A modification of chemical ionization mass spectrometry, which involves photoionization and cyclohexane as the source of the reagent ion, has been used to develop a technique for discriminatory "fingerprinting" of neat liquid fossil fuels. The method provides a 2-min turn-around time between samples and batch introduction, with no requirements for prior separation or fractionation. Depending upon the conditions chosen, the technique may also be extended to the partial quantification of aromatic and olefinic sample components.
other complex hydrocarbon mixtures. Realization of a tractable procedure would provide t h e unique benefits associated with MS instrumentation, including sub-minute analysis times, high sensitivity, a n d very small sample size requirements. This article describes a novel discriminatory technique for screening of batch hydrocarbon mixtures using a modification of CI mass spectrometry, a n d gives some preliminary supportive d a t a for extension of this technique to quantification of aromatic and olefinic components.
T h e recent literature of analytical chemistrv has been characterized by t h e exploitation and refinement of mass spectrometry (MS) as a versatile tool for identification of organic compounds. T h e major emphasis has been in the application areas of chemical ionization (CI), electron impact (EI), and field ionization (FI), particularly involving t h e interfacing of gas chromatographs for pre-separation (CI and E1 only). However, with t h e exception of an extensive feasibility study involving FI investigation of oil sampleq carried out a t the Stanford Research Institute (I),it appears that little. if any, effort has been directed toward the possible utilization of mass spectrometers, operating without a n ) auxiliary equipment (GCs, LCs, etc.), for forensic purposes such as the "fingerprinting" of liquid fossil fuels, industrial solvents, and
EXPERIMENTAI, Mass Spectrometric Instrumentation. The NBS high pressure photoionization mass spectrometer, which is described in detail elsewhere (2), was utilized as a test facility for development and refinement of this technique. Figure 1 shows a schematic of the heart of this unit, which consists basically of a sample introduction system, a photoionization and reaction chamber. an intense vacuum ultraviolet light, source to simulate low energy electron impact, and a quadrupole mass filter and associated E M detection system. The reaction chamber has a volume of approximately 3 cm' and LiF optical material, and may he heated to 200 "C. Since ionization is induced by photoabsorption there is no need for electron entrance and exit apertures in the chamber itself. Therefore, the loss of neutral flow components occurs only through the circular ion exit, pinhole (0.2 mm diameter in the present measurements). The chamber is also operated under field-free conditions; i.e., no repellers or imposed magnetic fields. The light sources are of the microwave-powered
This article not subject to U.S. Copyright. Published 1978 by the American Chemical Society
