Anal. Chem. 1988, 60. 2278-2282
2278
CONCLUSION
Table VI. Predicted Concentrations and ( % Relative Error) of Prediction for Test Samples Model
projection pursuit [K+], mM
simplex [K+], mM
sample
[Na+], M
1 2 3 4 5 6 7 8 9
0.1202 (0.2) 0.1999 (0.1) 0.1342 (0.6) 0.1341 (0.7) 0.1493 (0.5) 0.1498 (0.1) 0.1486 (0.3) 0.1668 (1.1) 0.1645 (0.3)
3.50 6.92 4.26 7.53 2.13 6.99 8.48 3.50 6.79
(8.4) 0.1198 (0.2) (1.1) 0.1199 (0.1) (11.5) 0.1351 (0.1) (7.6) 0.1349 (0.1) 0.1498 (0.1) (6.5) 0.1497 (0.2) (0.1) (1.0) 0.1486 (0.9) 0.1665 (0.9) (8.4) 0.1641 (0.5) (3.0)
3.73 6.86 3.89 7.15 2.03
(0.4)
(5.3)
(0.3)
(1.6)
av % re1 error
[Na+], M
(2.4) (2.0) (1.8) (2.1) (1.5)
7.00 (0.0) 8.45 (0.6) 3.69 (3.4) 7.03 (0.4)
is possible to improve the projection pursuit model once the log function is chosen to replace the smooth. One approach to calibration is to use projection pursuit to fiid the functional relationships and some other nonlinear regression technique to determine the model parameters, given the estimated functional form for the calibration model. Table VI lists the results of using the projection pursuit model to predict, from eq 17, the concentrations of analytes in the nine test samples. The resulta of the earlier study using the simplex model are also included for comparison. These results show that the model derived by using the simplex method yielded slightly better prediction than the model determined with projection pursuit. This is a reasonable result because of the approach to prediction that was followed. The projection pursuit smooth was assumed to equal the log transformation, and the a quantities calculated for the smooth were used with the log function. These a quantities were not exactly optimal for the log function, and therefore the projection pursuit model did not perform as well as the model derived from the simplex procedure. As stated earlier, a second approach would have been to use the smooth itself as a part of the calibration model (eq 5). Both of these are reasonable options, and either could be used, depending on whether the analyst has more confidence in the theory (in which case the functional model would be desirable) or the calibration samples (where the smooth would be used).
Projection pursuit has been presented along with an example of its use. Although ion-selective electrodes were used in this study, it is not to be inferred that this is the method of choice for the calibration of ISEs. ISE data were used to illustrate the effectiveness and capabilities of projection pursuit for calibration and model estimation in general. Many other possible applications can be imagined, and a variety of approaches to the data analysis can be used, depending on the particular type of data at hand. The most powerful aspect of the technique is that there is no need to assume any functional relationship between variables under investigation. The method can be used to verify assumed relationships, detect outliers, and determine functional relationships between variables that are unknown. Furthermore, the method is not tied to any fixed functional form. If the data do not follow some standard form, the more common modes of analysis cannot be used for model building. Using the smooths as the functional forms allows the analyst to build models that are uniquely characteristic of the system under investigation.
ACKNOWLEDGMENT The projection pursuit program used was written as a macro on the computer package S. We thank the University of Washington Department of Statistics for the use of both their computer and software. Registry No. Na, 7440-23-5; K, 7440-09-7.
LITERATURE CITED (1) Rams, L. S.; Beebe. K. R.; Carey, W. P.; Sanchez, M. E.; Erickson, 8. C.; Wilson, B. E.; Wangen, L. E.; Kowaiski, B. R. Anal. Chem. IQ86, 58, 294R. (2) Otto, M.; Thomas, J. D. R. Anal. Chem. lQ85, 5 7 , 2647. (3) Beebe, K. R.; Uerz, D.; Sandifer, J.; Kowaiski, B. R. Anal. Chem. iQ68, 60. 66-71. (4) Draper, N. R.; Smith H. Applied Regression Analysis; Wiiey: New York, 1981. (5) Geladi. P.; Kowaiski, 8. R. Anal. Chim. Acta 1986, 185, 1-17. (6) Friedman, J. H.; Stuetzle, W. J . A m . Stat. Assoc. lQ81, 76, 8 17-823. (7) Tukey, J. W. EDA Exploratory Data Analysis; Addison-Wesley: Readings MA, 1977. (8) Gans, P.; Gill, J. B. Appl. Spechosc. 1984, 38, 370-376. (9) Rosenbrock, H. H. Comput. J . IQ60, 3 , 175-184. (10) Box, G. E. P.; Hunter, W. G.; Hunter, J. S. Statistics forEk;oerimenters; Wiiey: New York, 1978.
RECEIVED for review June 19,1987. Accepted June 1, 1988. This work was supported in part by the Office of Naval Research.
A Photoelectrochemical Detector for High-pressure Liquid Chromatography Marye Anne Fox* and Tze-pei Tien Department of Chemistry, University of Texas, Austin, Texas 78712 A flow-through photoelectrochemical detector (TIO,/Pt) sultable for attachment to a hlgh-pressure llquid chromatograph Is described. The detector Is sensltlve to many oxldirable functional groups and Is capable of detectlng as little as 1.6 fig of anlllne. The relative peak helghts of signals engendered by palrs of elutlng compounds correlate roughly wlth the relative rates of photolnduced oxldatlon of Isomeric butanols on analogous semlconductor suspensions.
High-pressure liquid chromatography (HPLC) has become an indispensable tool of the preparative chemist. The easy
detection of the elution of compounds of interest can be accomplished by various methods, which vary in sensitivity and selectivity (1). By far, the most routinely employed detection methods involve absorption or emission spectroscopy or the shift in the refractive index of the eluting stream. While UV and fluorescencedetectors are sensitive for species with high extinction coefficients, they fail completely for compounds with low absorptivity. Furthermore, they offer little basis, apart from chromatographicretention volumes, for definition of the identities of similar compounds. Refractive index detectors provide a more general response, but with consequent loss of sensitivity and selectivity.
0003-2700/88/0360-2278$01.50/00 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 80, NO. 20. OCTOBER 15, 1988
2279
Table I. Comaarisan of Relative Photocurrent Simal Intensities fr& the Platizioed TiO, Photoel~ctrochcmicel Detector with Initial klalive Rates of Photocatalyzed Oxidation of Isomeric Rulanolr on Suspended PtITiO, Powders ~~~~~~~~
~
competing pair 2.hutanol:l butanol terr.butyl alcohol:l-butanol
terr-butyl alcohol:2-butanol
photmrrent relative rate of ratid photooxidationb 0.47 i 0.5 0.34 + 0.4 0.73 i 0.8
0.58 i 0.6 0.17 i 0.5 0.29 t 0.5
Ratio of photocurrents observed for injection of &PI. samples at a flow race of 2.0 mL, min with ethyl acetate as effluent. 'Ratio of initial rate of phutocatalyzed oxidative disappearance of equimolar mixtures of alcohols on Pt/TiO, particles suspended in oxygenated acetonit rile. -
The working electrode, which has been previously described
(13, was prepared by high-voltageanodization (7).It was fashioned 0.4-in. x %in. strip of 0.125 mm thick Ti, which had heen etched in a mixed solution of HF, HNOs. and water (1:1:40J for 5 min. After anodizntion. one surface of the sheet was etched to remove the TiO? layer. The resulting surface was sanded and sputtered with platinum metal to apparent smoothness. The counter electrode was made by sputtering a coating of Pt (up to 1.2 pm thick; Materials Research Corp. Rfi20 Sputtering System) on an optically flat quartz or Pyrex plate (1-in. X 3-in.) for up m 20 min. The thickness of the platinum roating was controlled hy varying the sputtering time: the sputtering rate was 10 A, s, with 309c error. Electrical connections were made from the working and counter elertrodes via silver-solderedwires toa Princetnn Applied Research Model 173 potenticatat controlled by a PAR 175 programmer. No potential was applied, and the connected cell had a zero resting potential. The TiO, working electrode was irradiated with a 250-W aircooled mediumpressuremercury lamp pitinned about 1-in.from the Pt/l'i02 surface. A Pyrex filter admitted only light of wavelengths longer than 310 nm. The cell and lamp were enclused in a copper mesh Faraday cage, which was wrapped with aluminum foil as a light shield. Photocurrents generated as the effluent passed over the photoactivated TiO, were measured on a Keithley 617 electrometer or a Princeton Applied Research Model 173 potentinsfat, galvanostat equipped with n Model 175 Universal programmer and were recorded on a Hcwlett-Packard 3385 integrator equipped with a strip chart recorder. Chemicals. Solvent8 (ethyl acetate and acetonitrile)employed in the liquid chromatographicanalysis were of HP1.C grade and were dried ior 3 days over 3-A molecular sieves. Reagents analyzed were obtained from Aldrich and were used without further purification. Steady-Stale Semiconductnr-Mediated Photooxidations. In 3U ml. of a solution of a pair of the isomeric butyl alcohols (ca 0.02 M each) in dry acetonitrile (Matheson-Coleman-Rell, chromatographic grade) was suspended 25 mg of TiO, powder (DeGussa, dried overnight in a vacuum oven at 125 "C). The mixture was sonicated for 30 min in an ultrasonic bath to ensure effectivedispersal of the particles. The resulting suspension WBS transferred to a k x ampule and was hubbled with a slow s h a m of nir to ensure oxygen adsorption on the photocatalyst surface. The ampules were irradiated in a Rayonet phomhemical reacmr equipped with phosphor-coatedlow-pressure mercwy arcs. blazed at 350 nm,for variable periods up to 8 h as a slow air stream gently xtirred the suspension. After irradiation, the ampules were filtered to remove the photocatalyst,and the resulting fdtrate WBP directly analyzed on a Hewlett-Packard gas-liquid chromatograph equipped with a 50-m HP.1 column. The relative rates of reaction were ohtained hy following the ratio of roncenmtions of the paired alcohols competing for photogenerated holes at various ronversions. The observed ratios were then extrapolated to zero conversion for the rate ratios reported in Table I.
on a
~
Figure 1. Photoelectrochemical detector for a liquid chromatograph (a) head-on view; (b) side view.
Electrochemical detectors measure current, voltage, capacitance, or resistance of an electroactive component present in the HPLC effluent. The detection of photocurrents in rationally designed photoelectroehemid cells (2)should allow for similar detection methods. Such cells involve hand gap excitation of a semiconductor surface that is immersed in an electrolyte containing an appropriate redox couple. Photocurrent is generated by interfacial hole capture hy an oxidizable donor adsorbed on the irradiated surface, as the photogenerated electron moves in the conduction band from the interface to the bulk and ultimately to a dark metallic counter electrode. The sensitivity of many organic functional groups to photoelectrochemical oxidation on irradiated semiconductor surfaces (3-5)suggests that such a detector may find general use for many organic mixtures. We describe in this article the construction and operation of a new type of photoelectrochemical detedor. Its use in the characterization of eluting components from mixtures of simple alcohole is described, and a brief survey of its use with other major functional groups is demonstrated. We also report on the use of the relative sensitivity of the technique as a method for rapidly scanning for photoelectrochemical activity under steady-state irradiation experiments.
EXPERIMENTAL SECTION Construction of the Cell. A two-electrode photoelectrochemical cell shown in Figure 1 was fashioned inside a polyethyleneblock through which a hole was cut to permit the prtssage of light. Into the cell body were inserted a TiO, on Ti working electrodeand either an opticallytransparent sputtered Pt counter electrode or a ptcoated glaae through which a transpamnt window had been etched, separated hy a 0.01-in. Teflon gasket with a channel positioned in its center. An external clamp was used to anchor the cell to the supporting block. Two holes were cut through the working electrode, through which the effluent from a Waters liquid chromatograph (Waters U6K injection valve and Waters 2000 solvent delivery system, equipped with a worasil or a Bondapak CISreversed phase analytical or semipreparative column) could flow into and out of the cell. The column effluent was analyzed directly without adding electrolyte. The dead volume of the cell (from extemal connections) was about 15 rrL, and the illuminated volume was about 8 r;L. The dead volume was measured empirically, and the illuminated area was estimated geometrically
RESULTS AND DISCUSSION A typiral photoelectrochemical detector trace of photocurrent shows resolution identical with that seen for the same mixture of oxidiieble substrates injected onto a reversed-phase
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 20, OCTOBER 15, 1988
Table 11. Magnitude of Photocurrent at Peak Maximum Observed with the Pt/Ti02 PhotoelectrochemicalDetector for Several Compoundsa,b
photocurrent, pA/mg 18%
compd
PhCHO PhNHz
19 23 2.7 0.70 0.15 0.50 0.35 28
CHSOH
PhCHzOH (CHJSCOH PhCOCHS cyclohexanone 1,3-cyclooctadiene
OElution with ethyl acetate at a flow rate of 2.0 mL/min. *Each reported value is the average of at least 10 separate injections.
Scheme I
TiOz
hu
eCB-+ hvB+
I
Illuminated TO
Liquid effluent
Metal Co,un te r electrode
Flgwe 2. Electron and hole trapping by donors and oxygen adsorbed from a Rowing siream in a photoelectrochemicaldetector: VB, valence band edge potential: CB, conduction band edge potential.
liquid chromatography column with UV or refractive index (RI)detectors. Thus, the peak width is controlled by column separation rather than by differential response by the photoelectrochemical detector. The same relative peak widths are observed in an analysis attained with a refractive index detedor, since the signal response shows an identical variance with each detector. The integrated area under each peak varied linearly with the concentration. The photocurrent response produced by various functionalgroups is summarized in Table 11. Although photocurrent is produced with many organic functional groups, the magnitude of the signal varies from compound to compound, allowing for desirable chemoselectivity. Mechanism. The operation of a photoelectrochemical detector relies on interfacial electron transfer induced by photocatalyticelectron-hole pair formation (Scheme I). When a semiconductiveTiOz electrode is illuminated with band gap photons (
