1318
Anal. Chem. 1982, 5 4 , 1318-1321
further fragmentations corresponding to the loss of HC1 and Clz are observed. In analyzing the mineral oil sample and the fish extract, it was not possible to detect ions from CP huls 70C, neither in the E1 nor in the PCI mass spectra. The resulb reported here imply that negative ion chemical ionization mass spectrometry is a suitable analytical technique for highly chlorinated paraffins. Chloroparaffin huls 70C and Witaclor 159, two commercial CP products containing 70% and 59% chlorine, both give simple and characteristic NCI mass spectra. Abundant fragment ions in the high mass region, interpreted either as (M - H)-or as (M - C1)- ions, dominate the spectra. Isomeric compounds are thus detected as one single compound. The high molecular weights and the specific chlorine isotopic patterns of the ion clusters minimize the risk for interference from other compounds. No contribution from other compounds present was found in analysis of biological samples. So far we have only used standard samples and samples from controlled experiments. It remains to see how useful this promising negative ion technique will be in the determination of chlorinated paraffins in environmental samples.
ACKNOWLEDGMENT We thank Elizabeth Baumann Ofstad, CIIR, for valuable discussions and the Brackish Water Toxicology Laboratory in Sweden for the fish extracts.
LITERATURE CITED Arnestad, K. G. CIIR report No. 7801 01-1, Oslo, 1979 (in Norwegian). Svanberg, 0.; LlndBn, E. Ambio 1979, 8, 206. Howard, P. H.; Santodonato, J.; Saxena, J. Document EPA-560/2-75007, Washington, DC, 1975. Lombardo, P.; Dennlson, J. L.; Johnson, W. W. J . Assoc. O f f .Anal. Chem. 1975, 58, 707.
Svanberg, 0.; Bengtson, B.-E.; LlndBn, E.; Lunde, G.; Baumann Ofstad, E. Ambio 1978, 7, 64. Benatson, B.-E.; Svanbera, 0.: LlndBn, E.; Lunde. G.: Baumann Ofstad. E. Ahbio 1979, 8 , 121. Campbell, I.; McConnell, G. Environ. Sci. Technol. 1980, 74, 1209. Madeley, J. R.; Birtley, R. D. N. Environ. Sci. Technol. 1980, 14, 1215 .Hollles, J. I.; Pinnington, D. F.; Handley, A. J.; Baldwin, M. K.; Bennett, D. Anal. Chim. Acta 1979, 7 1 1 , 201. Hunt, D. F.; Stafford, G. C., Jr.; Crow, F. W.; Russel, J. W. Anal. Chem. 1976, 4 8 , 2098. Horning, E. C.; Carroll, D. I.; Dzldic, I.; Lin, S.-N.; Stillwell, R. N.; Thenot, J.-P. J . Chrornatogr. 1977, 142, 481. Dougherty, R. C.; Roberts, J. D.; Blros, F. J. Anal. Chem. 1975, 4 7 , 54. Dougherty, R. C.; Piotrowska, K. J . Assoc. OM.Anal. Chern. 1976, 59, 1023. Hunt, D. F.; Harvey, T. M.; Russel, J. W. J . Chern. SOC.,Chem. Cornmun. 1975, 5 , 151. Hass, J. R.; Friesen, M. D.; Harvan, D. J.; Parker, C. E. Anal. Chem. 1978, 5 0 , 1474. Hass, J. R.; Friesen, M. D.; Hoffman, M. K. Org. Mass. Spectrom. 1979, 74, 9. Kuehl, D. W.; Dougherty, R. C.; Tondeur, Y.; Stalling, D. L.; Smith, L. M.; Rappe, C. "Environmental Health Chemistry"; McKinney, James D., Ed.; Ann Arbor Sclence: Ann Arbor, MI, 1980. Mitchum, R. K.; Molar, G. F.; Korfmacher, W. A. Anal. Chem. 1980, 52, 2278. Busch, K. L.; Norstrom, A.; Bursey, M. M.; Hass, J. R.; Nllsson, C.-A. Biomed. Mass. Spectrom. 1979, 6 , 157. Busch, K. L.; Norstr~m,A.; Nilsson, C.-A,; Bursey, M. M.; Hass, J. R. EHP, Envlron. Health Perspect. 1980, 36, 125. Kuehl, D. W.; Whitaker, M. J.; Dougherty, R. C. Anal. Chern. 1980, 52, 935. Dougherty, R. C.; Whltaker, M. J.; Smlth, L.; Stalllng, D. L.; Kuehl, D. W. EHP, Environ. Health Perspect. 1980, 36, 103. Crow, F. W.; BJorseth, A.; Knapp, K. T.; Bennett, R. Anal. Chern. 1981, 53,619.
RECEIVED for review February 2,1982. Accepted April 6,1982. This work was supported by the Norwegian Council for Scientific and Industrial Research under Contract No 0106.8147.
Minimization of Spin-Lattice Relaxation Time with Highly Viscous Solvents for Acquisition of Natural Abundance Nitrogen4 5 and Silicon-29 Nuclear Magnetic Resonance Spectra B. P. Bammel and R. F. Evllia" Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 148
The use of high viscoslty solutlon conditions to decrease T , of "N and %i nuclei so that natural abundance NMR spectra can be acquired In reasonable times is illustrated. Slgnlficant T , decreases wlth negligible increases In peak width are observed. No spectral shms are observed in any of the cases studied. Highly VISCOUS solutions are produced by using glycerol as a solvent for water-soluble molecules and a mixed solvent consisting of toluene saturated wlth polystyrene for organic-soluble molecules. The microviscosity In the latter solvent Is found to be much less than the observed macrovlscoslty. Hydrogen bonding of glycerol to the NH, of 2aminopyridlne results In a greater than predicted decrease In T , for this nitrogen. The technlque appears to be a useful alternative to paramagnetic relaxatlon reagents.
The long spin-lattice relaxation time encountered for some rare nuclei is a serious hinderance to the acquisition of natural
abundance nuclear magnetic resonance spectra of these elements. Perhaps the best known example of this problem is the important nucleus, 15N. A variety of approaches have been utilized in attempts to minimize the time necessary to acquire usable spectra in those cases where the inherently low sensitivity is aggravated by a long T I relaxation time. These approaches include the use of high field, wide bore magnets to maximize the available magnetization (1,2),optimization of flip angle and acquisition time for optimum signal to noise ratio ( 3 , 4 ) ,addition of paramagnetic relaxation reagents to shorten T I and to eliminate negative overhauser enhancements (5-7), and polarization transfer pulse sequences (8-11). One approach which appears to have been ignored to date involves the variation of solvent viscosity to obtain efficient T1relaxation without significant line broadening. Although the effect of viscosity and molecular correlation time on the efficiency of spin-lattice and spin-spin relaxation has been well studied and is understood (12, 13), the potential for utilizing viscosity effects to shorten Tl for optimization of the
0003-2700/82/0354-1318$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO.
acquisition time seems to have been overlooked (14). Viscosity variation has been used, however, to vary the nuclear Overhauser enhancement (16). Perhaps this potential has not been exploited because of a fear of line broadening such as is seen in proton spectra in viricous solvents. Because of the low magnetogyric ratio of l!jN and other rare nuclei, however, no significant Tz line broadening should be observed until extremely high viscosities are encountered even though Tl can be greatly shortened in these same solutions. This apparent contradiction occurs because even when T1 is as short as 1 s, the line width is determined by other factors such as the apodization function or the digital resolution. A T1 of 1 s, however, means that spectra can be averaged 100 times as fast as when Tl is 100 s, although the line width is not noticeably increased. Thus, a large decrease in acquisition time with minimal loss of spectral resolution is possible. Viscosity effects have, in principle, some advantages over paramagnetic reagents in that no specific interaction of reagents is necessary. Thus, all nuclei should be affected equally with no preference for those nuclei closest to the site of interaction. Also the reduction of T1 is not limited by the solubility of a paramagnetic compound and shifts of resonance positions should not occur in most cases. Obviously, since solvent-induced shifts do occur sometimes, one cannot expect zero shifts in all cases. The results of an investigation into the utility of viscosity variation as a method for decreasing T1for 15N and 29Si spectroscopy are reported in this paper.
-
EXPERIMENTAL SECTION All chemicals,except as specifically noted below, were obtained from commercial sourceci and used without further purification. Styrofoam coffee cups (AlWF240) from Alan I. W. Frank Corp. were used as the source of polystyrene for those experiments which utilized toluene/polystyrene mixed solvents. Water-soluble polyacrylamide polymers were obtained from C. L. McCormick of the University of Southorn Mississippi. All NMR spectra were recorded on a multinuclear JEOL FX 9OQ spectrometer in 10-mlm NMR tubes. A 1.7-mm D,O capillary was used as a lock signal in all cases. Optimal pulse widths and pulse delays were determined for the spectra presented here after determining Tl relaxation times for the analyte in the solvent being tested. Viscosity measurements were performed with a Cannon-Fenske calibrated Oswald viscoinieter thermostated at the probe temperature of the spectroimeter, -28 "C. Concentrations were determined by initial weights or volumes of analyte and solvents. The mixtures of analyte and solvent were stirred until homogeneous and then transferred to a sample tube. A vortex plug was used to hold the capillary in place.
RESULT13 AND DISCUSSION Under the conditionm where the extreme narrowing approximation is valid the rate of relaxation is proportional to , all common mechanisms except the correlation time, T ~for spin-rotation where the relaxation rate is inversely proportional to correlation time. The correlation time ki, in turn, directly proportional to the microviscosity via the 13tokes-Einstein equation. ha3
To
=-
3kT'
If the molecule is assumed to be spherical, a is the radius of the sphere, is the viscosity of the solvent, and the other symbols have their usual significance. This approximation assumes that there are no specific interactions (such as hydrogen bonding) between solute and solvent and that the microviscosity is equal to the macroviscosity. Although these approximations are not quantitatively met in this study, the desired qualitative reduction of T , is obtained. By use of eq 1 the maximum viscosity a t which the extreme narrowing
Table I. Viscosity at Which wore nucleus 'H I3C
29Si 'SN a
% v q - W + w - n * w *
'i
1319
- 1 for Various Nuclei
Ua
viscosity b
89.56 22.50 17.76
0.7 2.7 3.3
6.6
9.04
MHz assuming 2.11 T field.
c 04
8, JULY 1982
Poise.
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 8, JULY 1982
Table 11. Comparison of Predicted and Observed Relaxation Times predicted solvent
compound 2-aminopyridine -NH, ring N -NH, ring N 4-aminopyridine
viscositya 2.5 x 10-3 2.5 x 10-3 7.6g 7.6g
HZ0 HZ0 glycerol glycerol glycerol glycerol neat toluene/polystyrene toluene/polystyrene CCl, toluene/polystyrene STP
2"
ring N nitro benzene nitrobenzene nitro benzene te traethoxysilane cetraethoxysilane te traethoxysilane
TI
13.4e 30 e 0.12 2.0
7.6 7.6 0.02 46 78 0.01 45 96
0.12g 1.69 390' 170gJ 6ogJ 135m 7g 14
TI b , c
peak widthd
0.2 1.5
f f 3g 3g
-
-- 3g3g -- 0.1g 0.1g -- 0.1g -- 2g22 gg
0.2h 1.5h 26k,i 43k,i 14 30'
Calculated from literature value of TI assuming that extreme narrowing conditions hold except as a Poise. b Seconds. noted and that microviscosity = macroviscosity. Hertz. e Reference 4. Not reported. This work. Assuming TI 's for 4-aminopyridine are equal to T, 's for 2-aminopyridine. Estimated for 2.1 T field fromdata at 1.4 T (ref 18) and at Extreme narrowing conditions were not assumed. "Si spectra. 6.33 T (ref 19). Measured by progressive saturation. Reference 20.
'
J
A
Table 111. Temperature Dependence of a 2-Aminopyridine Spectra in Neat Glycerol -NH, tempa
T,
25 17 8
0.12 2.8 220
"C.
Seconds.
a
pyridine N Tzb9O
0.10 0.07
NOE
TI
T 2 b , CNOE
-4.1 -2
1.6 0.52 4.8
0.17
-2.9 -1.5
0.17
-1.0
0.3
Estimated from peak width.
microviscosity at 25 "C is very close to the maximum for the NH2group as a slight decrease in temperature results in a large increase in TI and large decrease in T2 The optimum viscosity for the pyridine nitrogen is, apparently, somewhat greater than that of neat glycerol as further improvement is noted at lower temperature. Thus, the experimental viscosity must be a compromise. In the studies reported here, the viscosity was made as high as possible and the spectra were run a t room temperature. Significant improvement was obtained in this fashion without the bother of carefully determining the true optimum for each case. Examination of Tables I1 and I11 shows that the T I values for the ring nitrogen of 2- and 4aminopyridines are equal to or longer than the calculated TI values but the NH2 nitrogens are shortened considerably more than expected. Such behavior is expected if a specific interaction between solute and solvent occurs which affects the movement of one nitrogen more than the other. Clearly hydrogen bonding is likely in this case and is probably responsible for the observed discrepancy. Figure 2 shows spectra of nitrobenzene obtained in about 2.4 h each of mixtures of 30:30:40 nitr0benzene:toluene: polystyrene and 305'0 nitrobenzene:toluene under instrumental conditions optimized for the estimated relaxation time in each case. Once again no line broadening is observed in the viscous solutions as the peak width is less than 0.1 Hz even though the viscosity of this solution is so great (-78 P) that it has the appearance of a gel. The gain in speed of acquisition is obvious. Although these solutions were prepared by weight using equal quantities of nitrobenzene the total volumes were almost identical after dissolving the polystyrene. Therefore, the observed increase in s;gnal to noise is not caused by a large difference in molar concentration. The major mechanisms of nitrogen T1relaxation in nitrobenzene are spin-rotation and chemical shift anisotropy (17, 18). Since spin-rotation is less efficient at high viscosities and the efficiency of chemical shift anisotropy is inversely pro-
Flgure 2. I5N spectra of nitrobenzene: (a) 30:70 nitrobenzene:toluene solution; pulse parameters 15' rf pulse, 25-s repetition rate; total of 2.4 h of data acquislion; (b) 30:30:40 nitrobenzene:toluene:polystyrene solutlon, 78 P pulse parameters 51' rf pulse, 25-s repetition rate; total of 2.4 h of data acquisition. The axes are in ppm from nitrobenzene.
portional to the square of the magnetic field strength, this molecule represents an extremely difficult test of the procedure discussed in this paper a t low field strengths. In spite of this difficulty, a significant (factor of >6) decrease in T I was obtained which allowed observation of this molecule in a reasonable time. This decrease in T1 demonstrates that it is possible to increase the efficiency of weak dipole-dipole and chemical shift anisotropy enough to get a useful decrease in acquisition time even in extremely unfavorable cases. The spectra shown in Figure 2 were obtained by dissolving polystyrene in a toluene/nitrobenzene mixture until the solution was saturated. No a priori calculation of the desired viscosity was performed nor does it appear necessary to do such a calculation. Thus it is clear that variation of solution viscosity can be a significant aid in the quest for rapid NMR spectra of difficult nuclei. We are currently investigating solutions with large viscosity increases at reduced temperatures in order to minimize sample handling problems associated with extremely viscous samples. Also it is hoped that the microviscosity will
Anal. Chem. 1982, 5 4 , 1321-1324
vary with temperature in the same way that the macroviscosity does so that less viscouii solutions can be prepared at room temperature and cooled in the NMR probe for spectral accumulation. Also the combination of solvent viscosity optimization and paramagnetic reagent addition as an approach to obtaining efficient relaxation without involving extreme solution conditions of either viscosity or relaxation reagent concentration is being studied. One obvious limitation of this method is that it cannot be used in cases where a specific solvent is required for some reason. This limitation may, however, be overcome to some extent by operation at temperatures near the freezing point of the solvent. Studies of this sort are in progress.
(8) Martin, G. J.; Martin, M. L.; Gouesnard, J. In “NMR Basic Principles (9) (10) (11) (12) (13) (14) (15) (16)
ACKN 0WLEDGMENT We thank C. McCormick of the University of Southern Mississippi for donation of the water-soluble polymers.
(17)
LITERATURE CITED
(19) (20)
Randall, E. W. I n “Nitrogen N.M.R.”; Witanowskl, W., Webb, G. A., Eds.; Plenum Press: London, 1973; pp 47-49. Gust, D.; Moon, R. B.; Roberts, J. D. froc. Nafl. Acad. Scl. U . S . A . 1975, 4698-4700. Becker, E. D.; Ferretti, $J. A,; Gambhir, P. R. Anal. Chem. 1979, 57, 14 13- 1420. Levy, G. C.; Lichter, R. I.. “Nltrogen -15 Nuclear Magnetic Resonance Spectroscopy”; Wiley: New York, 1979; Vol. 19. Lew. G. C.: Edlund, V.: Hexem, J. G. J . Maan. Reson. 1975, 19, 259-262. LaMar, G. N. Chem. f h y s . Lett. 1971, IO, 230-232. Freeman, R.; Pachler, H:. G. R.; LaMar, G. N. J . Chem. fhys. 1971, 55, 4566-4593.
1321
(18)
and Progress“; Riehl, P.; Fluck, E., Kosfeld, R., Eds.; Springer-Verlag: Berlin, 1981; Vol. 18, pp 28-35. Bertrand, R. D.; Moniz, W. B.; Garroway, A. N.; Chingas, G. C. J. Magn.Reson. 1978, 32, 465-467. Walborsky, H. M.; Murarl, M. P. J . Am. Chem. SOC. 1980, 102, 428-429. Balch, A. L.; Yow, J. R. J . Am. Chem. SOC.1980, 702, 1449-1450. Levy, G. C.; Lichter, R. L. ”Nitrogen-15 Nuclear Magnetic Resonance Spectroscopy”; Wiley: New York, 1979; Chapter 5. Bloembergen, R.; Purcell, E. M.; Pound, R. V. fhys. Rev. 1948, 7 3 , 679-712. Witanowski, M.; Stefanick, L.; Webb, G. A. In “Annual 9eport on NMR Spectroscopy”; Webb, G. A., Ed.; Academic Press, London, 1977; Vol. 7, 144-148. Williamson, M. P.; Wllllams, D. H. J . Chem. SOC.,Chem. Commun. 1981. 165-166. Panekon; G.-D.; Lindsey, C. P.; Alms, G. R. Macromolecules 1978, 1 7 . 1242-1244. Lyerla, J. R., Jr.; Levy, G. C. I n “Topics in Carbon-13 NMR Spectroscopy”; Levy, G. C., Ed.; Wiley: New York, 1974; pp 120-1 21. Lippmaa, E.; Saluvere, T.; Lalsaar, S. Chem. fhys. Lett. 1971, 1 7 , 120-123. Schweitzer, D.; Spless, H. W. J . Magn. Reson. 1974, 16, 243-251. Cargioll, J. D. In “Annual Report on NMR Spectroscopy”; Webb, G. A., Eds., Academic Press: New York, 1979; p 228.
RECEIVED for review November 30, 1981. Accepted March 26, 1982. Financial support from the National Science Foundation for the purchase of the FT NMR through Grant CHE 78-02081 is acknowledged. Support from a Merck Go. foundation faculty development grant (R.F.E.) is also acknowledged. Presented in part at joint SE/SW regional meeting, ACS, New Orleans, LA., Dec 1980.
Determination of Boron by Methyl Ester Formation and Flame Emission Spectrometry Darryl D. Siemer Exxon Nuclear Idaho Co., Idaho Fails, Idaho 83401
Soluble boron in aqueous nuclear fuel reprocessing plant streams resulting from the Rover process is determined by compiexing any fluoride present with aluminum chloride solution and then adding, first, sulfuric acid and, finally, methanol to form the volatile trimetthoxyboron ester. The gaseous ester is aspirated into a conve~ntlonalAAS air-acetylene slot burner through the sample pick-up tube normally used for soiutlon analysis. The resulting transient green boron oxide band emission signal at 548 nm is integrated. The detection limit of the method Is 0.2 pg and the precision Is on the order of 1-2% relative standard deviation. No matrix effects from any of the usual concomitants were observed.
The flame photometric determination of boron has an extensive history of both development and practical application work. Gilbert’s treathe on the flame spectroscopic determination of nonmetals gives a fascinating and comprehensive description of this work up to 1970 (1). In practice today, the analysis is usually done with the same solution nebulizerpremixed flame sourcei3 primarily designed for atomic absorption analyses. A t this installation, flame photometric boron determinations ate routinely performed on the tetran-butylammonium fluoborate ion pair extracted with methyl isobutyl ketone (MIBK) from sample solutions to which an excess of hydrofluoric acid has been added (2). The primary 0003-2700/82/0354-1321$01.25/0
drawback to this approach is that a number of transition elements also extract to some degree and give strong positive spectral interference from metal oxide band and incandescent particulate continuum emission. Another practical disadvantage is that the presence of nitrate in the sample requires an additional lengthy sample preparation step. The distillation of the methyl or ethyl borate ester from sample solutions to which alcohol and an acid have been added has long been used for the separation of boron from most sample concomitants (3). This paper describes a rapid analytical method combining the excellent separation inherent in the volatile ester formation process with the sensitivity and procedural simplicity inherent in determinations done with typical modern AAS/AES flame spectrometers. The conditions of the procedure were designed to rapidly form the volatile ester, quantitatively strip it from the sample solution, and then introduce it while still in the gaseous form to a flame spectrometer.
EXPERIMENTAL SECTION Written descriptions of the boron separation procedure involving the methyl borate distillation process typically call for large volumes of reagents and distillation times of as much as an hour. It was obvious that the classical “still”designs recommended for that purpose are totally impractical for rapidly transferring the boron in a small sample aliquot to a flame for a rapid analysis. Therefore, a reaction vessel with small heat capacity and minimum dead space and which is used with reagent volumes chosen t o 0 1982 Amerlcan Chemlcal Society
