Anal. Chem. 1988, 60,11-15
11
Determination of Chromium(I I I) and Chromium(V1) by Ammonium Pyrrolidinecarbodithioate-Methyl Isobutyl Ketone Furnace Atomic Absorption Spectrometry Kunnath S. Subramanian
Environmental Health Directorate, Health and Welfare Canada, Tunney's Pasture, Ottawa, Ontario K I A OL2,Canada
The solutlon conditions and other parameters affectlng the ammonlum pyrrolldlnecarbodlthloate-methyl Isobutyl ketone (APCD-MIBK) extractlon system for the graphlte furnace atomlc absorption spectrometrlc determhatlon of Cr( I I I ) and Cr(V1) have been studled In detall. The parameters studied Include pH of the aqueous phase prior to extraction, concentratlon of APDC, concentratlon of the potasslum hydrogen phthalate buffer, the length of time needed for complete extraction, and the ttme-stablllty of the chelate In the organlc phase. On the basis of these studles, procedures have been developed for the selectlve determlnatlon of Cr(VI), and the slmuttaneous determlnatlon of [Cr(III) Cr(VI)] wlthout the need to convert Cr(II1) to Cr(V1). The value of Cr(II1) is obtalned by difference. The methods have been applied to Cr(VI)] the determlnatlon of Cr(III), Cr(VI), and [Cr(III) In some natural and drlnklng water samples. The detectlon limits (3 standard devlatlons of blank) for Cr(II1) and Cr(V1) are the same and found to be 0.3 ng/mL in the MIBK phase. Data are presented on accuracy, precision, and Interference.
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The determination of Cr(II1) and Cr(V1) in environmental and biological systems is of considerable current interest because the toxicity of this metal to aquatic and terrestrial organisms including humans depends on its oxidation state (1). Cr(II1) is considered to be essential to mammals for the maintenance of glucose, lipid, and protein metabolism, but Cr(V1) is reported to be toxic because of its ability to oxidize other species and its adverse impact on lung, liver, and kidney (2).
Cr(II1) and Cr(VI) enter the waterways primarily as a result of effluent discharge from cooling towers, electroplating and tanning industries, oxidative dyeing, and leachings from sanitary landfills ( 3 , 4 ) . Cr(V1) may also enter the drinking water distribution system from the corrosion inhibitors used in water pipes (3). In addition to its existence in two main oxidation states (+3 and +6), chromium occurs in the aquatic environment at nanogram per milliliter levels or lower. At present few analytical techniques with sufficient sensitivity and selectivity are available for the direct determination and speciation of ultratrace levels of chromium in water samples. Some form of preliminary separation and preconcentration is required to determine the low levels of individual Cr species by sensitive analytical techniques such as graphite furnace atomic absorption spectrometry (GFAAS). One frequently used method is chelate solvent extraction with the dithiocarbamakmethyl isobutyl ketone system (5, 6). In general, the chelate extraction of Cr(II1) by the dithiocarbamate-MIBK system under conditions usually employed for the extraction of Cr(V1) has been found to be difficult. The nonextractability of Cr(1II) was ascribed to the difficulty of displacing the water of coordination from the strongly hydrated Cr(II1) ion by the dithiocarbamate ligand (6). At0003-2700/88/0360-00 1 1$01.50/0
tempts were made to reduce the stability of this Cr(II1) aquo complex by adding ethanol to water (7)or by increasing the reaction temperature and time (8,9) prior to dithiocarbamate complexation of Cr(II1). These approaches are not desirable in the ultratrace determination of Cr. For example, increasing the reaction temperature can cause decomposition of the chelating agent while addition of ethanol can cause coextraction of the excess ligand and subsequently lower the extraction yield of the chromium complex. It would be of interest to explore the feasibility of directly complexing both Cr(II1) and Cr(V1) by ammonium pyrrolidinecarbodithioate (APCD) at room temperature, extracting it subsequently into MIBK, and determining the Cr in the MIBK phase by GFAAS. This paper describes the results of such a study in aqueous medium and its applicability to the speciation of Cr in some water samples.
EXPERIMENTAL SECTION Apparatus. A Perkin-Elmer Model 603 atomic absorption spectrometer equipped with a microcomputer-controlled HGA-500 graphite furnace, an AS-1 autosampler incorporating a 10-pL pump, a PRS-10 printer, and a Perkin-Elmer Intensitron hollow cathode lamp operated at 25 mA and a resonance wavelength of 357.9 nm (spectral band-pass, 0.7 nm) was used for the determination of Cr. Argon served as the purge gas and its flow was interrupted during atomization. Nonatomic absorption measurements were made at the nonabsorbing Cr line of 358.8 nm (spectral band-pass, 0.7 nm) by using a Thermo-Jarrell-Ash uranium hollow cathode lamp operated at 25 mA. The HGA-500 graphite furnace was equipped with an optical temperature sensor for maximum power heating on the order of 2ooo OC/s. The sensor could be preset between 900 and 3000 "C. An Orion Model 901 Microprocessor-Ionalyzer was used for pH measurement while a Pye Unicam Model SP1800 double beam W-vis spectrophotometer was used for determining the solubility of MIBK in water and in potassium hydrogen phthalate solutions. All extractions were done in Pyrex glass separating funnels with poly(tetrafluoroethy1ene)stopcocks and polyethylene stoppers. Nalgene screw-cap bottles of 1-L capacity were used as containers for the water samples. Reagents. High-purity water was obtained passing tap water through a cellulose adsorbent and two mixed-bed ion exchange columns and finally by distillation in a Corning AG-11 unit. The quality of the deionized distilled water conformed to ASTM Type 1specification (IO). Also the Cr level in the high-purity water was found to be below the GFAAS detection limit (3 times standard deviation of the blank) of 0.3 ng/mL. A certified atomic absorption standard containing lo00 mg of Cr(VI)/L was obtained from Fisher Scientific. A stock solution containing lo00 mg of Cr(III)/L w a made by dissolving 7.6928 g of Cr(N03)3.9H20 (Baker Analyzed Reagent, J. T. Baker Chemical Co., Phillipsburg, NJ) in 1 L of 1%HC1. The concentration of Cr(1II) in the stock solution was ascertained by redox titration (11). An 8% solution of potassium hydrogen phthalate (Baker Analyzed Reagent) buffer was prepared by dissolving 40 g of the compound and 4 g of NaOH in 500 mL of high-purity water. The buffer was purified by extracting three times with 2.5 mL of 2% APCD and 20 mL of MIBK. The aqueous layer was stored in 0 1987 American Chemical Society
12
ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
a clean and dry 500-mL polypropylene bottle. APCD solutions (2% and 10%) were prepared by dissolving appropriate amounts of the compound (Baker Analyzed) in 1 L of high-purity water. The solutions were purified as described in a previous publication (5). All other reagents and solutions wed were of the highest purity available. Analytical Procedure. Effect of pH on Extraction. Phthalate buffer [0.3 mL for Cr(III), and 3 mL for Cr(VI)] was added to 25 mL of a 10 ng/mL solution of Cr(II1) or Cr(V1). The pH was adjusted to a value between 1.0 and 8.0 with nitric acid (0.02 M and 0.2 M) or sodium hydroxide (0.01 M and 0.1 M), 3 mL of APCD [lo% for Cr(II1) and 2% for Cr(VI)] was added, and the complex was extracted for 20 min with 5 mL of phthalate-saturated MIBK. A t pH values below 2.5, no buffer was added because the addition of APCD to the solution altered the pH only by 0.1-0.2. At pH values above 2.5, the addition of APCD changed the pH by 1-2 units and therefore the addition of a buffer was necessary to maintain the pH. Optimum Extraction Time and APCD and Phthalate Concentrations. The procedures were the same as in the pH-effect study except that the separating funnels were shaken for different time intervals (1-30 min) or different amounb of APCD (0.01-2%; final solution concentration) or phthalate (0.03-2%, final solution concentration) were added. In all the above cases the concentration of Cr was obtained by automatically transferring a 1O-wLaliquot of the MIBK phase into the pyrocoated graphite tube of the HGA-500 furnace atomizer. The Cr in the MIBK extract was atomized by raising the temperature from ambient to 900 "C in 30 s (ramp time) and holding there for 30 s (hold time) followed by rapid heating to 2500 "C under the temperature-controlled maximum power heating mode and holding for 6 s (ramp time, 0 s; hold time, 6 8). The results of triplicate determinations of each test solution were then averaged to obtain plots of percent extraction vs. pH for Cr(II1) and Cr(VI). Tests for completeness of extraction were made by multiple extractions of the same aqueous phase and monitoring the levels of Cr in each organic phase. Blanks were run regularly and their values were subtracted from the gross values to obtain the net values reported. Time Stability of Chelates in MIBK. The studies were done at the various pH values under the following conditions: (i) After extraction as in the pH-effect experiments, the MIBK phase was left in contact with the aqueous phase at room temperature in a lit room. (ii) Similar experiments were carried out with the two phases kept in contact in the dark. (iii) After extraction, the aqueous layer was drained off and the organic phase was shaken with 10 mL of water. After phase separation, one portion of the organic layer was transferred to a clean, dry Pyrex glass centrifuge tube which was kept in a lit room. Another portion of the organic phase was transferred to similar centrifuge tubes but kept in the dark. In all cases, the concentration of Cr(III), Cr(VI), or Cr(II1) + Cr(V1) was monitored at various time intervals by injecting 10 WLof the MIBK phase into the graphite furnace. Any temporal variations in the sensitivity of the graphite furnace were accounted for by injecting a freshly extracted MIBK solution of Cr once every 2 h. Effect of Foreign Zons. The studies were done at 0.25 and 0.5 ng/mL (in the MIBK phase these levels corresponded to 5 and 10 ng/mL, respectively at the 20-fold concentration factor employed) for Cr(III), Cr(VI), and [Cr(III) + Cr(VI)]. The concentration of each of the foreign ions tested was higher than is usually found fresh or potable waters and was progressively lowered until the response corresponded to 100 f 10% of the metal recovery. The ions tested are listed later. Calibration, Precision, and Accuracy. The range of linear calibration was determined by preparing a series of aqueous standards and extracting them under the optimum conditions and with aqueous to organic phase volume ratios of 1,5, 10, and 20. The calibration was done in triplicate for Cr(III), Cr(VI),and [Cr(III) + Cr(VI)]. The precision for Cr(III), Cr(VI), and [Cr(III) + Cr(VI)] was evaluated at concentrations corresponding to 5,10, and 20 times the detection limit. The extraction was done by preparing 10
solutions, each containing the same concentration of chromium. The accuracy was ascertained by analyzing the NBS multielement water standards 1643a and 1643b, and the National Research Council of Canada river water standard SLRS-1. The reliability of the extraction procedure was also assessed by doing recovery studies using some river, tap, and groundwater samples. Known aliquots of each sample were "spiked" with amounts of Cr(III), Cr(VI), or [Cr(III) + &(VI)] corresponding to 1 and 2 ng/mL, and recovery studies were made under the optimum extraction conditions described earlier. The analysis was done immediately after "spiking" in order to minimize the reduction of Cr(V1) to Cr(II1). Analysis of Water Samples. Determination of [Cr(ZZZ)+ Cr( VZ)]. To 25 mL of sample was added 0.3 mL of phthalate buffer, the pH was adjusted to 3.5, and 3 mL of 10% APCD and 5 mL of phthalate-saturated MIBK were added. The solution was extracted for 20 min and the concentration of [Cr(III) + Cr(VI)] in the organic layer was measured as described above. Determination of Cr( VZ). To 25 mL of sample contained in a 125mL separating funnel was added 3.0 mL of phthalate buffer, the pH was adjusted to 3.5, and 3 mL of 2% APDC and 5 mL of phthalate-saturated MIBK were added. The solution was extracted for 10 min and the Cr(V1) concentration in the MIBK phase was ascertained as described above. Determination of Cr(ZZZ). The value of Cr(II1) was obtained by the extraction procedure as the difference between [Cr(III) + Cr(VI)] and Cr(V1). The concentration of Cr(II1) in the water samples was also ascertained by an ion exchange-GFAAS procedure described previously (12). To 100 mL of water sample contained in a Nalgene polyethylene beaker was added 1 mL of 0.1 M ammonium acetate. The pH was adjusted between 4.5 and 4.9. The solution was passed through a Bio-Rad 30 X 1 cm polyethylene column, slurry-packed with a 50-100 mesh SM-7 resin (a polyacrylic ester resin supplied by Bio-Rad) to a bed height of 12 cm, at a flow rate of 1.0-1.2 mL/min. The Cr(II1) retained in the resin was eluted with 20 mL of 1% "0% A 1O-wL aliquot of the eluent was introduced into the graphite furnace and the Cr was atomized under the optimized operating conditions described elsewhere (12). The Cr(II1) content of the water sample was obtained by the method of standard addition.
RESULTS AND DISCUSSION Instrumental Parameters. Cr was determined in the present work by using the optimized dry/atomize program of 900 "C in 30 s (ramp), 30 s (hold), 2500 "C, in 0 s (ramp), 6 s (hold). The optimized parameters and the peak absorbance values were the same for both Cr(II1) and Cr(V1). Since the Cr content of the water samples analyzed in this work was expected to be in the nanogram per milliliter level, the "interrupt mode" (argon flow rate, 0 mL/min) was used throughout. The peak absorbance value at "interrupt mode" was 3.3 times that at an argon flow rate of 60 mL/min. The emission intensity of the deuterium arc is low at the chromium resonance line of 357.9 nm. In agreement with Halls and Fell (13),we found the hollow cathode lamp current had to be turned down to 10 mA from the recommended value of 25 mA and the deuterium arc energy had to be increased to its maximum value in order to balance the source (chromium hollow cathode) and background beams. Under these conditions the results were imprecise. Subsequent studies showed no difference in the peak absorbance value of Cr(II1) and Cr(V1) in MIBK solutions with or without background correction. The absence of nonatomic absorption was further confirmed by measuring background absorbance at the nonabsorbing line of 358.8 nm by using a uranium hollow cathode lamp and also at the chromium resonance line of 357.9 nm by using a Perkin-Elmer Model 5000 atomic absorption spectrometer equipped with a tungsten-halogen lamp capable of correcting up to 1.4 absorbance units (14).In both the cases, no significnat background absorption (20 were not posaible with MIK because of phase separation difficulties even after using phthalate-saturated MIBK. Concentration factors as high as 50 could be achieved with 2-methyl-3-heptanone, which is practically insoluble in water. However, extraction was nonquantitative (maximum yield, 61%) with heptanone and was not pursued further. The precision, expressed as percent coefficient of variation (% CV), at 5 , 10, and 20 times the detection limit of Cr was 33.3, 12.6, and 8.1, respectively. Considering the levels involved, the range of percision (33.3-8.170) is satisfactory. No significant differences were observed in the precision values for the measurement of Cr(III), Cr(VI), or [Cr(III) &(VI)]. A concentration factor of 10 was used in these measurements. The reliability of the proposed procedure was assessed by analyzing the National Bureau of Standards (NBS) multielement water standards, SRM 1643a and SRM 1643b, and the National Research Council of Canada (NRCC) river water standard, SLRS-1. (Note: Both the NBS and NRCC standards required neutralization with 1 M NaOH as a first
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 1, JANUARY 1, 1988
Table I. Determination of Chromium in Reference Water Samples by APCD-MIBK-GFAAS Procedure concentration (mean f SD), ng/mL total Crc Cr(V1)
ref sample
NBS SRM 1643a” NBS SRM 1643b NRCC SLRS-lb
certified value observed value no. of observations certified value observed value no. of observation certified value observed value no. of observations
18.1 f 0.6 10 17.8 f 0.3 10
17.3 f 2.0 [17.3 f 1.4Id [I01 18.9 f 0.4 [19.4 i 1.7Id [101
0.36 f 0.04 0.22 f 0.05 20
Cr(II1)
2.1 f 0.3 10
16 [14.6 f O.5le 10
1.4 f 0.3 15
16.4 [17.9 f O.5le
0.11 f 0.04
0.11
10
20
a SRM 1643a has now been discontinued. SLRS-1is a river water reference material certified for trace metals by the National Research Council of Canada. ‘Only total Cr values are certified. dThe values within brackets were obtained by direct injection of a fivefold diluted sample. OThe values within the bracket were obtained by the SM-7 ion exchange procedure. The values outside the bracket were obtained by subtracting the Cr(V1) value from the total Cr value.
Table 11. Analytical Recovery of Chromium Added to Some Water Samplesu % recovery at 1 ng/mL “spike”‘
sampleb tap water
well water river water treated water
Cr(II1)
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97.1 3.0 104.8 f 5.3 109.9 f 3.2 100.0 f 5.9
% recovery at 2 ng/mL “spike”
Cr(V1)
Cr(II1) + Cr(V1)
Cr(II1)
Cr(V1)
Cr(II1) + Cr(V1)
103.3 f 3.2 94.5 f 6.1 105.6 f 2.6 108.8 f 2.1
101.3 f 5.3 96.9 f 3.5 98.6 f 2.8 98.5 f 3.0
99.2 f 4.1 94.3 f 5.6 98.6 f 1.4 105.7 f 4.9
101.6 f 2.1 102.6 f 2.6 103.8 f 4.2 104.4 f 5.0
102.4 3.6 98.7 f 2.1 101.1 f 3.5 102.2 f 2.9
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a Values given represent the average of triplicate analyses of five tap water, five well water, two river water, and two treated water samples. bTheendogenous levels of Cr(III), Cr(VI),and [Cr(III) + Cr(VI)]in the samples were below the GFAAS detection limit. ‘The “spike”value refers to concentration in the aqueous phase. Since the aqueous to organic phase volume ratio used was 5, the concentrationsof pikes in the MIBK phase would be 5 and 10 ng/mL, respectively. The measure of precision is the standard deviation.
step.) The data obtained for Cr(II1) and Cr(V1) by the proposed APCD-MIBK-GFAAS procedure are given in Table I. Also given in the table are values for [Cr(III) + Cr(VI)] by APCD-MIBK extraction and direct injection and values for Cr(II1) by the SM-7 ion-exchange procedure (12). Both the NBS and NRCC specify certified values only for total Cr. Note from Table I the good agreement for total Cr with the certified values. Our analysis, however, revealed Cr(II1) to be the dominant species in the three strandards. This is confirmed by the good agreement obtained in the results for Cr(II1) by the APCD-MIBK extraction and the SM-7 ion exchange procedures. Cox et al. (17)using a microcolumn of activated alumina in a flow-injection analysis manifold separated Cr(II1) from Cr(V1) in the NBS standard 1643a prior to detection by inductively coupled plasma atomic emission spectrometry. They obtained a value of 14.8 f 1.0 ng/mL for Cr(III), which is in good agreement with our value (Table I), lending further support to the validity of our approach. The reliability of our APCD-MIBK procedure was also tested by doing recovery studies. The average percent recovery obtained for the addition of Cr(III), Cr(VI), and [Cr(III) + Cr(VI)] spikes to some water samples was quantitative as shown in Table 11. Effect of Foreign Ions. Extraction of Cr(III), Cr(VI), and [Cr(III) + Cr(VI)] a t 0.25 and 0.5 ng/mL levels showed no interference from a multielement solution containing (mg/L) the following: Ca, 150; Mg, 30; K, Na, 10 each; T1,5; Zn, 0.5; Sr, Fe(II), Fe(III), B, 0.3 each; Ag, AI, As(III), As(V), Ba, Be, Bi, Cd, Co, Cu, Mn, Ni, Pb, Se, Mo, V, 0.1 each. There was no interference from humic acid: up to 2 mg/L at a Cr(II1) concentration of 10 ng/mL; up to 40 mg/L for 10 ng/mL Cr(V1); up to 3 mg/L for 10 ng/mL Cr(II1) plus 10 ng/mL Cr(VI). At 3,5,10,20, and 40 mg/L of humic acid, the percent recovery of Cr(II1) a t the 10 ng/mL level was 81.5,72.2, 53.7, 35.2, and 20.4, respectively. In the case of 10 ng/mL Cr(II1) plus 10 ng/mL Cr(VI), the percent extraction was 92.1, 82.5, 74.6, and 63.5 a t 5, 10, 20, and 40 mg/L humic acid, respectively. The effect of humic acid at levels >40 mg/L could not
be tested because of emulsion formation during extraction. Also at the 10 ng/mL level, both Cr(II1) and Cr(V1) were unaffected at least up to 5 mg/L of dissolved chlorine added as NaOC1. In general the levels of interferents used in this test exceed those found in most fresh and drinking water samples and as such are not expected to cause any problems with the measurement of Cr(II1) and Cr(VI). The interference of humic acid in the APCD-MIBK extraction of Cr(III), however, cannot be discounted. The humic acid concentration in the solution to be extracted for Cr(II1) should be 1 2 mg/L. Application to Samples. The [Cr(III) + Cr(VI)] and the Cr(V1) contents of 1 tap water, 2 river water, and 20 groundwater samples were ascertained by the proposed APCD-MIBK-GFAAS procedures. The Cr(II1) values were obtained by subtracting the Cr(V1) value from the [Cr(III) + Cr(V1)J value but was also confirmed by its selective determination with the SM-7 ion exchange-GFAAS procedure
(12). Total soluble inorganic-Cr levels in all but six groundwater samples were 10.02 ng/mL at a concentration factor of 20 (40-mL water samples to 2 mL of MIBK). Detectable levels of total Cr were found in six groundwater samples: 0.17,0.25, 0.11, 0.18, 0.08, and 0.16 ng/mL, respectively. The sample with the 0.17 ng/mL level was found to contain Cr exclusibely in the +6 oxidation state, whereas in the remaining five samples, it was found to be in the +3 state.
ACKNOWLEDGMENT The author is grateful to John Connor for technical assistance. Registry No. APCD, 5108-96-3; MIBK, 108-10-1; Cr, 744047-3; water, 1132-18-5.
LITERATURE CITED (1) Stollenwerk, K. G.; Grove, D. 8. J . Env/ron. OM/.1985. 14, 396-399. (2) Langard, S.; Norseth, T. Handbook on the Toxicology of Metals: Friberg, L., et ai., Eds.; Elsevier/North-Holland Biomedical Press: Amsterdam, 1979 pp 363-397. (3) Orvini, E.; Zerlia, T.; Gallorini, M. Spezial, M. Radiochem. Radioana/. Lett. 1980, 4 3 , 173-184.
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Anal. Chem. 1988, 6 0 , 15-19 (4) Brown, D. A.; Qlasess, W. K.; Jan, M. R.; Mulders, R. M. W. Envlron. Technol. Len. 1986. 7 , 283-288. (5) Subramanlan, K. S.; Manger, J. C. I n t . J. Envlron. Anal. Chem. 1979, 7.25-40. (8) Tande, T.; Pattersen, J. E.; Torgrimsen, T. Chromafographia 1980, 73, 607-610. (7) Malissa, H.; Kotzlan, H. Talanta 1982, 9, 997-1002. (8) Schwedt, 0. Fresenius' 2.Anal. Chem. 1979, 2 9 5 , 382-387. (9) Bergmann, H.; Hardt, K. Fresenius' 2. Anal. Chem. 1979, 297, 38 1-383. (10) Annual Book of ASTM Standards; A.S.T.M.: Philadelphia, PA, 1981; Part 31, D1193-77, pp 29-31. (11) Vogel, A. I. A Textbook of Quantitative Inorganic Analysis, 3rd ed.; Longmans, Green: London, 1961; p 311.
(12) Subramanian, K. S.;Mbranger, J. C.; Wan, C. C.; Corsini, A. I n t . J. Envlron. Anal. Chem. 1985, 79, 261-272. (13) Halls, D. J.; Fell, G. S. J. Anal. At. Spectrom. 1988, 7 , 135-139. (14) Kayne, F. J.; Komer, G.; Labcda, H.; Van der Linde, R. E. Clln, Chem. (Winston-Salem, N . C . ) 1978, 2 4 , 2151-2154. (15) Ping, L.; Matsumoto. K.; Fuwa, K. Anal. Chim. Acta 1983, 747, 205-21 2. (16) Everson, R. J.; Parker, H. E. Anal. Chem. 1974, 4 6 , 2040-2042. (17) Cox, A. G.; Cook, I. G.; McLeod, C. W. Analyst (London) 1985, 170, 33 1-333.
RECEIVED for review March 19, 1987. Accepted September 10, 1987.
Use of 'H Nuclear Magnetic Resonance Longitudinal Relaxation Times in Structure Elucidation of Chlorinated Polyaromatic Compounds David L. Ashley,* Elizabeth R. Barnhart, Donald G. Patterson, Jr., and Robert H. Hill, Jr. Division of Environmental Health Laboratory Sciences, Center for Environmental Health, Centers for Disease Control, Public Health Service, US.Department of Health and Human Services, Atlanta, Georgia 30333
We describe a procedure for measuring relative longitudinal relaxatlon tlmes of protons In chlorinated polyaromatlc hydrocarbons. The results lndlcate that these times are dependent on Interproton dlstances and thus can be used to distlngulsh protons wlth ortho-proton neighbors from those wlthout ortho-proton nelghbors. I n addltlon, the longltudlnal relaxation tlme can also qualltatlvely descrlbe inter-rlng Interactkns under certain condnkns. The measurement is used for structural elucldatlon of three cases of unknown chlorlnated polyaromatlc hydrocarbons, and In each case an unambiguous amlgmnent Is possible. The use of relative proton longnudlnal relaxatkm tknes for stNctwal IdentMcatlon should be broadly applicable to many structural problems.
Since the discovery of the chemical shift in the early 195Os, numerous nuclear magnetic resonance (NMR) techniques have been used to measure characteristic molecular properties. The use of the appropriate experimental design, coupling constants, relaxation times, and chemical shifts has yielded information about electronic environments, nuclear distances, and bond angles. Measurement of carbon-13 longitudinal relaxation times of organic compounds has been widely applied (I, 2), but the proton counterpart has not, even though i t also has potential for molecular characterization (3). Polyaromatic halogenated hydrocarbons, including dibenzo-p-dioxins, dibenzofurans, biphenylenes, and pyrenes, are a significant public health concern. These compounds have been shown to be highly toxic to certain species, and health effects in humans have also been documented (4,5),although these findings are still controversial. In spite of the controversy, i t is widely accepted that the toxicities of these compounds are extremely isomer-specific. Thus, assessing risk from exposure is contingent on identifying substitution patterns and quantitating each isomer. The technique most frequently used to determine levels of these contaminants is mass spectrometry following gas chromatography (6), but that technique does not provide direct isomer differentiation in
all cases. Independent methods must be available to characterize standards used with this technique. NMR is particularly suited for identifying substitution patterns because of the dependence of NMR spectra on molecular level properties. Thus, NMR can substantially aid in characterizing these isomers. We investigated the possibility of using proton NMR longitudinal relaxation times to differentiate certain substitution patterns in chlorinated polyaromatic compounds. This technique was then applied to three examples of unidentified polychlorinated compounds that could not be determined through recognition of peak splitting patterns.
EXPERIMENTAL SECTION Chlorinated dibenzo-p-dioxinswere prepared by reacting the dipotassium salts of chlorinated catechols with chlorinated benzenes or chlorinated nitrobenzenes in anhydrous dimethyl sulfoxide at 175 "C as previously reported (7).These mixtures were purified by chromatography on silica gel with hexane as the eluting solvent. Chlorinated dibenzofurans were synthesized via palladium acetate cyclization of the appropriate chlorinated diphenyl ethers (8). Chlorinated pyrene and biphenylene derivatives were synthesizedby chlorinationof pyrene or biphenylene with S02C12reagent (reagent C, Perchlorination Kit, Analabs, Inc., New Haven, CT) (8). Polychlorinated biphenyl standards were acquired from Ultra Scientific (Hope, RI). Separation of mixtures of synthesis products sometimes required high-performanceliquid chromatography (HPLC). HPLC was carried out by using a Waters Associates M-6000 pump and Model 440 absorbance detector (254 nm) and a 25 X 2 cm i.d. Dynamax ODS column (Rainin Instrument Co., Woburn, MA). The derivatives were extracted into 50 MLof toluene, and this mixture was separated on the column with a flow rate of 9.5 ml/min. The eluting solvent varied from a 95% methanol/water mixture to 100% methanol. Purity and structural verification of standards were determined by using gas chromatography (GC) with flame ionization detection, GC coupled with mass spectrometry, and GC coupled with Fourier-transform infrared spectrometry. Proton NMR spectra were acquired on a Varian Associates XL-300 spectrometer that was equipped with a 7.0-T superconducting magnet and that employed the XL data system. Ap-
This article not subject to US. Copyrlght. Published 1987 by the American Chemical Society
