Anal. Chem. 1985, 57, 7237-1242
phorescent at room temperature. (vii) Finally, at low temperatures, of 5-65 "C, since the blank's phosphorescence is greater than at room temperature, background subtraction is much more critical. Since background subtraction can be carried out readily with the LS-5 and with the new sample compartment system (simply rotation of the sample and/or blanks into the excitation beam), reliable phosphorescence spectra-corrected for background-can be obtained.
ACKNOWLEDGMENT The authors thank Daily Burch, Chester Eastman, and Vernon Cook of the Department of Chemistry Machine Shop for construction of the sample compartment for room-temperature and low-temperature phosphorescence measurement. The authors also thank Edward Voigtman for his assistance in construction of the system. Registry No. 6-CHRA, 95646-96-1; PABA, 150-13-0.
LITERATURE CITED (1) Roth, M. J. Chromafogr. 1967, 3 0 , 276. (2) Paynter, K. A.; Wellons, S.L.; Winefordner, J. D. Anal. Chem. 1974, 46, 736. (3) Seybold, P. G.; White, W. Anal. Chem. 1975, 47, 1199. (4) Vo-Dinh, T.; Leu Yen, E.; Winefordner. J. D. Anal. Chem. 1978, 48, 1186. (5) Von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1976, 48, 1784. (6) Jakovljevlc, J. M. Anal. Chem. 1977, 49, 2048. (7) White, W.; Seybold, P. G. J. Phys. Chem. 1977, 87, 2035. (8) Vo-Dinh, T.; Lue Yen, E.; Winefordner, J. D. Talanta 1977, 2 4 , 146. (9) Schulman, E. M.; Parker, R. T. J. Phys. Chem. 1977, 81, 1932. (10) Niday, G. J.; Seybold, P. G. Anal. Chem. 1978, 50, 1577. (11) De Lima, C. G.; De M. Nicola, E. M. Anal. Chem. 1978, 50, 1658. (12) LUe Yen Bower, E.; Winefordner, J. D. Anal. Chlm. Acta 1978, 702, 1. (13) Aaron, J. J.; Kaleel, E.; Wlnefordner, J. D. J. Agric. Food Chem. 1978, 2 7 , 1233. (14) Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1980, 5 2 , 656. (15) Hurtubise. R . J. Talanta 1981, 2 8 , 145.
1237
(16) Ward, J. L.; Bateh, R. P.; Winefordner, J. D. Analyst (London) 1962, 107, 335. (17) Su, S. Y.; Winefordner, J. D. Can. J. Spectrosc. 1982, 2 8 , 21. (18) Bateh, R. P.; Winefordner, J. D. Talanfa 1982, 2 3 , 713. (19) Honnen, W.; Krablchler, G.; Uhl, S.;Oehkrug, D. J. Phys. Chem. 1983?8 7 , 4872. (20) Dalterio, R. A.; Hurtubise, R. J. Anal. Chem. 1984, 56, 336. (21) Senthlinathan, V. P.; Hurtubise, R. J. Anal. Chem. 1984, 5 6 , 913. (22) McAleese, D. L.; Dunlap, R. B. Anal. Chem. 1984, 56, 836. (23) Vannelli, J. J.; Schulman, E. M. Anal. Chem. 1984, 5 6 , 1033. (24) Vo-Dinh, T.; Winefordner, J. D. Appl. Spectrosc. Rev. 1977, 73, 261. (25) Parker, R. T.; Freedlander, R. S.;Dunlap, R. B. Anal. Chlm. Acta 1980, 179, 189. (26) Parker, R. T.; Freedlander, R. S.; Dunlap, R B. Anal. Chim. Acta 1980, 720, 1. (27) Aaron, J. J.; Winefordner, J. D. Analusis 1982, 10, 299. (28) Hurtubise, R. J. "Solid Surface Luminescence Analysis: Theory, Instrumentation, Applications"; Marcel Dekker: New York, 1981; Chapters 5 and 7. (29) Vo-Dinh, T. "Room Temperature Phosphorimetry for Chemical Analysis", Wiley: New York, 1984. (30) Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1978, 5 0 , 610. (31) McAleese, D. L.; Dunlap, R. B. Anal. Chem. 1984, 56, 600. (32) Sawlcki, E.; Johnson, H. Microchem. J. 1964, 8 , 85. (33) McCall, S. L.; Winefordner, J. D. Anal. Chem. 1983, 5 5 , 391. (34) Onoue, Y.; Hlraki, H.; Nishikawa, Y. Bull. Chem. Soc. Jpn. 1981, 5 4 , 2633. (35) Onoue, Y.; Hirakl, H.; Nlshikawa, Y. fiunseki Kagaku 1982, 3 1 , 169. (36) Ward, J. L.; Walden, G. L.; Bateh, R. P.; Winefordner, J. D. Appl. Spectrosc. 1980, 3 4 , 348. (37) Dewar, M. J. S.;Sampson, R. J. J. Chem. Soc. 1957, 2946. (38) Carruthers, W. J. Chem. Soc. 1953, 3486. (39) Ward, J. L.;Lue Yen-Bower, E.; Winefordner, J. D. Talanta 1981, 2 8 , 119. (40) Barnes, C. G.;Winefordner, J. D. Appl. Spectrosc. 1984, 3 8 , 214. (41) Becker, R. S. "Theory and Interpretation of Fluorescence and Phosphorescence": Wiiey-Interscience: New York, 1969; p 156. (42) Aaron, J. J.; Andino, M.; Winefordner, J. D., Talanfa, in press. (43) Nithipatikom, V.; Pollard, B. D. Appl. Spectrosc. 1985, 3 9 , 109. (44) Boutllier, G. D.; Wlnefordner, J, D. Anal. Chem. 1979, 57, 1391. (45) Goeringer, D. E.; Pardue, H. L. Anal. Chem. 1979, 5 1 , 1054. (46) Grlffin, R. N. Photochem. Photobiol. 1988, 7 , 159.
RECEIVED for review August 24,1984. Resubmitted January 18,1985. Accepted February 15,1985. This work supported solely by NIH-5RO1-GM-11373-21.
Remote Detection of Groundwater Contaminants Using Far-Ultraviolet Laser-Induced Fluorescence Wayne A. Chudyk* Department of Civil Engineering, Tufts University, Medford, Massachusetts 02155 Michael M. Carrabba and Jonathan E. Kenny* Department of Chemistry, Tufts University, Medford, Massachusetts 02155
We report the results of a successful test of remote fluorescence analysis of groundwater contaminants by uslng UV lasers and fiber optlcs. Several prlorlty pollutants (phenol, o -cresol, toluene, o -chlorophenol, p-nltrophenol, 2,441nitrophenol, and xylenes) as well as naturally occurring humlc acid, at environmentally signlflcant concentrations, were readily detected by using the technique. At an Instrument/ analyte dlstance of 25 m, which equals or exceeds the depth of many aquifers used as drinking-water supplies, lndlvldual compounds had detection llmlts at or below the parts per bllllon level. Phenollc and humlc acid contaminants In actual landfill and barkplle leachates were easily detected at 25 m after 10 000-fold dilutlon. Practical problems In lmplementlng a UV-laser-based system, such as efficient coupling, fiber damage threshold (-15 MW/cm2), scattered Ilght, shape of response curve, etc., have been examined and simple solutions demonstrated. 0003-270018510357-1237$0 1.5010
Remote sensors using fiber optics and spectroscopic techniques offer advantages in a variety of applications. These range from in vivo measurements of O2 concentrations ( I ) to conventional spectroscopic measurements (2) to monitoring groundwater contaminants ( 3 ) . Chemically modified (and functional-group-specific)fiber optic probes, or optrodes, have been recently reviewed (4). The development of a cost-efficient method of sampling groundwater supplies, allowing generic identification of low-level nonionic organic compounds, would in many ways be complementary to conductance probes (5), allowing early detection of pollutants escaping from a disposal site or approaching a drinking water supply. Other workers (3,6) in the field of groundwater monitoring have concentrated on the use of highly fluorescent tracers (e.g., laser dyes) to follow pollutant movement or, alternatively, the use of chemically specific sensors affixed to the tip of the fiber. In both cases, visible light (typically from an argon laser) has 0 1985 American Chemical Society
1298
ANALYTICAL
CHEMISTRY. VOC. 57, NO. 7, JUNE 1985
been used and good sensitivity (subparts per million) obtained a t distances of hundreds of meters. Our application calla for a different approach early-warning systems cannot depend on the presence of tracers, the specificity of optrodes is unnecessary and their complexity undesirable, and compounds of interest include small aromatic molecules like phenol and xylenes that have no visible absorption spectrum. However, most of these benzenoid pollutants (and others, as well) do absorb in the UV and have nonzero fluorescence quantum yields. The much poorer transmiasion of optical fibers in the UV is offset to a great extent by the fact that groundwater supplies are usually within 25 m of the surface. Thus, we elected to 888e98 the smpe and limitations of a remote detector based on UV-laser-induced fluorescence. T o our knowledge. no applications of remote fiber fluorimetry have been reported by using far-UV radiation. Some indication of the problems attendant with our choice is ob. tained from the following comparisons. (1)The absorptivity of the compounds to be detected is lower than that of typical tracera by a factor of -500 (t -100 vs. -50000). (2) The fluorescence quantum yields of the anal* are about a factor of 10 smaller than those of tracers (-0.10 VB. -1.0). (3) The attenuation of even the best fused silica fibers in the W (A = 266 nm) is about 40C-500 &/km vs.40 dB/km for viaihle light (argon lasers X = 514.5 nm). This is a result of the increased seattering (= v') and abeorption by fused silica in the UV. (4) Coupling W light to a fiber in more difficult. since the only practical sources of UV light are pulsed lasers, whose beam quality is poorer than that of C W lasers. (5) The short duration of the laser pulses means that the instantaneous power levela to which the fiber end is exposed are -lo7 aa high 88 for C W lasers with the same average power. Thus, fiber damage due to electric-field-induced dielectric breakdown is expected to provide an ultimate limit to sensitivity. (6)T o get the bent signal/noise and hence lowest detection limits,a pulsed murce requires a gated detector. An offsetting advantage is that thermal noise of a photomultiplier tube detector is totally negligible, even without cooling, when gate widths are short. Despite these drawhacks, feasibility studies indicated that the method would provide low detection limits a t reasonable distances, due to the inherent sensitivity of laser-induced fluorescence, which waa recently demonstrated to provide -lW3 M detection limits for rhodamine 6G with visible light (7).
In this paper, we describe the results of remote W-laserinduced fluorescence measurements on model and actual contaminated groundwater samples a t instrument/analyte distances from 1to 25 m. Detailed studies of signal vs. concentration and distance were carried out on phenol, 0-cresol, and humic acid; detection limits near or below 1 pph were obtained a t 25 m by using a relatively simple (and far from optimum) probe design. Other priority pollutants were also shown to be amenable to the technique, including toluene and xylenes, important components of gasoline. T h e method was also demonstrated for barkpile and landfill leachate samples; phenolic and humic components of these samples were ohservable after 1OWfold dilution at a distance of 25 m. Fiber properties, including transmissivity and damage threshold, were measured a t 266 nm. Other problems associated with low-level remote UV fluorimetry are discussed. EXPERIMENTAL SECTION Laser Sources. The source of W radiation was either the frequency-doubled output of an NRG nitmgen-pumped dye laser.
~lectronmiaqraphs of tused silica tibers uwxl h w shav: (a, top) Uxlarnaged Rber end after c!eavklg with razor blade and p a M of chdjing: (b. bottom) ntmr en3 damaged by YAG $ser p h e along top edge; defect along lower edge is due to the cleaving tech. nlqm. The scab of the micrographs is indicated by the *fie bar representing 100 pm which appears in the lower border area.
fbw 1.
I
lu.\
It.
Fbm 2. B W diamm of remote Ruaescence apparatus N. nC trcpn law:D. dye $aer; L lens; S. se2um%bmanicgeneratiar:F1, finer for hamxmlc saparath M, mirror: T. xyz translater fa flber poslaahg: E. exciiatbn Aber; P, pobe: W. water sample; C. rmecWar or detectkm fiber: F2, cuioff a bandgass finer: PMT. photomunipiier tube; B. boxcar (gated Integrator); V. voitmeter readout.
producing less than 1pJ of light per pulse at 270 nm, or the fourth harmonic of a NdYAG laser (both Molectron MY-34 and Quanta-Ray DCR-1were used). producing about 0.2 mJ at 266 nm. The pulse duration was 4-5 ns for the NRG and Quanta-Ray lasera and about 7-8 ns for the Molectron. Laser pulse energies were monitored by Laser Precision RjP 735 and RjP 765 laser energy probes. The UV output was passed through a black g l a s filter and/or a quartz prism to remove the lower harmonies and then foeused through a 2-in. f d length quartz lens onto the tip of the fiber. Fiber Properties. After preliminary tests on several fibera with fused silica cores,the f d choice, representing a compromise between high flexibility and coupling efficiency, waa a W-pm core diameter, Teflon-jacketed Superguide fiber from Fiberguide Industries. A sharp single-edge razor blade was used to cleave the fibers. Inspection with a low-power microscope indicated that this method produced acceptably square and uniform feces. An electron micrograph of such a fiber before use is shown in Figure la. The experimental system is shown in block diagram form in Figure 2. The excitation fiber was clamped into a small Vgrooved aluminum block on an xyz p i t i o n e r equipped with micrometem (0.001-in. divisions),to allow the fiber end to be placed at the focal point of the quartz lens. For transmission measurements, the other end of this fiber was directed at the laser energy probe.
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE
1985
0
1239
r
-
u c
c
270
320
370
4M
Wavelength
470 (nm)
380
360
520
Figure 3. Emission spectra of the principal compounds and samples studied. The spectra are arbitrarily normalized to unity, using the multipliers given below, and displaced for clarity. The excitation wavelength used was 266 nm; resolution was 4 nm. (a) Phenol, 94
mg/L (X0.2); (b) 0-cresol, 108 mg/L (X0.2); (c) humic acid 10 mg/L (X5.0); (d) bark leachate, undiluted (X3.6); and (e) landfill leachate, 10-fold dilution (X3.1). Fiber length was 1 m. Transmitted energy was optimized by using the positioner and then recorded and compared to the laser energy measured at the focal point in the absence of the fiber. Transmission measurements were made by using 1-,25-, and 50-m fiber lengths. The fiber damage threshold was determined by placing a fiber end a t the focal spot of the quartz lens and gradually increasing the laser pulse energy until a faint snapping was audible. The total pulse energy under these conditions was measured with the laser energy probe. In addition, a 50 ym diameter pinhole in a thin nickel shim (Ealing “utility pinholes”) was mounted a t the focal spot by using the xyz positioner, and transmitted laser energy was monitored with the probe as a function of position of the pinhole in the focal plane. Finally, an electron micrograph of the fiber damaged under these conditions was obtained (Figure lb). System Description. The remote fluorescence probe consisted of a small grooved aluminum clamp which held the excitation and detection fibers at an angle of 22O, immersed in the test tube or cuvette containing the sample (Figure 2). Fluorescence and scattered light were collected by the detection fiber and guided to an uncooled RCA 1P28A photomultiplier tube, operated at 1 kV. Cutoff or band-pass glass filters (Schott and Corning) could be inserted between the fiber and the photomultiplier tube. To determine the spectral properties of the detected light, we replaced the filters by a 0.3-m McPherson monochromator. The output current of the photomultiplier was monitored by a boxcar or gated integrator operated in the exponential averaging mode; both Evans 4130 and EG&G/PAR Model 162/166 gated integrators were used. The boxcar was triggered by a synchronizing electrical pulse from the laser, and sampled the PMT for a 5- or 50-11s interval when the signal was at peak intensity. The analog output of the boxcar was read by a digital voltmeter or a stripchart recorder. The laser power was monitored regularly by using a photodiode and showed negligible drift during the time required for an experiment. Sample Preparation. Solutions of phenol (Mallinckrodt), o-cresol (Fisher), humic acid (Aldrich), o-chlorophenol (Fisher), p-nitrophenol (Fisher), 2,4-dinitrophenol (Sigma),toluene (Fisher), and xylenes (Mallinckrodt)were prepared by dissolving a known weight of solute into a standard pH 7 phosphate bufferldistilled water solution (8) in a volumetric flask. Samples of bark leachate and landfill leachate were obtained from S.D.Warren Co., Westbrook, ME. Leachate samples were collected in acid-washed glass bottled with TFE-lined closures. Serial 10-fold dilutions of these leachates, as well as of phenol, o-cresol, and humic acid, were made by standard techniques in buffered distilled water. All solutions were made within 48 h of use and stored in a 4 “C refrigerator. Samples of distilled water and buffer solution were also prepared.
340
320
300
2EO
2E0
Y A V E L N G T H (rm)
Flgure 4. Emission spectrum of 0.108 mg/L solution of o-cresol, with (lower trace) and without (upper trace) Schott W G 3 2 0 cutoff filter in detection system. Two arrows indicate the water Raman line at 294 nm and the excitation laser line at 266 nm: resolution was 2 nm.
Table I. Relative Remote Fluorescence Signal Levels (in volts) for Selected Organic Contaminants in Aqueous Buffer Solution, Using 1-m Fibers. The Background Reading Was 0.238 f 0.002 V compound
concn, mg/L
signal background, V
phenol o-cresol humic acid o-chlorophenol p-nitrophenol to1uene 2,4-dinitrophenol xylenes
8.9 82 24 25 15 6.9 24
0.690 0.918 0.522 0.130
11
0.070
0.047 0.030 0.390
Characterization of Samples a n d System. Total organic carbon (TOC) levels were determined for the humic acid and both leachate samples by using a Xertex-Dohrman DC-80 total organic carbon analyzer. Available literature (9) indicates that all model compounds (with the exception of water and the buffer) studied in this experiment absorb strongly a t the wavelengths used, with molar absorptivities between lo2 and lo4 M-’ cm-’(30 g-’ cm-’ for humic acid). Emission spectra for several of the compounds studied were taken by using 1-m excitation and detection fibers and the monochromator described above (Figure 3). The excitation wavelength was 266 nm, and the spectral resolution was 4 nm. Similar spectra were taken with the probe immersed in the buffer solution and in an empty sample container; finally, the output of the excitation fiber was pointed directly into the monochromator to check for fiber fluorescence and inelastic Raman scattering. With the probe immersed in lo4 M o-cresol solution, various Schott WG-XXX series sharp-cutoff glass filters were placed between the output of the detection fiber and the entrance slit of the monochromator, and the emission spectrum was repeated (Figure 4). This allowed us to select the optimum filter for elimination of background signal and transmission of analyte emission (see Results section). Remote Total Fluorescence Measurements. Remote laser-induced fluorescence measurements were performed as follows. The system was assembled as in Figure 2, using two 1-m lengths of fiber with the low-power laser or two 25-m lengths of the same fiber with the high-power system. A WG-320 filter (or other filters when appropriate) was used in front of the PMT. With the probe immersed in solution, the output of the gated integrator was monitored long enough to obtain an average reading and a measure of the signal fluctuation. Typically, this was 3-5 min or -5000 laser shots. The beam was blocked, and a background reading
1240
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
l.]o
~, , ,
-5
LOG
,
1
,
L:,, , , , ,
5-5
0
0 L O U
, , ,
,;E,,
5-5
L3G
,
, 5
0
c
-0G c
Figure 5. Remote fluorescencesignal vs. log C (in milligrams/liter)for
phenolic compounds: (a) phenol, 1-m fibers and 270-nm excitation; (b) phenol, 25-m fibers and 266-nm excitation; (c) o-cresol, 25-m fibers and 266-nm excitation. For both phenol ( M , = 94 g) and o-cresol (M, = 108 g), log C (moVL) = log C (mg/L) - 5. In (b) and (c), a typical error bar, representing f 3 standard deviations, is shown. Table 11. Fraction of Light Transmitted, T,vs. Fiber Length, L , for 600 pm Diameter Fused Silica Fibers at 266 or 270 nm L, m
T
1 1
4.44 x 4.68 X 3.88x 4.60 X 4.93 x
25 25 50
A, nm
270 270 266 266 266
10-1
lo-' 10-2
lo-' 10-3
was obtained and subtracted from the signal. Values of fluorescence signal vs. concentration for several compounds are shown in Table I. Representative plots of signal vs. log of concentration are shown in Figures 5 and 6.
RESULTS The transmission data (Table 11) were analyzed according to the following model. Losses in transmitted power were of three types: (1) reflection losses at the fused silica-air interfaces; (2) absorption and scattering losses dependent on the length of the fiber; and (3) all other loss mechanisms. Three parameters and one variable (fiber length), then, describe the fraction T of incident light transmitted by a fiber
T = ERtLR = ER2tL
(1) where R is the fraction of light transmitted at an air-fiber or fiber-air interface where reflection also occurs, t is the internal transmittance per meter of fiber, L is the length of the fiber in meters, and E is an efficiency parameter that measures the extent of all other losses. It is expected that these are due primarily to improper matching of the focused laser beam to the diameter and acceptance angle of the fiber and imperfections in surface quality of the unpolished fiber ends. Since methodology and fiber type were identical for all measurements, it is assumed that t and E are constants to be determined experimentally while R may be calculated from the refractive index n. For fused silica, n = 1.500 at 265 nm (IO), and the formula (11)for reflection of light at normal incidence gives R 1 - ( n - 1)2/(n + 1)*= 0.960 (2) Taking the natural log of both sides of eq 1 gives In T = L In t In E 2 In R
+
+
(3)
A linear fit of In T vs. L yields a slope equal to In t and an intercept equal to In E + 2 In R. Knowing R, we can determine t and E. A linear regression of the transmission data (Table 11) yields In T = -0.0933L - 0.740 (4) with a correlation coefficient of -0.9984. Thus, the internal transmittance t is 0,911 f 0.003 which corresponds to an
LOGID:]
-0s I or I
Figure 6. Remote fluorescence signal vs. log of concentration for (a)
humic acid (concentration in milligrams/liter), (b) bark leachate, and (c) landfill leachate, concentrations given in terms of dilution factor, DF, of original samples. For humic acid, a typical error bar representing 1 3 standard deviations is shown; for the leachates, individual 3a error bars are smaller than the symbols used for the data points, but the multiple measurements at DF = lo-' indicate the scatter in replicate runs. attenuation of 405 dB/km, while E is determined to be 0.518 f 0.044. The good fit of the data suggests that the assumptions made in the model are reasonable. The laser beam profile in the focal plane was roughly elliptical with a full width at half-maximum intensity of about 100 pm in the short dimension and 600 fim in the long dimension, assuming a Gaussian distribution in each dimension. A fortunate coincidence makes the determination of the threshold for laser power damage particularly easy to calculate: the damage spot in Figure l b is almost exactly 50 pm in diameter; this equals the size of the pinhole used to measure the laser profile. Thus, the threshold intensity is equal to the maximum energy through the pinhole, 1.34 X lo* J per pulse, divided by the pinhole area and the laser pulse duration, 4.5 ns. A value of 15 MW/cm2 is obtained for the damage threshold. One must keep in mind that the instantaneous intensity is higher a t the temporal and spatial peaks of the laser pulse, so this number is more of a rough guide than an accurate measurement. The total pulse energy at the damage threshold was about 0.2 mJ. The monochromator scans provided essential information about the spectral distribution of signal and background. Fluorescence and/or Raman scattering from the fibers was negligible, even when the excitation fiber illuminated the entrance slit directly. When output of the detection fiber illuminated the monochromator, a signal at 266 nm, due to scattered laser light, was always present, even when the probe was placed in an empty sample container. When the probe was in any aqueous solution, including distilled water or buffer, a signal appeared at 294 nm, corresponding to a known Raman band of water (Figure 4). The laser and water Raman lines were of sufficient intensity to dwarf the fluorescence from analytes present in low concentration. However, they were effectively eliminated with a 1 mm thick Schott WG-320 sharp-cutoff long-wave pass filter, as shown in Figure 4, which depicts the spectral distribution of light detected from a lo* M solution of o-cresol with and without the cutoff filter in place. The emission spectra of several analytes, shown in Figure 3, afford insight into the scope and limitations of the technique with regard to selectivity. It is clear that similar compounds like phenol and o-cresol cannot be differentiated; however, even inexpensive glass band-pass filters could separate the signals from humic acids and phenolic contaminants. The similarities of the leachate samples to one or the other of these two classes are obvious. By far the most important results of this study are the response cwves of serial dilutions of analytes shown in Figures 5 and 6. In these figures, the fluorescence intensity was plotted on a relative scale, with the reading for distilled water or buffer
ANALYTICAL CHEMISTRY, VOL. 57, NO. 7, JUNE 1985
taken as zero and that for the largest signal in the series (less the buffer signal) taken as unity. The error bars on Figures 5 and 6 respresent &3 standard deviations in the measurement. Representative plots of signal vs. log of concentration are shown in Figure 5a and b for phenol. Figure 5a was obtained by using the low-power laser and 1-m fibers and Figure 5b, the high-power laser and 25-m fibers. The qualitative features of these plots indicate that relative response of the method does not change when laser power is increased so that more distant samples can be probed. Figure 5c shows signal vs. concentration for o-cresol, using the high-power laser and 25-m probe. Note the good agreement between repetitive measurements. Figure 6 shows response curves for humic acid, bark leachate, and landfill leachate. All response curves show the roughly exponential increase in signal with log of concentration predicted by Beer’s law; as expected, there are deviations from this simple model at high concentrations. Other factors affecting the details of the shape are discussed below. A comparison of the emission spectra of Figure 3 suggests identification of the fluorescent contaminant in landfill leachate as primarily a phenolic-type compound. The bark leachate, on the other hand, strongly resembles humic acid; this is not surprising, since one source of humic acid in nature is the breakdown of lignins (12). The results of TOC measurements, expressed in milligrams/liter, are as follows: 7600 for landfill leachate, 340 for bark leachate, and 4.9 for 10 mg/L humic acid. Thus, a 100-fold dilution of the bark leachate would have a TOC of 3.4 or 0.69 times the value for 10 mg/L humic acid. When the unnormalized remote fluorescence signal for bark leachate a t 100-fold dilution, 4.57 V, is compared to the unnormalized signal for 10 mg/L humic acid, 7.28 V, the ratio is 0.49. The approximate quantitative agreement between the two methods is encouraging, since the separation of phenolic and humic contributions to the total fluorescence signal may readily be achieved by using compact monochromators or even glass band-pass filters.
DISCUSSION In this discussion, we first assess the characteristics of the system as configured in the present work. Then we address the utility, limitations, and possible extensions of the technique. The sensitivity of the remote UV-laser-induced fluorescence technique is at or below parts per billion levels at an instrument/analyte distance of 25 m. The technique shows good reproducibility (Figure 5c), and the shape of the response curves shows insensitivity to fiber length (Figure 5a and b) if the laser power is increased accordingly (for constant response, the power required increases exponentially with length). Individual readings take at most a few minutes; operation is simple and easily automated. From the response curves shown in Figures 5b, 5c, and 6a, we estimate the detection limits (defined here as the concentration at which the signal is one standard deviation above the background) for phenol, o-cresol, and humic acid to be 10, 1, and 0.1 ppb by weight, respectively. These limits are well within or below the range typical for contaminant plume detection in current practice (13). They are also low enough to indicate the need for more thorough testing and/or remedial action. The leachates (Figure 6b and 6c) are easily detectable a t 25-m distances even after 10000-fold dilution. This is an impressive result for such “real-life’’ mixtures, since they may easily contain materials which would quench fluorescence (14). The deviation from Beer’s law-type response at high concentrations is readily apparent for phenol and o-cresol in Figure 5. Such qualitative deviations are expected in absorbance measurements (14). More quantitative and relevant to the present case, however, is the recent treatment of fiber
1241
optic fluorometers by Ratzlaff, Harfmann, and Crouch (15). They provide a quantitative treatment of the effects of scattering, absorption by fluorophores and nonfluorophores, and the collection efficiency of probes based on parallel excitation and detection fibers. Application of their correction provides a straight-line plot of log signal vs. log concentration, whereas the uncorrected response resembles ours a t comparable concentrations. The correction requires simultaneous absorbance measurements, however, and could not be used by us quantitatively. Identification of unknown contaminants using this technique holds the promise of modest selectivity. Our initial goal was to develop a generic, semiquantitative but sensitive technique to be used as an early warning system; this has been successfully demonstrated. However, low-level resolution (e.g., between phenolics and humic acids) may be obtained, even at the longest distances envisioned, by the trivial addition of a filter wheel. We are also confident that full spectral resolution at least as good as that shown in Figure 3 is achievable a t 25 m if some of the improvements mentioned below are implemented. Thus, specificity equal to that of conventional fluorometry, Le., limited by the bandwidths of the compounds themselves in aqueous solution, may be expected. Given the likely identity of the analyte (as in the barkpile leachate analyzed by remote fluorimetry and TOC measurements above), the quantitative accurcy of the method is also good. Serious limitations of the method are currently imposed by the properties of the available fibers, the method of coupling the laser onto the fiber, and the design of the probe. The latter two are largely artificial. Our low coupling efficiency was due to the focusing system available. However, coupling efficiencies of 90% are routine (10). The probe design was selected largely because its simplicity and inexpensiveness would emphasize the inherent sensitivity and utility of the method. Improvements that are currently available include (1) optimization of the angle between the excitation and collection fibers, (2) surrounding of the excitation fiber by a multitude of close-packed detection fibers, (3) use of collection optics, e.g., a spherical ball (6) or a microlens-ended fiber (16), and (4) use of commercially available probes, e.g., a 45” mirror for detection at right angles to the exciting light (10). The fiber limitations, on the other hand, represent the current state of the art. Mechanically, there are no serious problems; flexibility and strength are sufficient for the current application, even for fibers with core diameters as large as 600 pm. Absorption and scattering losses are higher than desired and represent a serious challenge in materials research (the dependence of Rayleigh scattering on v4 is not likely to change, although absorption losses may be reduced). The laser damage threshold is also a major limitation to the usable distance and/or achievable detection limits of the method. One possible solution is to spread more laser energy over multiple excitation fibers; another is to use a longer pulse like that of a flashlamp-pumped dye laser or, ideally, a CW laser. However, in the current market, high average power, reliable UV lasers are limited primarily to frequency-upconverted YAGs and excimers, both of which have pulse durations of a few nanoseconds.
CONCLUSIONS The sensitivity of remote far-UV laser-induced fluorescence detection meets or exceeds requirements for monitoring groundwater contamination. The method described herein is simple, fast, and reasonable economical. In addition, we have demonstrated that the method is potentially selective for specific families of compounds, especially when the distance to the sample is shorter than 25 m. Thus, many more applications requiring qualitative and quantitative analyses may be foreseen.
1242
Anal. Chem. 1985, 57, 1242-1252
Note Added in Proof. Improvements in fiber end preparation have allowed us to increase the laser power incident upon the fiber end to levels beyond that reported here. This observation is consistent with the manufacturer's reported damage threshold. ACKNOWLEDGMENT We thank Timothy M. Woudenberg and George Scherer for technical assistance, Lew Rubin of EG&G/Princeton Applied Research for use of equipment, Ray Pepin and Wallace M. Rogers of S.D. Warren Co. for providing leachate samples, and Karyn Moskowitz of the Harvard Museum of Comparative Zoology for preparation of the electron micrographs. Registry No. Phenol, 108-95-2; o-cresol, 95-48-7; toluene, 108-88-3;o-chlorophenol, 95-57-8;p-nitrophenol, 100-02-7;2,4dinitrophenol, 51-28-5; xylene, 1330-20-7;water, 7732-18-5. LITERATURE CITED (1) Peterson, J. I.; Fitzgerald, R. V.; Buckhoid, D. K. Anal. Chem. 1984, 56, 62-67. (2) McCreery, R. L.; Fleischmann, M.; Hendra, P. Anal. Chem. 1983, 55, 146- 148. (3) Hirschfeid, T.; Deaton, T.; Milanovich, F.; Kiainer, S. Opt. Eng. 1983, 22 (5),527-531. (4) Seitz, W. R. Anal. Chem. 1984, 56, 16A-34A. (5) Keith, S. J.; Wilson, L. G.; Fitch, H. R.; Esposito, D. M. Ground Water Monif. Rev. 1983, 3 (2) 21-32. (6) Hirschfeld, T.; Deaton, T.; Milanovich, F.; Klainer, S. M.; Fitzsimmons, C. Project Summary, "The Feasibility of Using Fiber Optics for Monitoring Groundwater Contaminants", Environmental Monitoring Systems Laboratory, US. E.P.A., January, 1984. (7) Dovichi, N. J.; Martin, J. C.; Jett, J. H.; Trkula, M.; Keiier, R. A. Anal. Chem. 1984, 56, 348-354.
M. A. H., Eds. "Standard Methods for the Examination of Water and Wastewater", 15th ed.; American Public Health Association, American Water Works Association, and Water Pollution Control Federation: Washington, DC, 1981; p 774. Beriman, I.B. "Handbook of Fluorescence Spectra of Aromtic Molecules", 2nd ed.; Academic Press: New York, 1971. Optical Systems and Components Catalogs, Oriel Corp., Stratford, CT 06497. Fowles, G. R. "Introduction to Modern Optics", 2nd ed.; Holt, Rinehart and Winston: New York, 1975; Chapter 2. Josephson, J. Environ. Scl. Technol. 1983, 17 (ll), 518A-521A. Hager, D. G.; Smith, C. E.; Loven, C. G.; Thompson, D. W. I n "Proceedings of the Third National Symposium on Aquifer Restoration and Ground Water Modeling"; Nielsen, D. M., Ed.; National Weii Water Association: Worthington, OH, 1983; pp 123-124. Willard, H. H.; Meritt, L. L., Jr.; Dean, J. A,; Settle, F. A,, Jr. Instrumental Methods of Analysis", 6th ed.; D. Van Nostrand Co.: New York, 1981; Chapter 4. Ratziaff, E. R.; Harfmann, R. G.; Crouch, S. R. Anal. Chem. 1984, 56, 342-347. Russo, V.; Righimi, G. C.; Sottini, S.; Trigari, S. Appl. Opt. 1984, 23. 3277-3283.
( 8 ) Greenberg, A. E., Conners, J. J., Jenkins, D., Franem,
(9) (10) (11) (12) (13)
(14) (15) (16)
RECEIVED for review December 26, 1984. Accepted February 14, 1985. This research was supported by a grant from Research Corp., NSF Grant PRM-8114621; and a Tufts Faculty Research Award. Part of this work was performed while we were visiting scientists at the M.I.T. Laser Research Center, Massachusetts Institute of Technology, Cambridge, MA 02139, which is a National Science Foundation Regional Instrumentation Facility. The Mallinckrodt Corp. provided a Graduate Research Fellowship for M.M.C. during part of this work. Part of this work was presented at the Northeast Regional Meeting of the American Chemical Society, June, 1984, in the Environmental Chemistry session.
Effects of Acid Type and Concentration on the Determination of 34 Elements by Simultaneous Inductively Coupled Plasma Atomic Emission Spectrometry Shane S. Que Hee,* Timothy J. Macdonald,' and James R. Boyle Department of Environmental Health, University of Cincinnati Medical Center, 3223 E d e n Avenue, Cincinnati, Ohio 45267-0056
A mlxed acld conslstlng of 11.6% HC1/2.8% HNO, proved superlor to 2 to 10% HCI, HNO,, and H2S04alone in chemical compatlblllty and storage characteristics for slmultaneous inductively coupled plasma atomlc emlsslon spectrometric (ICP-AES) determlnatlon of 33 elements admlxed up to concentrations of 100 pg/mL each. A 2 % aqua regla solution appeared to be adequate below 10 pg/mL of all these admixed elements plus sllver. Use of the mlxed acld generally also allowed for more reproduclble lnterelementalk factors. Less sensltlve elements and elements whose llnes were in the vacuum ultravlolet were not as reproducible. A two-point standardlratlon procedure was adequate, and k factor values agreed wlthln 10 % only over a speclflc concentratlon range. A practical procedure to deflne the range of determination was developed using the 11.6 % HCV2.8 % HNO, acid solvent.
'Present
ID 83415.
address:
EG&G
I d a h o Inc.,
P.O. Box 1625, I d a h o Falls,
Few investigators have studied the effects of different acids on the chemical compatibility, interelemental interferences, and spectral background for a large number of elements when analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). In contrast, there are many publications dealing with the effect of argon flow rate, rf power, and torch height (1-5), sample uptake (4, 5 ) , and determination of interelemental interferences (6-9). The importance of matrix matching to obtain accurate levels has been recognized (6,10,11). Investigators have attempted to obtain accurate answers in various ways: standard addition experiments can be performed; the sample can be diluted to match the matrix used for standards (at the sacrifice of sensitivity); or the matrix of the sample can be simulated artificially once the sample is screened. The matrix consists of contributions both from the acid and from the sample, and since the sample composition is often unique, many investigators have utilized the dilution method for screening purposes often with excellent results. In such a method, the acid solvent should be optimum in terms of chemical compatibility and
0003-2700/85/0357-1242$01.50/00 1985 American Chemical Society
