752
Anal. Chem. 1985, 57,752-758
for cupric ions and the complexing agents EDTA, citrate, D(+)-tartrate, and gluconate are presented (CcU:CL= 1:lO). The formation of dimeric species is accounted for (the formation constants used in the calculations are given in Table IV). The figure shows the ability of gluconate to mask copper(I1) efficiently in alkaline solutions, whereas citrate is a poorer masking agent. At neutral p H values citrate forms stronger complexes with copper(I1) due to its higher negative charge. EDTA is able to mask copper more efficiently than the mentioned hydroxycarboxylate ligands in neutral and weakly alkaline solutions. A comparison of the Cu-EDTA and Cugluconate log a curves for the two total concentrations shows the greater dilution effect occurring in systems containing dimeric species. Registry No. Cu2+,15158-11-9;EDTA, 60-00-4;tartaric acid, 87-69-4; citric acid, 77-92-9; gluconic acid, 526-95-4.
LITERATURE CITED (1) Sawyer, D. T. Chem. Rev. 1964, 6 4 , 833. (2) Stili, E. R.; Wikberg, P. Inorg. Chim. Acta 1980, 4 6 , 147. (3) Grenthe, I.; Wikberg, P.; Still, E. R. Inorg. Chim. Acta 1984, 91, 25. (4) Biomqvist, K.; Still, E. R. Inorg. Chem., in press.
(5) SiiiBn, L. G.; Martell, A. E. "Stability Constants of Metal Ion Complexes"; The Chemical Society: London, 1964 and 1972; Special Publications Nos. 17 and 25. (8) Bodini, M. E.; Wlliis, L. A.; Riechei, T. L.; Sawyer, D. T. Inorg. Chem. 1978, 15, 1538. (7) Coccioii, F.; Vicedominl, M. J . Inorg. Nucl. Chem. 1978, 4 0 , 2103, 2108. (8) Still, E. R. Anal. Chim. Acta 1980, 116, 77. (9) Ringbom, A. "Complexation in Analytical Chemistry"; Wiiey-Interscience, New York, 1963. (10) Baes, C. F., Jr.; Mesmer, R. E. "The Hydrolysis of Cations"; Wiley-Interscience: New York, 1976.
RECEIVED for review September 10,1984. Accepted November 27,1984.This work is part of a program financially supported by the Academy of Finland.
High-Sensitivity Spectroelectrochemistry Based on Electrochemical Modulation of an Absorbing Analyte Chwu-Ching Jan, Barry K. Lavine,l and Richard L. McCreery* Department of Chemistry, The Ohio State University, 140 West 18th Avenue, Columbus, Ohio 43210
When a laser beam passes parallel to an electrode surface, the ilght diffracted by the electrode samples the solution within less than 10 pm of the surface. I f an absorber Is generated electrochemlcaliy, the diffracted light is attenuated rapidly ( E
-40 a 3 c
v
-
a -60 J
4 -20
0
IO
20
30
40 50 60 70 80 M o d u l a t i o n F r e q u e n c y (Hz)
90
100
Figure 7. Effect of frequency on LIA output for a phase shift of 0' (curve a) and phase shlfts equal to the optimum for each frequency (curve b), diffraction angle 2.6'. Because of this convenient elimination of background, the phase angles for zero background were used to construct calibration curves. The LIA response was linear with conto 1 X centration over a t least the range from 3 X M for TAA. Slopes of log/log plots of response vs. concentration are of 1.011 (r = 0.9997), 0.996 (r = 0.9996), and 1.007 (r = 0.9990) at 5, 10, and 20 Hz, respectively, for phase angles of -15', -20°, and -40'. The detection limit for the conditions of Figure 4 was approximately 1 X lo-' M, but this value could be improved by changing experimental conditions. Since the background signal could be effectively eliminated by proper selection of phase, the detection limit was determined by noise rather than background. The diffracted intensity a t 2.6' is fairly weak, requiring high photometric gain and accompanying noise. A lower diffraction angle results in higher intensity a t the cost of a slower rise time of changes in the diffracted intensity and poorer linearity. In addition, a higher modulation frequency yielded less noise and improved signal/noise ratio. At a 1.4' diffraction angle, 30 Hz modulation frequency, and -94' phase shift, the signal decreased by about 40% compared to Figure 4,but the noise decreased more. By use of these conditions, the ratio of the signal for a 4.3 X M TAA solution to the standard deviation of the background was 6.0. Defining the detection limit as the concentration where the ratio of signal to background standard deviation is 2, the detection limit for TAA is less than 2 X lo-' M.
Ir0
V
Figure 9. Small amplitude ( A f = 70 mV, modulation frequency = 10 Hz) scanning experiment for mixture of TAA and benzoquinone: upper curve, LIA phase shift -20'; lower curve, phase shift = -109'. M TAA; peak at -0.50 V is Peak at +0.52 V is due to 1.57 X 2.57 X M benzoquinone. All results described so far were obtained with a large amplitude (500 mV) potential modulation centered approximately at E'. The results of a small amplitude experiment with potential scanning are shown in Figure 8 for three different modulation amplitudes. The modulation frequency was 10 Hz and the phase of the lock-in amplifier was set at that for minimum background (-20O). The 2 mV/s scan rate was slow relative to the modulation period and the time constant of the lock-in amplifier (0.3 s). Table I includes the peak output of the lock-in amplifier for modulation amplitudes from 5 to 100 mV. The peak outputs for scanning experiments are reported relative to the experimental diffusion limited values obtained with large AE. Both experimental responses are about 20% lower than ideal square wave response at -20' for a 10 Hz modulation frequency. The detection limit for a scanning experiment a t 10 Hz was 2 X lo-' M for a 60 mV modulation amplitude. A background interference was encountered when a high concentration of a nonabsorbing electroactive species was present. Benzoquinone is reduced to its anion radical at -0.5 V in acetonitrile, but both oxidized and reduced forms do not absorb at 633 nm. Figure 9 shows a scanning experiment for a mixture of TAA with a 16-fold excess of benzoquinone. The peak at -0.5 V is due to the quinone and is probably due to thermal or concentration induced refractive index gradients.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
757
Table 11. Comparison of Spectroelectrochemical Techniques conditionsc
technique optically transparent electrode ( 7 ) optically transparent thin-layer electrode ( 7 ) internal reflection spectroelectrochemistry (23)
fiber optic thin-layer cell (15)
long path thin-layer cell (13)
multiple specular reflection (24) sinusoidally modulated ac reflectance (19) glancing incidence reflection (25) parallel absorption (12) diffraction with signal averaging (I7 ) modulated diffraction (this work)
b = 0.02 cm Amin= 1 X
Aminb-'
0.002
b = 1.0 cm
Aminb-'
0.0012
b = 1.62 cm
[AminT(cos a)/4L](r/Dt)'/'
1.5 X lo4 calcdd 0.028 exptld
N = 10, 6 = 4000 A
w = 100 Hz,AE = 20 aRmin= 1 x mV, E = El/', 0 = 45'
[ 0 . 8 7 ~ l(cos / ~ 0) RTJD1/2AEnF]ARmin0.025
(Aminsin @/4)(7r/Dt)'/'
0.0015
p - 1'
Ami&' Ami,,b-'
0.0044 0.002 (calcd) 0.013 (exptl)
b = 0.45 cm exptl Amin= 0.015 b = 1.1 cm
Mmin(2.31ob~)-'
2.2 x
(exptl)e
Minimum value for eC derived from equations for absorbance given in original references. Calculated value for conditions listed, unless indicated as an experimental value. Reported experimental values were always higher than calculated values. Amin,minimum measurable cm2/s, and t = 10 s unless absorbance; D, diffusion coefficient of reactant. For the purposes of comparison, Amin= 0.002, D = 1.0 X stated otherwise. dCalculatedvalue for T = 100 pm, L = 4 cm, = 45', t = 10 s, Amin = 0.002. Observed value was lowest reported. Corresponds to minimum measurable M / I of 8 X lo4. (Y
These effects have a different phase relationship than the absorbance signal, since they are transient effects. Figure 9 also shows a scan carried out at a phase angle which minimizes the quinone signal. While the TAA signal is decreased, the quinone peak is virtually eliminated.
DISCUSSION The rapid response to a oonstant absorbance shown in Figure 2 demonstrates that diffracted light is effectively sampling a region very close to the electrode. (Dt)lI2at 20 ms is 4 pm, implying that the diffracted light a t 2.6" is sampling a less than 4 pm thick portion of the diffusion layer adjacent to and driven by the electrode. The faster response of absorbance over previous work (17)is a consequence of the higher diffraction angle allowed by the shorter electrode. The 168-pm optical path length is being achieved after only 4 pm worth of diffusion, since the diffusion direction is across rather than parallel to the beam. In addition, the diffraction experiment is theoretically applicable to cases where both reactant and product absorb, with only the change in molar absorptivity being important to the response. For a modulation experiment producing an ideal square wave, the lock-in amplifier output would be predicted to be half of the peak to peak signal calculated from eq 2 or 3. Furthermore, an ideal diffracted intensity vs. time wave form would track the applied potential and the phase shift would be zero. Since the rise time of the attenuation of the diffracted light is finite, the change in diffracted intensity always lags the applied potential and the demodulated signal is slightly smaller than that predicted from eq 2 or 3. Notice that phase shifts 4 will be in the range of 0' to -50' or -180' to -230' depending upon whether Ox or Red is the absorber but are always reported as the lower range here. The signal magnitudes and phase shifts depend on several experimental variables in predictable fashions, as shown in Figures 5-7. First, larger diffraction angles increase the signal because the rise time of the change in diffracted intensity is faster a t higher angles. In effect, the larger angles sample regions closer to the electrode and the electrode process can more rapidly generate and remove absorber. Second, higher modulation
frequencies decrease the signal and increase the magnitude of the phase shift. For a fixed rise time of the attenuation of the diffracted light determined by the diffraction angle, an increase in frequency will produce a more distorted square wave, lower demodulation signal, and larger phase shift. As shown in Figure 6, the phase of maximum signal shifts more negative at higher frequencies. The net result of these effects is a demodulated signal which is 10-30% lower than that predicted from eq 3. As noted in the results section, the LIA response was linear with concentration over the range from to 1 x M TAA. Higher concentrations could 3X certainly be determined, although some nonlinearity is likely because the shape of the wave form will change when the absorbance exceeds about 0.3. Given such high absorbance values, a modulation technique is probably unnecessary and single step experiments would be a simpler approach. The TAA detection limit of 2 X lod8M corresponds to an absorbance of 4 X lo4 in a 0.0168-cm cell. Electrochemical modulation of the absorber permits such low absorbances to be measured, and the combination of small absorbance and relatively long path length results in low detection limits. An attempt to improve the detection limits by increasing the electrode length b did result in an increased signal, but also more noise. Larger b leads to lower diffracted intensity and increased solution noise, and detection limits were comparable for b = 0.05 cm and 0.0168 cm. The origin of the background signals shown in Figure 3 is not clear, but they are likely to result from refractive index gradients generated by ion motion during double layer charging. The transient charging current would cause temporary concentration gradients near the electrode, and the resulting refractive index gradients would affect the diffraction pattern. If a Faradaic process involving nonabsorbers occurs, these gradients would be larger and would lead to the signal observed in Figure 9. It is important to note that absorber generation leads to a constant attenuation of diffracted light (Figure 2), while a refractive index gradient is a transient phenomenon associated with current flow. Therefore the two effects have different phase relationships, and the background can be decreased relative to the absorbance signal with proper
758
Anal. Chem. 1985, 57, 758-762
choice of phase. Thus an absorber should be distinguishable modulation. from a nonabsorber even if they have the same redox potential, Registry No. TAA, 13050-56-1. but care must be taken when determining an absorbing redox system in the presence of a large concentration of a nonabLITERATURE CITED sorbing electroactive species. (1) Roston, D. A.; Shoup, R. E.; Kissinger, P. T. Anal. Chem. 1982, 5 4 , 1417A. The experiments in Figures 8 and 9 and eq 5 and 6 are very Wlghtman, R. M. Anal. Chem. 1982, 5 4 , (2) Caudili, W. L.; Howell, J. 0.; similar to the corresponding expressions for ac polarography, 2532. and the two methods have the same peak width. The two (3) Vydra, F.; Stulik, K.:Julakova, E. "Electrochemlcal Stripping Analysis"; Halsted Press: New York, 1977. techniques are conceptually similar, in that their potential (4) Flato, J. D. Anal. Chem. 1972, 4 4 , 75A. dependence is derived from the fraction of a redox system in (5) Adams, R. N. Anal. Chem. 1978, 4 8 , 1126A. one redox state at the electrode surface. An important dif(6) Wightman, R. M. Anal. Chem. 1981, 5 3 , 1125A. (7) Heineman, W. R.; Hawkridge, F. M.; Blount, H. N. I n "Electroanalytical ference is that ac voltammetry measures the flux of redox Chemistry"; Bard, A. J., Ed.: Marcel Dekker: New York, 1984; Vol. material reaching or leaving the surface, while the spectroe13, pp 1-113. (8) McCreery, R. L. I n "Physical Methods in Chemistry"; Rossiter, B., Ed.; lectrochemical method measures the concentrationat or near Wiiey: New York, in press. the surface. In addition, the spectroelectrochemical method (9) Robinson, R. S.; McCurdy, C. W.; McCreery, R. L. Anal. Chem. 1982, 5 4 , 2356. possesses the added selectivity of a spectroscopic absorption (10) Bancroft, E. E.; Sidwell, J. S.; Blount, H. N. Anal. Chem. 1981, 5 3 , measurement of redox activity over more common monitoring 1390. of current. (11) Tyson, J. F.; West, T. S. Talanta 1980, 27, 335. (12) Pruiksma, R.; McCreery, R. L. Anal. Chem. 1981, 5 3 , 202. A direct comparison of the present method with other (13) Zak, J.; Porter, M. D.; Kuwana, T. K. Anal. Chem. 1983, 55, 2219. spectroelectrochemical techniques is not straightforward due (14) Porter, M. D.; Kuwana, T. Anal. Chem. 1084, 5 6 , 529. (15) Brewster, J. D.; Anderson, J. L. Anal. Chem. 1982, 5 4 , 2560. to differences in time scale and instrumental variables. In (16) Rossi, P.; McCurdy, C. W.; McCreery, R. L. J . Am. Chem. SOC. 1981, several cases, techniques were designed for examining reaction 103, 2524. mechanisms, and analytical sensitivity was not optimized. (17) Rossi, P.; McCreery, R. L. J . Necfroanal. Chem. 1983, 757, 47. (18) Bard, A. J.; Fauikner, L. R. "Electrochemical Methods"; Wiley: New With these caveats in mind, Table I1 compares several techYork, 1980; p 161. niques with respect to their ability to measure the minimum (19) Hinman, A. S.; McAleer, J. F.; Pons, S. J . Nectroanal. Chem. 1983, product of molar absorptivity and bulk concentration, ( C C ~ ) ~ ~ 154, 45. (20) Bard, A. J.; Faulkner, L. R. "Electrochemical Methods"; Wiley: New This quantity provides the most relevant comparison of deYork, 1980; pp 331-332. tection limits since it is composed of variables related only (21) Nelson, R. F.; Adams, R. N. J . Am. Chem. SOC. 1968, 9 0 , 3925. (22) Winograd, N.; Kuwana, T. J . A m . Chem. SOC. 1971, 9 3 , 4343. to the analyte and not the instrument or technique. The (23) Winograd, N.; Kuwana, T. Anal. Chem. 1971, 4 3 , 252. spectroelectrochemical techniques listed vary significantly in (24) Baumgartner, C. E.; Marks, G. T.; Aikens. D. A,; Richtol, H. H. Anal. Chem. 1980, 52, 267. both their optical path lengths and their minimum measurable (25) Skuily, J. P.; McCreery, R. L. Anal. Chem. 1980, 52, 1885. absorbance, Amin. With comparable Amin,those techniques with long path lengths will have better detection limits. Furthermore, those methods which are amenable to signal RECEIVED for review July 27, 1984. Accepted December 3, averaging will improve detection limits by decreasing Amin. 1984. Major support for this work was provided by the The low detection limits of the modulated diffraction techchemical analysis division of the National Science Foundation, nique presented here result from both relatively long path with additional support from the Technicon Instrument length and signal to noise enhancement provided by potential Corporation.
Pulsed Infrared Laser Thermal Lens Spectrophotometry of Flowing Gas Samples Scott L. Nickolaisen and Stephen E. Bialkowski* Department of Chemistry and Biochemistry, Utah State University, Logan, Utah 84322
The behavlor of the pulsed Infrared laser thermal lens slgnal Is studied for flowing gas samples. Posslble sources of decreased TLS slgnal are dlscussed lncludlng flow rate and cell temperature. The TLS slgnal is found to be Independent of the flow rate at all flow rates studled. I t Is calculated that absorbances as low as 2.5 X In argon and 1 X In llquld carbon dlsulflde should be detectable. The posslbllltles of using this technlque as a chromatography detector for ultratrace amounts of analyte are also dlscussed.
The use of laser analytical techniques has produced interesting results both in terms of sensitivity and in the se-
lectivity of the specific technique ( I ) . This progress has for the most part been due to the high spectral brightness and good optical characteristics of the laser source (2). There are effectively two major classes of laser analytical techniques for spectrophotometric analysis. The first class relies on the luminescent relaxation mechanism. Among this class is laser-induced breakdown spectroscopy (3), single and multiphoton laser excited fluorescence ( 4 ) , and light scattering spectroscopies. The second class derives the analytical signal from dark relaxation pathways of the laser excited analyte (5). The latter category includes photoacoustic (6),photothermal lensing (7) and deflection (8),and interferometry (9). Absorption spectrophotometry utilizing laser light sources may also be placed in this category.
0 1985 American Chemical Society 0003-2700/85/0357-0758$01.50/0
