Anal. Chem. 1982, 5 4 , 1615-1623
higher spectrometer sensitivity of higher static magnetic field strengths. One point that must be kept in mind in judging the value of resolution-enhancement approaches like convolution difference and double exponential multiplication is the inevitable and obvious intensity distortions that they bring to the spectra. By emphasizing narrow-line components of the spectrum at the expense of broad-line components, one gives up the hope of having integrated intensities that are analytically directly and straightforwardly proportional to the abundances of the various carbon types. Iin return, one gains information on, or confirmation of, specific peaks that may be obscured in the normal spectra. Although one can envision ways in which empirical corrections could be developed for extracting quantitative information from such resolution-enhanced spectra, it appears that tlhe most straightforward applications of these techniques will be concerned with elucidating of confirming the specific resonances and carbon structural moieties that contribute to a broad resonance band. ACKNOWLEDGMENT The authors acknowledge the assistance of N. Szeverenyi of Colorado State University and L. Petrakis of Gulf Research and Development Company. LITERATURE CITEYD (1) (2) (3) (4)
(5) (6) (7)
(8)
Pines, A.; Glbby, M. 0.; Waugh, J. S. J . Chem. Phys. 1973, 59, 569. Schaefer, J.; Stejskal, E. 0. J . Am. Chem. SOC. 1978, 98, 1031. Andrew, E. R. Prog. Nud. Magn. Reson. Spectrosc. 1971, 8 , I . Schaefer. J.; Stejskal, E. 0. "Topics in Carbon-13 NMR Spectroscopy"; Levy, 10.C., Ed.; Wiley-Interscience: New York, 1979; Vol. 3. p 283. Llppmaa, E. T.; Alia. M. 8.;Pehk, T. J.; Englehardt, G. J . Am. Chem. SOC. 1978, 100, 1929. Maclel, G. E.; Bartuska, V. J.; Mlknis, F. P. Fuel 1979, 58, 391. Mlknis, F. P.; Sulllvan, M.; Bartuska, V. J.; Maclel, G. E. Org. Geochem. 1981, 3 , 19. Maciei, 0. E.; Sullivan, M. J.; Sreverenyl, M. M.; Mlknls, F. P. I n "Chemistry and Physics; of Coal Utilization-1980"; Cooper. V. R., Pe-
(9) (10) (11) (12) . . (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)
1615
trakis, L., Eds.; American Instltute of Physlcs: New York, 1981; pp 66-81. Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 5 4 , 0000. Sullivan, M. J. Ph.D. Thesls, Colorado State University, Sept 1981. Maciel, G. E.; Sulllvan, M. J.; Petrakls, L.; Grandy, D. W. Fuel 1982, 61, 411. Zllm, K. W.; Pugmire, R. J.; Grant, D. M.; Wlser, W. H.; Wood, R. E. Fuel 1979, 58.-11. Zllm, K. W.; Pugmire, R. J.; Larter, S. R.; Allan, J.; Grant, D.M. Fuel 1681. 8 0 . 717. VanderHart, D. L.; Retcofsky, H. L. Fuel 1978, 55, 202. Retcofsky, H. L.; VanderHart, D. L. Fuel 1978, 57, 421. Gersteln, B. C.; Ryan, L. M.; Murphy, P. D. Prepr. Pop.-Am. Chem. SOC.,Dlv. FuelChem. 1979, 24, 90. Buchdahl, R. Macromolecules 1977, 10, Schaefer, J.; Stejskal, E. 0.; 384. Opella, S.J.; Frey, M. H. J . Am. Chem. SOC. 1979, 107, 5854. Earl, W. L.; VanderHart, D. L. Macromolecules 1979, 72, 762. Jeener, J.; Meier, B. H.; Bachman, P., Ernst, R. R. J . Chem. Phys. 1979, 71, 4546. Szeverenyi, N. M.; Sullivan, M. J.; Maciel, G. E. J . Magn. Reson., 1982, 47, 462. Szeverenyl, N. M.; Sullivan, M. J.; Bronnlmann, C.; Maclel, G. E. J . Chem. Phys., to be submitted. Ernst, R. R. "Advances in Magnetlc Resonance"; Waugh, J. S., Ed.; Academic Press: New York, 1971; Vol. 2, p 1. Campbell, 1. D.; Dobson, C. M.; Wllliams, R. J. P.; Xavier, A. V. J. Magn. Reson. 1973. 11, 172. Stothers, J. B. "Carbon-I3 Nuclear Magnetlc Resonance Spectroscopy"; Academic Press: New York, 1972. Levy, G. C.; Nelson, G. L. "Carbon-I3 Nuclear Magnetic Resonance for Organic Chemlsts"; Wiley-Interscience: New York, 1972. VanderHart, D. L.; Earl. W. L.; Garroway, A. N. J. Magn. Reson. 1981. 44, 361. Dixon, W. T. J . Magn. Reson. 1981, 4 4 , 220. Dlxon, W. T.; Schaefer, J.; Sefcik, M. D.; Stejskal, E. 0.: McKay, R. A. J.Magn.Reson. 1981, 45, 173. Maricq, M. M.; Waugh, J. S . J . Chem. Phys. 1978, 70, 3300.
RECEIVED for review December 18,1981. Accepted April 29, 1982. The authors are grateful for support of this work by the U.S. Department of Energy under Contract No. DEAT20-81LC10652 from the Laramie Energy Technology Center and Contract No. DE-AC22-79ET14940 from the Pittsburgh Energy Technology Center.
Spin Dynamics in the Carbon- 13 Nuclear Magnetic Resonance Spectrometric Analysis of Coal by Cross Polarization and Magic-AngIe! Spinning Mark J. Sullivan' andl Gary E. Maclel" Department of Chemlstry, Colorado State University, Fort Collins, Colorado 80523
A detailed reiaxatlon study has been carried out on an eastern bltumlnous coal from thle Powhatan No. 51mine. Emphasis is on the relaxation parameters that determlne the analytlcai reilablilty of I3C CP/MA!S experiments. The desired condition, T,, < contact time < T l p Hwas , found flo hold, lndlcating a good prognosls for the I3C CP/MAS analysls of coal. The apparent aromaticity, fa', obtalned wlth a contact time of 1 ms Is in aggreement wlth the value obtained from a Blochdecay experknent. The effklency of the "C CP/MAS analysis of coal Is discussed, including consideration of iow-temperature work. 'Present address: JECK (USA), Inc., Cramford, NJ 07016. 0003-2700/82/0354-1615$01.25/0
13C NMR, using cross polarization (CP) and magic-angle spinning (MAS) (Id), has become an important analytical tool for coals (6-18). The line-narrowing influence of MAS (for eliminating the effect of chemical shift anisotropy) and high-power lH decoupling (for eliminating the effect of lH--l3C dipolar interactions) are coupled with the CP method of generating measurable 13Cnuclear magnetization to provide a technique with great utility in the analysis of many types of solid samples (19, 20). In the CP technique, shown schematically in Figure 1, 'H nuclear magnetization is transferred to the I3C manifold in a double-resonance experiment under the Hartmann-Hahn condition (21);this greatly reduces the relaxation time constraints of the normal Bloch-decay experiment (pulse, observe, 0 1982 American Chemlcal Society
1616
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 1
H- 1
I
a
CONThCT TIME
Flgure 1. (a) Pulse diagram of the single-contact cross polarization experiment ( 7): ACQ, acquisltiin tlme; CT, contact time. (b) Behavior of transverse I3C magnetlration with tlme during the contact period (a to b).
wait). This is extremely advantageous in natural-abundance 13CNMR experiments, in which the delay between repetitions in synchronous time-averaging constitutes one of the main parameters that determine the achievable signal-to-noiseratio. One of the most important questions in the 13C CP/MAS analysis of a solid fossil fuel concerns the reliability of integrated intensities, i.e., the quantitative reliability of the data. It is important to known the extent to which the 13C magnetization generated in a CP process represents faithfully the concentrations of different types of carbon in the sample, as has been discussed previously (6-10,14-18). Various approaches to this problem have been addressed in different laboratories during the past few years. VanderHart and Retcofsky (14-16) have compared 13C integrated intensities obtained by Bloch decays on a coal-derived liquid with data obtained by CP on a solidified sample of the same material. Maciel and co-workers (6-10)have reported CP/MAS 13Cstudies on coals in which the results of experiments at various CP contact times have been compared with each other and with corresponding results obtained from Bloch decays (with MAS and high-power lH decoupling). In these Bloch-decay experiments the 13C magnetization is generated by 13Cspin-lattice relaxation, a much different process than the CP process that generates observable 13C magnetization in a lH-13C CP/MAS experiment. The fact that integrated intensities of aromatic carbons relative to aliphatic carbons in CP experiments appeared to reach a plateau in contact-time dependence a t about 1ms and that these results compared within a few percent to values obtained in the Bloch-decay experiments has been interpreted as implying that both the CP and non-CP results provide analytically useful integrations. According to this view, the major disadvantage to the Bloch-decay approach is simply that it takes much longer to achieve a given signal-to-noise ratio by that method, assuming that proton Tl values of coals are typically much shorter than 13C Tl values. Gerstein has estimated on the basis of carbon-counting experiments that about 60-100% of the carbon is accounted for in a 13C CP/MAS experiment on coal (17, 18). If this conclusion proves to be characteristic of coals generally, then one needs to know whether or not the fraction of carbons that are observed in 13C CP/MAS is truely representative of all of the carbon in the sample. Perhaps the main reason why one might question the extent to which the 13Cnuclei in a coal sample are observed completely, or even representatively, is the large concentration of paramagnetic centers typical of
coals. If a significant fraction of the 13C resonances are sufficiently broadened and/or shifted by paramagnetic centers to render them outside the limits of detection of a typical CP/MAS spectrometer, then major distortions of integrated intensities could result. These points are being explored here and elsewhere by electron-nuclear dynamic polarization approaches (22), which have the promise of elucidating the nature of the paramagnetic centers in coal and the effeds they have on 13C CP/MAS spectra. It may be worth recalling that the present uncertainties regarding the quantitative analytical reliability of the 13C CP/MAS approach for coal are reminiscent of the uncertainties and doubts surrounding early pulse-Fourier transform (FT) 13C NMR work on liquid samples, Le., questions regarding relaxation, saturation, and nuclear Overhauser effects, and how they influence intensities in the liquid-state pulse-FT experiment. Such issues have now been well characterized, and ways of circumventing or overcoming these difficulties have become commonplace. Before a similar stage can be reached for 13CCP/MAS work on coal, or on any other class of sample, the following three related, but somewhat separate, issues must be characterized fully: (1) the extent to which the CP/MAS intensities discriminate against one type of carbon relative to another or miss entire collections of carbons; (2) the extent to which the condition employed in 13CCP/MAS experiments on coals (in relation to the pertinent spin dynamics) would be expected to provide quantitatively useful 13Cintensities for those peaks that are obtained; (3) the time efficiency of the 13C CP/MAS experiment. All of these issues depend upon the spin dynamics of the 13CCP/MAS experiment on coal, which is the subject of this article. For this study an eastern bituminous coal, Powhatan No. 5, was chosen. For this same coal some other aspects of the NMR spin dynamics concerned more with structural resolution than with general questions of analytical quantitation are presented in another article (9). EXPERIMENTAL SECTION The specific coal chosen for this study is an eastern bituminous coal from the Powhatan No. 5 mine, provided by the Gulf Research and Development company. Analysis (dry): C, 72.25%; H, 4.94%; N, 0.83%;C1,0.07%; S, 3.66%; 0,6.68% (difference); ash, 11.57%. Polystyrene (mol wt 3 X lo6) was obtained from Polysciences, Inc. Cr(acac)3 was obtained from MacKenzie Chemical Works, Inc. Cr(a~ac)~-polystyrene samples were prepared by dissolving both materials in an excess of benzene with gentle heating, resolidifying the mixture by quickly removing the bulk of the solvent on a rotary evaporator, yielding a thin film, followed by evacuation for 24 h at about 2 torr to remove solvent further. Nevertheless, some solvent remained in the final product; hence, the undoped polystyrene sample used for comparison with the paramagnetic doped samples was also resolidified from a benzene solution and subjected to evacuation. The majority of the 13CCP/MAS experiments were performed at 15.1 MHz on a homebuilt spectrometer based on a 12-in., 14 kG Varian electromagnet. The system included a time-shared 2H external field-frequency lock and quadrature detection (23). Amplifier Research Model 200L and EN1 Model 3100L broadband rf power amplifiers were used in the 13C and 'H channels, respectively. This spectrometer was equipped with a Nicolet 293A programmable pulser that is software-controlled by a Nicolet 1180 computer, which permitted automatic variation of all pulse parameters. Data were stored on a Diablo Series 30 disk drive. Some additional CP/MAS 13CNMR measurements were made at 15.1 MHz on a JEOL FX-6OQ NMR spectrometer modified for solids. One set of experiments was carried out at 50.3 MHz on a home-modified Nicolet NT-200 spectrometer. The magic-angle spinnii probes used were of the double-tuned, single coil design, with a bullet-type rotor (24). The 13Cand 'H rf fields employed in the Hartmann-Hahn match of the single contact experimenta were 44 G and 11G, respectively. The Kel-F rotor held a sample volume of about 0.75 om3 and was operated
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
I SL-----I---
H-1
I
~DECOUPL
rI h
I i
oa
b
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I
-
A
d
C w
Figure 3. Pulse diagram for the proton T I , relaxation experiment (32). t -0
t=a
t= b
DECOUPL I I
I I
I
ai
I
EN0 OF C P CONTACT, d
Figure 2. (a) Pulse diagram of TIH determinatiori via I3C CP/MAS ( 8 , 2 7 ) DECOUPL, decoupling; S L , spin lock. (b) Behavior of spins during the sequence.
at spinning speeds of 24010-2600 Hz. A variable-temperature version of the probe was built by adding suitable insulation and heating elements, with the temperature of the sample region varied by the temperature of the spinner driving gas. Temperatures from -160 OC to 100 OC were achieved. Details of the probe are described elsewhere (IO). All 13C spectra were externally referenced to liquid tetramethylsilane (Me4Si),basedl on substitution of hexamethylbenzene (HMB) as the secondary reference and assigning 132.3 and 16.9 ppm to the shifts of the aromatic and aliphatic carbons, respectively, of HMB relative to liquid Me4Si. All chemical shifts are given in parts per milliion with respect to liquid Me4Si;shifts to higher shielding are given as algebraically smaller numbers. The pulse sequences used in this project are shown schematically in Figures 1-4. The basic single-contact cross polarization sequence presented by Pines, Gibby, and Waugh is shown in Figure 1. A brief descriptioin of this important experiment is given here. This pulse sequence starts with a a / 2 pulse (during the period 0 to a), after which (a) there is a 90° phase shift in the 'H rf field (€IlH)to place the protons1 in a spin-lock condition along H ~ H , and in a condition of very low spin temperature in the 'H rotating frame (I). At the beginniing of this 'H spin--lock condition ( a ) the 13Crf field is applied under conditions of the Hartmann-Hahn match (1,21)for the period a to b. The conditions of this match require that the rf field intensities for 'H anid 13C are adjusted so that the precessional frequencies of 'H and 13Cabout H ~ and H €Ilc,respectively, are the same. This condition allows proton spin polarization to be transferred to 13Cspin polarization, as the 13C spin temperature in the 13C rotating frame approaches the low 'H spin temperature. The buildup of 13Cmagnetization along Hlc is represented by the dotted curve in Figwe l a and proceeds at a rate determined by the cross polarization relaxtion time constant, TCH.When the desired level of 13C spin polarization and magnetization has been obtained (time b, vide infra), the 13C rf field is turned off, and the decay of the 13C magnetization in the transverse plane is observed during the racquisition period, b to c. During this collection of the free induction decay ( b to c ) , which accounts for ca. 60 ms in most of the work reported in this paper, the 'H rf field is maintained for 'H decoupling. Figure l a implies that the strength of the 'H rf field is kept constant during the entire period, 0 to c. This is the single-level case employed in the present wlork; but a different value of H1H can be used during the periods! a to b and b to c. Figure l b shows the typical dependence of the amplitude of the 13CNMR signal obtained in a CP experiment as a function of the contact time (CT), b-a. The initial slope of the curve for short contact times is deteirmined by the value of TCH. At long contact times one sees a fall-off in 13Cintensity; this is associated
END OF CP CONTACT, b
Id
t =c
d
I
t me Flgure 4. (a) Pulse diagram for the I3C TI inversion recovery experiment using cross polarization (70). (b) Behavior of I3C magnetization during the sequence.
with a decrease in the 'H magnetization spin-locked along H ~ H . This decay in spin-locked 'H magnetization results from proton "spin-lattice relaxation in the rotating frame", characterized by ~ Hence, ideally one should be able a relaxation time T I C(26). ~ by systematically varying the to estimate TcH and T l pvalues contact time and analyzing a plot like that shown in Figure lb. Figure 2 shows a pulse sequence designed to measure the 'H spin-lattice relaxation time (TiH)indirectly via detection of 13C resonances (8, 27). In the proton channel the first part of the experiment, from 0 to c, is a 180-~-90Osequence of the type one would use in liquid-state proton T1experiments (28). The differences come in the approach to detection. Direct observation of 'H resonances in a solid either would produce very broad lines, because of 'HJH dipolar interactions or would require a multipulse sequence of the WAHUHA (29) or MREV (30,31)type. In the experiment reported here the 'H magnetization remaining at the end of the relaxation period ( T ) at b is monitored via 13C by initiating a CP sequence of the type shown in Figure l a at time b in Figure 2a. Then, by observing the 13Cmagnetization generated during the contact time (c to d ) as a function of T, one indirectly obtains a measure of 'H relaxation during the T period ( a to b). Figure 3 gives the timing diagram of the experiment developed by Schaefer and Stejskal (32) for measuring proton T1,values in a CP experiment. In the sequence shown in Figure 3 one sees that a relaxation period (a to b) of length T is inserted between the beginning of the 'H spin-lock condition at a and the beginning of the contact period at b (at which time the Hartmann-Hahn condition is established). During the period T rotating-frame 'H spin-lattice relaxation occurs; hence, by observing the 13Cintensity at the end of the contact period as a function of T, one can obtain a measure of T~,H. A scheme for measuring 13C Tl values is shown in Figure 4 (IO). The time period from 0 to b simply involves the usual CP tech-
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
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INVERSION RECOVERY
Table I. NMR Relaxation Data for Polystyrene (30 000 mol wt) Doped with Crlll(acac),a espins/gX lo-'* T ~ Hs , T l p ~ms , T I C ,s undoped 1.7 3.8 19 8.5 0.03 0 4.1 1.0 42 0.006 2.3 85 0.004 1.4 0.04 9
TIME
150MS
a Relaxation parameters have standard deviations of about A 10%in all cases given here.
-
125MS _w
60MS __r_-
25MS
5 lOMS 1 MS
,
,
,
I
'
'
I
"
,
PPM Flgure 5. Stack plot of proton T I inversion recovery data (observed via lac)for Powhatan No. 5 coal. Each spectrum was obtained from 2000 acquisltlons, with a pulse delay of 1.0 8, a contact time of 1 ms, and 50-Hz line broadening. 200
I
,
0.0
,
,
I
,
,
,
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,
!
,
0
1
,
,
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,
,
,
,
,
,
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-
2.0
SEC
Flgure 6. Plot of the proton T inversion recovery curve measured from the aromatic peak in the I4Cspectrum of Powhatan No. 5 coal. T,(arom) = 95 ms. Arbltrary Intensity scale.
nique for generation of 13C magnetization along Hlc. A 90" 13C pulse between b and c reorients the 13Cmagnetization along the -z direction at time c. Then, during the relaxation period (7) between c and d, 13C spin-lattice relaxation occurs, and the remaining 13C magnetization along z at time d is monitored by reorienting it into the transverse plane by the 90" pulse at time d . Data acquisition occurs during the time period, e to f , with 'H decoupling. Experiments in which 7 is varied give a set of data from which the 13C Tl value can be obtained by analysis.
RESULTS The result of applying the proton Tl measurement scheme of Figure 2 is shown in Figure 5, where the results obtained at 7 values ranging from 1 ms to 1 s are displayed. Peak heights were taken at the apparent maxima of the aromatic and aliphatic regions at approximately 128 and 34 ppm, respectively; and these data obtained as a function of the inversion recovery time (7)were fitted to an exponential function from which TIH was determined. A plot of proton Tl data is shown in Figure 6. Proton spin-lattice relaxation times of 95 and 110 ms were derived from the aromatic and aliphatic carbon regions, respectively (standard deviation h5 ms for these results). For a pure organic solid, this kind of multiple proton T1 behavior is not observed, since a uniform proton
spin temperature is maintained throughout the sample by rapid spin diffusion, and only one proton Tl value is measured for all protons in a given compound. The small, but significant difference found in the proton Tl values indicates that the aliphatic and aromatic protons in coal are not in intimate spin contact. This finding indicates that coal is heterogeneous, consisting to some degree of separate aliphatic-rich and aromatic-rich domains. This is consistent with the contention of Hatcher et al. (33)who have concluded that the aromatics in coal are derived largely from lignin, while most of the aliphatic intensity is due to algal or microbial residues. Another interesting aspect of the proton Tl data on Powhatan No. 5 coal is that the relaxation rates are faster than expected based on known data on pure organic systems. Nevertheless, the values of 95 and 110 ms measured for Powhatan No. 5 coal are in line with TIH values obtained by the same technique on a series of coals of varying rank (34). The presence of paramagnetics in crystals is known to shorten their spin-lattice relaxation times (35). One of the mechanisms postulated for proton spin-lattice relaxation in coal is based on the free radical content (36-38). Powhatan No. 5 coal has been found to contain 1.02 X 1019 electron spins/g, as determined by ESR measurements (39). The nature and distribution of the free radicals in coal have been studied extensively, but many details remain undefined. The measured g values in coal are similar to those found for organic free radicals (40, 41). The g value measured for Powhatan No. 5 is 2.00287 (39). In addition, Wind et al. have indicated from their dynamic-polarization/ cross-polarization experiments that a greater portion of the free radicals in coal are located in the aromatic region (22), in agreement with suggestions from ESR work (40, 4 1 ) . In order to explore the consequences of the above considerations, a set of experiments was carried out to determine the effect of the paramagnetic metal ion, Cr(III), on the relaxation behavior in polystyrene. Only one T1H value was found for all of the 13C resonances of any given polystyrene sample. The proton T1results are given in Table I for various Cr(II1) concentrations, along with the corresponding T l pand ~ TIC data. A dramatic decrease in the proton Tl value from 1.7 s to 30 ms is seen for the sample of polystyrene containing 0.85 X 1019electron spins/g, compared to the undoped sample. This evidence lends support to the belief that the paramagnetic centers in coal contribute to its NMR relaxation behavior and can account for the short proton spin-lattice relaxation times observed. The proton Tl value measured via the aromatic carbons in Powhatan No. 5 coal at -141 "C was found to be 124 ms. This represents an increase of 29 ms over the T1H value found at 21 "C. An accurate T1H value could not be measured via the aliphatic carbons at this temperature because of the problem of spinning side-band overlap. In nuclear spin relaxation by paramagnetics the local magnetic field due to the electronic spin may be modulated both by electronic relaxation processes and by molecular motion. In crystalline solids, where motion is highly restricted, the electron relaxation process usually dominates (42). Since electron spin-lattice relaxation times
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 CONTACT TIME
2 0 0 MHz
II //
I\
1619
7ms
^cc
A
~ O M H ~
7 ,correlation timeC
Flgure 7. Qualitative plot of proton T , value vs. correlation time,
7,,
as a function of temperature, and H,. I\
SPIN-LOCK TIME
ZOO
0
PPM
Flgure 9. Stack plot of variable contact-time data for Powhatan No. 5 coal. Each spectrum was obtained from 2000 acquisitions, a 0.5s
w
pulse delay, and 50-Hz line broadening.
d L E - - J L Z di- - - . 7MS
200
0
PPM
Figure 8. Stack plot of proton T,, data for Powhatan No. 5 coal. Each spectrum was the result of 5000 acqulsitions, a pulse delay of 0.5 8, a I - m s contact time, and 30-Hz line broadening. The sharp peak at -2 ppm is due to a methylsllicone reference standard.
for solids typically are not very temperature dependent (35), the paramagnetic contributions to T lin~solids are expected to be largely independent (of temperature. The change in the proton Tl value with temperature that is observed for the Powhatan No. 5 coal indicates that molecular motions, as well as spin fluctuations, contribute to T1H. An increase in T1H with decreasing temperature indicates that the molecular motions in this temperature range have correlation times (73which are longer than 7,for the minimum in the T1 vs. 7,curve (43). On the basis of this analysis one would predict that the proton Tl values measured in a spectrometer with a higher static magnetic field should be larger. An increase in TIIHwas indeed observed for the relaxation experiment performed at 4.7 T, relative to that performed at 1.4 T. The relaxation time for the photons coupled to the aromatic carbons in Powhatan No. 5 coal measured a t 21 "C with a 4.7 T field was 184 ms. Spinning side-band overlap again pirohibited the measurement of the proton Tl via the aliphatic carbons under these conditions. This qualitative analysis is summarized symbolically in Figure 7. Figure 8 presents a stack plot that summarizes an experiment of the type represented in Figure 3, for measuring TIDH.
Careful inspection of the spectra in Figure 8 shows that, not only do the absolute intensities vary as 7 is varied but also there are significant changes in the shapes of the aromatic and aliphatic bands. It would be difficult to quantify these band-shape changes with the signal-to-noise ratios available in these experiments. Consequently, we have analyzed the data, using a fit to an exponential expression, only in terms of the peak intensities at the maxima in both regions. In this way values of 3.8 f 0.5 ms and 4.4 f 0.5 ms were obtained for the T1,H values corresponding to the aromatic and aliphatic bands, respectively. These results are compatible with the results of a variable contact-time experiment. Figure 9 gives a stack plot obtained by varying the contact time between 20 ps and 7 ms. The general behavior predicted in Figure l b is seen in this experiment, with the maximum 13C signal intensity occurring a t about 1ms. The increasing intensity as the contact time is varied from 0 to 1 ms is primarily a manifestation of the dynamics of the cross polarization and is characterized by TCH.The loss of overall 13C intensity a t contact times longer than 1ms is due to proton rotating-frame spin-lattice relaxation, characterized by TIp" Again changes in band shapes make it difficult with the available signal-to-noise ratio to extract quantitatively accurate TipHor TCH values. Nevertheless, by makmg use of the spectra corresponding to the longer contact times it can be estimated that the TipHvalues for the detectable carbon atoms are about 4 ms. By utilizing the spectra corresponding to shorter contact times, one can estimate that TCHis less than about 70 ps for aromatic carbons and less than 25 ps for aliphatic carbons. Figure 10 shows a series of spectra taken with the I3C T1 inversion recovery CP sequence diagrammed in Figure 4. Examination of the figure shows that the aromatic band relaxes rather homogeneously, without drastic changes in line shape. This permits the determination of an average Tl value for aromatic carbons by fitting to an exponential. A value of 5.8 f 0.8 s is obtained. Relaxation of the aliphatic band occurs with major changes in line shape; while a detailed discussion of this kind of behavior is deferred to another paper (changes in line shape are discussed together with analogous line shape changes in other relaxation experiments) (9),it is clear from a cursory examination of Figure 10 that 13C spin-lattice relaxation of the aliphatic carbons in Powhatan No. 5 coal is
1620
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 INVERSION RECOVERY T I M E
-
+
',
T, "C intens ratio
T, K TK-'/T,,'
56 21 -19 - 50 -82 -107 -141
3 29 294 254 223 191 166 13 2
0.91 1.00 1.23 1.35 1.58 1.84 2 20
a
0. 89 1.00 1.15 1.32 1. 54 1.77 2 20
fa'&
0.72 0.70 0.75 0.70 0.72 0.79 0.72
Magnetization: M = xH,,where x = Nr2/4kT. b fa' = (aromatic integral)/(total integral ). Standard deviations range from about 0.03 o r higher temperatures to about 0.06 for lower temperatures.
lGUS
\
Table 11. Intensity and Apparent Aromaticity Data from the Variable Temperature Experiment on Powhatan No. 5 Coal
'
J
200
0
PPH
10. Stack plot of the inversion recovery ( T , ) data on Powhatan No. 5 coal. Each spectrum was obtained using 4000 acquisitions, a pulse delay of 1 s, a contact time of 1 ms, and 50-Hz line broadening. The sharp peak at -2 ppm Is due to an external methylsilicone reference standard. Flgure
-1
lo
01
0
, , ,
,
2
,
, , ,
, , ,
, , ,
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4
6
Contact
Time, MS
8
, , , , 10
12
Figure 12. Plot of apparent aromaticity ( f t ) vs. contact time for polystyrene (30000 mol wt, not cross-linked),0; polystyrene doped with Cr"'(acac), to a concentration of 8.5 X 10" e- spins/g, A; Powhatan No. 5 coal, +. Dashed line corresponds to the 0.75 value expected for polystyrene.
200
0
PPM
11. Variable-temperature I3C CPIMAS spectra of Powhatan No. 5 coal. Each spectrum was the result of 400 acquisitlons, a 1-s Figure
pulse delay, a I-ms contact time, and 50-Hz line broadening. even faster than that of the aromatic carbons. These 13C Tl values are remarkably short for organic solids, especially for condensed-ring aromatic carbons; and these results again suggest an important role for relaxation induced by paramagnetic centers. T o examine the plausibility of this suggestion, we carried out analogous 13C T1 experiments on polystyrene samples doped with varied amounts of Crm(acac)3. These results, which gave essentially only one, uniform reduced T1 value for all carbons, are summarized in Table I. Figure 11 shows the 13CCP/MAS spectra of Powhatan No. 5 coal taken at various temperatures between 56 "C and -141 "C, each obtained in 400 repetitions. The most salient feature of this series is the increase in overall intensity that occurs with decreasing temperature. The relative total integrated
areas of these spectra, normalized to the room-temperature spectrum, are shown in Table 11. The calculated relative change in magnetization expected on the basis of the temperature dependence is also expressed in the table (as the ratio of the inverse temperatures). The calculated and experimental results agree closely. This appears to show that the observed enhancement is due to the increased Boltzmann factor at low temperature, rather than to changes in relaxation behavior or to changes in the Q factor of the sample coil with cooling. A variable contact time experiment was carried out at -141 "C. At this temperature the optimal contact time (maximum signal-to-noise ratio) was found to be 1ms. This is the same result as was found at room temperature. There is apparently no major change in the relationships among the relevant relaxation times a t -141 "C in Powhatan No. 5 coal, relative to 21 "C. As a type of calibration on the 13C CP/MAS approach, a series of variable contact time experiments was carried out on polystyrene and on polystyrene doped with Cr111(acac)3to a concentration of 8.5 X 10l8e- spins/g. The data were analyzed in terms of the apparent aromaticity, f,', the ratio of intensity in the aromatic region (including relevant spinning sidebands) to the total integrated carbon intensity. It is this parameter that has received the most attention in previous 13C CP/MAS studies of coal (6-18). A plot of apparent aromaticity vs. contact time for polystyrene is shown in Figure 12. The results of this experiment showed the expected behavior of increasing f,' at longer contact times. This occurs because an aromatic carbon without an attached hydrogen atom has the longest TcH of all of the carbons in the sample. In the range of contact times from 0.5 to 5.0 ms, f,' reaches a plateau value of 0.74, which is within experimental error of the theoretical value of 0.75. At longer contact times scatter
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
in the data increases as tlhe overall signal intensity decreases due to TlpHrelaxation. The analogous results on the doped polystyrene are also given in Figure 12. Inspection of the spectra (not given) shows that addition of these paramagnetic centers renders no measurable increase in the observed line widths. It did, however, cause a drastic ireduction in the carbon and proton Tl’s in polystyrene in coimparison to those for the undoped sample, as shown previously in Table I. Nevertheless, these data yield a curve that is virtually indentical with the analogous plot for the pure polystyrene sample. In both cases the measured apparent aromaticity reaches a plateau value that approaches the theoretical value to within the limits of experimental error. It is inkresting to find that the introduction of paramagnetics in a concentration sufficient to shorten the relaxation times in a model system did not cause intensity distortions. The apparent aromaticity of Powhatan No. 5 coal measured as a function of contact time is also displayed in Figure 12. As in the case of polystyrene an initial sharp increase in f,’ at short contact times is followed by a flattening out at longer contact times. Bloch-decay experimenls, although time-consuming because of the long delays betweein repetitions, are important in that they provide an independent check on the quantitation of cross polarization NMR experiments. These experiments were performed by using mag ic-angle spinning and high-power proton decoupling just a13 in the CP experiments. The difference in the Bloch-deca:y experiment is that the observable magnetization is created by applying a 7r/2 pulse directly to the carbon magnetization generated by spin--lattice relaxation, rather than through cross polarization with the protons. The uncertainties associated with FT experiments are not necessarily fewer than those atstdated with CP experiments; rather, they are derived from different sources. In Bloch-decay experiments, the uncertainties are associated with nuclear Overhauser effects (44) and differing 13Crelaxation times (45). A bloch-decay 13CNMR spectrum of Powhatan No. 5 coal was obtained in 2484 accumulations, using a pulse delay of 40 s (total time 27.6 h). The pulse sequence employed a gateddecoupling scheme designed to suppress NOE effects by turning the decoupler off during the pulse delay and on only during data acquisition. The pulse delay was set to be greater than 6T1’~for the slowest-relaxing carbons in the sample, 5.8 s (vide supra). Although the resulting signal-to-noise ratio was relatively poor in comparison to CP spectra, the spectrum could still be integrated, and f,’ was found to be 0.76 f 0.07. This number is an agreement with the values obtained from the CP experiments using contact times >:Ims.
DIBCUSSION The fact that the f,’ value obtained by a Bloch-decay experiment with MAS and high-power lH deeoupling is within experimental uncertainty the same as that obtained by an optimal CP/MAS experiment is an excellent indication that both approaches provide relative NMR intensities that represent the relative concentrations of the various carbon types. Of course, the possibility exists that with both techniques some fraction of the carbons are missed, e.g., by large paramagnetic shifts or broadening. This possibility is being addressed by electron-nuclear polarization experiments (22). In any case, the excessive time requirements of the Bloch-decay technique dictated by 13C T1values of a few seconds preclude its consideration as a serious competitor for CP/MAS in coal analysis. On the practical analytical level, there are three major conclusions that can be drawn from the results presented here. These conclusions apply strictly only to Powhatan No. 5 coal. However, until analogous data on other coals become available, these results can serve as guidelines for 13C! CP/MAS work
1621
on nonanthrocitic coals in general. The fiist conclusion, drawn from the measured lH T1values, 95 and 110 ms, is that the CP experiment can be repeated with about 0.5-s intervals. This corresponds to a repetition rate that is probably as fast as can be tolerated by the allowable duty cycles of most spectrometers, especially probes. The second conclusion, which more or less confirms previous experience (&IO), is that a contact time of about 1 ms provides the maximum signalto-noise ratio in a 13CCP/MAS experiment on coal. The third conclusion is that the measured relaxation parameters present a favorable prognosis for obtaining quantitatively useful 13C intensities from 13C CP/MAS experiments, at least for those carbons that are the primary contributors to the 13Cspectra. This last conclusion is based on considering the nature of the CP experiment, as diagrammed in Figure 1. One readily sees that the contact time (CT) should be much longer than the largest TCH value in the system, in order that all carbon types are cross polarized to an equal extent. Furthermore, one sees that CT should be much less than the T i p H values, so that the spin-locked lH magnetization does not decay during the contact period. Thus, the desired regime is
For a contact time of 1ms, with TCH values in the range below 70 ps and with TiPH values in the range of about 4 ms, we see that these conditions are reasonably well satisfied. Perhaps larger T i p H values would be even better. However, unless there are major differences in T l p H values for different protons-differences not found in this work-a small amount of decay of the spin-locked lH magnetization will diminish all cross polarized 13C magnetization by approximately the same fraction. This would lead to spectra in which relatiue carbon intensities are correct, while absolute intensities may not be. Hence, it appears that those carbons that are the major contributors to the CP spectrum are governed by spin dynamics that facilitate a straightforward quantitative interpretation of integrated intensities. There are other pointa that can also be made regarding the efficiency of 13C CP/MAS experiments. Selections of the proper pulse repetition rate and contact time are crucial to optimizing the efficiency of cross polarization experiments in solids. Choice of the pulse repetition rate is determined solely by the proton spin-lattice relaxation time, while the contact time depends on both the proton-carbon cross relaxation times and the proton rotating-frame relaxation time. In choosing the pulse delay (repetition interval) for cross polarization experiments on solids, the proton Tl relaxation time must be taken into account. Because T2*relaxation times are usually short in comparison to T~H’sin solids (9),the acquisition time needed to collect the complete FID is short in comparison to the pulse delay. Under these conditions the full FID is collected, since little time savings can be gained from truncation, as is sometimes done to enhance sensitivity in solution NMR. The optimal pulse-delay time from the point of view of maximizing signal-to-noise can be determined from a straightforward calculation. For a given total experimental time, T, the NMR signal can be acquired an integral number of times equal to T/PD, where PD is the pulse delay. For a given sample size and CP contact time the expression for the signal-to-noise ratio in CP experiments is given by
S/N
(&)”‘[l
- exp(-PD/TIH)]
(1)
That is, the signal-to-noiseratio is proportional to the square root of the number of scans, times a proton relaxation term. This latter term approaches unity for PD > 5TlH. Since the recovery of magnetization is exponential, the initial slope of
1622
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
the function is steep, but gradually flattens out as the spin system approaches equilibrium. It is, therefore, not efficient to allow the protons to relax back to equilibrium between pulse repetitions. Rather, over a period of several repetitions the proton magnetization reaches a steady state. The fraction of the total proton magnetization that is not lost due to progressive saturation is given by the second factor in eq 1. By factoring terms this equation becomes
The substitution, x = (PD/TlH), and elimination of the constant factor, (T/T1H)1/2,gives
S/N
a
x-lI2[1 - exp(-x)]
(3)
The derivative of this expression for S I N with respect to x , set equal to zero, yields a transcendental equation. An extremum of this transcendental function is found that occurs near x = 1.25. Therefore, the optimal S I N is obtained for a repetition rate given by PD 1.25TlH. In most pure organic solids rapid spin diffusion maintains a uniform spin temperature among all of the protons in the sample. Thus, they exhibit a single T1H relaxation time. Under these circumstances the relative intensities of the various carbon resonances are independent of the pulse repetition rate. In organic materials with weakly coupled protons or in highly inhomogeneous materials such as coal, multiple T1H behavior may be exhibited through different resolved carbon resonances. Under these conditions the pulse repetition rate can have an effect on relative carbon intensities and a pulse delay of 4 to 5 TlH’8 for the slowest relaxing protons in the sample should be used. Previous authors (46,47) have described the problem of molecular motions that are comparable or large compared to the proton decoupling field strength. The beating of the decoupling and motional frequencies gives rise to dipolar interactions that are essentially static. When this occurs, the proton decoupling effectiveness may be significantly decreased, and 13C line broadening is the result. In addition, these motional frequencies in the mid-kilohertz range may greatly accelerate proton and 13C Tlpdecay. When the loss of proton and carbon rotating-frame magnetization is sufficiently rapid, the cross polarization process is foiled and the signal intensity suffers. This complication can be circumvented either by increasing the decoupling field strength or by altering the correlation time of molecular motion-e.g., by lowering the temperature of the sample. An example of this has been seen in the CP spectrum of anthracene. With a proton decoupling field of 50 kHz no signal was observed at room temperature. At -133 OC, however, a spectrum was rapidly obtained (48). Because coal is thought to contain condensed polynuclear aromatics, it was feared that there might be regions of the sample that would be susceptible to the above effects and hence would give diminished or no 13C resonance signals. Encouragingly, no major changes in the spectral features of coal were observed as the temperature was changed (see Figure 11 and Table 11). In light of the fact that proton spin-lattice relaxation does not slow dramatically at low temperature, the prospects for obtaining enhanced sensitivity in NMR spectra of coal at low temperature are very good. The problem of slow spinning at low temperature can probably be solved by using helium in place of nitrogen as the driving gas and/or by other technical developments aimed at faster spinning or sideband suppression (49). Tegenfelt and Haeberlen (50) have suggested preserving the spin-locked proton magnetization that remains at the end of the acquisition time by tipping it back up along the Ho direction. This process facilitates the return of proton
=
magnetization back to equilibrium and “savesn lH magnetization in a way different from the multiple-contact approach ( I ) . If one is to apply this method, however, the TlpH value must be long in comparison to the acquisition time; otherwise the spin-lock ‘Hmagnetization has decayed during the acquisition period. Rotating-frame proton spin-lattice relaxation times for the coals studied in this laboratory are typically too short (4-5 ms for Powhatan No. 5) to permit taking advantage of this technique. Another way in which relaxation experiments on a coal may provide useful information can be appreciated by close inspection of the 13Cinversion recovery results given in Figure 10. As the relaxation period, 7,is varied, definite changes in the aliphatic line shape are discernible. It can be seen that resonances on the high-shielding side of the dashed line relax faster than those on the lower-shielding side. The difference in the T1cvalues for these spectral regions can be interpreted in terms of molecular motion. Carbon spin-lattice relaxation depends on high-frequency motions at wo and 2w0, where wo is the resonant NMR frequency of 15 MHz at the field strength employed. Methyl groups, which fall in the 15-20 ppm range, are expected to have much larger Fourier components of motion at wo than the more rigid branched alkanes that appear in the region from 40 to 50 ppm. The methylene-type carbons at 30-40 ppm fall between these two extremes. The carbon T1 results observed in Powhatan No. 5 coal are qualitatively consistent with these spectral assignments. What this example shows is that it may be possible to make structural distinctions of different carbon types by differential relaxation behavior in a broad resonance band in which a distinct resolution of peaks is not directly available in the standard 13C CP/MAS spectrum. This approach is considered in greater detail in another paper (9). ACKNOWLEDGMENT The assistance of L. Petrakis and the Gulf Research and Development Company is acknowledged. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7) (8)
(9) (10) (11) ’
(12) (13) (14) (15) (16) (17) (18) . . (19)
Pines, A.; Gibby, M. G.; Waugh, J. S. J. Chem. fhys. 1973, 5 9 , 569. Lowe, I . J. fhys. Rev. Lett. 1959, 2 , 285. Kessemeler, H.; Norberg, R. E. fhys. Rev. 1967, 155, 321. Andrew, E. R. f r o g . Nucl. Magn. Reson. Spectrosc. 1971, 8, 1. Schaefer, J.; Stejskai, E. 0. J. Am. Chem. SOC. 1976, 08, 1031. Maciel, G. E.; Bartuska, V. J.; Mlknis, F. P. Fuel 1979, 58, 391. Mlknis, F. P.; Sullivan, M.; Bartuska, V. J.; Maciel, G. E. Org. Geochem. 1981, 3 , 19. Maclel, G. E.; Sullivan, M. J.; Szevereny, N. M.; Miknis. F. P. “Chemistry and Physics of Coal Utillzatlon-1980”; Cooper, B. R., Petrakls, L., Eds.; American Instltute of Physlcs: New York, 1981; pp 66-6 1. Sullivan, M. J.; Maclel, G. E. Anal. Chem., in press. Sulllvan, M. J. Ph.D. Thesls, Colorado State University, Sept 1981. Zllm, K. W.; Pugmire, R. J.; Grant, D. M.; Wiser, W. H.; Wood. R. E. Fuel 1979, 58,-11. Maciel, G. E.; Sulllvan, M. J.; Petrakis, L.; Grandy, D. W. Fuel 1982, 81, 411. Zllm, K. W.; Pugmlre, R. J.; Larter, S. R.; Allan, J.; Grant, D. M. Fuel 1981, 80, 717. VanderHart, D. L.;Retcofsky, H. L. Fuel 1976, 55, 202. Retcofsky, H. L.; VanderHart, D. L. Fuel 1978, 57, 421. VanderHart, D. L.; Retcofsky, H. L. frepr. Coal Chem. Workshop 1976, 202. Gerstein, B. C.; Ryan, L. M.; Murphy, P. D. f f e p r . fap.-Am. Chem. SOC., Div. FuelChem. 1979, 2 4 , 90. Wemmer, D. E.: Pines, A.; Whitehurst, D. D. Phllos. Trans. R . SOC. London, Ser. A 1981. 300, 15. Schaefer, J.; Stejskal, E. 0. “Topics in Carbon-I3 NMR Snectrosconv”: New York. _r...._..r , , Lew. ~. ,, G. C... Ed.:. Wilev-Intersclence: 1979; Vol. 3. p 283. Ballmann, G. E.; Groombridge, C. J.; Harris, R. K.; Packer, K. J.; Say, B. J.; Tanner, S. J. fhllos. Trans. R . Soc. London, Ser. A 1981. 299, 643. Hartmann, S. R.; Hahn, E. L. fhys. Rev. 1952, 728, 2042. Wlnd, R. A,; Trommel, J.; Smidt, J. Fuel 1979, 58, 900. SteJskal, E. 0.; Schaefer, J. J. Magn. Reson. 1974, 14, 160. Bartuska, V. J.; Maciel, G. E. J. Magn. Reson. 1981, 4 2 , 312. Sullivan, M. ?,.;Szeverenyi, N. M.; Maciei, G. E., unpublished work. Mehrlng, M. NMR Spectroscopy In Solids”; Springer-Veriag: Berlln 1976; p 138. Szeverenyl, N. M.; Sullivan, M. J.; Maciel, G. E. 21st ExperimentalNMR Conference, Tallahassee, FL, March 1980. I
(20) (21) (22) (23) (24) (25) (26) (27)
1623
Anal. Chem. 1982, 5 4 , 1623-1627
(30) Rhim, W.-K.; Elleman, D. ID.: Vaughan, R. W. J . Chem. Phys. 1973, 5 8 , 1772. (31) Mansfleld, P. J . fhys. Chim. 1971, 4 . 1444. (32) Schaefer, J.; Stejskal, E. 0.;Buchdahl, R. Macromolecules 1977, 70, 384. (33) Hatcher, P. G.; Breger, I. A.; Dennis, L. W.; Maclel, G. E. Prepr. Pap .-Am. Chem. SOC., Div. Fuel Chem., in press. (34) Maciel, G. E.; Szeverenyl, W. M.; Sullivan, M. J., to be submltted for publication. (35) Bloembergen, N. Physlca (Amsterdam) 1949, 15, 386. (38) Gerstein, B. C.; Chow, C.; Pembleton, R. G.; Wllson. R. C. J . Phys.
(42) Abragam, A. "The Prlnclples of Nuclear Magnetism"; Oxford University Press: London, 1961; p 378. (43) Farrar. T. C.; Becker, E. D. "Pulse and Fourler transform NMR"; Academlc Press: New York, 1971; p 57. (44) Noggle, J. H.; Shirmer, R. E. "The Nuclear Overhauser Effect"; Academlc Press: New York, 1971. (45) Farrar, T. C.; Becker, E. D. "Pulse and Fourier Transform NMR"; Academic Press: New York, 1971. (46) Schaefer, J.; SteJskal, E. 0.;Buchdahl, R. Macromolecules 1975, 8 , 291. (47) VanderHart, D. L.; Earl, W. L.; Garroway, A. N. J . M a p . Reson. 1961, 44, 381. (48) Szeverenyl, N. M.; Sulllvan, M. J.; Maciel, G. E., unpublished work. (49) Dlxon, T. W. J . Magn. Reson. 1981, 44, 220. (50) Tegenfeldt, J.; Haeberlen, U. J . Magn. Reson. 1979, 36, 453.
(37) Yokono, T.; Sanada, Y. Friel 1978, 5 7 , 334. (38) Yokono, T.; Miyazawa, K.; Sanada, Y.; Marsh, H. Fuel 1979, 5 8 , 896. (39) Qandy, D. W., Gulf Research and Development Co., private communlcatlon. (40) Retcofsky, H. L.: Stark, J. M.;Friedel, R. A. Anal. Chem. 1968, 40, 1899. (41) Retcofsky, H. L.; Hough, M. R.; Maguire, M. M.; Clarkson. R. B. "Advances in Chemistry Series"; Gorbaty, M. L., Ouchl, K.. Eds.; Amerlcan Chemical Society: Washington, DC, lg8l;No. 192, Chapter 4.
RECEIVEDfor review December 9, 1981. Acepted April 19, 1982. The authors are grateful for support of this work by the U.S.Department of Energy under Contract No. DEAT20-81LC10652 from the Laramie Energy Technology Center and Contract No. DE-AC22-79ET14940 from the Pittsburgh Energy Technology Center.
(28) Farrar, T. C.; Becker, E. D. "Pulse and Fourler Transform NMR"; Academic Press: New York, 1971; Chapter 2. (29) Waugh, J. S.; Huber, L. M.;Haeberlen, V. Phys. Rev. Lett. 1966, 20,
180.
-. .-... . .- .. - . - - -.
Chem 1077, R I , S A 5
Determination1 of Elements in National Bureau of Standards' Geological Standard Reference Materials by Neutron Activation Ana lysis Chrlstopher C. Graham, Mlchael D. Glaecock, James J. Carnl, James R. Vogt," and Thomas G. Spaldlng Research Reactor Facility, Un,iversityof Missouri, Columbia, Missouri 652 1 I
Instrumental neutron activcitlon analysts (INAA) and prompt gamma neutron activation slnalysis (PGNAA) have been used to determine elemental concentrations in twai recently issued National Bureau of Standards (NBS) Standard Reference Materlais (SRM's). The rersults obtained are in good agreement wlth the certlfled and lnformatlon values reported by NBS for those elements In each materlal for which comparisons are avallabie. Average concentratlons of 35 elements in SRM 278 obsidian rock and 32 elements in SRM 688 basalt rock are reported for comparison with results that may be obtained by other laboratories.
With the certification of SRM 1571 orchard leaves in 1972, the National Bureau of Standards began a program to produce Standard Reference Materials (SRM) for use in biological and environmental analytical chemistry. The purpose of this program is to provide mate:rials of known elemental composition in a wide variety of natural matrices for use in analytical development and quality a,ssurance ( I ) . The usefulness of these materials, however, is, often limited by the number of elementa that NBS is able to certify. Consequently, numerous investigators have, independently of NBS, published a large body of data on the elemental concentrations of the various SRMs, thus extending their usefulness to the scientific community. In 1981, NBS issued two new materials, SRM 688 basalt rock (2)and SRM 278 obsidian rock (3),both of which should prove to be extremely useful in applications involving geological materials. At the prtssent time, NBS has certified 14 elements in basalt rock and 1,B elements in obsidian rock. This
study is an attempt to extend the data on the elemental concentrations of these SRMs and is similar in nature to those studies conducted by Ondov e t al. ( 4 ) for SRM 1633 fly ash and by Germani et d.(5) for SRM 1632a bituminous coal and SRM 1635 subbituminous coal. The study presented here utilized different nuclear analysis methods and several analysts working independently. EXPERIMENTAL SECTION INAA. All samples were freeze-dried for 24 h and stored in a desiccator prior to irradiation. The details of the irradiation procedures varied according to the experiment; but, in general, two types of irradiations were performed. For the determination of Na, Mg, Al, Ca, Ti, V, Mn and K, samples of approximately 100 mg were encapsulated in high density polyethylene vials and irradiated at a flux of 1 x 1014neutrons cm-2 s-l for periods of 6 and 10 s,2 and 5 min, and 1h. For the determination of Na, Ca, As, Br, Ba, La, Nd, Sm, Lu, Au, U, Sc, Cr, Fe, Co, Ni, Zn, Rb, Zr, Sb, Cs, Ce, Eu, Tb, Yb, Hf, Ta, and Th, samples of approximately 200 mg were encapsulated in quartz vials and irradiated in an automatically rotated position at a typical flux of 5 x l O l S neutrons cm-2 s-l for periods varying from 30 to 235 h. Following irradiation, samples and standards were carefully transferred and weighed into nonradioactive polyvials so that it was not necessary to make a correction for the activity of the irradiation container. The well-known geological standard materials (4,6-10) NBS SRM 1633, NBS SRM 1633a,USGS 6 2 , and USGS BCR-1 were used as multielement comparator standards and were treated in exactly the same manner as the samples. The newer USGS rocks ( I I , 1 2 ) RGM-1 and QLO-1 were analyzed along with the samples and standards for quality control. The data from the samples irradiated for 6 and 10 s were taken at constant decay times of 10 and 13 min, respectively. The 2and 5-min irradiations were permitted to decay for 10 h and the 1-h irradiation was pemitted to decay for -3 days. Mea-
0003-2700/82/0354-1623$01.25/00 1982 American Chemical Society
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