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than one faradaic process is ongoing. The insensitivity of the technique to nonfaradaic charge-consuming processes suggests its use in the determination of analytes in matrices such as physiological fluids which normally exhibit high background currents (32). The use of DLSVA and DCVA with multiple wavelength detection permits simultaneous multicomponent analysis with a single potential scan (32). The DCVA technique also holds considerable promise for mechanism diagnosis and kinetic characterization of homogeneous chemical reactions which are coupled to the primary electrode reaction (33). In addition, heterogeneous electron transfer kinetic parameters can also be evaluated with DLSVA and DCVA (33).
ACKNOWLEDGMENT Stimulating discussions with P. F. Seelig and the experimental assistance of E. R. Summers are gratefully acknowledged. This work was supported in part by grants from the University of Delaware Center for Catalytic Science and Technology, the IJniversity of Delaware Institute of Neuroscience (NIH Biomedical VII), and the North Atlantic Treaty Organization.
LITERATURE CITED (1) Kuwana, T.; Dariington, R. K.; Leedy, D. W. Anal. Chem. 1964, 3 6 , 2023. (2) Kuwana, T.; Winograd, N. Electroanal. Chem. 1974, 7. (3) Kuwana, T. Ber. Bunsenges. Phys. Chem. 1973, 77, 858. (4) Kuwana, T.; Heineman, W. R. Acc. Chem. Res. 1978, 9 , 241.
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(5) Heinernan, W.R. Anal. Chem. 1978, 50, 390A. (6) Aibertson, D. E.; Biount, H. N.; Hawkridge, F. M. Anal. Chem. 1979,
51, 556.
(7) Bowden, E. F.; Hawkridge, F. M.; Biount, H. N. Bloelectrochem. Bloenerg. 1980, 7, 447. (8) Bowden, E. F.; Wang, M.; Bailey, J. W.; Hawkridge, F. M.; Biount, H. N. J . Nectrochem. Soc., in press. (9) Winograd, N.; Biount, H. N.; Kuwana, T. J . Phys. Chem. 196S, 73, 3456. (10) Deiahay, P.; Chariot, G.; Laitinen, H. A. Anal. Chem. 1960, 32 (6), 103A. (11) Armstrong, N. R.; Vanderborgh, N. E.; Quinn, R. K. J. Phys. Chem. 1976,80,2740. (12) Randies, J. E. B. Trans. faraday SOC. 1948, 44, 327. (13) Sevcik, A. Collect. Czech. Chem. Commun. 1948, 73, 349. (14) Reinrnuth, W. H. Anal. Chem. lS62, 34, 1446. (15) Oidham, K. B. J . Nectroanal. Chem. 1979, 705,373. (16) Nicholson, R. S.;Shain, I. Anal. Chem. 1964, 3 6 , 706. (17) Bellsteln, E I11 13, 484. (18) Weiiand, H.; Wecker, E. Ber. Dfsch. Chem. Ges. 1910, 4 3 , 699. (19) Evans. J. F.; Blount, H. N. Anal. Lett. 1974, 7, 445. (20) Shang, D. T.; Biount, H. N. J . Electroanal. Chem. 1974, 5 4 , 305. (21) von Benken, W.; Kuwana, T. Anal. Chem. 1970, 4 2 , 1114. (22) Grant, G. C.; Kuwana, T. J . Electroanal. Chem. 1970, 2 4 , 11. (23) Hawkridge, F. M.; Kuwana, T. Anal. Chem. 1973, 4 5 , 1021. (24) Strojek, J. W.; Kuwana, T. J. Nectroanal. Cbem. 1968, 76, 471. (25) Evans, J. F.; Biount, H. N. J . Phys. Chem. 1976,80, 1011. (26) Savitzky, A.; Goiay, M. J. E. Anal. Chem. 1964, 36, 1627. (27) Ito, M.; Kuwana, T. J . Electroanal. Chem. 1971, 32,415. (28) Kuwana, T.; Strojek, J. W. Dlscuss. faraday SOC. 1968, 45, 134. (29) Petek, M.;Neal, T. E.; Murray, R. W. Anal. Chem. 1971, 43, 1069. (30) Osa, T.; Yiidiz, A.; Kuwana, T. J . Am. Chem. SOC. 1969,91, 3994. (31) Matsuda, H.; Ayabe, Y. 2.Nektrochem. 1955,59, 494. (32) Biount, H. N.; Bancroft, E. E., unpublished work. (33) Biount, H. N.; Sidweii, J. S.,unpublished work.
RECEIVED for review February 17,1981. Accepted May 4,1981.
Digital Alternating Current Polarography with Microprocessor-Based Instrumentation J. E. Anderson and A. M. Bond” Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 32 17, Victoria, Australia
The technique of dlgital ac polarography is described. In this technlque, a small amplltude digltal sine wave is applied to the cell instead of an analog sine wave. This slgnal is obtained from a microprocessor-based functlon generator and data acquisition system. Assuming that the 36-step sine wave produces a response simllar to that in conventlonal ac polarography, current data are collected every 10’ relative to the applied slgnal. By simulating the various electronic components found in conventional ac instruments, phase-sensltive detection of fundamental and second harmonics Is also posslble. A qualitative comparlson between the absolute current polarograms and of the phase-sensitive fundamental harmonlcs is in excellent agreement wlth ac polarographic theory for a reversible system.
The instrumentation used to perform ac polarography is somewhat more extensive than that used in other forms of polarography. Even in the wide variety of pulse polarographic techniques which have emerged, the only additional circuitry required (besides a basic potentiostat) is a sample and hold amplifier if a microprocessor-based function generator is used. With the exception of the more recently developed fast Fourier
transform (FFT)techniques ( I , 2) most ac instruments must also be equipped with: (1) a small amplitude ac signal source (and reference signal), (2) a high pass filter or tuned amplifier input, (3) a lock-in amplifier (phase sensitive detector), (4) phase shifting circuitry (if phase-sensitive detection is used), and finally (5) a low pass filter output (3-5). Although more and more instrument companies are providing these features as options to their polarographic instruments, they are often inflexible (with respect to ac amplitude, frequency, and phase selectivity) as well as costly. The use of on-line FFT in ac polarography is quite different from the above approach in that the only analog components which remain are the potentiostat and a low pass filter(s) ( I ) . This technique is quite elegant and a wealth of information may be obtained from a single experiment. However, a t present the necessity of a minicomputer or the equivalent in computing power still restricts the number of users of this technique. We report on here the technique of digital ac polarography which uses a microprocessor data acquisition system to mimic the analog devices mentioned above which are necessary for conventional ac polarography. As in the FFT technique, the only analog component of the instrument is the potentiostat. Unlike the FFT technique, the software required is quite simple and may be easily accomplished in assembler language.
0003-2700/81/0353-1394$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
E
,,me
Flgure 1. Waveform used in digital ac polarography
It is apparent that the components necessary to perform digital ac polarography are the same as are required to perform the pulse techniques. From a commercial viewpoint, the cost and ease of use of this ac technique are essentially the same as for the pulse methods. It should be mentioned that the software simulation of the electronic components in ac polarography has previously been suggested by Smith (4). However, this branch in the evolution of ac polarography was bypassed by this worker in favor of the FFT techniques. EXPERIMENTAL SECTION Instrumentation. The microprocessor-had instrumentation used in this work is identical with that used to perform variable-amplitude normal-pulse polarography (6)except that it was necessary to increase the memory capabilities to 16K of RAM. This system based on a Motorola 6800 D2 microprocessor “kit” has also been described in part elsewhere (7).Two 12-bitdigital to analog converten (DAC) (Analog Devices DAC80-CB1-V) and a 12-bit analog to digital converter (ADC) (Analog Devices AD74JD) were interfaced to the system via three peripheral interface adapters (PIA). One of the DACs was used to generate the staircase ramp potential and the other to generate the digital ac sine wave as described later. The ADC was used to collect current data during the experiment. A homemade potentiostat was used in all experiments (6). Further information on the hardware or software may he obtained by writing to the authors. Data were recorded on a Y-t recorder (Houston Instruments Model zo00) via the DAC after the experiments. A static mercury drop electrode (SMDE) Model 303 from E.G.+G. Princeton Applied Research Corp., Princeton, NJ, was used in all experiments. A platinum auxiliary electrode and a Ag/AgCI (saturated KCI) reference electrode were wed in conjunction with the SMDE. Reagents. Analytical reagent grade chemicals were used throughout 88 was distilled water. All solutions were degassed 4 min with high-purity nitrogen (two distilled water bubblers) prior to undertaking polarographic experiments. All polarograms were recorded at ambient temperatures of 21 (*I) OC. RESULTS AND DISCUSSION Figure 1illustrates the digital sine wave superimposed on to a staircase ramp potential waveform. The two waveforms were summed via the summing amplifier type controller of the potentiostat. In addition, the output of the sine wave generated was scaled down from 8.19 V p-p to 0.0819 V p-p by the summing amplifier to reduce significant losses in resolution. Additional scaling of the sine wave may be achieved by software division by two (81.90,40.95,20.48,10.24, 5.12 mV). Alternatively, intermediate amplitudes of sine waves could he obtained by adjusting the gain of the DAC output amplifier. Each cycle of the digital sine wave was made up of 36 potential steps. The sine wave was obtained by stepping through a look-up table containing the appropriate codes for the DAC. This method was found to be faster than calculating the appropriate code prior to each step. This procedure could be carried out as many times as desired to produce multiple
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cycles. Each potential step in the sine wave consisted of: (1) a software delay, (2) sampling of the resulting current, and (3) a software delay (same as 1). The frequency of the resulting sine wave could be.changed by varying the software delay times. Since the current was sampled once per step, 36 current measurements were made per cycle. Assuming that this waveform adequately approaches a true sine wave, the collection of data may be considered to be in the frequency domain rather than in the time domain. This means the current is measured once every 10’ with respect to the applied potential (360°/36 steps). Therefore in the following discussion the sampling of the current will he considered to be a t a particular phase angle rather than a t a particular position or step on the sine wave. However, from an instrumental point of view, time domain measurements are being made which have parallek with those in dc and pulse techniques. In subsequent discussion this mode of measurement is referred to as total alternating current polarography. As is apparent from Figure 1, more than one cycle was applied at each ramp potential. In the work presented here a total of 18 cycles were applied on each ramp step. This was done for the twofold purpose of allowing the ac current to reach equilibrium after the initial application of the digital sine wave and for signal averaging purposes. It was assumed that transients would have vanished and that equilibration was achieved after two cycles of the sine wave and data collected during these cycles was discarded. The data from the remaining 16 cycles (576 points) were averaged as described below to eliminate the dc current and to improve the signal to noise ratio. Since the dc component of the signal is a function of time ( t P with the SMDE) (8, 9 ) , it is difficult to obtain a mean dc current to subtract from the signal, particularly a t low frequencies. T o obtain the mean dc current, we averaged the data from one cycle (36 points) and subtracted from each of the individual points making up that particular cycle. This procedure was carried out on each of the 16 cycles of data. The data from the 16 cycles were then averaged to obtain a representative value (36 data points) at the given applied potential ramp step. This approach to eliminating the de component assumes that the dc component is constant during the time of one cycle and will obviously be most effective a t high frequencies. However, this filtering technique appeared to he quite effective in the frequency range studied (5-102 Hz). In analogy to conventional ac polarographic instrumentation, this procedure represents the high pass filter or tuned amprier at the input. The data manipulation described above is summarized in the flow diagram in Figure 2. Three facton determine the minimum drop time which may be achieved by using this procedure: (1) time before the sinusoidal signal is applied (determined by the staircase ramp generator), (2) time required to apply the sine waves, and (3) time required for data manipulation. By use of a 0.5 s delay before the sine wave was applied, a minimum drop time of -1 s (total) could be used at 102 Hz. At 5 Hz the minimum drop time was -3.5 8. These drop times are determined primarily by the time required to apply 18 cycles. Further experiments indicate that the number of cycles averaged could be reduced to eight or four without seriously lowering the signal to noise ratio. This approach could be useful in decreasing the total time required to perform an experiment. Indeed, this reduction in cycles may be neceSSary if this digital ac technique is used to study electrode kinetics. Since the surface concentration in other than reversible systems may still he time dependent, it may be necessary to know the time of measurement more accurately (rather than an average time).
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 FACTOR AC SIGNAL FULL SCALE ( 8 2 . 5 mV) OR D I V I S I O N S BY 2
I I
I
APPLY S I N E WAVE AND COLLECT DATA (648 P O I N T S )
Flgure 3. Digital total current ac pohrograms at 0 to 360"; 5 X lo4 M Cd(I1) in 1.0 M KCI. Digital sine wave frequency of 83.3 Hz with an amplitude of 6.3 mV peak-to-peak. Dc potential scan from -0.45 to -0.85 V vs. Ag/AgCi with 2 mV ramp steps. Electrode area was 0.0156 om2. The sold horizontal line is the point of zero current. Also
F I R S T TWO CYCLES
shown is the expanded ac polarogram at 90' relative to the applied Dotential.
R-4 ALL CYCLES SUMMED?
NUMBER OF CYCLES
I
(161
( 3 6 POINTS) FROM T H I S POTENTIAL
STORE CYCLE
1
.(r.> ALL S T E P S DONE?
i ves
I
H A L T
I
Flgure 2. Flow diagram of software used in digital ac polarography.
As mentioned above, the maximum frequency studied was 102 Hz. This maximum frequency is a limitation of the microprocessor and data acquisition system used. In this case, the Motorola 6800 based system used has a 614.4-kHz clock. More recent experiments indicate that the use of a 1-mHz clock in the same system increases the maximum frequency to 194 Hz. Although this frequency is still quite low compared to the capabilities of analog ac polarographic instrumentation, the work presented here was to test the basic concept of digital ac polarography. Analytical work in fact is usually confined to frequencies less than 100 Hz. Higher frequencies could easily be obtained by using a faster processor with a more extensive instruction set and faster data acquisition (ADC, etc.). The use of direct memory access or even decreasing the number of steps per sine wave are but two other possibilities. Therefore, the limited frequency range of the particular system described in this work should not be considered anywhere near the upper limit of this technique. Once a particular experiment has been completed according to the potential region scanned, the data may be displayed in a number of ways via the DAC and Y-t (X-Y) recorder. Figure 3 shows 37 polarograms which are obtained from a single scan from 0.5 to -0.8 V vs. SCE with 2 mV ramp steps of 5 X M Cd(I1) in 1.0 M KC1. In this display method, the phase angle was set to 0" (step 1 on the sine wave) and the current data from 0" from each ramp step is successively output via the DAC. Once the data from the entire scan has been output, the phase angle is incremented by 10" to display the next polarogram. The complete curve in Figure 3 consists
-
of 7200 points. It is also apparent that in this digital approach the reference ac signal and the applied signal are one in the same. Also shown in Figure 3 is an expanded polarogram at 90'. In this case, the phase angle was specified and only one polarogram output (at a slower speed). Two important aspects of Figure 3 are that: (1)the amplitude of the charging current is at a maximum at -0" and -180" (zero at -90" and -270", note solid line at zero current), (2) the amplitude of the faradaic current is a t a maximum a t -45" and -225" (zero a t -135" and -315"). These observations are consistent with polarographic theory (for a reversible system) ( 4 , 5). It is possible some phase shift may exist in our data from iR drop since a positive feedback circuit to eliminate uncompensated resistance was not used. The use of the difference in phase of the capacitance and faradaic current is the basis of phase-sensitive detection as a means of minimizing the capacitance current from faradaic measurements (3). The above technique of ac polarography measures what we refer to as total alternating current in a slightly different mode to that normally associated with ac polarography. An alternate method of displaying the data involves the use of software to mimic the last two components of a conventional phase-selective ac polarograph: a lock-in amplifier and finally a low pass filter. Up to this point the dc component and some unwanted frequencies have been minimized by averaging (high pass filter). Lock-in amplifiers provide a very effective means of frequency discrimination in measurement systems and are presently used in most commercial ac polarographs. The lock-in amplifiers operation was simulated by summing the first 18 points with the 2 s complement of the last 18 points of a given cycle and dividing by 36. This operation is more closely related to synchronous rectification of the signal followed by application of a low pass filter. It may also be considered as multiplying the data of a cycle by a square wave of the same frequency with an amplitude of kl.0 followed by averaging of the resulting 36 points. In this case the square wave is the reference signal. The net result of this operation enables the phase-selectiveac polarograms to be produced with an improved signal to noise ratio. As in the previous output method, the phase relationship between the signal and reference may be changed via software (phase shifter) and phase-sensitive detection achieved. Figure 4 shows the fundamental harmonic polarograms a t phase angles of 0" to 180" using data shown in Figure 3. Again as expected, the capacitance current is now essentially zero at -0" and -180" and at a maximum amplitude at -90' and -270". A comparison of Figures 3 and 4 indicates the expected decrease in noise obtained by using the lock-in am-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
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0'
'/;:0 0
Figure 4. phase selective fundamental harmonic digital ac pohrograms at 0 to 180' using simulated lock-in amplifier. Same conditions as in Figure 3. Also shown is the fundamental harmonic at '0 expanded along the potential axis.
1100
Figure 5. Phase selective second harmonic digital ac polarograms at 0 to 180'. Same conditions (data)as in Figure 3 and 4. In expanded form is the second harmonic response at 110'. plifier approach. The Faradaic current now has its maximum amplitude a t -135' and -315'. This technique is referred to as phase-selective ac polarography, because of its close relationship to the conventional analog instrumentation. In essence, the data are shifted -90' with respect to the total ac current version and the response is slightly decreased in amplitude. Carrying the "lock-in amplifier" approach one step further, one may obtain higher harmonics of the ac polarogram by multiplying the signal by a reference square wave an integer multiple of the signal frequency. Since the digital sine wave used consisted of 36 steps, harmonics higher than the third may not be readily obtained. However, harmonics higher than the second are seldom of any practical use. The nonlinearity of the faradaic response which gives rise to higher harmonics is not shared by the capacitance component so the second harmonic is often of interest because of the near absence of this capacitance component ( 3 ) . Figure 5 shows the second harmonic response of the data (in Figures 3 and 4) at various phase angles. It is clear that the capacitance current is absent in the second harmonic a t all phase angles and that the appropriate wave shape is obtained for phase-sensitive second harmonic ac polarography. When the third harmonic was observed, it was apparent that the wave shape was not as predicted (derivative of the second harmonic). Due to the smaller amplitude of the third harmonic, the output had to be expanded to the point where the resolution of the ADC and DAC distorted the data. Therefore, the third harmonic is not shown here. It should be noted that up to this point no mention has been made concerning the assumption that the digital sine wave used adequately simulates an analog sine wave. Qualitatively, the results suggest that the digital sine wave gives the predicted results for the fundamental and second harmonics based on a theoretical model assuming a pure sine wave. I t is undoubtedly at higher harmonics that the deviation caused by the use of a step function will first appear. Apparently resolution based upon 36 steps is inadequate for accurate resolution of the third harmonic. Quantitative comparison between the digital ac method and ac theory for the fundamental harmonic indicates excellent
SO
100
W"
I _ _
110
ZOO
210
Flgure 6. Plot of peak current (I,) of total ac current (90') and phase-selective fundamental harmonic (0') pohrograms vs. the square root of the angular velocity (u"*):total current, 0 ;phase-selective fundamental harmonic, 0; 5 X lo4 M Cd(I1) in 1.0 M KCI. Sine wave amplitude was 6.3 mV peak to peak. agreement. Figure 6 shows a plot of peak ac current vs. the square root of the angular velocity for both methods of measurement with the Cd(I1) system. As expected for a reversible system (in this frequency range) a straight line is obtained with a zero intercept (3). Cd(I1) reduction is known to show this behavior in conventional ac polarography in this frequency range. Finally, an additional check for reversibility is a plot of E vs. log [(Zp/Z)1/2 f ( ( I , - Z)/Z)1/2] which should be linear, where Zp is the peak current (3). This procedure is based on the fact that for a small applied ac potential, the shape of the ac response appears as the first derivative of the dc wave. These plots of the absolute current a t 90' and the in-phase first harmonic of digital ac data yield slopes of 61 and 59 mV, respectively, compared to the theoretical value of 118.2/n mV (3). These features, coupled with the observation of the expected phase-angle response are indicative of a close approximation to results obtained with a conventional analog ac polarograph.
CONCLUSIONS The technique of digital ac polarography appears to yield results in excellent qualitative agreement with its theoretical analog counterpart. The microprocessor-based instrumentation required is the same as that for the various pulse polarographic techniques. Therefore, the software simulation of the components of ac instrumentation reduces the hardware requirements necessary for ac polarography. I t is also apparent that much more information is collected in a single digital ac experiment than is possible using analog instrumentation. For example, once an experiment has been performed either the total current or fundamental ac polarograms may be examined. The phase angle may then be easily adjusted to minimize the charging current. This technique is somewhat more closely related to FFT ac polarography in that the only analog circuitry needed is the potentiostat. Although the F F T approach has many advantages, it is at present out of the range of most users' financial resources and therefore it is only used in a very limited number of laboratories. The digital ac polarographic technique described appears to be a viable intermediate between analog and FFT ac polarography primarily becuase of its simplicity and low cost. ACKNOWLEDGMENT The authors thank Fred L. Walter and Howard B. Greenhill for their work on the hardware and helpful discussion. LITERATURE CITED (1) Smith, D. E. Ana/. Chem. 1978, 48, 221A-240A. (2) Smith, D. E. Compuf. Chem. Instrum. 1972, 2, 369-422. (3) Smith, D. E. In "Electroanalytical Chemistry": Bard, A. J., Ed.; Marcel Dekker, New York, 1966; Vol. I, Chapter 1. (4) Smith, D. E., CRC Crit. Rev. Anal. Chem. 1971, 2, 247-343.
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(5) Bond, A. M. "Modern Polarographic Methods in Analytical Chemistry"; Marcel Dekker: New York and Basel, 1980; Chapter 7. (6) Anderson, J. E.; Bond, A. M. Anal. Chern. 1981, 53, 504-508. (7) Anderson, J. E.; Bagchi, R. N.; Bond, A. M.; Greenhill, H. B.; Henderson, T. L.; Waiter, F. L. Am. Lab. (Fa/rf/eM, conn.) w81, 13(2), 21-32. (8) Bond, A. M.; Jones, R. D. Anal. Chlm. Acta 1880, 121, 1-11. (9) Anderson, J. E.; Bond, A. M.; Jones, R. D. Anal. Chem. isel, 53, 1016-1020.
(IO) Bond, A. M.; O'Halloran, R. J.; Ruzic, I.; Smith, D. E. J . Ebctroanal. Chem. 1978, 90, 381-388.
RECEIVED for review December 29, 1980. Accepted April 20, 1981. The financial assistance of the Australian Research Grants Committee in support of this work is gratefully acknowledged.
Multielement Analysis of Human Blood Serum by Neutron Activation and Controlled Potential Electrolysis Kari Jerrstad," Brit Salbu, and Alexis C. Pappas Department of Chemistry, University of
Oslo, Oslo, Norway
The combination of neutron activation analysis and controlled potential electrolysis has proved itself to be a useful muitielement method for the determination of 14 elements in a 500-pL sample of human blood serum. Afler freezedrying and irradiation, the samples are decomposed and electrolyzed for 3 h. The radioactive species deposited on the mercury cathode allow simultaneous determination of eight elements (Ag, Au, Cd, Co, Fe, Hg, Sb, Zn). Another six elements (Br, Ca, Cs, Na, Rb, Sc) are determined by measuring the activities in the residual solution. Data illustrating the precision of the method are given. The determination limits are of the order 10-'-1O3 pg/L for the nine trace elements investigated.
Table I. Elemental Content of Seronorm, Batch No. 139 this work 3.64 i 0.07 0.12 0.02 1.48 + 0.04 108 3 9.2 r 0 . 3 1 . 3 9 i 0.03 2.68 + 0.09 1.54 0.05 2 . 6 1 2 0.08 3.06 0.03 350 t 7 0.85 t 0 . 0 2 0.053 0 . 0 0 3 803 i 9
* * * *
*
According to the World Health Organization (I) 14 trace elements, Le., Co, Cr, Cu, F, Fe, I, Mn, Mo, Ni, Se, Si, Sn, V, and Zn, are considered to be essential to the human organism. Several other elements (e.g., Br, Ba, Sr) are supposed to be added to the list as more knowledge about their metabolic functions is obtained. The concentrations of the trace elements in body tissues and fluids are frequently in the parts-per-billion range or lower. The metabolism of a healthy individual maintains the concentrations of trace elements within narrow limits. In addition, the concentration ranges between essential and toxic levels for different chemical forms of elements are rather narrow. Moreover, species of several elements (e.g., As, Cd, Hg, Pb) are assumed to be hazardous even in very low concentrations. Thus, investigation of trace elements in human tissues and body fluids has become a field of great emphasis in recent years, and attempts have been made to relate changes in concentration of specific elements to various diseases. In some cases such relationships are firmly established (2-6). Human blood serum supplies all parts of the body with necessary components, and it can be obtained easily and frequently in routine procedures. Thus, analysis of human blood serum is very useful in studies of trace elements in man (5-8). However, the number of useful methods for multielement analysis of human blood serum is rather limited, due to the low concentrations involved and to interfering effects of the matrix. Application of instrumental neutron activation analysis (INAA) to whole blood and serum has recently been reported (8-12). After irradiation with thermal neutrons, whole blood and blood serum show a y-spectrum in which the predominant 0003-2700/81/0353-1398$01.25/0
nuclide 15.0 h "Na prevents determination of other elements. The background activity is also increased by the presence of 35.4 h 8%r and 14.3 days 32P(bremsstrahlung). Most applications are therefore hampered by these effects and the resulting necessity of singling out groups of elements. This requires many samples, a variety of irradiation times (10 s-10 days) and decay times (10 s-6 months), or introduction of radiochemical separation procedures. In this way only, a number of elements can be determined by neutron activation analysis; however, it is expensive and time-consuming. The elimination of Na, Br, and P without affecting the concentration of activation products of other elements of interest would not only permit simultaneous determination of a greater number of trace elements but also reduce the irradiation time and avoid the long decay times mentioned above. The present paper describes a technique based on a combination of neutron activation and controlled potential electrolysis for the simultaneous determination of eight elements (Ag, Au, Cd, Co, Fe, Hg, Sb, Zn) in human blood serum 3 days after irradiation. During the electrolysis, reducible species in solution will be deposited on the cathode, while the interfering elements Na, Br, and P will remain in solution. In addition, another six elements (Br, Ca, Cs, Na, Rb, Sc) may be determined in the residual solution of the sample by means of their activation products, 3-4 weeks later. METHOD OF ANALYSIS Reference Standard. Seronorm (batch no. 139,standard reference material from NYCO, Oslo ( 1 3 ) )with certified values for Fe, Zn, Ca, and Na was used as a standard. On the basis of 0 1981 American Chemical Society
