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Analyst, December, 1978, Vol. 103, $9. 1233-1238
1233
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Determination of Trace Organic Pollutants in Water by Proton Magnetic Resonance Spectroscopy of Solvent Extracts J. K. Becconsall I m f e r i a l Chemical Industries Limited, Corporate Laboratory, P.O. Box 11, Runcorn, Cheshire, W A7 4QE
Extraction with carbon tetrachloride followed by proton magnetic resonance spectroscopy, using a pulsed Fourier transform spectrometer with external field-frequency lock, has been investigated as a method for identifying and determining trace organic pollutants in water. The resulting spectra can be used as fingerprints in order to identify pollutants, and for quantitative measurements on specific compounds. The method is useful for all hydrogencontaining compounds that are efficiently extracted by the solvent, including hydrocarbons, halogenated hydrocarbons and organochlorine pesticides. Measurements can be made over a wide range of pollutant concentrations, from heavily polluted effluents to waters containing as little as 15 pg1-l of alkanes.
Keywords : Organic pollutant determination ; ?roton magnetic resonance spectroscopy ; water analysis
The relatively low sensitivity of nuclear magnetic resonance spectrometers has in the past excluded the technique from being seriously considered for routine measurement and identification of organic water pollutants. The objective of this paper is to evaluate the prospects opened up in this field of analysis by the much more sensitive pulsed Fourier transform nuclear magnetic resonance spectrometers now in use in many laboratories. The use of proton nuclear magnetic resonance spectroscopy in this application is broadly similar to that of infrared absorption spectroscopy, as both techniques involve a solvent extraction step for pre-concentration and elimination of the water band, which would otherwise obscure large regions of the spectra. Methods for quantifying oil in water by measurement of infrared absorption in solvent extracts have already been in use for several years,l and fingerprint identification by infrared is also well known.2 Previously reported applications of nuclear magnetic resonance spectroscopy to water pollution analysis have involved phosphorus-31 measurements,3,4 and proton nuclear magnetic resonance spectroscopy of ether extracts from concentrated sewage effluent.5
Solvent Extraction By extracting organic pollutants with a suitable solvent that is immiscible with water, the water band is eliminated from the spectra, or a t least greatly reduced, and where the partition coefficients are favourable a useful pre-concentration is achieved. Carbon tetrachloride is a useful solvent for both nuclear magnetic resonance and infrared spectroscopy, as in its pure state it contributes nothing to the proton nuclear magnetic resonance spectrum and gives good infrared transmission, particularly between 1650 and 4000 cm-l. It will extract very efficiently all types of hydrocarbons, halogenated hydrocarbons including polychlorinated biphenyls, vinyl chloride and chloroform, hexachlorophene, organochlorine pesticides such as DDT, aldrin and dieldrin, and organophosphorus pesticides such as malathion. At the opposite extreme, hydroxyl-containing pollutants such as phenols, alcohols and organic acids are not extracted efficiently by organic solvents, and the limits of detection for these are consequently much higher than for oily substances. As an alternative extraction solvent we have tried carbon disulphide, in the hope that the residual dissolved water peak would be shifted from its position in carbon tetrachloride (see below), but this turned out not t o be so. Carbon disulphide has the disadvantages of being more volatile and less dense than carbon tetrachloride and it cannot conveniently be purified by distillation on account of its flammability. A better alternative solvent,l
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BECCONSALL: DETERMINATION OF TRACE ORGANIC POLLUTANTS IN
Artalyst, Vol. 103
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
which should be considered because of the much lower toxicity risk compared with carbon tetrachloride, is fluorocarbon 113 (C,F,Cl,). If a volume Vw of polluted water is contacted with a volume Vs of solvent and equilibrium is attained before separation of water and solvent, the pollutant concentration in the solvent is enhanced by a factor
compared with the initial concentration in water, Cgw being the partition coefficient for a particular solute. The objective for spectroscopic purposes is to maximise f, which is evidently achieved by using as high an extraction ratio, Vw/Vs, as is practicable. As V s is fixed by the requirements of the spectrometer sample cell, this volume of solvent should be contacted with as large a volume of water as can conveniently be handled with effective recovery of most of the separated solvent. We have used various extraction volume ratios from 40: 1 to 200: 1. The true solubility of oil in water is very low, and most of the oil present in environmental samples is either emulsified or carried on suspended solids. This results in high effective values for the partition coefficient Csw, giving to a good approximation f = Vw/Vs; under these circumstances virtually none of the pollutant is left behind in the water or on solid particles. For measuring trace amounts of pollutants, purity of the extracting solvent is clearly important. Reagent-grade carbon tetrachloride usually contains small amounts of chloroform and dissolved grease, which can present difficulties where low pollutant levels (of the order of less than 1 mg 1-1 in water) are to be measured. We have used fractionally distilled carbon tetrachloride, thus eliminating the grease, but not the chloroform. The presence of the latter would hinder the measurement of very low concentrations of aromatic compounds, or of chloroform itself. Polluted waters usually contain surfactants that make it difficult to separate the solvent from the water after the mixture has been shaken for a few minutes. We have found that the dense carbon tetrachloride - water emulsion that typically settles out in 5-10 min is stable under normal conditions, but can be broken down rapidly by freezing it to liquid nitrogen temperature in a small phial and allowing it to warm up to room temperature; this operation is repeated several times if necessary. As an alternative to batch extraction in the laboratory, continuous on-site solvent extractors have been described and applied, for example, to the recovery of organochlorine pesticides and polychlorinated biphenyls at low levels in river or sea water.6 By this means the effective volume of water contacted with the solvent can be greatly increased, and the results obtained are average pollutant levels over the period of hours or days during which the solvent is exposed to the moving body of water. Other possible methods for increasing the over-all concentration factor include a preliminary freeze concentration step carried out on a large water sample,' or further concentration of the solvent extract by evaporation. These methods could reduce the limits of detection considerably, although at the expense of greater labour and risks of contamination or loss of volatile components. Dissolved or emulsified hydrophobic substances such as hydrocarbons and organochlorine compounds can alternatively be extracted from water by retention in a column containing an immobilised hydrophobic surface phase, from which they are then desorbed by using a small amount of solvent. Methods using silicic chromatographic supports with a chemically bonded silicone coatings or commercially available copolymer resins9 have been described. Although hitherto used exclusively in conjunction with gas - liquid chromatography, these methods could be readily adapted for nuclear magnetic resonance spectroscopy. Any appreciable amounts of suspended solids must, however, be filtered from the water sample and treated in a separate solvent extraction step, whereas the solvent extraction method treats the whole sample. Careful attention to standardisation of the preparation of the column packing seems likely to be needed, in order to avoid variations in extraction efficiency, whereas the solvent extraction method has fewer variables and should more easily give
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December, 1978 WATER BY PMR SPECTROSCOPY O F SOLVENT EXTRACTS 1235 reproducible performance. On the other hand, the column retention method is claimed9to give higher extraction efficiencies for slightly polar compounds whose solvent - water partition coefficients are unfavourable. Another possible advantage is that the column extraction process may leave less water in the final nuclear magnetic resonance spectroscopy sample than does the method described here.
Sample Presentation and Nuclear Magnetic Resonance Spectroscopic Measurements The solvent extract contains dissolved water, which gives a troublesome, unwanted peak in the nuclear magnetic resonance spectrum. In carbon tetrachloride its position is about 1.05 p.p.m. downfield from the silicone grease reference peak (see below), and thus it is in the middle of the saturated hydrocarbon region. The water can be removed by using phosphorus(V) oxide or other drying agents, but there are risks of organic contaminants being introduced or of reactions with other compounds present. We have reduced the size of the water peak by shaking each extract briefly with 0.5-1.0 times its volume of deuterium oxide (99.8% D), then drawing off the deuterium oxide with a pipette and adding an amount of Linde 4A molecular sieve sufficient to increase the total volume by about 25% t o the solvent extract. The molecular sieve immobilises any remaining free deuterium oxide, and the extract can then be drawn off with a pipette and transferred into the spectrometer sample tube. Most pulsed Fourier transform nuclear magnetic resonance spectrometers rely for precise field stabilisation on a deuterium lock signal, which is usually provided by a deuterated solvent such as deuterochloroform. For the present purpose an appropriate amount of deuterochloroform or deuterobenzene can be added to the solvent beforehand or to the separated extract. Attention must then be paid to the purity of the locking additive as well as to that of the solvent itself. The additive inevitably produces a large peak in the proton nuclear magnetic resonance spectrum, the deuteration level usually being in the range 99.5-99.9%, and for deuterochloroform or deuterobenzene this peak will prevent or greatly hinder analysis for aromatic compounds and for chloroform itself. These problems can be avoided if the spectrometer has an external lock facility, where the field stabilisation is independent of the analytical sample and no additive is needed. Our measurements were made on a JEOL FX-100 spectrometer (proton nuclear magnetic resonance frequency 100 MHz) with external lock attachment. This feature is a considerable advantage for environmental analysis and we recommend that an external lock is used whenever possible for such work. Maintenance of adequate field homogeneity, despite the absence of the usual internal deuterium signal for optimisation, was not a serious problem, at least for samples in 5 m m diameter tubes; a test sample of 10% deuterobenzene in carbon tetrachloride was substituted for the analytical sample before the start of the run, its magnetic susceptibility being close enough to that of the analytical sample to allow effective optimisation of field homogeneity. With this system of analysis using an external lock the only solvent additive needed is a small amount of a spike compound to serve as a reference for comparing peak intensities, and also as a chemical shift reference if needed. We have found that about 30 mg 1-1 of silicone stopcock grease added to the solvent stock is a suitable reference for our work on river and canal waters, as it gives an appropriate peak width and is not lost by evaporation. This addition is intended only for relative intensity measurements, not for absolute Calibration. For reliable quantitative measurements, the intensity scale in relation to the spike should be calibrated by making up standard solutions in water containing known concentrations of the compounds to be measured, or of related compounds, and preparing solvent extracts as for the field samples. We have used two different nuclear magnetic resonance probes, accommodating 5 and 10 mm diameter sample tubes. The 10-mm probe was found to give an advantage of about 2 x in effective signal to noise ratio for a given solvent extract compared with the 5-mm probe. Disadvantages of 10-mm sample tubes are the unavoidable sacrifice of resolution (accentuated by having to rely on a sample substitution method for field homogeneity adjustment), and possible inconvenience in scaling up the volumes of sample that must be handled.
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Results and Discussion We have used the nuclear magnetic resonance spectroscopic method to examine various river and canal waters that receive industrial and municipal effluents. Fig. 1 shows a spectrum obtained in a 5 mm diameter sample tube using a 1 :150 carbon tetrachloride extraction from the Weaver Canal, which receives water from the Manchester Ship Canal and is polluted with a wide variety of mainly industrial effluents. The most prominent feature, as in nearly all of the samples that we have examined, is the straightchain alkane CH, peak at S = 1.30 p.p.m. (relative to the silicone grease reference peak, the position of which is close to that of the more usual tetramethylsilane). The peak at 6 = 0.90 p.p.m. is assigned to alkane CH, groups. By comparing the CH, peak with that of a standard, made by adding a weighed amount of heating fuel oil to distilled water and extracting similarly with carbon tetrachloride, we estimated the alkane concentration in this particular Weaver Canal sample to be 2.4 & 0.2 mg 1-l. The presence of unsaturated hydrocarbons is shown by the downfield CH, peaks at 8 = 2.06 and 2.35 1p.p.m. and by olefinic proton resonances in the region S = 4-5 p.p.m. To identify other features confidently would need additional information, e.g., from detailed surveys relating water movements to known effluent outfalls, aided if necessary by other analytical techniques.
I
I
L
Fig. 1. Spectrum of 1 to 150 carbon tetrachloride extract from a Weaver Canal sample. Internal reference 4 mg 1-1 silicone stopcock grease; pulse width 10 ps (90' approx.); free induction decay spectrum width 1 kHz; free induction decay filter band width 1 kHz; 8192 data points positive exponential window, EX = 8 ; quadrature detection not used ; 5-mm sample tube. Duration of run 1 h, 720 pulses recorded.
The nuclear magnetic resonance spectroscopic method could thus be used as a convenient fingerprint to trace the fate of individual pollutants in a water system. For example, samples from the River Weaver downstream of its junction with the Weaver Canal, when examined by the nuclear magnetic resonance spectroscopic method, showed the expected large dilution effect for most of the peaks seen in Fig. 1. The peak at S = 3.80 p.p.m., whose position indicates a methyl ester, was an exception, being comparatively stronger in
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
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December, 1978 WATER BY PMR SPECTROSCOPY O F SOLVENT EXTRACTS 1237 the River Weaver samples, showing that the compound concerned enters the river a t some point upstream of the junction. The spectrum in Fig. 1 was obtained by a 60-min nuclear magnetic resonance spectroscopic run of 720 pulses, to illustrate the potential of the method for showing detail. In fact, we found that nearly all of the features seen here could be satisfactorily identified and measured in a run of only 10 min (120 pulses). For most of our work we have used only 10-min standard runs. Fig. 2 shows a spectrum of an extract prepared under identical conditions from an industrial water supply reservoir (abstracted from local streams) in which it was of interest to know the hydrocarbon content. Measurement of the CH, peak intensity indicated that 1.3 0.2 mgl-1 of alkanes are present. The spectrum also illustrates the effect of residual dissolved water in the extract, giving a peak a t 6 = 1.05 p.p,m.
Fig. 2. Spectrum of 1 to 150 carbon tetrachloride extract from an industrial water supply. Internal reference 4 mg 1-1 silicone stopcock grease. External reference hexamethyldisilazane in carbon tetrachloride in a 2 mm 0.d. capillary. Other instrumental conditions and Fourier transform parameters as for Fig. 1. (The external reference, which is the more intense peak a t 6 = -0.04 p.p.m., is not normally needed; it was used here as a quantitative standard instead of the inadequately small internal reference peak.) Five-millimetre sample tube with capillary insert. Duration of run 10 min, 120 pulses recorded.
The above two examples are taken from our recent surveys of the River Weaver and its tributaries and connecting waterways. Alkane contents varying between 0.1 and 2.4 mg 1-1 were measured. These results were compared with measurements made by the infrared absorption technique,l also using carbon tetrachloride extracts, and the agreement was generally good. The limits of detection of the nuclear magnetic resonance spectroscopic method with our spectrometer can be determined from measured signal to noise ratios. An environmental sample containing 100 pg 1-1 of alkane (as measured by comparison with a standard of 2 mg 1-1 of fuel oil in water) gave a CH, peak signal to noise ratio of about 5 in a 10-min run (120 pulses) using 100: 1 solvent extraction ratio and a 10 mm diameter sample tube. Thus, a 90-min run with an extraction ratio of 200: 1 would be expected to give the same signal to noise ratio for a concentration of one sixth of that amount, i.e., approximately 17 pg 1-l. An alkane concentration of 15 pg 1-1 would be detectable with a signal to noise ratio of at least 4 under the latter conditions.
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I n marked contrast to this figure, phenol is an example of a pollutant for which the extraction efficiency is very unfavourable. A 40 :1 carbon tetrachloride extract from water containing 100 mg 1-1 of phenol gave easily detectable ring proton peaks in a 33-min run (in this instance with a 5-mm sample tube), but when the phenol concentration was reduced to 10 mg 1-1 no identifiable peaks were seen even in an overnight nuclear magnetic resonance spectroscopic run.
Conclusions Pulsed Fourier transform nuclear magnetic resonance spectroscopy offers a versatile, straightforward and reasonably quick method for measuring organic compounds in water, and is especially suitable for hydrocarbons and chlorinated hydrocarbons. The sensitivity allows measurements down to tens of micrograms per litre in favourable instances, which is useful for analysis of a wide range of polluted waters and effluents, but falls well short of gas chromatography and gas chromatography - mass spectrometry. Because the method gives a composite spectrum of all of the extractable organic pollutants present, it is much less powerful for positive identification of individual compounds than is gas chromatography - mass spectrometry, but the spectra can be used in a fingerprint fashion to identify pollution sources, and could provide a convenient comparison technique when the pollutants of interest had been identified with the help of other information.
I thank E. F. Thurston and other colleagues in Imperial Chemical Industries Limited, Mond Division, for co-operation and useful discussions on sampling in the River Weaver area, and B. W. Cook of this Laboratory for infrared measurements on environmental samples and for discussion of these results. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
Coles, G. P., Dille, R. M., and Shull, D. L., P r e p . Div. Petrol. Chem. A m . Chem. SOG.,1975, 20, 641. Brown, C. W., Lynch, P. F., and Ahmadjian, &I., Envir. Sci. Technol., 1974, 8, 669. Gurley, T . W . ,and Ritchey. W. M., Analyt. Chem., 1975, 47, 1444. Gurley, T. W., and Ritchey, W. M., Analyt. Chem., 1976, 48, 1137. Adams, V. D., Middlebrooks, E. J., and Nance, P. D., Wat. Sewage W k s , 1976, 122, 98. Ahnhoff, M., and Josefsson, B., Analyt. Chem., 1974, 46, 658. Shapiro, J., in Zief, M., Editor, “Purification of Inorganic and Organic Materials: Techniques of Fractional Solidification,” Marcel Dekker, New York, 1969, p. 147. Aue, W. A., Kapila, S., and Hastings, C. R., J . Chromat., 1972, 73, 99. Osterroht, C., J . Chromat., 1974, 101, 289. Received March loth, 1978 Accepted June 12th, 1978
Analyst, December, 1978, Vol. 103, $9. 1233-1238
1233
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
Determination of Trace Organic Pollutants in Water by Proton Magnetic Resonance Spectroscopy of Solvent Extracts J. K. Becconsall I m f e r i a l Chemical Industries Limited, Corporate Laboratory, P.O. Box 11, Runcorn, Cheshire, W A7 4QE
Extraction with carbon tetrachloride followed by proton magnetic resonance spectroscopy, using a pulsed Fourier transform spectrometer with external field-frequency lock, has been investigated as a method for identifying and determining trace organic pollutants in water. The resulting spectra can be used as fingerprints in order to identify pollutants, and for quantitative measurements on specific compounds. The method is useful for all hydrogencontaining compounds that are efficiently extracted by the solvent, including hydrocarbons, halogenated hydrocarbons and organochlorine pesticides. Measurements can be made over a wide range of pollutant concentrations, from heavily polluted effluents to waters containing as little as 15 pg1-l of alkanes.
Keywords : Organic pollutant determination ; ?roton magnetic resonance spectroscopy ; water analysis
The relatively low sensitivity of nuclear magnetic resonance spectrometers has in the past excluded the technique from being seriously considered for routine measurement and identification of organic water pollutants. The objective of this paper is to evaluate the prospects opened up in this field of analysis by the much more sensitive pulsed Fourier transform nuclear magnetic resonance spectrometers now in use in many laboratories. The use of proton nuclear magnetic resonance spectroscopy in this application is broadly similar to that of infrared absorption spectroscopy, as both techniques involve a solvent extraction step for pre-concentration and elimination of the water band, which would otherwise obscure large regions of the spectra. Methods for quantifying oil in water by measurement of infrared absorption in solvent extracts have already been in use for several years,l and fingerprint identification by infrared is also well known.2 Previously reported applications of nuclear magnetic resonance spectroscopy to water pollution analysis have involved phosphorus-31 measurements,3,4 and proton nuclear magnetic resonance spectroscopy of ether extracts from concentrated sewage effluent.5
Solvent Extraction By extracting organic pollutants with a suitable solvent that is immiscible with water, the water band is eliminated from the spectra, or a t least greatly reduced, and where the partition coefficients are favourable a useful pre-concentration is achieved. Carbon tetrachloride is a useful solvent for both nuclear magnetic resonance and infrared spectroscopy, as in its pure state it contributes nothing to the proton nuclear magnetic resonance spectrum and gives good infrared transmission, particularly between 1650 and 4000 cm-l. It will extract very efficiently all types of hydrocarbons, halogenated hydrocarbons including polychlorinated biphenyls, vinyl chloride and chloroform, hexachlorophene, organochlorine pesticides such as DDT, aldrin and dieldrin, and organophosphorus pesticides such as malathion. At the opposite extreme, hydroxyl-containing pollutants such as phenols, alcohols and organic acids are not extracted efficiently by organic solvents, and the limits of detection for these are consequently much higher than for oily substances. As an alternative extraction solvent we have tried carbon disulphide, in the hope that the residual dissolved water peak would be shifted from its position in carbon tetrachloride (see below), but this turned out not t o be so. Carbon disulphide has the disadvantages of being more volatile and less dense than carbon tetrachloride and it cannot conveniently be purified by distillation on account of its flammability. A better alternative solvent,l
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BECCONSALL: DETERMINATION OF TRACE ORGANIC POLLUTANTS IN
Artalyst, Vol. 103
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
which should be considered because of the much lower toxicity risk compared with carbon tetrachloride, is fluorocarbon 113 (C,F,Cl,). If a volume Vw of polluted water is contacted with a volume Vs of solvent and equilibrium is attained before separation of water and solvent, the pollutant concentration in the solvent is enhanced by a factor
compared with the initial concentration in water, Cgw being the partition coefficient for a particular solute. The objective for spectroscopic purposes is to maximise f, which is evidently achieved by using as high an extraction ratio, Vw/Vs, as is practicable. As V s is fixed by the requirements of the spectrometer sample cell, this volume of solvent should be contacted with as large a volume of water as can conveniently be handled with effective recovery of most of the separated solvent. We have used various extraction volume ratios from 40: 1 to 200: 1. The true solubility of oil in water is very low, and most of the oil present in environmental samples is either emulsified or carried on suspended solids. This results in high effective values for the partition coefficient Csw, giving to a good approximation f = Vw/Vs; under these circumstances virtually none of the pollutant is left behind in the water or on solid particles. For measuring trace amounts of pollutants, purity of the extracting solvent is clearly important. Reagent-grade carbon tetrachloride usually contains small amounts of chloroform and dissolved grease, which can present difficulties where low pollutant levels (of the order of less than 1 mg 1-1 in water) are to be measured. We have used fractionally distilled carbon tetrachloride, thus eliminating the grease, but not the chloroform. The presence of the latter would hinder the measurement of very low concentrations of aromatic compounds, or of chloroform itself. Polluted waters usually contain surfactants that make it difficult to separate the solvent from the water after the mixture has been shaken for a few minutes. We have found that the dense carbon tetrachloride - water emulsion that typically settles out in 5-10 min is stable under normal conditions, but can be broken down rapidly by freezing it to liquid nitrogen temperature in a small phial and allowing it to warm up to room temperature; this operation is repeated several times if necessary. As an alternative to batch extraction in the laboratory, continuous on-site solvent extractors have been described and applied, for example, to the recovery of organochlorine pesticides and polychlorinated biphenyls at low levels in river or sea water.6 By this means the effective volume of water contacted with the solvent can be greatly increased, and the results obtained are average pollutant levels over the period of hours or days during which the solvent is exposed to the moving body of water. Other possible methods for increasing the over-all concentration factor include a preliminary freeze concentration step carried out on a large water sample,' or further concentration of the solvent extract by evaporation. These methods could reduce the limits of detection considerably, although at the expense of greater labour and risks of contamination or loss of volatile components. Dissolved or emulsified hydrophobic substances such as hydrocarbons and organochlorine compounds can alternatively be extracted from water by retention in a column containing an immobilised hydrophobic surface phase, from which they are then desorbed by using a small amount of solvent. Methods using silicic chromatographic supports with a chemically bonded silicone coatings or commercially available copolymer resins9 have been described. Although hitherto used exclusively in conjunction with gas - liquid chromatography, these methods could be readily adapted for nuclear magnetic resonance spectroscopy. Any appreciable amounts of suspended solids must, however, be filtered from the water sample and treated in a separate solvent extraction step, whereas the solvent extraction method treats the whole sample. Careful attention to standardisation of the preparation of the column packing seems likely to be needed, in order to avoid variations in extraction efficiency, whereas the solvent extraction method has fewer variables and should more easily give
View Article Online
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
December, 1978 WATER BY PMR SPECTROSCOPY O F SOLVENT EXTRACTS 1235 reproducible performance. On the other hand, the column retention method is claimed9to give higher extraction efficiencies for slightly polar compounds whose solvent - water partition coefficients are unfavourable. Another possible advantage is that the column extraction process may leave less water in the final nuclear magnetic resonance spectroscopy sample than does the method described here.
Sample Presentation and Nuclear Magnetic Resonance Spectroscopic Measurements The solvent extract contains dissolved water, which gives a troublesome, unwanted peak in the nuclear magnetic resonance spectrum. In carbon tetrachloride its position is about 1.05 p.p.m. downfield from the silicone grease reference peak (see below), and thus it is in the middle of the saturated hydrocarbon region. The water can be removed by using phosphorus(V) oxide or other drying agents, but there are risks of organic contaminants being introduced or of reactions with other compounds present. We have reduced the size of the water peak by shaking each extract briefly with 0.5-1.0 times its volume of deuterium oxide (99.8% D), then drawing off the deuterium oxide with a pipette and adding an amount of Linde 4A molecular sieve sufficient to increase the total volume by about 25% t o the solvent extract. The molecular sieve immobilises any remaining free deuterium oxide, and the extract can then be drawn off with a pipette and transferred into the spectrometer sample tube. Most pulsed Fourier transform nuclear magnetic resonance spectrometers rely for precise field stabilisation on a deuterium lock signal, which is usually provided by a deuterated solvent such as deuterochloroform. For the present purpose an appropriate amount of deuterochloroform or deuterobenzene can be added to the solvent beforehand or to the separated extract. Attention must then be paid to the purity of the locking additive as well as to that of the solvent itself. The additive inevitably produces a large peak in the proton nuclear magnetic resonance spectrum, the deuteration level usually being in the range 99.5-99.9%, and for deuterochloroform or deuterobenzene this peak will prevent or greatly hinder analysis for aromatic compounds and for chloroform itself. These problems can be avoided if the spectrometer has an external lock facility, where the field stabilisation is independent of the analytical sample and no additive is needed. Our measurements were made on a JEOL FX-100 spectrometer (proton nuclear magnetic resonance frequency 100 MHz) with external lock attachment. This feature is a considerable advantage for environmental analysis and we recommend that an external lock is used whenever possible for such work. Maintenance of adequate field homogeneity, despite the absence of the usual internal deuterium signal for optimisation, was not a serious problem, at least for samples in 5 m m diameter tubes; a test sample of 10% deuterobenzene in carbon tetrachloride was substituted for the analytical sample before the start of the run, its magnetic susceptibility being close enough to that of the analytical sample to allow effective optimisation of field homogeneity. With this system of analysis using an external lock the only solvent additive needed is a small amount of a spike compound to serve as a reference for comparing peak intensities, and also as a chemical shift reference if needed. We have found that about 30 mg 1-1 of silicone stopcock grease added to the solvent stock is a suitable reference for our work on river and canal waters, as it gives an appropriate peak width and is not lost by evaporation. This addition is intended only for relative intensity measurements, not for absolute Calibration. For reliable quantitative measurements, the intensity scale in relation to the spike should be calibrated by making up standard solutions in water containing known concentrations of the compounds to be measured, or of related compounds, and preparing solvent extracts as for the field samples. We have used two different nuclear magnetic resonance probes, accommodating 5 and 10 mm diameter sample tubes. The 10-mm probe was found to give an advantage of about 2 x in effective signal to noise ratio for a given solvent extract compared with the 5-mm probe. Disadvantages of 10-mm sample tubes are the unavoidable sacrifice of resolution (accentuated by having to rely on a sample substitution method for field homogeneity adjustment), and possible inconvenience in scaling up the volumes of sample that must be handled.
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AnaZyst, VoZ. 103
Results and Discussion We have used the nuclear magnetic resonance spectroscopic method to examine various river and canal waters that receive industrial and municipal effluents. Fig. 1 shows a spectrum obtained in a 5 mm diameter sample tube using a 1 :150 carbon tetrachloride extraction from the Weaver Canal, which receives water from the Manchester Ship Canal and is polluted with a wide variety of mainly industrial effluents. The most prominent feature, as in nearly all of the samples that we have examined, is the straightchain alkane CH, peak at S = 1.30 p.p.m. (relative to the silicone grease reference peak, the position of which is close to that of the more usual tetramethylsilane). The peak at 6 = 0.90 p.p.m. is assigned to alkane CH, groups. By comparing the CH, peak with that of a standard, made by adding a weighed amount of heating fuel oil to distilled water and extracting similarly with carbon tetrachloride, we estimated the alkane concentration in this particular Weaver Canal sample to be 2.4 & 0.2 mg 1-l. The presence of unsaturated hydrocarbons is shown by the downfield CH, peaks at 8 = 2.06 and 2.35 1p.p.m. and by olefinic proton resonances in the region S = 4-5 p.p.m. To identify other features confidently would need additional information, e.g., from detailed surveys relating water movements to known effluent outfalls, aided if necessary by other analytical techniques.
I
I
L
Fig. 1. Spectrum of 1 to 150 carbon tetrachloride extract from a Weaver Canal sample. Internal reference 4 mg 1-1 silicone stopcock grease; pulse width 10 ps (90' approx.); free induction decay spectrum width 1 kHz; free induction decay filter band width 1 kHz; 8192 data points positive exponential window, EX = 8 ; quadrature detection not used ; 5-mm sample tube. Duration of run 1 h, 720 pulses recorded.
The nuclear magnetic resonance spectroscopic method could thus be used as a convenient fingerprint to trace the fate of individual pollutants in a water system. For example, samples from the River Weaver downstream of its junction with the Weaver Canal, when examined by the nuclear magnetic resonance spectroscopic method, showed the expected large dilution effect for most of the peaks seen in Fig. 1. The peak at S = 3.80 p.p.m., whose position indicates a methyl ester, was an exception, being comparatively stronger in
Published on 01 January 1978. Downloaded by McMaster University on 28/10/2014 21:13:42.
View Article Online
December, 1978 WATER BY PMR SPECTROSCOPY O F SOLVENT EXTRACTS 1237 the River Weaver samples, showing that the compound concerned enters the river a t some point upstream of the junction. The spectrum in Fig. 1 was obtained by a 60-min nuclear magnetic resonance spectroscopic run of 720 pulses, to illustrate the potential of the method for showing detail. In fact, we found that nearly all of the features seen here could be satisfactorily identified and measured in a run of only 10 min (120 pulses). For most of our work we have used only 10-min standard runs. Fig. 2 shows a spectrum of an extract prepared under identical conditions from an industrial water supply reservoir (abstracted from local streams) in which it was of interest to know the hydrocarbon content. Measurement of the CH, peak intensity indicated that 1.3 0.2 mgl-1 of alkanes are present. The spectrum also illustrates the effect of residual dissolved water in the extract, giving a peak a t 6 = 1.05 p.p,m.
Fig. 2. Spectrum of 1 to 150 carbon tetrachloride extract from an industrial water supply. Internal reference 4 mg 1-1 silicone stopcock grease. External reference hexamethyldisilazane in carbon tetrachloride in a 2 mm 0.d. capillary. Other instrumental conditions and Fourier transform parameters as for Fig. 1. (The external reference, which is the more intense peak a t 6 = -0.04 p.p.m., is not normally needed; it was used here as a quantitative standard instead of the inadequately small internal reference peak.) Five-millimetre sample tube with capillary insert. Duration of run 10 min, 120 pulses recorded.
The above two examples are taken from our recent surveys of the River Weaver and its tributaries and connecting waterways. Alkane contents varying between 0.1 and 2.4 mg 1-1 were measured. These results were compared with measurements made by the infrared absorption technique,l also using carbon tetrachloride extracts, and the agreement was generally good. The limits of detection of the nuclear magnetic resonance spectroscopic method with our spectrometer can be determined from measured signal to noise ratios. An environmental sample containing 100 pg 1-1 of alkane (as measured by comparison with a standard of 2 mg 1-1 of fuel oil in water) gave a CH, peak signal to noise ratio of about 5 in a 10-min run (120 pulses) using 100: 1 solvent extraction ratio and a 10 mm diameter sample tube. Thus, a 90-min run with an extraction ratio of 200: 1 would be expected to give the same signal to noise ratio for a concentration of one sixth of that amount, i.e., approximately 17 pg 1-l. An alkane concentration of 15 pg 1-1 would be detectable with a signal to noise ratio of at least 4 under the latter conditions.
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I n marked contrast to this figure, phenol is an example of a pollutant for which the extraction efficiency is very unfavourable. A 40 :1 carbon tetrachloride extract from water containing 100 mg 1-1 of phenol gave easily detectable ring proton peaks in a 33-min run (in this instance with a 5-mm sample tube), but when the phenol concentration was reduced to 10 mg 1-1 no identifiable peaks were seen even in an overnight nuclear magnetic resonance spectroscopic run.
Conclusions Pulsed Fourier transform nuclear magnetic resonance spectroscopy offers a versatile, straightforward and reasonably quick method for measuring organic compounds in water, and is especially suitable for hydrocarbons and chlorinated hydrocarbons. The sensitivity allows measurements down to tens of micrograms per litre in favourable instances, which is useful for analysis of a wide range of polluted waters and effluents, but falls well short of gas chromatography and gas chromatography - mass spectrometry. Because the method gives a composite spectrum of all of the extractable organic pollutants present, it is much less powerful for positive identification of individual compounds than is gas chromatography - mass spectrometry, but the spectra can be used in a fingerprint fashion to identify pollution sources, and could provide a convenient comparison technique when the pollutants of interest had been identified with the help of other information.
I thank E. F. Thurston and other colleagues in Imperial Chemical Industries Limited, Mond Division, for co-operation and useful discussions on sampling in the River Weaver area, and B. W. Cook of this Laboratory for infrared measurements on environmental samples and for discussion of these results. References 1. 2. 3. 4. 5. 6. 7. 8. 9.
Coles, G. P., Dille, R. M., and Shull, D. L., P r e p . Div. Petrol. Chem. A m . Chem. SOG.,1975, 20, 641. Brown, C. W., Lynch, P. F., and Ahmadjian, &I., Envir. Sci. Technol., 1974, 8, 669. Gurley, T . W . ,and Ritchey. W. M., Analyt. Chem., 1975, 47, 1444. Gurley, T. W., and Ritchey, W. M., Analyt. Chem., 1976, 48, 1137. Adams, V. D., Middlebrooks, E. J., and Nance, P. D., Wat. Sewage W k s , 1976, 122, 98. Ahnhoff, M., and Josefsson, B., Analyt. Chem., 1974, 46, 658. Shapiro, J., in Zief, M., Editor, “Purification of Inorganic and Organic Materials: Techniques of Fractional Solidification,” Marcel Dekker, New York, 1969, p. 147. Aue, W. A., Kapila, S., and Hastings, C. R., J . Chromat., 1972, 73, 99. Osterroht, C., J . Chromat., 1974, 101, 289. Received March loth, 1978 Accepted June 12th, 1978
