Comparative Study of Photovoltaic and Photoconductive Detection Methods for Liquid-Phase Photoionization

Comparative Study of Photovoltaic and Photoconductive Detection Methods for Liquid-Phase Photoionization

J O H N W. J U D G E * a n d V I C T O R I A L. M c G U F F I N t Department of Chemistry, Michigan State University, East Lansing, Michigan 48824

The photovoltaic method for the detection of liquid-phase photoionization in the absence of an applied electric field is compared to the traditional photoconductive method. Each system is characterized by means of signal and signal-to-noise ratio measurements with respect to laser pulse energy, applied voltage, electrode separation distance, solute concentration, and solvent composition. Although both techniques exhibit similar detection limits (1 x 10 7 M ) for solutes in polar solvents such as methanol and water, the photovoltaic technique has a more extensive linear dynamic range (103). Moreover, this technique allows detection of solutes in ionic solutions with comparable detectability and linearity.

Index Headings: Photoionization; Photoconductive detection; Photovol- taic detection; Spectroscopic techniques; Instrumentation.

INTRODUCTION

Photoionization phenomena in the liquid phase have been used for the determination of important physical and chemical constants I as well as for analytical measurements. 2 A vast majority of such measurements have been performed in nonpolar solvents, due to the inherent difficulties in detecting photoionization in dielectric media. In this paper, we compare from a theoretical and an experimental perspective the traditional photoeonduc- tance detection technique (see Refs. 1 and 2 and refer- ences therein) with a novel photovoltaic method 3,4 for photoionization in polar solvents such as water and methanol.

T H E O R Y

When a molecule in solution absorbs a photon of sufficient energy, it undergoes ionization to produce a geminate cation/electron pair. The threshold ionization energy of the molecule is strongly dependent upon the physical properties of the solvent. Jortner and co-workers 5,6 have proposed a relationship between the gas-phase (It) and liquid-phase (IL) ionization energies that reflects this solvent dependence:

It. = I t + P+ + Vo. (1)

In this expression, P+ represents the electronic polariza- tion energy of the cation in the solvent, and Vo is the energy of the electronic conduction level (electron affinity) of the solvent. The magnitude of the P+ term, estimated with the Born approximation for the solute N,N,N',N'- tetramethyl-l,4-phenylenediamine (TMPD), is approximately - 1.54 eV in hexane, - 1.42 eV in methanol, and

Received 16 December 1993; accepted 23 May 1994. * Cuirent address: Department of Chemistry, Culver-Stockton College,

Canton, MO 63435. t Author to whom correspondence should be sent.

- 1.42 eV in water. 4,7 Hence, the cation is stabilized to a greater extent in nonpolar media that can solvate the nonpolar organic functional groups more effectively. In contrast, the electron is most strongly stabilized by polar solvents and is unaffected or destabilized by nonpolar media; the Vo term is estimated relative to vacuum as +0.04 eV in hexane, - 1.0 eV in methanol, and - 1.3 eV in water. 8,9 The combined effect of the cation (P+) and electron (Vo) stabilization energies generally results in a reduction in ionization energy of 1-2 eV in nonpolar solvents and 2.-4 eV in polar solvents, compared with that in the gas phase.

A majority of the geminate pairs created upon irradiation will recombine in a very short time (<10 -~° s), whereas only a small fraction will dissociate into solvated cations and electrons. The probability P(0) of generating free ions from a geminate cation/electron pair with an initial separation distance r is given by Onsager 1° as:

P(0) = exp 4~- (2)

where e is the charge of the electron, ~o is the permittivity of vacuum, ~ is the relative permittivity (dielectric constant) of the solvent, k is the Boltzmann constant, and T is the absolute temperature. As a result of the strong dependence on dielectric constant, the free-ion yield for molecules in nonpolar solvents may be several orders of magnitude smaller than that in polar solvents.

P h o t o c o n d u c t i v e Detec t ion . The dissociation of the geminate ion/electron pairs into free ions is most fre- quently detected by the traditional photoconductive method. In this method, a large voltage is applied across the solution to induce migration of the ions to a collection electrode of opposite charge, where the net charge flux (current) is measured. Since the photoconductive detection technique requires application of an external electric field, the probability P(E) of free-ion formation is enhanced by charge separation within that field. In the low- field limit, it can be shown that: 1°

-- e 2 p ( E ) = [ e x p ( ~ ) ] [ 1 + e3V ]

8 r , 0 ~ 2 T i j (3)

where the electric field strength is the applied potential (V) per unit length (L), and all other parameters are as previously defined. The relative enhancement of free-ion formation, expressed as the probability ratio P(E)/P(O) from Eqs. 2 and 3, is shown as a function of electric field strength in Fig. 1. For nonpolar solvents such as hexane, a substantial increase in free-ion yield can be obtained through the application of an external electric field. How- ever, little or no advantage is to be gained for polar sol-

1 102 Volume 48, Number 9, 1 994 0003-7028/94/4809-110252.00/0 APPLIED SPECTROSCOPY © 1994 Society for Applied Spectroscopy

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METHANOL WATER

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ELECTRIC FIELD STRENGTH (V/crn)

Relative enhancement of free-ion formation in nonpolar and polar solvents examined as a function of electric field strength, calculated by using Eqs. 2 and 3. Dielectric constants (0 are 1,88, 32.63, and 78.54 for hexane, methanol, and water, respectively, at 25°C."

vents, such as methanol and water, with intrinsically higher ionization yields.

A comprehensive model of the photoconductive detection technique has been reviewed by Schmidt 12 and extended for analytical applications by Voigtman, Winefordner, and co-workers, la,~4 The photocurrent (I) observed after irradiation may be described by the following semi-empirical relationship::~

I = K[Ep"ezC, P(E)] [V Z aizi~'igi] .-~ j . (4)

In this equation, K is an instrument-dependent constant that includes factors such as the geometry of irradiation, reflection and refraction losses, size, shape, and position of the electrodes, etc. The first parenthetical term represents the overall efficiency of forming free ions in solution, which is dependent upon the total number of solute molecules absorbing radiation as well as the probability of free-ion formation after absorption. The number of absorbing molecules may be determined from the inci- dent pulse energy (Ep) and the number (n) of photons required to induce ionization, together with the molar absorptivity (ez) and molar concentration (C,) of the solute Z. The second parenthetical term in Eq. 4 represents the efficiency of collecting and detecting the free ions after they have been formed, which is expressed as the ratio of the ion lifetime to the transit time required for the ion to reach the electrode at a distance (L) in a uniform electric field (V/L). If more than one ionic species is formed by photoionization of the solute, each species contributes to the overall photocurrent according to its relative fraction (a), charge (z), lifetime (h), and mobility (#). Experimental results reported by Locke et alJ S for one-photon ionization and by Voigtman, Winefordner, and co-workers ~3J4 and Yamada et al. 2J6 for two-photon ionization show

:~ Equation 4 may be evaluated by using any internally consistent system of units, for example, by using the International System of Units (SI) with the mksa convention: I (A), K ( A m j- l ) , E~ (J), ~, (m 2 mol ~), C, (tool m-3), V(V), r~ (s), ~ (m 2 V ~ s '), L (m), whereas P (E), a~, and z~ are unitlessJ 4

OPTICAL

. . . . 4 s - . . . . . . . . . - 1 - - - -

~ . ~ OS CII~ t L o OXCAR

INTEGRATOR I

FIG. 2. Schematic diagram of the photoconductive detection system for liquid-phase photoionization: M, mirror; PD, photodiode.

good agreement with the parametric form of this relationship.

Because of the increase in ionization efficiency (Eq. 3) and in ion collection efficiency (Eq. 4) with applied electric field strength, the photoconductive detection technique is well suited for photoionization in nonpolar solvents. Unfortunately, these advantages are not applicable in the case of polar solvents. The intrinsically high conductivity of such solvents leads to a substantial increase in the background current; for example, the background for water and methanol is more than six orders of magnitude greater than that for hexaneJ 3 In addition, any native ions or ionic impurities will also increase the background current, thereby restricting the use of buffers and other solutions of high ionic strength. Although improvements in the detection systems have enabled the use of polar organic solvents and solvent mixtures containing low concentrations of water, the high background current and the fluctuations therein remain a primary limitation in the use of photoconductive detection. 2,~3,~4

Photovoltaic Detection. One means to circumvent the problems encountered in the photoconductive technique is to use a different method of signal acquisition. Coleman et al. 3 reported the detection of solute photoionization by measuring the photopotential induced at the electrode/ solution interface in the absence of an applied field. In this technique, the sample is enclosed between two optically transparent electrodes and is irradiated transverse- ly by a laser. The photopotential developed across the unbiased electrodes after illumination is measured relative to external ground. By elimination of the bias voltage, contributions from intrinsic solvent conduction and from the photoelectric effect are substantially reduced or elim- inated. Consequently, measurements of the photovoltaic response can be readily accomplished in polar solvents where a photoconductive measurement would be diffi- cult.

However, the absence of an applied electric field has several detrimental consequences on the measurement of liquid-phase photoionization. First, the probability that the electron will escape from the geminate ion/electron pair is reduced according to Eq. 2. In addition, a smaller fraction of these ions and electrons will be collected at

APPLIED SPECTROSCOPY 1103

the electrode, since diffusion is the only means of mass transport. As a result, the photovoltaic technique may not be suitable in nonpolar solvents where the intrinsic ionization efficiency is low and/or the ions are short-lived.

A theoretical model describing the response of the photovoltaic detection method is developed herein by exten- sion of the work of Albery and Archer 17 and Quickenden and Yim. 18 In this model, the photoelectrochemical system is presumed to consist of two inert metal electrodes; one of the electrodes is illuminated by a pulsed source at low average irradiance and the other remains dark. The electrodes are immersed in a solution containing two re- dox couples; the photosensitive electron-donor couple (Y/ Z) is electrochemically irreversible, whereas the electron- acceptor couple (A/B) is reversible. Upon pulsed illumination, the steady-state concentration of each species is perturbed, which leads to a transient change in the open-circuit potential between the illuminated and dark (reference) electrodes. The photopotential (V) may be described by the following relationship:§

(kl/k_l)

where R is the gas constant and F i s the Faraday constant. In this equation, K is an instrument-dependent constant that includes factors such as the geometry of irradiation, refraction, and reflection losses, as well as the illuminated area, transmittance, and resistance of the optically transparent electrodes. The first parenthetical term represents the efficiency of forming free ions in solution, which is dependent upon the total number of solute molecules absorbing radiation as well as the probability of free-ion formation after absorption. The second parenthetical term represents the efficiency of collecting and detecting the free ions, which is related to the rate constants for mass transport of Z to and from the electrode surface (kl and k i, respectively), mass transport of B to and from the electrode surface (k2 and k_2, respectively), and electron transfer from B to the electrode (k3). This model was developed with the assumptions that the signal arises from photoionization at or near the surface of the illuminated electrode, and that the concentrations of all species at the electrode are diffusion limited. It is also assumed that the perturbation in the concentrations caused by pulsed illumination is small in comparison with the steady-state concentrations. As a consequence with these assumptions, this model may not be rigorously applicable under all circumstances.

In this paper, we compare and evaluate the photoconductive and photovoltaic techniques for detection of liquid-phase photoionization. The response of each system is characterized with respect to laser pulse energy, electrode separation distance, applied potential, and other relevant experimental parameters. In addition, the detection limit and linear dynamic range are compared in methanol, water, and simple electrolyte solutions.

§ Equation 5 may be evaluated by using any internally consistent system of units, for example, by using the International System of Units (SI) with the mksa convention: V(V), R (J K-' mol 9. T(K), F(C mol-~), K(molJ ~s -~m-2),Ep(J),ez( m2mol 9, CzandCB(molm 3),k~ to k3 (s-t), and P(0) is unitless.

OPTICAL

I

I

FARADAY CAGE

BOXCAR

INTEGRATOR

I Fie. 3. Schematic diagram of the photovoltaic detection system for liquid-phase photoionization: M, mirror; PD, photodiode.

EXPERIMENTAL

Optical System. The experimental apparatus for these studies is shown schematically in Figs. 2 and 3. The excitation source is a Lambda Physik EMG 101 MSC laser, operated with KrF exciplex as the lasing medium at 248 nm (5.0 eV), 200-mJ pulse energy, 23-ns pulse width, and 10-Hz repetition rate. The laser radiation enters an optical attenuation box, where a fraction (~ 8%) is diverted and attenuated by a series of fused-silica plates. This attenuated beam is used to produce a trigger signal using a fast-response photodiode (Hamamatsu Corp., Model S 1722-02). The remainder of the beam is reflected from another fused-silica plate and is directed to the photoionization cell. For the photoconductive cell, two lenses are used to focus the radiation from the fused-silica plate to a spot size of 2-mm diameter between the electrodes. The pulse energy reaching the cell is approximately 6.8 mJ with a corresponding integrated irradiance of 210 m J/ cm 2. For the photovoltaic cell, no focusing optics are used, and the integrated irradiance is approximately 3.9 mJ/ c m 2.

Photoconductive Detection System. A static cuvette cell, constructed from 12.7-mm-i.d. square-wall quartz tubing with a total cell volume of 3 mL, is used in all studies. The platinum foil electrodes (0.21-mm thickness, 8-mm width, 3$-mm length) are positioned in the cuvette by using Teflon~ blocks to control the separation distance (3, 5, 7, 9, and 11 mm).

A high-voltage power supply (Keithley Instruments, Model 240) is used to provide the bias voltage to the platinum electrodes. The photocurrent, which is modulated at the repetition rate of the laser, is detected with an ac-¢oupled amplifier similar to that described by Ya- mada et a1.19 The amplifier has been modified by changing the input capacitor from 1 to 3 pF, resulting in a time constant of approximately 1 #s.

Photovoltaic Detection System. The electrodes used in these studies are optically transparent gold films (~ 100 ~), produced in-house by vapor deposition or electron- beam sputtering on fused-silica substrates (Esco Products Inc., 2.54-cm diameter, 0.159-0.476 cm thickness). A thin layer of silver conductive epoxy (TRA-CON Inc.) is

1104 Volume 48, Number 9, 1994

50-

40-

30-

z L,J C.d O~ D 0 0 0 li_ O_

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F i G . 4. Logarithmic dependence ofphotocurrent (A) and photocurrent signal-to-noise ratio (B) on laser pulse energy. Experimental conditions: K r F excimer excitation using platinum foil electrodes at 5-mm separation distance; 5 0 0 V applied potential; 10 -4 M TMPD in methanol.

coated from the electrode surface onto the side of sub- strate, to which a copper wire is attached for electrical connection. The electrodes are positioned in a sandwich- type cell with a silicone rubber O-ring or a Teflon ® gasket to control the separation distance (0.125, 0.25, 0.5, 1.6, and 4.7 ram).

The photopotential developed between the electrodes, which is modulated at the repetition rate of the laser, is amplified by a two-stage, ac-coupled differential amplifier that was manufactured in-house. The preamplifier, lo- cated immediately adjacent to the cell inside a Faraday cage, has a nominal gain of 10, while the second-stage amplifier has a nominal gain of 40. The time constant of the complete circuit is approximately 2 us.

Data Acquisition. A boxcar integrator (Stanford Re- search Systems, Model SR250) is used to sample the amplifier output at a fixed time interval after the trigger pulse. This sampling interval, which corresponds to the signal maximum, occurs after 100 ~s (gate width 50 ns) for the photoconductive method and after 12 tss (gate width 15 ns) for the photovoltaic method. An analog-to-digital converter (Stanford Research Systems, Model SR245) is used in conjunction with an IBM PC-XT computer to acquire data. Both the boxcar integrator and the A/D converter are controlled by a commercially available soft- ware package (Stanford Research Systems, SR265). An

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APPLIED POTENTIAL (V)

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20-

10- ~ u m

0 0 1;0 2 ;0 ' ~ 0 4~0 5 ;0 600

APPLIED POTENTIAL (V)

F r o . 5. Dependence of photocurrent (A) and photocurrent signal-to- noise ratio (B) on applied potential. Experimental conditions: laser pulse energy 6.8 mJ; other conditions as given in F i g . 4.

oscilloscope (Tektronix Inc., Model 2235) is used to mon- itor the photoionization signal in real time during the experiment.

Both photoconductive and photovoltaic measurements are performed by averaging 50 data points collected at the specified time interval. The response is measured under illuminated (signal) and nonilluminated (background) conditions. Corrected signals are calculated by subtract- ing the background, which is predominantly due to amplifier and other electronic offsets as well as radio-fre- quency (r0 interference, from the signal measurements. The photocurrent or photopotential is then reported as the average of five corrected data sets, with the confidence interval expressed by the standard deviation.

Reagents. N,N,N',N'-Tetramethyl- 1,4-phenylenediamine (Aldrich, 98%) is purified by vacuum sublimation at 50°C and stored in the dark. Stock solutions (10 -3 - 10 -7 M) of this reagent are prepared freshly as needed in the appropriate solvent. High-purity methanol is distilled-in-glass grade (Baxter Healthcare Corp., Burdick and Jackson Division); water is deionized and doubly distilled in glass (Coming Glass Works, Model MP-3A).

RESULTS AND DISCUSSION

N,N,N',N'-Tetramethyl- 1 , 4 - p h e n y l e n e d i a m i n e (TMPD) was chosen as the analyte for these studies be-


cause it provides visual verification of the photoionization process; the neutral molecule is colorless, whereas the cation has an intense and characteristic blue color. In the gas phase, the one-photon ionization threshold of TMPD has been reported at 6.2 eV. 2° One-photon ionization has also been observed in liquid solvents, where the ionization threshold varies from a maximum of 5.0 eV in nonpolar solvents such as hexane 21 to 3.5 eV in polar solvents such as methanol. 4,22 In addition to one- photon excitation, ionization can also occur via simultaneous 23 or sequential 24 two-photon excitation as well as by ionization of a solute/solvent complex. 25 Under the experimental conditions employed in this study, only one-photon ionization is expected in methanol and water solutions.

Characterization of the Photoconductive Detection Sys- tem. The response of the photoconductive detection system is dependent on a number of experimental parameters, including the laser pulse energy, the applied potential, the separation distance between the electrodes, and the analyte concentration.

The dependence of the photocurrent on the laser pulse energy is shown in Fig. 4A for a solution of 10 -5 M TMPD in methanol. The logarithmic graphic ofphotocurrent vs. pulse energy is linear with a slope of 0.98 (R 2 = 0.99) in the range from 4.2 to 6.8 mJ. The slope indicates that the photoconductive signal arises from a one-photon ionization process, which is consistent with previous studies 26 and with Eq. 4. The signal-to-noise (S/N) ratio is shown as a function of the laser pulse energy in Fig. 4B. The predominant sources of noise in photoconductive detection have been determined to arise from the amplifier circuit or from fluctuations in the background current in polar solvents. 27 Because neither of these noise sources is influenced by the laser pulse energy, the S/N ratio increases proportionally with the signal. These results suggest that the highest laser pulse energy that avoids satu- ration and photodecomposition of the solute should be utilized to achieve maximum sensitivity in polar solvents.

The photocurrent is also a function of the potential applied to the electrodes. This dependence is shown in Fig. 5A for a solution of 10 -5 M TMPD in methanol for applied potentials from 50 to 500 V. In agreement with the theory described in Eq. 4, the photocurrent is linearly dependent on the applied potential up to the limit of the experimental system (500 V). As shown in Fig. 5B, the S/N ratio increases with potential to approximately 200 V, above which it remains essentially constant. At potentials less than 200 V, the noise is constant and is independent of applied voltage, which suggests that the noise probably arises from the amplifier circuit. Above 200 V, an additional contribution to the noise, probably arising from fluctuations in the background current, increases at the same rate as the signal. Because the SIN ratio remains relatively constant, no real advantage is gained by operating the photoconductive detection system at high voltages with polar solvents.

The magnitude of the photocurrent is also dependent on the separation distance between the electrodes at constant voltage. Equation 4 predicts an inverse relationship between the photocurrent and the square of the separation distance. Figure 6A demonstrates this relationship for a solution of 10 -s M TMPD in methanol for separation

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FIG. 6. Dependence of photocurrent (A) and photocurrent signal-to- noise ratio (B) on inverse separation distance. Experimental conditions as given in Fig. 5.

distances from 3 to 11 mm. The observed increase in signal magnitude results from greater ionization efficiency (Eq. 3) and enhanced collection efficiency (Eq. 4) of the ions at higher electric field strength. Figure 6B indicates that the S/N ratio improves with decreasing separation distance due, in part, to this increase in signal magnitude. In addition, the noise is reduced because of the smaller volume of sample and, hence, the smaller number of native and contaminant ions that cause interference. Therefore, the separation distance should be decreased insofar as possible to achieve the maximum S/N ratio in polar solvents.

The dependence of photocurrent on sample concentration has also been investigated, as shown in Fig. 7A for solutions of TMPD in methanol and water. The calibration curve for the methanolic solutions is linear for concentrations below 10 -4 M, with a linear dynamic range of three orders of magnitude (R 2 = 0.99). The calibration curve for the aqueous solutions is linear for concentrations below 10 -5 M, with a linear dynamic range of two orders of magnitude ( R 2 = 0.93). Above the linear range, the signal decreases with increasing TMPD concentration for both methanol and water solutions. At high concentration, the excited-state solute molecules have a greater probability of interaction with other solute molecules, allowing triplet-triplet annihilation 2s as well as other photochemical reactions to occur. These reactions decrease

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F[c. 7. Logarithmic dependence ofphotocurrent (A) and photocurrent signal-to-noise ratio (B) on TMPD concentration in methanol (D) and water (11). Experimental conditions as given in Fig. 5.

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FIG. 8. Logarithmic dependence of photopotential (A) and photopotential signal-to-noise ratio (B) on laser pulse energy. Experimental conditions: KrF excimer excitation using 100-A gold-film electrodes at 0.250-mm separation distance; 10 4 M TMPD in methanol.

the ion abundance and lifetime, both of which reduce the signal magnitude according to Eq. 4.

The S/N ratio is shown in Fig. 7B as a function of the TMPD concentration in methanol and water solutions. The methanolic solutions show a large increase in the S/N ratio, whereas the aqueous solutions show a smaller increase over the same concentration range. The noise level is greater in aqueous solutions due to dissociation of the solvent, which increases the background current. The minimum detectable concentration is 1 × 10 -7 M TMPD in both methanol and water (S/N ~- 2). This detection limit is similar to that reported by other research- ers using TMPD in alcoholic solvents and is poorer by several orders of magnitude than that in nonpolar solvents?

Characterization of the Photovoltaic Detection System. In this study, the response of the photovoltaic detection method is characterized with respect to a number of experimental parameters, including laser pulse energy, electrode separation distance, and solute concentration.

The dependence of the photovoltaic signal on laser pulse energy is shown in Fig. 8A for a solution of 10 -4 M TMPD in methanol. The logarithmic graph ofphotopotential vs. laser pulse energy is linear with a slope of 0.98 (R 2 = 0.99) in the range from 4.3 to 6.8 mJ. This slope confirms that the photovoltaic signal arises from a one-photon ionization process. When these data are replotted in a semilog-

arithmic form [photopotential vs. log(laser pulse energy)], the resulting graph is also linear ( R 2 = 0.98), as expected from Eq. 5. The S/N ratio is shown as a function of laser pulse energy in Fig. 8B, which demonstrates that the noise is independent of the pulse energy. The predominant source of noise has been identified as the amplifier circuit under all experimental conditions. These results imply that the highest laser pulse energy that does not cause photodecomposition of the solute or damage to the optically transparent electrodes should be utilized to achieved maximum sensitivity.

The maximum photopotential is also a function of separation distance between the electrodes. Figure 9A demonstrates this relationship for a solution of 10 -4 M TMPD in methanol for separation distances between 0.125 and 5 mm. The photopotential decreases in an exponential manner with increasing separation distance until approximately 0.5 mm, after which it remains relatively constant. In the theoretical model (Eq. 5), it is assumed that the separation distance is sufficiently large to ensure that the back electrode remains nonilluminated. These conditions appear to be achieved at separation distances greater than 0.5 mm, where the signal magnitude is constant. At shorter separation distances, where the rear electrode may be illuminated, the observed increase in signal magnitude is not readily explained and is contrary to initial predictions. 29 Figure 9B indicates that there is a


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substantial improvement in the S/N ratio due to the increase in signal magnitude with decreasing separation distance. The noise, which arises predominantly from the amplifier circuit, is independent of the separation distance. These results suggest that a substantial increase in sensitivity can be achieved by miniaturization of the detection cell.

The relationship between signal magnitude and sample concentration, shown in Fig. 10A, is observed to be semilogarithmic, as expected from Eq. 5. The calibration curves are linear for both methanol (R 2 = 0.90) and water solutions (R 2 = 0.85) at concentrations greater than 10 -6 M with linear dynamic ranges that exceed three orders of magnitude. Figure 10B shows a large increase in the S/N ratio with increasing concentration in both methanol and water solutions. This enhancement is due to an increase in signal magnitude, while the noise, predominantly due to the amplifier circuit, remains constant. The minimum detectable concentration is 5 x l0 -7 M TMPD in methanol and 1 x 10 -7 M TMPD in water (S/N = 2). This detection limit represents a fivefold improvement over that reported previously for the photovoltaic detection technique. 4

Comparison of Photoionization Detection Techniques. From the results presented herein, it is apparent that the photoconductive and photovoltaic detection techniques have inherent advantages and disadvantages. In many circumstances, the different operating characteristics of

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CONCENTRATION (M)

FIG. 10. Semilogarithmic dependence ofphotopotential (A) and photopotential signal-to-noise ratio (B) on TMPD concentration in methanol (E3) and water (11). Experimental conditions as given in Fig. 9.

these detection techniques make them complementary, rather than competitive, approaches for the detection of liquid-phase photoionization.

Under the conditions employed in this study, both techniques show similar detection limits of approximately 1 x 10 -7 M TMPD in both methanol and water solvents. The linear dynamic range of the photoconductive system is less than three orders of magnitude and is limited to concentrations less than 10 -4 M TMPD in methanol and 10 -5 M in water. In contrast, the photovoltaic method has a linear range in excess of three orders of magnitude, extending from the detection limit to the solubility limit of TMPD.

Although both of these detection techniques can be utilized to detect photoionization in methanol and water solutions, their performance in other solvents differs substantially. The photovoltaic technique shows small signal magnitude for TMPD in solvents of intermediate polarity such as chloroform, but no response in nonpolar solvents such as hexane. This lack of response may arise because the photon energy (5.0 eV) is barely sufficient to cause one-photon ionization of TMPD in hexane. 2~ The photoconductive technique, by using high electric field strengths that enhance both the ionization efficiency and the collection efficiency, has a reported detection limit of 1 x 10 -9 M for a similar aromatic amine (1-aminonaph- thalene) in hexane. ~9


10-

8 -

c

6 - I - - Z Ld

4 -

0

0 2 - I - - 0 I

O_ O -

- 2 . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . . i . . . . . . . 10-8 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2

0 . . . . . . .

1 0 - 8 1 0 - 3 1 0 - 2

C O N C E N T R A T I O N ( M )

FIG. 11. Semilogarithmic dependence ofphotocurrent on TMPD concentration in aqueous 10 -4 M KCI solution. Experimental conditions as given in Fig. 5.

500-

~ " 4 0 0 -

. .J

.~ 300- Z taJ

2 0 0 [1- © I - - o 3Z o_ 1 0 0 -

. . . . . . . . i . . . . . . . . I . . . . . . . . i . . . . . . . . i . . . . . . . . i

1 0 - 7 1 0 - 6 1 0 - 5 1 0 - 4

CONCENTRATION ( M )

FIG. 12. Semilogafithmic dependence of photopotential on TMPD concentration in aqueous l0 4MKClsolution. Experimentalconditions as given in Fig. 9.

Conversely, the photovoltaic method is capable of detecting photoionization in ionic solutions, where a photoconductive measurement is not possible. As shown in Fig. 1 1, the photoconductive system shows no discernible response above the background for varying concentrations of TMPD in aqueous 10 -4 M KCI solutions. The photovoltaic system shows detectability and linearity in these ionic solutions (Fig. 1 2) that compare favorably to those in distilled water solutions (Fig. 10A). In addition, the photovoltaic method can be used to detect ionic species, such as the permanganate ion, that undergo photodecomposition or other photochemical reactions. 4

Both of the detection techniques exhibit an increase in signal magnitude and signal-to-noise ratio with decreasing separation distance. Consequently, sensitive detection can be achieved in very small sample volumes. The photoconductive technique has been utilized in static systems of 1 0-#L volume 3° and in flowing systems of 1 4-nL volume. 31 The photovoltaic technique has been utilized in static systems of 2-uL volume (Fig. 9A), and preliminary studies have been performed in flowing systems of sub- microliter volume.

A final point of comparison concerns the laser pulse energy required to induce ionization. The photovoltaic method can detect solute photoionization using moderate integrated irradiance (3.9 mJ/cm2), whereas the photoconductive technique requires significantly higher irradiance (2 1 0 mJ/cm0 to obtain comparable results. If the irradiance is increased significantly for the photovoltaic method, the optically transparent electrode material may be damaged or destroyed. This restricts the photovoltaic technique to one-photon ionization schemes; however, this limitation may be overcome by the use of more robust electrode materials such as overcoated or minigrid gold electrodes. 4 In contrast, the photoconductive technique is compatible with both one- and two-photon ionization s c h e m e s . 21-24

CONCLUSIONS

The photoconductive and photovoltaic detection techniques have been compared from both theoretical and experimental perspectives. These results provide guid-

ance for the selection of optimal experimental conditions for the detection ofphotoionization in polar solvents. On the basis of their inherent advantages and limitations, the photoconductive and photovoltaic methods appear to be highly complementary rather than competitive techniques.

ACKNOWLEDGMENTS

We thank Dr. Richard D. Sacks (University of Michigan), Dr. Stephen W. Brewer (Eastern Michigan University), and Jim Kulman (University of Michigan) for help in producing the gold-film electrodes, and Martin Rabb (Michigan State University) for design and construction of the photovoltaic amplifier circuit. This work was supported in part by the Dow Chemical Co.

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Comparative Study of Photovoltaic and Photoconductive Detection Methods for Liquid-Phase Photoionization