1646 Volume 53, Number 12, 1999 APPLIED SPECTROSCOPY0003-7028 / 99 / 5312-1646$2.00 / 0

q 1999 Society for Applied Spectroscopy

Fiber-Optic Resonance-Enhanced Multiphoton IonizationProbe for in Situ Detection of Aromatic Contamination

BRIAN M. CULLUM, SLADE K. SHEALY, and S. MICHAEL ANGEL *Department of Chemistry and Biochemistry, The University of South Carolina, Columbia, South Carolina 29208

A simple ® ber-optic probe suitable for remote analysis with the useof resonance-enhanced multiphoton ionization has been developed

and demonstrated by measuring vapor-phase toluene and gasoline

above aqueous solutions. An optical ® ber transmits a high-powerlaser pulse (266 nm or 532 nm) to the sample, ionizing it, and the

subsequent ions are collected with a platinum electrode mounted at

the ® ber tip. Measurements take approximately 100 s to perform,require no sample preparation, and have been demonstrated over

a distance of ® ve meters. The limit of detection for toluene in water

with the use of this probe is 1.47 6 0.02 ppb (wt/wt). In addition totwo-photon excitation at 266 nm, the feasibility of a 21 2 excitation

scheme using 532 nm has also been shown.

Index Headings: Resonance-enhanced multiphoton ionization;

REMPI; Fiber-optic; sensor; Volatile organics.

INTRODUCTION

Volatile organic chemicals (VOCs) are found in almostevery natural water source on earth.1 This presence isprimarily due to the poor disposal practices of the pastand to accidental spills. For example, in 1988 it was es-timated that 33.1 million pounds of benzene and 344.6million pounds of toluene were released into the envi-ronment.1 Once these chemicals have been introducedinto the ground, they begin to partition into water sup-plies and collect in wells and other storage areas. Sinceclean-up of contaminated water supplies is very expen-sive, it is desirable to develop techniques that can be usedon-site to rapidly identify organic pollutants as well asthe amount of contaminant present.

Gas chromatography (GC) is the most commonly usedtechnique for the detection of trace volatile organic com-pounds in water.2,3 For GC analysis of VOCs, there is typ-ically a preconcentration step that extracts the sample fromthe water, requiring a great deal of time and effort. Ex-traction methods include activated charcoal, liquid/liquidextraction, and solid-phase microextraction (SPME).4,5

Also, GC measurements are often dif® cult to make. Thusa relatively rapid method of analysis for in situ monitoringof ground water and vadose-zone contaminants is desir-able.

Resonance-enhanced multiphoton ionization (REMPI)is potentially a useful in situ technique for obtaining bothqualitative and quantitative information about volatile or-ganic pollutants. This technique has been used extensive-ly in the last couple of decades as a selective ionizationsource in mass spectrometry 6±12 and for spectroscopiccombustion analysis.13±16 In the REMPI technique, a high-powered, tunable pulsed laser is used to electronicallyexcite a speci® c molecule either directly or through a

Received 17 December 1998; accepted 25 June 1999.* Author to whom correspondence should be sent.

multiphoton process. The subsequent absorption of ad-ditional photons ionizes the molecule. Due to the nonlin-ear nature of this technique, it is important to have highlaser power densities near the collection electrode. A highpower density increases the probability of ionizing themolecule of interest. Once ionized, an electrode biased ata high positive or negative potential is used for electronor ion collection. The resulting current is then measuredin an electrical circuit across a ``leak resistor’ ’ , and theresulting voltage is proportional to the concentration ofthe molecular species being ionized.

Because of its sensitivity and selectivity, REMPIwould appear to be an excellent technique for analysis ofVOCs. The purpose of this paper is to demonstrate thatthe technique is also suitable for remote in situ environ-mental measurements of a model VOC, toluene, by usingoptical ® bers. We have demonstrated previously the useof optical ® bers to introduce high-power laser pulses toa remote area for laser-induced breakdown spectroscopymeasurements.17,18,20 In the case of REMPI spectroscopy,a small electrode is placed at the ® ber probe tip near thefocus of the laser. The addition of the 1 mm diameterelectrode increases the size and complexity of the probeonly minimally. In this paper, we describe the ® ber-optic-based REMPI probe and demonstrate it by measuring tol-uene- and gasoline-spiked water samples. Both two(1 1 1) and four (2 1 2) photon excitation schemes aredemonstrated with toluene detection limits of 1.47 6 0.02ppb (wt/wt) and 8.5 6 0.4 ppb (wt/wt), respectively.

EXPERIMENTAL

REMPI System. The REMPI system (see Fig. 1) con-sists of a Q-switched Nd:YAG laser operating at eitherthe second or fourth harmonic (Continuum Model Sure-lite III operating at 532 nm, or New Wave Research Mod-el MiniLase-20 operating at 266 nm), a high-voltagepower supply (Gamma High Voltage Model RC5-30P),the probe/electrode electronics, and a fast digital oscil-loscope (LeCroy Model 9350L). The details of launchinga high-power laser pulse into a relatively small-diameteroptical ® ber has been described in detail in our previousworks.17,18 This process basically involves a precision pin-hole placed on the ® ber tip. Once launched, the laser lightis transmitted through the ® ber to the probe where it isfocused to a point by an f /1.5 lens, ionizing the toluene.Electrons are collected by the platinum electrode that isbiased at 1 1000 V with respect to the aluminum probehousing. The current passes through a 250 k V resistor,and the resulting voltage drop is measured with the useof a 1 M V coupled 500 MHz digital oscilloscope. All thedata shown are the average signal from 1000 laser shotsat 10 Hz for a total acquisition time of 100 s. The laser

APPLIED SPECTROSCOPY 1647

FIG. 1. Schematic diagram of REMPI apparatus showing detail of the® ber-optic REMPI probe. The lenses are 1 in. diameter f /1.5 planoconvex, and the platinum electrode is biased at 1 1000 V with respectto the aluminum housing. L , lens; R, resistor; V, power source.

power at the probe tip was typically 4.0 and 20 mJ/pulsefor 266 and 532 nm wavelength excitation, respectively.

Fiber-Optic Probe. Most sensitivity and detectionlimit measurements were made by using the ® ber-opticprobe, while optimization steps including electrode biasand effect of ® ber diameter and type were performed witha sealed cell similar in design to the ® ber-optic probe. Across section of the probe is shown in Fig. 1. The exci-tation ® ber is located in the top of an aluminum probehousing, and the transmitted light from this ® ber is col-limated by using an f /1.5 fused-silica lens. Another f /1.5 fused-silica lens is used to focus this collimated lightto a small spot within 1 mm of the electrode tip. Threedifferent excitation ® bers were tested with the use of thesealed cell, with core diameters of 1000, 910, and 550m m (3M Models FG-1.0-URT, FG-910-UER, or FG-550-UER). The 910 and 550 m m ® bers were high ±OH con-tent while the 1000 m m ® ber was low ±OH content, andall ® bers were 5 m in length.

Preparation of Samples. Aqueous solutions were pre-pared by placing equal volumes of toluene (Aldrich), orgasoline (Shell 93 octane), and deionized water in asealed container and stirring vigorously for several days.The water portion of the sample was then extracted andplaced in a sealed container. A concentration of 525 ppb(wt/wt) toluene was measured for this solution by absor-bance at 261 nm. This measurement is close to the sat-uration concentration of 550 ppb (wt/wt) toluene.19 Tocover the concentration range of toluene that was mea-sured, 0 ppb to 525 ppb, we diluted this stock solutionwith deionized water. For measurements using the probe,sample solutions were placed in a petri dish and coveredwith a plastic air-tight lid with a small hole in the center.The probe was placed over this hole, and the headspaceabove the sample was allowed to equilibrate for 20 minbefore measurements were taken. All solution concentra-tions are reported in ppb (wt/wt). For optimization ex-periments, neat toluene or a toluene/methanol solutionwas injected into the sealed cell and allowed to evaporate

completely. For sealed cell measurements the concentra-tion of toluene is reported in ppb (v/v) with respect tothe air.

Analysis of Data. All of the data are shown withoutaveraging or smoothing other than what was done by theaccumulation of the signal from 1000 laser pulses. Linearleast-squares calculations were performed and data plot-ted with the use of Igor Pro Version 3.1 data analysissoftware from Wavemetrics, Inc. For all calculations, theintensity of the REMPI signal is based on peak height,and all error bars displayed represent 6 1 standard devi-ation of ® ve replicate measurements. The detection limitwas determined by using a linear least-squares ® t of thedata and is reported as three times the standard deviationof the blank divided by the slope (e.g., a ``3 s ’ ’ detectionlimit).

RESULTS AND DISCUSSION

Since REMPI is a nonlinear process, it is important tolaunch as much power down the ® ber as possible. How-ever, focusing the laser on the tip of the ® ber can chipthe face, or in some cases completely destroy the ® bertip. In addition, too much laser power can cause cata-strophic damage to the core/cladding interface. By usinga pinhole at the ® ber tip and placing the ® ber just beyondthe focus of the laser, we have shown that 1000 m m di-ameter optical ® bers can withstand approximately 140mJ/pulse at 1064 nm.17,20 This launch procedure workedvery well for REMPI experiments with virtually no ® berdamage after repeated measurements.

The REMPI signal intensity for a particular sampledepends on several parameters such as electrode voltage,type and diameter of excitation ® ber, laser power, andexcitation wavelength. We have investigated the effect ofchanging some of these parameters for ® ber-optic mea-surements of aqueous toluene and gasoline solutions. The® rst of these parameters, electrode voltage, is very im-portant for ef® cient signal collection, and is primarily de-pendent on the probe design. One of the main reasonsfor the great sensitivity of REMPI is because chargedparticles, ions or electrons, can be collected with nearly100% ef® ciency.13 The effect of electrode bias was stud-ied by injecting 1.78 ppt of toluene into the cell, allowingit to evaporate, and then monitoring the ion signal as theprobe voltage was varied from 500 to 1200 V. Three dis-tinct regions were evident from these data. Below 800 V,there is a dramatic increase in signal with increasing volt-age due to enhanced collection ef® ciency and reducedelectron/ion recombination. Between 800 and 1100 V, aplateau region exists where the maximum collection ef-® ciency occurs with little change in signal as the voltageis increased. Above 1100 V, the ion signal increases dra-matically with increasing probe voltage, because of a cas-cading effect. A bias of 1 1000 V was used for all probemeasurements, to maximize the collection ef® ciencywhile minimizing signal ¯ uctuations caused by smallvoltage ¯ uctuations.

Figure 2 shows oscilloscope traces of the time responseof the ® ber-optic REMPI probe to the measurement of525, 105, and 52.5 ppb (wt/wt) aqueous-phase toluenesolutions. For these measurements the probe was placedin the headspace above each solution. The initial, almost

1648 Volume 53, Number 12, 1999

FIG. 2. Oscilloscope traces showing time response of ® ber-optic REM-PI probe to the measurement of 525, 105, and 52.5 ppb (wt/wt) aqueous-phase toluene solutions. All measurements were made in the headspaceabove the solution. Note: vertical scale is in arbitrary units.

FIG. 3. The effect of optical ® ber type and diameter on sensitivity ofREMPI signal. Circles (V) denote the 1000 m m low ±OH content ® ber,squares (M) denote the 910 m m high ±OH content ® ber, and triangles( n ) denote the 550 m m high ±OH content ® ber. Excitation of the samplewas accomplished by launching 6.1 mJ/pulse of 266 nm light into each® ber. Each point represents the average of ® ve measurements with errorbars showing 6 1 sample standard deviation.

FIG. 4. Dynamic range of REMPI technique for measurement of tol-uene. A 550 m m high ±OH content ® ber was used for these measure-ments with the collection electrode biased at 1 1000 V and 266 nmexcitation. Each point in this curve represents the average of ® ve mea-surements with error bars showing 6 1 sample standard deviation. Note:most of the error bars are smaller than the data points.

vertical, increase in signal occurs almost immediately af-ter the laser pulse. The slower decay occurs over hun-dreds of milliseconds, the exact decay time being depen-dent on the distance between the electrode and the fo-cused laser beam. There was also considerable electronicnoise when the laser was ® red that was due to poorshielding of the probe electronics. This noise was re-moved by smoothing. For the calibration data shown inFigs. 5 and 6, 1000 laser pulses were averaged to mini-mize this noise. The highest concentration shown here,525 ppb, is well above the linear working range of theprobe.

Laser power density is also crucial to obtaining qualityREMPI signals and high sensitivity. It is important bothto transmit the maximum amount of excitation lightthrough the ® ber and to have a small spot size near theelectrode tip. Optimizing the power density of the spotnear the electrode is dependent on the diameter and typeof excitation ® ber used as well as the focusing optics. Onlythree suitable ® bers were available for these experiments;a low ±OH content 1000 m m ® ber, and two high ±OHcontent, UV transmitting, 910 and 550 m m ® bers (see Fig.3). The low ±OH content ® ber has its greatest transmissionef® ciency between 350 and 800 nm. This ® ber also costssigni® cantly less than the UV transmitting ® bers of com-parable diameter and is preferable in terms of cost. How-ever, the 266 nm transmission of the low ±OH content® ber is signi® cantly worse than for the high ±OH content® bers.

To test the ® ber transmission at 266 nm, 6.1 mJ waslaunched into each ® ber and the power was measured atthe ® ber tip. The output of the ® bers was 2.0, 4.0, and4.0 mJ for the 1000, 910, and 550 m m diameter ® bers,respectively. Figure 3 shows a calibration curve for tol-uene in the vapor phase with the use of the sealed cell.From these calibration data, the sensitivity was found tobe 0.045 6 0.004, 0.064 6 0.001, and 0.091 6 0.001mV/ppb of toluene, corresponding to detection limits of4.4 6 0.4, 1.01 6 0.01, and 0.858 6 0.001 ppb (v/v) oftoluene in air for the 1000, 910, and 550 m m ® bers, re-spectively. The use of the 550 m m ® ber allowed for atighter focus near the electrode, with no loss of laser pow-er, and therefore a greater power density at the electrode.As a result, an increase in sensitivity was obtained byusing the 550 m m ® ber compared to the other two. The

550 m m ® ber is also relatively inexpensive, only a fewdollars per meter. When the laser is properly launched,the ® ber lasts for many weeks or even months and needsto be replaced only when the tip is burned by the laser.Degradation in the ® ber that results from UV excitationis minimal. Also, most burn spots on the tip of the ® bercan be quickly removed by polishing the ® ber.

A measurement of the dynamic range of this techniquefor toluene was performed by using the 550 m m excita-tion ® ber. For this measurement, different amounts of tol-uene were injected into the sealed cell and allowed toevaporate, creating a concentration range from 1 ppb to10 ppm of toluene (v/v). Five replicate measurementswere made at each concentration. The results, shown inFig. 4, indicate that the REMPI signal remains linear overapproximately 3.5 orders of magnitude for vapor-phasetoluene. This ® nding indicates that good quantitativeREMPI measurements can be made remotely for tolueneat ppb and ppm concentrations.

Calibrations of the REMPI probe for toluene and gas-oline were obtained with aqueous solutions. Shown inFig. 5a is a calibration curve for aqueous toluene solu-tions from 5.25 to 105 ppb (wt/wt) with the use of the

APPLIED SPECTROSCOPY 1649

FIG. 5. Calibration of (a) toluene- and (b) gasoline-spiked water sam-ples. For both calibrations a 550 m m high ±OH content ® ber was usedfor the measurements with the collection electrode biased at 1 1000 Vand 266 nm excitation. Each point in this curve represents the averageof ® ve measurements with error bars showing 6 1 sample standard de-viation. Note: most of the error bars are smaller than the data points.

1 1 1 excitation scheme at 266 nm. These concentrationsrefer to the solution-phase concentration and not to theconcentration in the headspace. It is dif® cult to establishtrue equilibrium in the headspace above a solution. Forthese experiments, conditions were used that producedreproducible results, without an attempt to reach trueequilibrium. Since the purpose of this study was to dem-onstrate an in situ measurement capability, the gas-phaseconcentrations were not determined directly. In an effortto make it easier to reproduce these results, the solutionvolume was the same as the headspace volume above thesample. After approximately 20 min, equilibrium be-tween toluene in solution and vapor phase was reached,and a measurement was taken. From the calibration dataa sensitivity of 0.144 6 0.002 mV/ppb, corresponding toa detection limit of 1.47 6 0.02 ppb of toluene in water,was obtained.

The probe was also tested by using water samplesspiked with gasoline. Gasoline is often found in groundwater samples, and a major constituent of gasoline is tol-uene (5±7%).2 These samples were prepared from a stocksolution of water saturated with gasoline, the same pro-cedure used for toluene solutions. The concentrations oftoluene in the gasoline-saturated stock solution was as-sumed to be the same as the saturated toluene solution(e.g., 525 ppb). A calibration curve is shown in Fig. 5b,and the concentration listed on the abscissa is the esti-mated concentration of toluene, based on a concentrationof 525 ppb (wt/wt) in the extracted water. It is obviousfrom this calibration that the sensitivity and detection

limit of toluene from the gasoline-spiked samples aresimilar to those for toluene-spiked water samples (0.1396 0.005 mV/ppb and 1.79 6 0.06 ppb). This ® ndingseems to indicate that most of the REMPI signal comesfrom the toluene in the sample and not much comes fromthe other constituents. Gasoline is a very complex mix-ture, and it is not the purpose of this paper to test allpossible interferences. However, a few of the more com-mon constituents were tested, including benzene, hexane,heptane, and methanol. With the use of the probe with266 nm excitation, no appreciable signal from neat so-lutions of these compounds was observed. This result oc-curs partly because the solubility of toluene in water ismuch greater than that of all other major constituents ofgasoline except for methyl tertbutyl ether and benzene.19

The lack of interference from these samples also pertainsin part because the vibronic bands for these species inthe vapor phase are suf® ciently resolved to minimize ion-ization with the experimental conditions used here.

The gasoline results should not be interpreted as anindication that the REMPI probe is selective for toluenewith the use of 266 nm excitation. It has been shown that266 nm light ionizes a large number of aromatics.21±23

The reason the observed response to gasoline is so similarto that of toluene alone is not clear. This response mightpartly result from the extraction technique used, with tol-uene being preferentially extracted. It also might be thatthe signal comes from several different aromatics, and itis just a coincidence that the response matches so wellthat observed for toluene alone. A complete interferencestudy was not the intention of this work.

Although excellent results have been obtained by using266 nm excitation, all fused-silica, low or high ±OH, op-tical ® bers transmit more ef® ciently in the visible than inthe ultraviolet. Because of this consideration, we are in-vestigating the use of higher order excitation schemeswith this probe to minimize ® ber losses of the excitationlight. In one experiment, 23 mJ/pulse of 532 nm lightwas launched into the 550 m m ® ber, and approximately20 mJ/pulse was measured at the distal end. In this case,two photons excite and two photons ionize the toluenevia a 2 1 2 excitation scheme. The results, shown in Fig.6, demonstrate that this higher order excitation schemeworks very well for toluene, showing feasibility for veryremote measurements.

Fiber transmission at 532 nm can be several hundredtimes higher than at 266 nm for 100 m lengths. Whilethe sensitivity, 0.014 6 0.007 mV/ppb, and detection lim-it, 8.5 6 0.4 ppb (wt/wt), for this experiment are nearlyan order of magnitude lower than for the 1 1 1 excitationscheme, these results are very promising and the detec-tion limit is still quite good. In addition, since it is pos-sible to launch much greater laser powers at 532 nmthrough ® bers larger than 550 m m, comparable sensitivitywith the use of very long optical ® bers and increasedmeasurement distances is likely. Another potential use of2 1 2 excitation is to take advantage of the different re-sponse factors at 532 nm relative to 266 nm to obtainadditional chemical information and improve selectivity.

POTENTIAL APPLICATIONS

With appropriate probe modi® cations, and a ® eld-por-table tunable pulsed laser, the technique described here

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FIG. 6. Excitation of toluene spiked water samples using 532 nm light.A 550 m m high ±OH content ® ber was used for these measurementswith the collection electrode biased at 1 1000 V. Each point in this curverepresents the average of ® ve measurements with error bars showing6 1 sample standard deviation. Note: one set of error bars is smallerthan the data point.

could be used to monitor toluene or gasoline in watersamples (by headspace measurements), soil samples, orair samples. By suitable choice of laser wavelength, itseems possible that other volatile organic pollutants inthe environment such as benzene, xylenes, trichloroeth-ylene (TCE), perchloroethylene (PCE), and others couldalso be measured. Molecules such as TCE and PCE thatabsorb below 266 nm could be measured by using visibleexcitation as demonstrated for toluene with the use of a2 1 2 scheme. The use of a tunable laser would also allowfor REMPI excitation spectra to be obtained, giving a``® ngerprint’ ’ of the molecules for identi® cation. Smallcommercially available, tunable, pulsed laser sourcespresently exist that weigh under ® fty pounds and run on115 V ac, providing outputs up to 200 mJ, suf® cient formany REMPI measurements. These lasers would be idealfor ® eld analyses.

CONCLUSION

In situ REMPI measurements using a ® ber-optic probehave been demonstrated by measuring vapor-phase tolu-ene in the headspace of toluene and gasoline solutions.The demonstrated detection limits are suf® cient for manyenvironmental measurements. In addition, the feasibilityof using longer, visible, wavelength excitation schemeshas been shown for UV-absorbing molecules, through theuse of 532 nm light for toluene. Negligible interferences

are observed for toluene in aqueous gasoline solutionswith the use of 266 nm excitation. To understand the fullpotential of this technique for small nonaromatic mole-cules such as PCE and TCE, it will be necessary to fur-ther investigate the use of higher order excitationschemes.

ACKNOWLEDGMENT

Support of this work was provided in part by the Federal AviationAdministration, Grant No. 95-P-0022. Additional funding was providedby the Department of Energy, Grant Number DE-FG0799ER62881.

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