Anal. Chem. 1091, 63,2564-2570
2564
Photovoltaic Detection Method for Condensed-Phase Photoionization J o h n W. Judge a n d Victoria L. McGuffin*
Department of Chemistry, Michigan State University, East Lansing, Michigan 48824
A novel method Is examined for the detection of photoinduced ions In the absence of an applied electric field. The analyte solution Is enclosed between two optlcally transparent electrodes and Is lrradlated transversely by a pulsed laser. Afler Irradiatlon, the extent of photoionlration differs at the two electrodes and a translent photopotential is developed. This photovoltaic response Is characterlred wlth respect to eiectrode composition and film thickness, electrode separation distance, illuminated area, and laser pulse energy. I n addltion, the dependence of the photopotential on solute concentratlon Is examined in a variety of polar solvents. Detection limits of 5 X lo-' M potassium permanganate In water are reported, with a linear range exceeding 3 orders of magnltude.
INTRODUCTION Photoionization phenomena in the condensed phase have been investigated for several decades and continue to be of great interest. Previous studies have been concerned with the determination of ionization spectra, ionization energies, mechanisms of ionization, and the nature of ionic products for pure aromatic liquids and for aromatic compounds in alkane solvents (1-3). Important physical parameters such as solvated ionic radii ( 4 ) ,electron mobility (5, 61, electron transport and reactivity (6, 7) have also been examined. In addition, the fluorescence emission produced during ionization and from highly excited states has been investigated ( I , 8). Although analytical applications have been reported in both static and dynamic systems (see refs 9-13 and references therein), the high sensitivity and selectivity of this technique have yet to be fully exploited for analytical purposes. Photoionization in the condensed phase is complicated by many problems that do not occur to a great extent in the gas phase, where much of our present knowledge resides. In the condensed phase, the excess kinetic energy of the ejected electron is dissipated more rapidly by collision ( 4 ) . Consequently, it is possible to have geminate ion recombination, formation of radical anions, electron capture, molecular rearrangement or fragmentation, in addition to simple solvation of the product ions. Another important difference is that the ionic products are stabilized by solvation, which causes a reduction of the ionization threshold by approximately 2-4 eV for most pure organic compounds in the liquid phase when compared to the gas phase (9). If the compound is not pure but dispersed in another medium, the ionization energy and efficiency are highly dependent on the physical properties of the solvent (14). Furthermore, many polar solvents are themselves more readily photoionized in the condensed state; most notably, the photoionization of water occurs in the gas phase a t 12.6 eV and in the liquid phase at 6.05 eV (9). The interference of solvent photoionization complicates the acquisition and interpretation of spectral information for the analyte molecule. Consequently, quantitative detection of
* To whom correspondence should be addressed. 0003-2700191/0363-2564$02.50/0
photoionization in the condensed phase may be complicated and difficult. The most commonly employed detection technique for condensed-phase photoionization is the measurement of electrical conduction. In this technique, large bias voltages (up to several thousand volts) are applied across a solution to drive photoinduced ions to a collection electrode, where the net charge flux (current) is measured (2-4,15,16). Since any naturally occurring ions or ionic impurities are also detected, extensive purification procedures may be required (2, 16). In addition, solvents with intrinsically high conductivity, such as water and other highly associated liquids, cannot be readily employed due to the high background current. A novel alternative to conduction measurement, proposed by Coleman and co-workers (19,is to detect the photopotential induced a t the electrode-solution interface in the absence of an applied electric field. By elimination of the bias voltage, contributions from intrinsic solvent conduction are substantially reduced or eliminated. Consequently, measurements of the photovoltaic response may be accomplished in polar solvents and in solutions of relatively high ionic strength, where a photoconduction measurement may not be possible. Furthermore, this detection method may reduce background current from the photoelectric effect, which is caused by electron ejection from the electrodes when directly irradiated in an applied electric field. Coleman and co-workers (17) have reported such photovoltaic measurements for the photoreduction of potassium permanganate in aqueous solution. In these studies, the sample was enclosed between two optically transparent electrodes (n-type tin oxide semiconductor overcoated with an insulating quartz layer) and was irradiated transversely by a nitrogen-pumped dye laser. The photopotential developed across the unbiased electrodes after illumination was measured with an oscilloscoperelative to external ground. The signals were characterized by a rapid rise (
