Anal. Chem. 1902. 5 4 . 1437-1439
receiver plates (no amplification) could be achieved routinely and were, to our knowledge, the largest signals yet observed in FTMS studies. I t has not been possible to generate ICR signals of such quality using an electron beam, even in a quench off mode. It is also interesting to note that in this experimental setup ions were injected into the analyzer cell transverse to the magnetic lines of force from the sample probe tip located approximately 0.5 cm outside the analyzer cell. Considering that cyclotron radii of‘ ions expected in this experiment are less than 0.5 cm, the ions cannot be formed in a collision-free environment. We postulate that the laser pulse produces a “plume” of ions and neutrals, or perhaps a neutral plasma, which allows ions to diffuse several millimeters transverse to the magnetic field within a few milliseconds. As this plume expands into the analyzer cell, neutral and ionic components of polarity opposite that of the trap voltage are rapidly removed, leaving sample ions of desired polarity trapped for mass analysis. On the basis of the results of our preliminary study, we suggest that the LD/F’TMS system is suitable for the analysis of “nonvolatile” and/or thermally labile polar organic molecules and organic salts. The mechanism of ion injection and its possible relation to the large signal intensities observed are subjects of further study. Future work will also include synchronizing the laser pulse to the FTMS pulse sequence to facilitate time-resolved experiments, studying high molecular weight samples and investigating pretreatment procedures. The lack of Na+ and K+ ions in cation spectra, the reason for relatively high negative ion abundance, and the extraordinarily large 1:CR signal intensities observed in the LD/FTMS experiments will also be topics of investigation. ACKNOWLEDGMENT We wish to thank the group of N. Nogar for the use of the laser. LITERATURE C I T E D (1) Comisarow, M. 6.; Marshall, A. G. J . Chem. Phys. 1978, 6 4 , 110-1 19.
1437
White, R. L.; Ledford, E. B., Jr.; Ghaderi, S.; Wilklns, C. L.; Gross, M. L. Anal. Chem. 1980, 5 2 , 1525-1527. Comisarow, M. 6.; Marshall, A. G. Presented at the 23rd Annual Conference on Mass Spectrometry and Allied Topics, Houston, TX, May 1975, Paper R-5. Bingham, R. A.; Salter, P. L. Anal. Chem. 1980, 4 8 , 1735-1740. Stoll, R.; Rollgen, F. W. Org. Mass Spectrom. 1979, 14, 642-645. Cotter. R. J. Anal, Chem. 1980. 52. 1767-1770. Heresch. F.; Schmid. E. R.; Huber, J. F. K. Anal. Chem. 1980, 52, 1803-1807. Heinen. H. J.; Meler, S.; Vcgt, H.; Wechsung, R. A&. Mass Spectrom. 1979, 8A , 942-953. Kaufmann, R.; Hillenkamp, F.; Nltsche, R.; Schurmann, M.; Wechsung, R. Microsc. Acta 1978, 2 , 297-306. Kupka, K.-D.; Hillenkamp, F.; Schiller, Ch. A&. Mass Spectrom. 1979, 8 A , 935-941. Salvati, L., Jr.: Hercules, D. M.; Vogt, H. Specfrosc. Lett. 1980, 13, 243-251. Balasangmugan, K.; Dang, T. A.; Day, R. J.; Hercules, D. M. Anal. Chem. 1981, 53, 2298-2298. Posthumus, M. A., Kistemaker, P. G.; Meuzelaar, H. L. C.; Ten Noever de Brauw, M. C. Anal. Chem. 1978, 50, 985-991. Kistemaker, P. G.; Lens, M. M. J.; van der Peyl, G. J. Q.; Boerbwm, A. J. H. A&. Mass Spectrom. 1979, 8A, 928-934. Cody, R. 6.; Burnier, R. C.; Reents, W. D., Jr.; Carlin, T. J.; McCrery, D. A.; Lengai, R. K.; Freiser, B. S. Nit, J , Mass Spectrom, Ion Phys . 1980, 33, 37-43. Cody, R. 6.; Burnier, R. C.; Freiser, B. S. Anal. Chem. 1982, 54, 96-101. Ledford, E. B., Jr.; Ghaderi, S.; White, R. L.; Spencer, R. 6.; Kulkarni, P. S.;Wllkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 463-468. Ledford, E. B., Jr.; Whlte, R. L.; Ghaderl, S.: Wllkins, C. L.; Gross, M. L. Anal. Chem. 1980, 52, 2450-2451. Issachar, D.; Yinon, J. Biomed. Mass Spectrom. 1979. 6, 47-56. Bradley, C. V.; Howe, I.; Beynon, J. H. Biomed. Mass Spectrom. 1981, 8. 85-89. Shabanowitz, J.; Brynes, P.; Maellcke, A,; Bowen, D. V.; Field, F. H. Biomed. Mass Spectrom. 1975, 2, 164-167.
David A. McCrery E d w a r d B. Ledford, Jr. Michael L. Gross* Department of Chemistry University of Nebraska-Lincoln Lincoln, Nebraska 68588
RECEIVEDfor review February 2, 1982. Accepted April 23, 1982. This research was supported by the National Science Foundation (Grant CHE-80-18245)and by the Environmental Protection Agency (Grant No. R807251010).
Determination of Diffusion Coefficients by Chronoamperometry with UnshieIded P’Ia na r Stationary EIect rodes Sir: Diffusion coefficients of electroactive species can be accurately determined from chronoamperometry using shielded planar stationary electrodes (1). In this case, the one-dimensional diffusion model applies and the Cottrell equation predicts that the product of the current and the square root of time is ai constant, nFAD’W*, from which the diffusion coefficient, 13, is determined knowing the concentration, C*, and the electrode area, A . However, in chronoamperometry using unshielded planar electrodes the product itlIz is not a constant but is a function of time. It has been suggested (2, 3) that this is a iresult of edge effects (laterial diffusion in the vicinity of the electrode) and that for planar disk electrodes it1/2should Ibe given by
[
(I);)’”] itlf2 = -tnFAD1l2C* l + b .p
(l)
where R is the radius of the electrode and the empirical constant b has a value of approximately 2. In principle, both D and n can be estimaited from the intercept and slope of a linear plot of it112 vs. t1/2. Recently numerical simulation
studies of the edge effect of unshielded electrodes in chronoamperometry ( 4 , 5 )showed that the it1i2 vs. t 1 / 2plots are curved, due to b also being a function of t ( 4 ) . Therefore, a linear fit of the data in it112vs. t1I2plots may lead to some inaccuracies. Since we are interested in obtaining accurate diffusion coefficients for organic compounds, we report here the comparison of methods for obtaining D by chronoamperometry, employing the reduction of ferricyanide as a test system. EXPERIMENTAL SECTION Chemicals. Reagent grade potassium ferricyanide,K,Fe(CN),, and the supporting electrolyte, potassium chloride, were used as received. Apparatus. A three electrode potentiostat was used to perform the chronoamperometric experiments. The current-time curves were recorded with an X-Y recorder. The electrochemical cell, described elsewhere (6), was thermostated to 25 f 0.2 “C. The solution was deaerated with argon. Before each the solution was dowed to for 5 min. The working electrode was a platinum disk mounted in a Kel-F rod. The plane of the electrode disk was horizontal so that
-
This article not subject to US. Copyright. Published 1982 by the Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
plot can be used in eq 2 to determine D. L 62 L
L
501
0
,
,
,
,
I 5
10
I"?, (sec)''Z
Flgure 1. Plot of it1/' vs. t"* for the reduction of 4.03 mmol/L K,Fe(CN), in 0.1 mol/L KCI. Each point is the average of five runs. The dashed line represents the least-squares fit of the data between 15 and 75 s. The solid line represents the least-squares fii of eq 3 to the
same data. diffusion was upward. The electrode surface was given a mirror finish by polishing with 6 pm diamond polishing compound, followed by light buffing with 0.05 pm alumina. The electrode was subsequently treated with concentrated nitric acid, flushed with distilled water, and allowed to dry. The electrode area (0.079 cm2)was determined from its diameter, which was measured by optical microscopy. The auxiliary electrode was a platinum foil located directly opposite and parallel to the plane of the working electrode. A saturated calomel electrode (SCE) served as the reference electrode. The background current was negligibly small. The potential of the working electrode was such that the surface concentration of Fe(CN)63-is essentially zero and the current was diffusion controlled.
RESULTS AND DISCUSSION Chronoamperograms for five runs, using 4.03 mmol/L K,Fe(CN),, were obtained. The average current of the five runs a t a given time was calculated. The standard deviation of the measured current, ranging from ca. 20 to 7 pA, is f0.05 MA. The data are replotted in Figure 1 as it1I2vs. t1/2. If diffusion were strictly one dimensional, a horizontal straight line would fit the entire range of the it1i2vs. t1/2plot. In Figure 1,the data are most nearly on a horizontal line for 1 < t < 10 s. For this range oft, the average value of it1/2gives upon substitution in the Cottrell equation an estimated value of the diffusion coefficient, D = 9.1 X lo* cm2/s. This value of D is relatively high and is surely the result of unreliable data a t small t. Measurements at very small t are often unreliable, generally because of electronic and cell time constant restrictions. We have observed that the it1l2 vs. t1/2 curve tends to reach a shallow minimum for t equals approximately 2 to 3 s and tends to increase slightly at t < 1 s. The charging current was estimated to be an insignificant fration of the total current a t t > 0.5 s. The cause of this artifact is as yet unexplained but may be instrumental. Data at long times are equally unreliable since the current decreases to such an extent that precision suffers. The most accurate range of measurement of both current and time appears to be in the range of 10-80 8.
Method I: it 1/2 vs. t 1/2 Linear Extrapolation. By taking into account the edge effect of a finite unshielded planar disk electrode, Lingane (7) has shown that it1I2,as a linear function of t1/2 (eq I), accounted for most of the deviation from the Cottrell equation. In this case, the intercept of an it1/' VS. t1l2
The dashed curve in the it1i2vs. t1i2plot (Figure 1)represents the least-squares straight line fit of the data in the range of 15 < t < 75 s. By use of the intercept at t = 0 of the dashed curve of Figure 1 in eq 2, the value of the diffusion coefficient was calculated to be 7.68 X lo* cm2/s. From the slope and intercept of the same dashed line, a value of 2.02 was estimated for b of eq 1,which is in the range of values reported in the literature (5, 7). Method 11: it1/2vs. t1I2Curve Fit. Recently two reports of numerical simulation of chronoamperometry using unshielded planar electrodes have appeared in the literature (4, 5). The results indicate that curvature in the it1I2vs. t1I2plots is to be expected. To facilitate regression analysis, the tabulated values of simulated current functions reported by Heinze (Table 1of ref 4) were used to generate a least-squares second degree polynomial fit for (Dt)1/2/R15 0.25, which spans the range of our experiment. The maximum percent deviation of the fitted curve to the tabulated data was less than 0.02%, which is less than the average percent deviation of our experimental points. With this second degree polynomial (eq 3) in a least-square regression analysis of our experimental (Dt)lI2
it1/2
nFAC*D112
= 0.5642 + 1.018-
+ 0.2471-Dt
R
(3)
R2
data between 15 < t < 75 s, treating D as the only parameter, an estimated value of 7.88 X lo4 cmz/s was obtained for D. The least-squares line is shown as the solid curve in Figure 1. Method 111: i vs. t-'i2 Linear Extrapolation. The diffusion coefficient can be estimated from the limiting current at t m, iL, by plotting i as a function of and extrapolating to infinite time. In this case the diffusion coefficient is given by -+
D112= RiL/4FAC*
(4)
Using only data for t 2 45 s, for which the i vs. t-ll2 plot appears to be approximately linear within experimental uncertainty, we obtained, by linear extrapolation to t-lIz = 0, iL = 2.0 pA and estimated D = 7.8 X lo4 cmz/s. Method I V i vs. t-1/2Curve Fit. A plot if i vs. tf1I2shows only slight curvature for a limited range of t, t2-tl. On the basis of their numerical simulation results, Kakihana et al. (5)have suggested an iterative method for obtaining both D and nC* from this plot, utilizing a regression equation relating i to R/(Dt)'/'. In this method, a straight line is drawn through points (xl, il) and ( x 2 , i2),where x1 = (3t2-lI2 tl-1/2)/4and x 2 = (3tl-'i2 t2-lI2)/4and il and i2 are currents at x1 and x2, respectively. The values for tl and tz were chosen to be 15 s and 75 s, respectively, to correspond to the range used for the methods I and 11. From the slope and intercept at t-lI2 = 0 of this straight line, D is determined to be 7.54 X lo6 cm2/s.
+
+
CONCLUSIONS All the D values determined by the methods above have an estimated uncertainty of f0.3 X lo6 cm2/s; they are within 5% of each other and the accepted value of 7.62 X lo4 cm2/s (1). However, since D is concentration dependent, the D values reported herein may not properly be compared directly with the accepted value which corresponds to infinite dilution. Moreover the correct electrode A is probably greater than the geometric area we used in the calculations of D , and this systematic error would make all the D values somewhat smaller by the same factor. One of our aims in this study is
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Anal. Chem. 1982, 5 4 , 1439-1441
to see the effect on the D value by the application of different methods of calculation, especially those which take edge effect into account explicitly. I t appears that all the above mentioned methods give essentially the same value for D, within the estimated uncertainty. None of these methods is clearly superior. Methods I, 11, and IV allow one to use the data in the middle range of the i-t curves, thus avoiding the current artifacts a t early times. However, methods I, 11, and IV also introduce some degree of arbitrariness in choosing the range of data to use. The methods that utilize it1Iz in the ordinate axis appear slightly less precise as a result of error mixing in forming the it1/2prodiict from the two independently measured variables, current and time. The methods derived from i vs. t-1/2plots are free of this complication and are preferred by us. Method I11 is sinnple in both calculation of D and fitting a linear line. However, the procedure of Kakihana et al. (5) (method IV) utilizes the most reliable portion of the data. It is obvious from Figure 1that the geometrical edge effect correction for unshielded electrodes did not completely explain
the experimental results. Other physical factors such as convection might have to be considered.
LITERATURE CITED (1) von Stackelberg, M.;Pllgram, M.; Toome, V. Z . Electrochem. 1953, 5 7 , 342-350. (2) Soos, Z. G.; Llngane, P. J. J . Phys. Chem. 1964, 6 8 , 3821-3828. (3) Flanagan, J. B.; Marcoux, L. J . Phys. Chem. 1973, 77, 1051-1055. (4) Heinze, J. Ber. Bunsenges. Phys. Chem. 1980, 8 4 , 785-789. (5) Kaklhana, M.; Ikeuchi, H.; Sato, G. P.; Tokuda, K. J . Electroanal. Chem. 1981. 117, 201-211. (6) Biubaugh, E. A.; Doane, L. M. And. Chem. 1982. 5 4 , 329-331. (7) Lingane, P. J. Anal. Chem. 1964, 3 6 , 1723-1726.
William T. Yap* Lawrence M. Doane Organic Analytical Research Division National Bureau of Standards Washington, D.C. 20234 RECEIVED for review January 18, 1982. Accepted April 13, 1982.
AIDS FOR ANALYTICAL CHEMISTS Microprocessor-'ControlledInfrared Data Acquisition for Liquid Chromatography Sharon L. Smith* Eli Lily and Company, Inidianapolls, Indiana 46285
Claude E. Wllson Perkin-Elmer Corporation,,Lombard, Illinois 60 148
The use of an infrared (IR) spectrometer as a detector for liquid chromatography (LC) is a potentially very powerful analytical tool. With a well-designed flow cell, virtually any good IR spectrophotometer can be used to monitor a specified frequency as a function of time. However, recent advances in computer-aided spectroscopy have significantly extended the capability of such systems. For selective LC detection, signal averaging the transmittance readings and digital smoothing of the resultant data can lead to improved signal-to-noise and lower detection limits. Modern data handling techniques also aid in tlhe abstraction of the infrared spectrum of a separated component. In this work, a high-performance liquid chromatography (HPLC) system was coupled with a dispersive IR spectrophotometer and microcomputer data system. This system was used to obtain signal-enhanced chromatograms and s,pectra of the separated components. IR selective detectors for LC have been used in a variety of applications (1-3). The infrared systems used include single-beam, filter monochromator (1,3), double-beam grating spectrophotometers (49,and Fourier transform IR (2,5). The recovery of spectra of the Components of an LC eluant is also applicable to viirtually all forms of LC. However, the majority of published applications have utilized gel permeation chromatography (GPC!) (5-7) because of the freedom to use solvents that are largely transparent in the IR and columns with high capacity. Two methods of obtaining component spectra in LC-IR have been employed. The oldest is to isolate fractions, remove the solvent, and then obtain the IR spectra. Perhaps the most sophisticated use of this procedure has been developed by Griffithei (8)where fraction collection is combined with a diffuse reflectance technique. A novel method using micro multiple internal reflection (6,9)has also been
developed. Recently, the availability of dedicated computers and powerful data manipulation programs has provided a means of abstracting a component spectrum from a mixture. All previous literature which applied this computational technique for subtracting the LC mobile phase spectral interference has involved use of Fourier transform spectrometers (2,lO). An excellent review of this work is given by Vidrine (11). Although Vidrine's review focuses on FT-IR, much of the discussion applies to infrared spectroscopy in general. The principle of solvent subtraction in dilute solution applications is not, however, unique to the FT-IR method and can also be performed with modern dispersive IR systems (12). A good review of computerized signal enhancement techniques as applied to chromatography is given by Annimo (23).
EXPERIMENTAL SECTION All separations were performed on a 0.46 cm X 25 cm Perkin-Elmer RP18 column (Perkin-Elmer, Norwalk, CT). A Per!+-Elmer high-pressure LC pump, Series 3B, was used as the solvent delivery system. The methylene chloride mobile phase, distilled in glass (Burdick and Jackson Laboratories, Inc., Muskegan, MI), was pumped at a flow rate of 1.0 mL/min. Injections were made by filling a 20 WLloop injector (Rheodyne, Model 7125). Tsuda et al. (14) described one of the earliest liquid flow cells for IR use. The criteria for flow cell selection for LC-IR was discussed by Vidrine (11). In the present work, a conventional demountable NaCl liquid cell with 0.2-mm spacer (part number 0816-0072, Perkin-Elmer) was modified to function as a liquid flow cell. A 1.5-mm hole was drilled in each of the Teflon stoppers of the IR cells. Flexible Teflon tubing, 1.7 mm 0.d. and 0.8 mm i.d., served as the connection line between the column outlet and the bottom of the IR cell. The cell's dead volume was minimized by extending the tube into the cell through the drilled window
0003-2700/82/0354-1439$01.25/00 1982 American Chemical Society
