Table I. With five of these solvents, the transmission curve after the final sieve drying was indistinguishable from that of the water-free reference.

ACKNOWLEDGMENT

This work was performed in the course of studies supported by the National Science Foundation under Contracts GP27928 and MPS74-17322. Their assistance is gratefully acknowledged.

1. W. F. Edgell, J. Lyford IV, A. Barbetta, and C. I. Jose, J. Am. Chem. Soc. 93, 6403 (1971); W. F. Edgell and A. Barbetta, 96, 415 (1974); W. F. Edgell, S. Hegde, and A. Barbetta, J. Am. Chem. Soc. in press.

2. H. Cordes and C. Tait, Anal. Chem. 29, 485 (1957). 3. D. Chapman and J. Nacey, Analyst (London) 83,377 (1958). 4. H. G. Streim, E. A. Boyce, and J. R. Smith, Anal. Chem. 33, 85 (1961). 5. R. Meeker, F. Critchfield, and E. Bishop, Anal. Chem. 34, 1510 (1962). 6. R. Jasinski and S. Carroll, Anal. Chem. 40, 1908 (1968).

Detection of Water Pollutants by Laser Excited Resonance Raman Spectroscopy; Pesticides and Fungicides

RICHARD J. THIBEAU, LAURENT VAN HAVERBEKE * and CHRIS W. BROWN University of Rhode Island, Department of Chemistry, Kingston, Rhode Island 02881

The feas ib i l i ty of resonance Raman spectroscopy in the detection of hazardous chemicals in water has been tested on some nitrophe- nol-based pesticides and fungicides. In most cases, detection limits below the parts per million level were obtained. The method was also tested on an artifically polluted river water sample.

Index Head ings : R a m a n spectroscopy; Water pollution.

INTRODUCTION

Raman spectroscopy has been shown to be a useful analytical tool for the detection of ionic and molecular species in aqueous solution. Bradley and Frenzel I de- tected benzene in water at 50 ppm. Brown and co- workers have shown that inorganic anions can be de- tected in the range 25 to 75 ppm with conventional laboratory instrumentation 2 and between 100 and 200 ppm when using remote detection equipment2 More recently, Cunningham e t a l . 4 were able to lower the detection limit for ionic and molecular species to 4 to 40 ppm under ideal conditions.

To lower the minimal detectable concentration, the intensity of the solute spectrum has to be increased, without changing that of the solvent, water. This may be achieved by taking advantage of the resonance Raman effect. This phenomenon occurs when the excit- ing frequency approaches the frequency of an electronic absorption frequency. The Raman bands corresponding to vibrations which contribute to the vibronic absorp- tion band may be enhanced several orders of magnitude relative to the nonresonance condition. A preresonance Raman effect occurs when the exciting frequency is situated within the absorption band envelope, but not near the center. Although the effect is not as great, signal enhancement still occurs.

The study of Brown and Lynch 5 on the identification

Received 17 May 1977; rev is ion received 29 A u g u s t 1977. * N A T O Resea rch Fellow. P r e s e n t address : R i j k sun i ve r s i t a i r Cen-

t r u m A n t w e r p e n , L a b o r a t o r i u m voor A n o r g a n i s c h e Sche ikunde , Groenenbo rge r l aan , 171, B2020, An t werp , Be lg ium.

of FD&C dyes in sodas and juice mixes clearly showed the sensitivity of resonance Raman spectroscopy. In a more recent study, Van Haverbeke e t a l . s were able to detect industrial dyes in water at concentrations be- tween 30 and 300 ppb.

In this paper, we have investigated the feasibility of preresonance Raman spectroscopy to detect and identify hazardous chemicals present in water at low concentra- tions. A number of pesticides and fungicides, all based on the nitrobenzene structure, were chosen because of their partial solubility in water. Although none of these chemicals are strongly colored in aqueous solution, they have absorption bands in the visible violet or ultraviolet region; thus, it is possible to take advantage of the preresonance Raman effect when using Ar ÷ ion laser lines.

I. EXPERIMENTAL

Raman spectra were measured on a Spex Industries model 1401 double monochromator using a photon counting detection system. Excitation was provided by a Coherent Radiation Laboratories model 52A argon ion laser. Both the 488.0 nm (1500 mW at the sample) and the 457.9 nm (200 mW at the sample) laser lines were used. The samples were contained in cylindrical cells (10 mm inner diameter) mounted in the Spex standard illumination chamber. A slit width of 8 cm -1 and a scan speed of 50 cm-i/min were used.

Visible and ultraviolet absorption spectra of the solu- tions were measured on a Cary model 15 uv-visible spectrophotometer using standard 5 cm quartz cells.

2-Nitrophenol and 2,4-dinitrophenol were supplied by the Aldrich Chemical Company, Milwaukee, WI. The other pesticides and fungicides were obtained from Chem Service Inc., West Chester, PA. Purities varied from 97 to 99%. The compounds were used without further purification. Solutions were prepared at concen- trations approaching the limit of each compound's solu-

98 Volume 32, Number 1, 1978 APPLIED SPECTROSCOPY

bility. Lower concentrations were obtained by consecu- tive dilutions.

To avoid possible confusion, all compounds are re- ferred by their chemical composition following the IUPAC convention ruling instead of by their commer- cial names. Some of the most common tradenames are, however, included in Table I.

II. RESULTS AND DISCUSSION

The pesticides and fungicides examined in this study have aqueous solubility limits of 50 to 150 ppm. Before Raman spectra were measured, visible-uv absorption spectra were recorded. O,O-Dimethyl-O-4-nitrophenyl phosphorothioate solution had an absorption maximum at 278 nm, whereas maxima for the other solutions ranged from 357 to 414 nm.

Raman spectra were recorded using the 488.0 and the 457.9 nm Ar + laser lines. Since the absorption maxima are farther from the 488.0 line than from the 457.9 nm line, a greater resonance enhancement is to be expected for the latter. This is confirmed by the fact that the relative height of the solute Raman bands vs the height of the 1640 cm-' water band is larger for 457.9 nm excitation. On the other hand, the 488.0 nm excited spectra give a better signal/noise ratio due to more intense incident light. Similar results were obtained with both lines; however, the spectra were slightly better with the 457.9 nm excitation

In addition to dependence on the excitation wave- length, we also studied relative intensity changes caused by changes in the absorption spectrum. 2-Nitro- phenol has an absorption maximum at 350 nm when dissolved in water at low pH. However, by making the solution basic the absorption maximum shifts towards 414 nm because of the formation of the 2-nitrophenolate anion. Since the latter is closer to the 488 nm laser line, a greater enhancement is observed. This is clearly demonstrated in Fig. 1, where the spectra of a 31 ppm solution at pH 4.9 and 10.9 are compared with a 308 ppm solution (pH = 4.4) in neutral water.

Spectra of solutions of several compounds at concen- trations near 2 ppm are shown in Fig. 2. Each spectrum is bounded on the left by the 1640 cm -1 band of water. At these concentrations, each compound exhibits sev- eral distinguishable bands. Since all compounds are based on nitrobenzene, they all show strong bands in

the 1320 to 1360 cm-' region, due to the symmetric NO2 stretching vibration. Despite their similarity, their spectra show considerable differences at these low con- centrations. Due to different symmetry properties and differences in vibronic transition contributions, both their Raman frequencies and their intensities may vary considerably.

The minimum detectable concentration was consid- ered to be the smallest concentration which gave a Raman spectrum such that the stronger bands were clearly visible, i.e., they had a signal/noise ratio of 3 or more. A typical example is the spectrum of a 0.5 ppm

488o

i , t i , , l h i , , I 15(X) IOOO 500

FREQUENCY. CM-'

Fia. 1. R a m a n spec t ra of 2 -n i t rophene l so lu t ions wi th 488.0 n m exci ta t ion, a, 308 ppm, pH = 4.4; b, 31 ppm, pH = 4.9; c, 31 ppm, pH = 10.9.

T A B L E I. P e s t i c i d e s / f u n g i c i d e s , a b s o r p t i o n b a n d m a x i m u m , m i n i m u m d e t e c t a b l e c o n c e n t r a t i o n , a n d o b s e r v e d R a m a n b a n d s .

M i n i m u m de- X"~) Eo" tec table con- R a m a n ban d s observed

C o m p o u n d name" (nm) E ' c cen t r a t i on (cm ') (ppm)

2-Ni t rophenol 414 2 ,4-Dini t rophenol 357 2-Methyl-4 ,6-dini t rophenol (DNOC) (Ditrosol) 366 O,O-d imethy l -O-4-n i t ropheny l phosphoro th ioa te 278 (methy l pa ra th ion ) 2,6-Dichloro-4-ni t roani l ine (Dichloran) 364 4,6-Dinitro-2-sec-butylphenol (Dinoseb) (DNBP) 376

1.43 x 103 7.85 x 102 0.8 824, 881, 1083, 1255, 1338 1.02 × 104 9.81 x 10 ~ 0.7 801,964, 1318, 1350 7.29 × 103 1.43 x 103 0.9 1279, 1330 2.86 × 103 3.74 × 10' 7.0 1361

1.19 × 104 2.42 × 10 ~ 0.4 1338 1.45 × 104 4.15 × 103 0.5 943, 1272, 1327

a Some t r ade n a m e s are g iven in p a r e n t h e s e s . ') ~, = posi t ion of absorp t ion band m a x i m u m . c E , = mola r absorp t iv i ty a t band m a x i m u m . " Eo = mola r absorp t iv i ty a t exc i ta t ion w a v e l e n g t h , 457.9 nm.

APPLIED SPECTROSCOPY 99

L I I I i I I I I I

1500 I000 FREQUENCY, CM -I

FIG. 2. Raman spectra of pesticide/fungicide solut ions in 5 x 10 -4 M NaOH, 457.9 n m excitation, a, 1.2 ppm of 4,6-dinitro-2-sec-butyl- phenol; b, 1.8 ppm of 2,6-dichloro-4-nitroaniline; c, 3.7 ppm of 2,4- dinitrophenol.

, i I ' i I I I ' I

1500 I000

FREQUENCY, CM - I

FIC. 3. Raman spectrum of 0.5 ppm of 4,6-dinitro-2-sec-butylphenol in 2 x 10 -4 M NaOH us ing 457.9 n m excitation.

solution of 4,6-dinitro-2-sec-butylphenol shown in Fig. 3. The bands at 943, 1272, and 1327 cm 1 are easily distinguishable from noise. The minimum detectable concentrations for all compounds used in this study are given in Table I. The table also includes the bands that are visible at this concentration. The detection levels are below 1 ppm, except for O,O-dimethyl-O-4-nitro- phenyl phosphorothioate. The high detection limit for the latter is due to the fact that the absorption maxi- mum is relatively far from the excitation frequencies. It is clear that the minimum detectable concentrations depend on the absorption spectra of the samples. The use of excitation wavelengths closer to absorption band

I I I I I I L I 1500 1200 900

FREQUENCY , CM "~

FIG. 4. Raman spect rum of r iver water , polluted wi th 4,6-dinitro-2- sec-butylphenol to 3 ppm, 488.0 n m excitation.

maxima (approaching the resonance Raman condition) should allow detection of all of these compounds at still lower concentrations.

In order to investigate the use of this technique in actual situations, we simulated a real world sample. Two hundred and fifty milliliters of water taken from a nearby river were "polluted" with 10 ml of a 77 ppm solution of 4,6-dinitro-2-sec-butylphenol in 1 x 10 -3 M NaOH, resulting in a 3 ppm final concentration. The spectrum obtained from this sample is shown in Fig. 4. The spectrum shows considerable fluorescence, proba- bly due to other compounds already present in the river water sample. The bands at 943, 1272, and 1327 cm -1 are nevertheless clearly visible. The band around 1100 may be due to other compounds in the river water sample.

The samples used in this study showed no decomposi- tion and consequently no degradation of the spectra, although the spectra were measured without rotating or circulating sample cells proposed by other resonance Raman investigators. 7' s This is due to the fact that none of the solutions were strongly absorbant at the exciting wavelengths.

This research shows that use of resonance or prereso- nance Raman spectroscopy can allow detection of some hazardous chemicals in water at concentrations below 1 ppm. The use of lasers with output in the uv region may extend this technique to a broader range of com- pounds.

ACKNOWLEDGMENT

This work was supported by the National Sea Grant Program (National Oceanic and Atmospheric Administration), and by the Office of Water Re- sources Research (Department of the Interior). One of us (L.V.H.) gratefully acknowledges financial support from the State University Center of Antwerp and a NATO Research Fellowship.

1. F. B. Bradley and C. A. Frenzel, Water Res. 4, 125 (1970). 2. S. F. Baldwin and C. W. Brown, Water. Res. 6, 1601 (1972). 3. M. Ahmadjian and C. W. Brown, Environ. Sci. Technol. 7, 452 (1973). 4. K. M. Cunningham, M. C. Goldberg, and E. R. Weiner, Anal. Chem. 49,

70 (1977). 5. C. W. Brown and P. F. Lynch, J. Food Sci. 41, 1231 (1976). 6. L. Van Haverbeke, P. F. Lynch and C. W. Brown, Anal. Chem., to be

published. 7. W. Kiefer and H. J. Bernstein, Appl. Spectrosc. 25, 500 (1971). 8. W. M. Woodruff and T. G. Spiro, Appl. Spectrosc. 28, 74 (1974).

100 Volume 32, Number 1, 1978