-tremote - 0.6 Aremote tlab
Alab
giving tremote= 24 s for the case discussed above. Although .xb can never be zero, any practical remote sensor will require a very low value for x b , so that whatever counting time is necessary to achieve the desired sensitivities may be used. At present, these conditions can best be achieved with pulsed laser and gated TV-detector equipment.
LITERATURE CITED (1)M. C. Goldberg and E. R. Weiner, Proc. 4th Annu. Earth Resour. Program Rev. 81. 1-14 (19721. (2)R:deeves, Ed.,' "Manual of Remote Sensing", Vol. (I, American Society of Photogrammetry, Falls Church, Va., 1975,pp 1496-1500. (3) A. Davis, M. Bristow, and J. Koningstein, "Remote Sensing and Water Resources Management", Am. Water Res. Assoc., Proc., 17, 239-246
,t i.a-7. w
I,.
(4)D. E. Irish, H. Chen, Appl. Spectrosc., 25, 1-6 (1971). (5) G. Braunlich and G. Gamer, Water Res., 7, 1643-1647 (1973). (6) E. B. Bradley and C. A. Frenzel, Water Res., 4, 125-128 (1970). (7) S.F. Baldwin and C. W. Brown, WaterRes., 6, 1601-1604 (1972)
(8)M. Ahmadjian and C. W. Brown, Environ. Sci. Techno/., 7, 452-453 (1973). (9)R. Reeves, Ed., "Manual of Remote Sensing", Vol. 1, pp 221-223 and Vol. 11, Chap. 21,American Society of Photogrammetry, Falls Church, Va., 1975. (IO)B. G. Oliver and A. R. Davis, Can. J. Chem., 51, 698 (1973). (11)M. C. Tobin, J. Opt. SOC.Am., 58, 1057 (1968). (12)G. E. Walrafen, J. Chem. Phys., 36, 1035 (1962). (13)S.L. Meyer, "Data Analysis for Scientists and Engineers", John Wiley and Sons, New York, 1975,Chap. 24. (14)R. R. Alfano and N. Ockman. J. Opt. SOC.Am., 56, 90 (1968). (15)S.L. Meyer, "Data Analysis for Scientists and Engineers", John Wiley and Sons, New York, 1975,p40. (16)R. L. Schwiesow and M. J. Post, Rev. Sci. Instrum., 46, 413 (1975). (17)W. Kiefer, Appl. Specfrosc., 27, 253 (1973). (18)R. L. Schwiesow, J. Opt. SOC.Am., 59, 1285 (1969). (19) P. P. Young, J. Opt. SOC.Am., 62, 1297 (1972). (20) W. H. Woodruff and G. H. Atkinson, Anal. Chem., 48, 186 (1976). (21)T. A. Nieman and C. G. Enke, Anal. Chem., 48, 619 (1976). (22)Y. Talmi, Anal. Chem., 47, 658A (1975). (23)Y. Talmi, Anal. Chem., 47, 697A (1975).
RECEIVEDfor review April 7, 1976. Accepted October 5. 1976. The use of brand names in this report is for identification purposes only and does not imply endorsement by the U.S. Geological Survey. Publication authorized by the Director, U S . Geological Survey.
Pulsed Nuclear Magnetic Resonance Spectrometry for Nondestructive Determination of Hydrogen in Coal 6. C. Gerstein" and R. G. Pembleton Ames Laboratory-ERDA
and Department of Chemistry, Iowa State University, Ames, Iowa 500 1 1
Initial values of free induction decays are shown to yield a rapid, nondestructive quantitative measure of protons in coals provided: 1) an appropriate In A( f ) vs. time plot is made; 2) the zero of time is chosen to be the center of the pulse; 3) the pulse wldth for a 90' pulse Is not longer than T2*/4; 4) recovery time of the receiver-dc amplifier system is of the order of the pulse width; 5) both coal sample and calibration sample are exposed to a uniform t i field; and 6) the caiibratlon sample is measured under the same experimental conditions as is the coal sample.
The use of nuclear magnetic resonance spectrometry (NMR) for quantitative analysis of elements in solids is not common for a number of reasons. For elements with sufficiently high gyromagnetic ratios and abundances to give good signal to noise ratios, e.g., lH, 19F,and 31P,dipolar broadening, which can be of the order of tens of kHz, makes most lines unobservable by continuous wave techniques. Commercial pulse instruments in general do not have sufficiently rapid receiver recovery times to provide facile data recovery in the tens of microsecond range. In a great many solids, spin lattice relaxation times, TI,are sufficiently long to make rapid accumulation of data unfeasible. The work of Vaughan and Schreiber ( I ) is one example of the power of pulse techniques for quantitative determination of nuclei in solids. With relatively inexpensive and straightforward modification, commercial pulse NMR spectrometers may be arranged to have sufficiently rapid receiver recovery times such that decays of tens of microseconds may be recovered for measurements in the 60-MHz or higher range. Coals generally have a sufficient concentration of free radicals such that electron-nuclear spin coupling leads to T I ' Sof the order of
tenths of seconds, thus allowing for rapid accumulation of data. The present work describes the application of a rapid recovery receiver-dc amplifier system to the quantitative determination of protons in coals, and the conditions to be met for such determinations to be quantitatively valid.
EXPERIMENTAL The pulsed NMR spectrometer, designed and constructed in this laboratory, is similar to units discussed by Waugh ( 2 ) ,and by Vaughan ( 3 ) .It is a single sideband unit capable of broadband operation, operating a t a nominal frequency of 56 MHz for protons. The receiver system is limited only by probe ring-down time; it is capable of recovery in less than 0.1 ps after a power pulse, and is described in a separate communication (4). Briefly, the receiver consists of three stages, the first two being Spectrum Microwave Model SMLD-10 limiting amplifiers, and the third being an Avantek Model UTO-502 amplifier. The unit was tuned to the operating frequency by inserting series tuned LC circuits between the first and second, and the second and third stages. The transient recorder is a Biomation 805, with minimum sampling time of 0.2 ps per channel. The memory of the unit contains 2048 eight-bit words. The Biomation is interfaced to a PDP 11/10 computer for signal averaging and data manipulation. The transmitter is an IF1 model 404 distributed amplifier system, capable of 500 watts in pulsed operation. We note that this is less power by a factor of two than that available in common commercial pulse NMR spectrometers. With proper matching of transmitter to 2 can be attained in 1.3 probe, and a probe Q of less than 30, ~ / pulses ps. The homogeneity of the HI field is indicated by the decay ( 5 ) in Figure 1,taken on a spherical sample of water and sphere diameter of 6 mm. The figure exhibits the response to 256 7/2 pulses over a time scan of 20 ms. Variations in the pulse width seen a t the beginning of the scan are due to 60-Hz modulation of the transmitter output. The dc supply of the transmitter is essentially unregulated. Seven coal samples were used in the present determinations. Two were Virginia vitrains (Pocahontas No. 4 and Powellton) obtained from H. L. Retcofsky a t the Pittsburgh Energy Research Center of the U.S. Energy Research and Development Association. Five were obtained from three mines in Iowa. All samples were crushed to 200 mesh and ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
75
T a b l e I. Comparison of Q u a n t i t a t i v e Analysis of H y d r o g e n i n Coals by Pulse NMR a n d Combustion Analysis” %H rf pulse length, ws
Coal sample Pocahontas No. 4 Vitrain Powellton Vitrain Star Vitrain Star Vitrain Star Vitrain Lovilia Vitrain Upper Mich. Vitrain Lovilia Channel Mich. Run of Mine
Figure 1. Homogeneity of HIfield; 256; 7~/2 pulses. Total time scan, 20 ms
I
I
I
6.4
6.2
6.0
5.6
I\
1
Comb. analysis
1.25
4.72(03)
4.12(07)
1.25 1.25 3.0 5.0 1.25 1.25 1.25 1.25
4.87 (03) 5.63(0.5) 6.31 (50) 5.60(40) 5.88(03) 5.05(03) 5.29(07) 6.20(02)
4.80 (05) 5.53(04) 5.53(04) 5.53(04) 5.54(03) 5.32 (04) 5.40(11) 5.63(11)
Uncertainties are mean square deviations from least square
fits. to choose for the case where nonnegligible dephasing is occurring during the pulse is the center of the pulse. The second problem is an appropriate function of A ( t )to use in order to extrapolate A ( t )to zero time. Abragam ( 8 ) has suggested that for the case of solids, the FID should decay exponentially as t2.This choice has been used successfully by Lowe in studies on single crystal CaF2 (7). For quantitative determination of hydrogen in solid coals, the zero of time was chosen to he the center of the pulse, and the initial amplitude of the FID was chosen as the zero intercept of a In A ( t )vs. t 2 plot.
RESULTS AND DISCUSSION
1 t tz(psec)
Flgure 2.Typical In A(f) vs. fz for 500 scans
analyzed “as received”. No attempt was made to dry the samples, or to remove volatile matter by heating at reduced pressure. A sample transfer method was used to determine relative concentrations of protons in coals to that of a standard water sample. The number of protons in the water sample was determined to he 3.94(2) X loz1by weighing the NMR tube containing the water standard before and after sealing in the water. The lengths of the coal samples in the NMR coil were kept the same size or smaller than that of the water sample in order to ensure that the coals were exposed to the same homogeneous H Ifield as was the water. Under this condition of H I homogeneity, it was not necessary to have a coal sample of accurately known packing density. An alternative would have been to have both the coal and water samples long compared t o the NMR coil. While this would have resulted in better signal to noise because of the higher filling factor, a determination of the packing density would have been necessary for a quantitative comparison of numbers of protons in the standard and the coals. The basis of utilizing sample transfer to determine relative numbers of protons in coals compared to a standard is that the polarization of the protons in the sample is proportional to the total number of protons in the sample. If the pulse width of the rf field is small compared to the effective spin-lattice relaxation time of the system, T2*,or, if appropriate corrections are made when this condition is not fulfilled, the initial value of the free induction decay (FID) is proportion1 to this polarization, and therefore to the number of protons in the sample. Proton-proton dipolar coupling in solid coals has been found to be of the order of 30 kHz ( 6 ) ,which is to say that spin-spin relaxation times as short as 10 1 s are observed. With pulse widths of 1.3 j ~ s it , is clear that the pulse width is a nonnegligible fraction of the decay, and that two problems must he confronted. The first is the choice of the zero of time from which to calculate the initial amplitude of the FID, A ( 0 ) .Lowe has shown (7) that the appropriate zero of time 76
a
NMR
ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
Hydrogen contents of seven coals were obtained using the initial value of t h e FID as described i n t h e experimental section. A typical plot of In A ( t) vs. t 2is given for 500 data accumulations in Figure 2. A ( t )are the extrema in t h e oscillations of t h e FID. Comparison of hydrogen c o n t e n t s determined chemically, a n d via t h e initial value of t h e FID is given in T a b l e I. T h e values of “% H b y NMR” a r e those obtained from t h r e e s e p a r a t e least squares fits of In A ( t )vs. t 2for all samples except t h e S t a r vitrain at 3.0 and 5.0 p5 pulse widths. W i t h t h e exception of the latter, values i n parentheses represent m e a n square deviations of t h e three values of t h e initial amplitudes. Values in parentheses for the Star vitrain at 3.0- a n d 5.0-ps pulse width are mean square deviations of one set of extrema t o a linear least squares fit of In A ( t ) vs. t 2 . Chemical d e t e r minations of hydrogen in t h e coals were made b y combustion gravimetry, according t o ASTM s t a n d a r d D271. I n order t o determine the effect of pulse width u p o n the results, hydrogen content of one coal sample, Star vitrain, was determined as a function of pulse width. Pulse widths of 1.25, 3, a n d 5 ps were used. T h e results a r e not a function of pulse width t o within t h e precision of t h e measurements, as c a n b e seen from T a b l e I. Because t h e initial a m p l i t u d e of the FID is a sinusoidal function of the pulse width, with decreasing a m p l i t u d e as the pulse w i d t h Hamiltonian becomes comparable in size to the offset Hamiltonian (9),it was impossible t o obtain a reasonable signal to noise ratio for pulse widths more than 5 ps with offsets of 400 kHz. This result m a y be shown utilizing a straightforward calculation of the t i m e evolution of the density m a t r i x under t h e influence of the Hamiltonian H = A d , wJX. For this reason, it appears that t h e technique is limited t o those cases in which the pulse width is less than a q u a r t e r of t h e decay time. W e note that t h e only factors preventing commercially available Fourier T r a n s f o r m NMR spectrometers from utilizing this technique for t h e quantitative analysis of hydrogen in solids i n which spin lattice relaxation times a r e sufficciently s h o r t t o allow for r a p i d d a t a accumulation are: 1) lack of a sufficiently r a p i d receiver recovery time, and 2) lack of a
+
sufficiently rapid analog to digital conversion for recovery of signals disappearing in less than 40 ws. The former may be avoided by the use of the receiver described herein, which is easily constructed from commercially available units a t a cost less than $700. The latter problem may be circumvented by the use of an appropriate transient recorder, a number of which are commercially available with sampling rates useful in the microsecond range of data accumulation.
ACKNOWLEDGMENT The authors are indebted to H: L. Retcofsky for supplying two of the coal samples used in the present analysis, and to C. R. Dybowski for conversations regarding anlysis of the data. The magnet and power supply were donated by Dow Chemical Company.
LITERATURE CITED L. B. Schreiber and R. W. Vaughan, J. Catal., 40, 226 (1975). (2) J. D. Ellet, M. G. Gibby, V. Haeberlen,L. M. Huber, M. Mehring, A. Pines, and J. S . Waugh, Adv. Magn. Res., 5 , 117 (1971). (3) R. W. Vaughan. D. D. Ellman, L. M. Stacy, W.-K. Rhim. and J. W. Lee, Rev.
(1)
Sci. Instrum.. 43, 1356 (1972).
(4)
D. J. Adduci, P. press,.
A.
( 5 ) VV.-K. Rhim, D. D.
Hornung. and D. R. Torgeson, Rev. Sci. Instrum. (in
Elleman, and R. W. Vaughan, J. Chem. Phys.,59, 3740
(1973). (6) B. C. Gerstein, Chee Chow, R. G. Pembleton, and P.I C. Wilson. J. Phys. Chem. (inpress). (7) D. E. Barnaal and I.J. Lowe, Phys. Rev. Lett., 11, 258 (1963). (8) A. Abragam, "The Principles of Nuclear Magnetic Resonance", Oxford Univ. Press, London and New York, 1961. Chap. 4. (9) C. R. Dybowski, personal communication.
RECEIVEDfor review November 12, 1975. Accepted September 24,1976. A portion of this work was sponsored by the Iowa Coal Project.
Compositional Analyses of Methyl Methacrylate-Methacryl ic Acid Copolymers by Carbon- 13 Nuclear Magnetic Resonance Spectrometry Duane E. Johnson,* James
R. Lyerla, Jr.,
Teruo T. Horikawa, and Lester A. Pederson
IBM Research Laboratory, San Jose, Calif. 95 193
The use of carbon-I3 Fourier transform nuclear magnetic resonance for quantitative analysis of the composition of methyl methacrylate-methacrylic acid copolymers has been Investigated. It was found that in pyridine solutions of these copolymers, the resonances arising from acid carboxyl and ester carbonyl carbons were Sufficiently resolved to allow the determination of relative Integrals. Compositlonal data obtained using 13C NMR compared favorably with those obtained by standard titration techniques.
In titrating copolymers of methyl methacrylate and methacrylic acid P(MMA/MAA), with standard base to determine composition, a number of deficiencies were encountered. For example, there were two common sources of "contamination'' that gave rise to underdetermination of the acid content: 1) the copolymers tended to be hygroscopic and hence, could contain ca. 5% absorbed moisture and 2) they could, on occasion, retain solvents and/or monomers. Additionally, the method was found to be inapplicable to P(MMA/MAA) of high molecular weight (MW Z 1 000 000) and/or high acid content (>6W?h acid) because of the tendency of such systems to reprecipitate during the titration procedure. Because of the nondestructive nature of the technique and the potentially routine character of analyses, proton nuclear magnetic resonance (IH NMR) appeared to be a viable alternative. By using the integral of the ester methoxy protons and combining this result with the total integral for CH2 and ( Y - C Hprotons ~ (the overlap between CH2 and a-CH3 resonances was enough at 100 MHz to prevent separate determination of these integrals), the copolymer composition could be ascertained; however, it was necessary to carry out the determinations at 100 "C or higher to attain resolution sufficient for reliable integrals. Although the temperature requirement was an unattractive feature of these analyses, the
central difficulty was that the reaction solvents (e.g., toluene and hexane) and comonomers had resonances that overlapped those of the CH2 and a-CH3 of the copolymers. Thus when solvents and/or monomers were retained, considerable inaccuracy was introduced in the total CH2/(r-CH3integral. Because of the greater spectral dispersion and narrower resonance lines (1-3) obtained with carbon-13 NMR relative to proton NMR, problems associated with resonance overlap in l H spectra are often resolved. Thus, we examined the use of Fourier transform I3C NMR as a probe for the compositional analysis of P(MMA/MAA) and herein are reported the results.
EXPERIMENTAL Synthesis of Poly(methy1 methacrylate-co-methacrylic Acid). The copolymers were synthesized by using well known free radical techniques. Reactions were carried out in pressure bottles which were oven dried, cooled under Nz, and all reagents were added under a Nz blanket. As one example, 100 ml of THF, redistilled from LiAlH4,14 g (0.14 M) MMA, passed through neutral A1203,12.2 g (0.14 M) MAA, vacuum redistilled, and 0.1 g tert-butyl peroxide were added to a bottle. The capped bottle was placed in a 70 "C oven over the weekend and the product recovered by pouring the viscous solution into excess n-hexane or diethyl ether. The product was washed with nonsolvent and vacuum dried a t 60 "C. The acid content was 48%. Other polymerizations were carried out in toluene, which had been redistilled from Na. Acid contents were varied by adjusting monomer ratios in the monomer feed stocks. The emulsion polymerizations were carried out by forming an emulsion with 20 g Triton X-200,100 g MMA, and 86 g MAA in about 1 1. of DI water. The reaction was initiated by adding 3.8 ml of an F e S 0 ~ 7 H 2 0solution (0.15 g of sulfate in 100 ml HzO), 1.7 g of (NH&Sz08 in 25 ml HzO, and 0.85 g NaHS03 in 10 ml HzO. The copolymer was precipitated by adding i-PA. Titration. Samples of the copolymers were analytically weighed into a beaker and dissolved in 50 ml ethanol, followed by the addition of 50 ml HzO. Sample weights were chosen to require a titer of 1.5 mequiv or more. The solution was titrated to a phenolphthalein end point (5 min) with standardized 0.15 N aqueous KOH. Carbon-13 NMR Analyses. Samples were prepared by dissolving ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
77
