Anal. Chem. 1992, 6 4 , 349-353
Table 111. Release of Fluoride from VF,/HFP Copolymer at 335 OF by Selected Combinations of Compounding Ingredients
microname F released per gram of the copolymer
ingredients" added to VF2/HFPcopolymer a b C
d e
f g
h i j
none
BPAF PC1 Ca(OH), Ca(OH)2,BPAF MgO MgO, BPAF MgO, PC1
Ca(OH)2,PCl Ca(OH)2,PCl, BPAF
55 46 61 347 269 807 677 1729 3399 4531
tions. Thanks are also due to D. L. Stanek and L. D. Winter for helpful discussions. NO. BPAF, 147861-1; F, i6w-488; H ~ O7732-185; , (FzC=CFCFS-H~CECF~), (copolymer), 9011-17-0.
REFERENCES (1) (2) (3) (4) (5) (6) (7) (8)
(9) (IO) (11) (12) (13) (14)
" Concentrations of ingredienta (parts per hundred) added to the elastomer were BPAF, 1.5;PC1, 0.475;Ca(OH),, 6.0;and MgO, 3.0. (15)
fluoride release (Table 111). These results demonstrate the usefulness of the present procedure.
ACKNOWLEDGMENT I wish to thank R. A. Prokop, R. A. Guenthner, and G. H. Millet for their interest and support during these investiga-
349
(16) (17)
Singer, L.; Armstrong, W. D. Anal. C h . 1954, 2 6 , 904-906. Stegm"a, H.: Jung, G. F. 2. Anal. Chem. 1959, 375, 222-227. Hall, R. J. AnaW~t1963, 88, 76-83. Frere, F. J. Anal. Chem. 1961, 33, 646-645. Wharton, H. W. Anal. Chem. 1962, 34, 12961298. Venkateswarlu, P.; Sita, P. Ana/. Chem. 1971, 49,758-780. Singer, L.; Armstrong, W. D. Anal. Bbchem. 1965, 10, 495-500. Baeumler, Von J.; Glinz, E. MM. Geb&te Lebensm. Wg7. 1964, 55, 250-264. Taves, D. R. Talenta 1868, 15, 969-974. Qreenhalgh,R.: Riley, J. P. Anal. Chlm. Acta 1961, 2 5 , 179-188. Taves, D. R. Talenta 1968, 75, 1015-1023. Frant, M. S.; Ross, J. W., Jr Science 1966, 154, 1553-1555. ORION RESEARCH Instructkm Menuel, Fluoride Elsctrodea, 1982. Taves, D. R.; Forbes, N.; Slbwman, D.; Hicks, D. In FluorMee: Effects on V q t a t k m , AnhnelsandHumns; Shupe, J. L., Peterson, H. B., Leone, N. C., Eds.; Paragon Press: Sat Lake City, 1983; pp 189-193. Singer, L.; Armstrong, W. D.; Vogel, J. J. J . Lab. Clln. M .1969, 74, 354-358. Singer, L.; Armstrong, W. D. Anal. Chem. 1968, 40. 613-614. Venkateswarlu, P.; Koib, R. E.; Guenthner, R. A. Po@w preprtnts 1990, 37.360-361.
RECEIVED for review July 25,1991. Accepted November 15, 1991.
Detection and Determination of Dilute, Low Molecular Weight Organic Compounds in Water by 500-MHz Proton Nuclear Magnetic Resonance Spectroscopy D. Bruce Fulton, Brian G. Sayer, and Alex D. Bain* Department of Chemistry, McMaster University, Hamilton, Ontario, Canada U S 4Ml Harold V. Malle Wastewater Technology Centre, 867 Lakeshore Road, P.O. Box 5068, Burlington, Ontario, Canada L7R 4L7 The feasiblltty of using hlgMleM proton NMR spectroscopy to analyze aqueous solutlons of organlc solutes at mlcromolar concentrations has been evaluated. N-Nltrosodhnethylamlne (NDMA) and benzene served as model compounds. The WATR (water attenuatlon by transverse relaxatlon) method of solvent suppresslon was optknlzed to permlt detectlon of the protons of analytes In the submicromolar concentratlon reglme. All data were collected In blocks to permlt quantltatlve estlmatlon of detectlon llmlts and experimental error. Externally referenced peaks heights, rather than peak Integrals, were used to measure solute concentration. The detection limn for NDMA, uslng a 500-MHz Instrument for 2 h, was 510 ng/mL and, for benzene, ushg a 4 4 acgulsltbn, was 35 ng/mL. The technlque was used to monltor the effect of UV lrradlatlon on a 10 pg/mL sample of NDMA. The NDMA was ellmlnated; the predomlnant photoproduct was dlmethylamine (DMA).
INTRODUCTION Concern about the contamination of freshwater systems by organic compounds is increasing, particularly where public water supplies may be affected. For example, the Ministry of the Environment in the Province of Ontario, Canada, has 0003-2700/92/0384-0349$03.00/0
recently lowered the allowable limit of N-nitrosodimethylamine (NDMA) from 14 parts per trillion to 8 ppt. The purpose of this work is to develop a method for the detection of low level concentrations of specific organic compounds directly in water without the requirement of separation or preconcentration. Proton nuclear magnetic resonance (NMR) spectroscopy is not usually considered as a method for analyzing aqueous solutions of very dilute (4Hg/mL) organic solutes, since it is perceived to suffer from certain disadvantagea. First, NMR spectroscopy is inherently less sensitive than other types of spectroscopy because it involves radio-frequency transitions. Second, proton NMR spectra of aqueous solutions are dominated by the water signal (effectively 110 M), which must be suppressed to bring micromolar solute signals within the dynamic range of detection. Third, quantitative measurement of NMR intensities requires very careful data acquisition and analysis.'P2 On the other hand, the rich information content of the spectra gives NMR certain advantages over other analytical techniques. In particular, the high resolution of chemical shifta allows for the analysis of a mixture of similar species without the need for chemical separation. Because NMR is nondestructive, the sensitivity problem can in principle be overcome by accumulating a sufficiently large number of transients. However, in practice one is limited 0 1992 American Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 4, FEBRUARY 15, 1992
by fiiite instrument stability and the availability of instrument Table I. Delay Time T Required To Suppress Water Signal time. One goal of this project was to establish practical deIntensity by a Desired Factor in a Spin-Echo Experimenta tection limits of a 500-MHz instrument for detecting small water water organic molecules in water within a realistic time frame. suppression suppression Considering the typically high demand for access to a 500factor T , ms factor T , ms MHz spectrometer, we think that the complete analysis of a sample, including the running of blanks and standard addi10 20 105 98 102 39 106 117 tions, should not exceed 8-12 h. Therefore, the maximum 103 59 107 137 acquisition time per spectrum was limited to 2-4 h. 104 78 108 157 Extremely strong solvent suppression was crucial for this method to work. A number of solvent suppression methods LI These values are valid for aqueous solutions containing 0.5 M NHXl with DH = 6.5. were evaluated for use in this project: simple presaturation, selective e~citation,~ WEFT,4and WATR."7 All introduced lutions were prepared to minimize decomposition of the NDMA amplitude and phase distortion to the spectrum, to some (exceptfor the irradiation experiment;see below). Proton NMR extent. Only WATR (water attenuation by transverse respectra (Figure 4) showed only two lines at 2.88 and 3.53 ppm laxation) could be pushed to give sufficient suppression to relative to tetrakis(trimethylsiy1)methane (?TSM), corresponding detect solutes in the submicromolar regime. to the two inequivalent methyl groups, and further purification The method involves adding ammonium chloride to the of the NDMA was deemed unnecessary. sample solution and adjusting the solution pH to ca. 6.5. The The benzene solution was prepared from reagent grade benzene ammonium ion and water signals coalesce into a single broad purchased from Baker. The benzene spectrum consisted of a line due to rapid proton exchange. The pH adjustment single line at 6.21 ppm relative to TTSM. The line is shifted maximizes the width of this line; Le., the spin-spin relaxation considerably from the position for neat benzene (7.19 ppm), because of interaction with the solvating water molecules. time T2is minimized. The NMR signal is acquired as a spin In preparation for the WATR technique 0.67 g of Aldrich echo, employing the pulse sequence 90,-7/2-180,-7/2-acq~ire.~ high-purity ammonium chloride was added to 25 mL of each A sufficiently long delay time 7 is used to allow the water sample to produce a 0.5 M solution. The pH was adjusted to proton magnetization to relax but still permit the detection within the range 6.45-6.50 by the addition of small amounts of of the desired analyte magnetization, which have relatively sodium hydroxide and hydrochloricacid solutions. NMR spectra long T,'s. The simple Hahn echo pulse sequence was chosen were recorded using 0.5 mL of solution in a 5-mm tube, also over the Carr-Purcell-Meiboom-Gill (CPMG) sequence containing a concentric insert tube. The insert tube contained (which consists of a whole series of refocusing 180' p u l ~ e s ~ , ~ ) a solution of 4 pg/mL TTSM in a deuterated organic solvent. for several reasons. First, experimental comparisons with the Benzene-d6was used for the NDMA work; acetone-$ was used CPMG sequence produced better results with the Hahn echo. for the benzene work. 'M'SM served as a chemical shift reference and an external intensity standard; the deuterated solvent proSecond, although the CPMG sequence refocuses scalar couvided a lock signal for field/frequency stabilization. TTSM was pling (vide infra), small errors in the pulse calibration can lead chosen over TMS as a reference material because of its low to non-zero steady-state magnetizations,l0so the CPMG exvolatility. TTSM is shifted to high frequency relative to TMS periment is less robust.'l Finally, the Hahn echo does not by 0.37 ppm. suppress self-diffusion effects, whereas the CPMG experiment All spectra were acquired on a Bruker AM-500 spectrometer does.s Because of proton exchange, the diffusion coefficient with an 11.75-T magnet and at a proton frequency of 500.13 MHz. in water is unusually high, compared to other liquids. This The proton coil of a Bruker 5-mm inverse detection probe was phenomenon helps in suppressing the water signal relative used for rf transmission and reception. The signals were digitized to the solutes in the Hahn echo experiment. with a 16-bit AD converter, and a receiver gain of 800 was used. The transmitter frequency was set on resonance with the water Precautions are essential when this method is used. Care proton frequency, and a sweep width of 5000 Hz was employed. must be taken to avoid introducing impurities with the amThis swept the region between -0.5 and 9.5 ppm and gave a digital monium chloride and the acid and base solutions. The signals resolution of 0.6104 Hz/pt for a 16 K data set. of all exchangeable hydrogens are lost (though these are at UV irradiation was performed directly on a prepared sample best very broad and weak). Finally, only uncoupled protons in a glass 5-mm sample tube. A spectrum was first recorded of are usually observable for very dilute solute molecules. The the unirradiated sample. The tube was then placed in the center WATR method has low sensitivity for coupled protons for of a group of Rayonex 250-nm UV lamps and irradiated for 1 h, three reasons: (1)the signal intensity is divided among the after which a second NMR spectrum was recorded. members of the multiplet; (2) scalar coupling introduces an RESULTS AND DISCUSSION additional Tz mechanism;12(3) the relative phases of multiplet A set of spectra were recorded on a standard 5 Mg/mL (6.7 members are modulated by the scalar coupling (see ref 9, p X M) sample of NDMA, prepared as was described in the 208). Experimental Section, to optimize the choice of delay time N-Nitrosodimethylamine (NDMA) and benzene both occur 7 for a desired degree of suppression of the water signal. In as contaminants of potable water. Both are suspected carFigure 1the logarithms of the signal intensities, measured as c i n o g e n ~ The . ~ ~ existing methods of analysis for these compeak heights, are plotted versus 7. It is evident that a very pounds involve solvent extraction and preconcentration high level of suppression is possible with only modest atten~ t e p s . ' ~AJ ~corroborative method in which the analytes are uation of the NDMA signal. The Tz's estimated from the detected directly in the aqueous solution would be useful. slopes of the lines in Figure 1 are 8.5 ms for water and 273 EXPERIMENTAL SECTION ms for NDMA. The 7 values needed to attain various degrees All solutions were prepared with water that was purified by of suppression are listed in Table I. A solvent suppression distillation and subsequent passage through a Millipore filtration factor of lo7is required to detect solutes in the submicromolar system, consisting of ion-exchange, Carbon-1, and Org-Q carregime, corresponding to a value of about 140 ms for 7. At tridges. The water was routinely monitored with a Dohrmann this delay time the NDMA signal has only decreased by about DC-180 carbon analyzer, employing UV digestion and IR dehalf (see Figure l),whereas the water signal was completely tection. The total dissolved organic carbon was consistently less eliminated (see Figures 3 and 4). The absence of baseline than 400 ng/mL. distortion from the solvent peak greatly simplified the data The standard solutions were prepared from NDMA purchased analysis. from Aldrich. Spectra were recorded immediately after the so-
ANALYTICAL CHEMISTRY, VOL. 64, NO. 4, FEBRUARY 15, 1992
351
benzene d-5
TTSM
1J
1 1
I
40
60
100
80
120
140
160
Delay Time (ms)
~
9
8
7
6
9
4
3
2
~~
1
0
Figure 1. Natural logarithm of TTSM and N W A peak heights plotted
Chsmlcal S h l t t lppm)
against delay time T (Ume to the top of the echo) to optimize solvent suppressbn. All data were recoTded using a standard 5 pg/mL NDMA sample.
Flgure 3. 'H NMR spectrum of 1 bg/mL NDMA In water showing the complete suppression of the water signal. The NDMA peaks are not
'c; 0.12
a
;t
6-
0.041 I
I y 0
visible at thls scale.
O
O
r/ F
'
0.1
Q)
n 4
I n z -0.02
0
500
1000
1500
2000
2500
0
NDMA Concentration (ng/mL) 'c; 0.12 ; t z
5
0.1
1 I
1b
I
I
I
I
8
I . .
I
I
I
I
I
I
3.8 3 . 7 3.6 3.5 3.4 3.3 3 . 2 3 . 1 3.0 2.9 2.8 2 . 7 2.6
4
Chemical S h i f t (ppm)
Flgure 4. 'H NMR average (bottom) and standard deviation (top) spectra of 1 pg/mL NDMA In water at expanded scale showing the NDMA methyl peaks.
Table 11. Peak Height Data for the High-Frequency (3.53 ppm) NDMA Peak (Denoted by Pl) and the Low-Frequency (2.88 ppm) NDMA Peak (Denoted by P2) NDMA conc, pg/mL TTSM Detection llmlt
= 5 10
ng/mL
NDMA Concentration (ng/mL) FIgm 2. (a)Calibration cuye of the highfrequency (3.53ppm) methyl peak of NDMA. Spectra were recorded using the WATR solvent suppression method, with a 140-ms delay Ume. The IntensMes are block average values calculated from 24 blocks. The lntenslties were measured as peak heights normallzed to the peak height of a n S M external reference. The error bars represent a 99% confidence Interval, calculated from the block standard deviation. (b) Callbratlon curve of the low-frequency (2.88 ppm) methyl peak of NDMA. Data were acquired and processed as In Figure 2a.
A set of standard solutions of NDMA was prepared in order to construct calibration curves (Table 11,Figure 2a,b) and to estimate the limit of detection by extrapolation to zero concentration. Each data point in Figure 2a,b was derived from
0.503 1.081 1.909 2.823
9.52 14.66 14.28 15.82
NDMA conc, pg/mL TTSM 0.503 1.081 1.909 2.823
9.52 14.66 14.28 15.82
peak heights, arbitrary units
P1/ P1 0.19 0.55 0.95 1.59
absolute
%
SIN
TTSM stddev stddev (RMS) 2.00 3.75 6.68 10.05
0.15 0.15 0.14 0.09
79 27 15 5.7
4.6 10.0 13.7 26.2
peak heights, arbitrary units P2l absolute % SIN P2 TTSM std dev std dev (RMS) 0.08 0.561 0.871 1.741
0.84 3.83 6.10 9.05
0.17 0.16 0.17 0.14
210 29 20 8.0
1.9 10.5 12.5 28.7
24 blocks, where each block was an average of 128 transients. Each data point corresponds to 2 h of acquisition time. The partitioning of data into blocks requires no increase in acquisition time. It is somewhat consumptive of spec-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 4, FEBRUARY 15, 1992
I
6.3
,
I
6.2
6.1 C h e m i c a l S h i f t (ppm)
6.0
5.9
Figure 5. 'H NMR average (bottom) and standard deviation (top) spectra of 50 ng/mL benzene in water showing the benzene peak.
trometer disk space and processor resources (although less so than a typical 2D experiment). The spectra were therefore transferred and processed off line on IBM-compatible computers, Using the programs NMR-286 and NMRSTAT. NMR-286 is a general-purpose program purchased from Softpulse Software for processing and displaying NMR spectra. NMRSTAT is a program that compiles a set of blocks into average and standard deviation spectra. The data were subjected to a minimal amount of processing. The free induction decays (FIDs) of a block set were first averaged, Fourier-transformed (without multiplication by a window function), and phase-corrected. The half-height line widths of the solute signals were measured from the resultant block average spectrum. The individual FIDs of the block set were then individually processed with an exponential window function using the block average line width as a matched filter constant for optimum sensitivity (see ref 9, p 152). The set of match-filtered spectra were processed by NMRSTAT to yield an average spectrum and a standard deviation spectrum (for example, see Figure 4). For the average spectra the baseline in the immediate neighborhood of a solute line was corrected by taking the average value of an eight-point region of peak-free baseline on either side of the line, estimating the true baseline by linearly interpolating between the two average values, and subtracting this estimated baseline from the spectrum. These same eight-point regions of the baseline were used to calculate RMS noise values and signal to noise ratios. The standard deviation of a peak height was taken by averaging the 21 points in the standard deviation spectrum centered at the corresponding point of maximum intensity in the average spectrum. When operating the system on the verge of the detection limit, where S I N is low, one expects the error to be predominantly random, rather than systematic. This was reflected in the standard deviations. The frequency distribution of standard deviation fluctuated randomly about a constant mean and was not correlated with the average spectrum (Figures4 and 5). Also, the absolute standard deviations did not scale with the NDMA peak height over the concentration range studied (Table 11). The solute concentration was measured by taking the peak height of the NMR line in question divided by the peak height of the TTSM external reference line. The relative merits of using peak heights and peak integrals have been considered in previous work.I6 In the present case, where one is working
near the detection limit, externally referenced peak heights are the superior method because the baseline is obscured by noise. For example, the 1 pg/mL NDMA peak heights had S I N values of about 1 0 1 and standard deviations of about 30%. A theoretical calculation of the correspondingstandard deviation for peak integrals gives a value of about 70%.17 Implicit in the use of peak heights is the assumption that the line widths are constant over the concentration range in question. The linearity of the plots in Figure 2a,b confirms this assumption. The error bars represent 99% confidence levels calculated from the standard deviations by use of the Student's t distribution.18 The upper and lower error limits were extrapolated to zero concentrationby linear regression. The detection limit was estimated by extrapolating the upper error limit to zero concentrationand extrapolatingto that value in the lower error limit (see ref 18, chapter 4). The average of the detection limits for the two NDMA methyl peaks was 510 ng/mL (6.9 X lo+ M). Figure 5 shows the average and standard deviation spectra of a 50 ng/mL (6.4 X M) sample of benzene. The spectra were calculated from 24 blocks, each the average of 256 transients, which required 4 h to acquire. This constitutes the practical limit of instrument time, since the analysis of a real sample would normally require running a blank and at least one standard addition. The standard deviation at the position of the benzene line was 23% of the peak height; the detection limit, estimated as three standard deviations was therefore 35 ng/mL (4.5 X M). The lower detection limit observed for benzene (relative to NDMA) may be attributed to the longer T2of benzene, the fact that the signal arises from six equivalent hydrogens, and the longer acquisition time. The width at half-maximum of the NDMA and benzene peaks was 2.4-2.5 Hz. This corresponds to at least four data points per line width, since the digital resolution was 0.6104 Hz/pt, which should not be a significant source of e r r ~ r . ~ J ' However, to assure that the precision was not limited by digital resolution, the original benzene FID's were zero-filled from 16 to 32 K and reprocessed. The standard deviation at the benzene frequency was then found to be 22% (versus 23% for the 16 K data sets), which is not a signifcant improvement. The practical limit of sensitivity (for this N M R instrument) has been approached. It may be possible to increase sensitivity by going to a larger sample volume. However, we were unable to obtain results from a 10-mm probe because of difficulties in tuning the highly lossy samples. As probe technology improves higher sample volumes may become feasible. A modest increase in sensitivity may result from degassing the samples, which would increase the solute T,'s. However, this advantage would probably be offset by increased error in the concentrationcaused by evaporation of solvent and/or solutes. There may be room to improve the precision of this method. In spite of essentially complete suppression of the water signal, the standard deviation of the peak heights still appears to be limited by the dynamic range of digitization. The dominant signal in the solvent-suppressedspectra is due to the residual protons in the deuterated solvent. One possible solution would be to use high-purity, highly deuterated solvents. This would yield, at best, a 10-fold increase in dynamic range. Altemately, it may be useful to incorporate a presaturation pulse at the solvent frequency into the pulse sequence. No increase in acquisition time would result, and an even higher receiver gain might be possible. However, we have so far found this difficult to implement in practice because the gain became so large that the receiver was overloaded by the slight offsets in dc level arising from imperfections in the tuning of the receiver and amplifier circuits.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 4, FEBRUARY 15, 1992
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may have higher detection limits, since paramagnetic impurities could shorten solute Tz’s. The nondestructive nature of NMR spectroscopy allowed the effect of radiation on a dilute aqueous NDMA sample to be conveniently monitored and the predominant photoproduct (DMA) to be identified. While ‘H NMR spectroscopy will not replace other analytical methods on a routine basis for analyzing dilute solutions, its ability to resolve the chemical components of a solution without an extraction or chemical separation makes it a useful technique for analyzing mixtures of low molecular weight organic compounds.
ACKNOWLEDGMENT 3.8
3.6
3.4
3.2
3.0
2.8 Chemical S h i f t (ppml
2.6
2.4
2.2
Flgure 6. ‘H NMR spectra showing the effect of 1 h of 250-nm UV irradiation on a sample of 10 pg/mL NDMA in water. The baseline distortion is due to incomplete suppression of the water peak, since for these spectra a shorter delay time of 40 ms was used. (a) Before irradiation, the NDMA peaks are visible. (b) After irradiation, the dimethylamine (DMA) peak Is visible. The DMA peak was identified by subsequently spiking the sample with DMA.
W irradiation has been proposed as a method for removing nitrosoamines from water.19 NMR spectroscopy provides a convenient method for studying the effect of radiation on dilute aqueous solutions of organics. Figure 6 shows the effect of irradiation on a 10 pg/mL sample of NDMA. Prior to irradiation the two methyl lines of NDMA were observed. After 1h of 250-nm irradiation the NDMA signals vanished and a new line a t 2.44 ppm appeared. This signal was determined (by spiking) to be due to dimethylamine (DMA). These data suggest that W irradiation is effective in removing NDMA from water and that DMA is the predominant photoproduct and are consistent with what has been observed for other nitrosoaminea: W irradiation cleaves the N-N bond.20z1 However, water that has been treated by irradiation should be carefully monitored, since DMA in the presence of nitrosonium ions can reform NDMA.22 CONCLUSION We have demonstrated the feasibility of using high-field lH NMR spectroscopy as a method for the analysis of dilute aqueous organics and have established that practical detection limits are well below 1pg/mL. The WATR method provided the necessary solvent suppression and contributed minimally to experimental error. The use of externally referenced peak heights and block data acquisition produced linear calibration curves. It should be recognized that the detection limits reported here represent the best case scenario. Real samples
This work was supported by the Natural Science and Engineering Research Council of Canada. Registry NO.NDMA, 62-75-9; DMA,124-40-3;C&, 71-43-2; HzO,7732-18-5.
REFERENCES (1) Kasler, F. Qualitative Analysis by NMR; Academic Press: New York, 1973. (2) Rabenstein, D. L.; Keire, D. A. In Modern N M R Techniques and Their ApprrCelion in chemistry; Popov, A. I., Hailenga, K., Eds.; Marcel Dekker: New York, 1991. (3) Hore. P. J. J . Magn. Reson. 1083, 55, 283-300. (4) Pan, S. L.; Sykes, 0. D. J . Chem. f h y s . 1072, 56, 3182-3184. (5) Rabenstein, D. L.; Fan, S.; Nakashima, T. T. J . Magn. Reson. 1085, 64, 541-546. (6) Rabenstein. D. L.; Fan, S. Ana/. Chem. 1088, 58, 3178-3184. (7) Connor, S.; Everett, J. R.; Nicholson, J. K. Anal. Chem. 1987, 59, 2885-2891. (8) Can,H. Y.; Purcell, E. M. f h y s . Rev. 1054, 9 4 , 630-638. (9) Emst, R. R.; Bodenhausen, G.; Wokaun, A. prlnclples of N u c k r Magnetic Resonance in One and Two Dlmensions ; Clerendon Press: Oxford, U.K., 1987. (10) Vold, R. L.; Vold. R. R.; Simon, H. E. J . Megn. Reson. 1073. I f . 283-298. (11) Baln, A. D. J . Magn. Reson. 1088. 77, 125-133. (12) Shou~.R. R.; Vander Hart. D. L. J . Am. Chem. Soc. 1071.. 93.. 205312054. (13) Lijinsky, W.; Epstein, S. S. Nature (London) 1070, 225, 21-23. (14) Takatsuki, T.; Kikuchi, T. J . Chromatcgr. 1090, 508, 357-362. (15) Environmental Protection Agency. Method 607; GPO: Washington,
Dc. (16) Baln, A. D.; Fahie, B. J.; Kozluk, T.; Leigh, W. J. Can. J . Chem. 1001. 69, 1189-1192. (17) Weiss, G. H.; Ferretti, J. A. J . Magn. Reson. 1083, 55, 397-407. (18) Sharaf, M. A.; Iiiman, D. L.; Kowalski, B. R. Chemometrics; J. Wiiey: New Y ork, 1986. (19) Ballweg, H.; Schmiihi, D. Naturwissenscheffen 1067, 5 4 , 116-120. (20) Axenrod, T.; Mllne, G. W. A. Tefrahedron 1988, 2 4 , 5775-5783. (21) Crumrine, D. S.; Brodbeck, C. M.; Dombrowski, P. H.; Haberkamp, T. J.: Kekstas, R. J.; Nabor, P.; Padieckas, H. A.; Suther, D. J.; Yonan, J. P. J . Org. Chem. 1082, 4 7 , 4246-4249. (22) March, J. Advanced Organic Chemistry, 2nd ed.; Mc&aw-Hili: New York, 1977.
RECEIVED for review August 21,1991. Accepted November 1, 1991.
