Anal. Chem. 1990, 62, 2527-2532
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Noise Characteristics in Inductively Coupled Plasma Optical Emission Spectrometry Using Slurry Nebulization and Direct Powder Introduction Techniques Werner A. Van Borm' and Jos6 A. C. Broekaert* Institut fur Spektrochemie und angewandte Spektroskopie (ISAS),Bunsen-Kirchhoffstrasse 11,D-4600 Dortmund 1, Federal Republic of Germany
By use of Fourler analysls, the noise sources of Inductively coupled plasma optlcal emlsslon spectrometry ( ICP-OES) were examlned for two common types of torches, a Greenfield and a Fassel type torch. Furthermore, the dominant nolse types occurrlng by the nebullzatlon of solutions and suspenslons uslng a Bablngton (G.M.K.) nebulizer and by the direct lntroductlon of powders In the plasma were compared. Flrstly, R was found that In contrast to a Fassel type torch a GreenfleM type torch exhlbits no discrete audb frequency (af) nolse band In the 150-400 Hz region. The noise types, Including af, whlte, fllcker ( W), and pumplng noise were examlned as a function of rf power, outer gas flow rate, observatlon height, and pumplng rate. Upon the Introduction of solld partlcles In the plasma, elther as slurries or as a dry powder, a small but slgnlflcant Increase (about 5 % ) of the af noise band frequency was observed. The preclslon for the analysls of suspenslons (1.9 % ) was found to be somewhat lower than for solutions (1.4%), whlch may result from a small Increase In flicker noise.
INTRODUCTION Inductively coupled plasma optical emission spectrometry (ICP-OES) using slurry nebulization as a direct sample introduction technique has recently become quite popular for the analysis of a variety of materials such as pulverized biological tissues, geological material, and also ceramic powders ( 1 ) . In all cases, the analytical gain lies in the ease of sample preparation, being relatively fast and contamination-free as compared to methods requiring sample dissolution. Apart from analytical applications, studies concerned with slurry nebulization have concentrated on the evaporation behavior of solid aerosol particles in the hot plasma (2). In this respect, the particle size has been found to be the most critical parameter for the success of the method, other related parameters being the particle residence time in the plasma, sample throughput, thermochemical properties of the sample matrix, and plasma temperature. The noise properties of an analytical method determine the quality of the signal, and one usually aims at reducing the noise components such that optimum values for the precision and the power of detection can be obtained. In order to optimize both, the origin and the type of the dominant noise must be determined. Other than with solutions, the noise of the ICP-OES when applied to slurries has hardly been investigated as a function of the instrumental operating parameters. Indeed, the noise characteristics of ICP-OES for solutions have already been discussed in a number of studies (3-11). Three major types of noise have been identified in ICP-OES, in relation to their frequency and
* Author to whom correspondence should be addressed.
'Present address: DSM Research, Division FA-AE, P.O. Box 18, NL-6160 MD Geleen, The Netherlands. 0003-2700/90/0362-2527$02.50/0
intensity range, i.e. white noise (random fundamental), flicker noise (random nonfundamental), and discrete noise (systematic nonfundamental). White noise is due to completely random variations in intensity, giving rise to a smooth background in the noise spectrum, whereas flicker noise (l/f noise) is related to slow fluctuations, e.g. in the plasma power, detection system, or sample introduction system, and only shows up at low frequencies. Discrete noise sources such as whistle noise, audio frequency noise, or line frequency noise are found at discrete frequencies and have distinct sources (such as the use of 50 Hz ac power). The audio frequency noise type especially has been given much attention in ICP-OES; its sources are up to now not completely understood (9). This study focuses on the noise characteristics of ICP-OES when introducing particulate solids either as slurries or as a dry powder aerosol, i.e. without the usual water matrix in comparison to the introduction of solutions. Fourier analysis was used to detect and determine the noise components in the frequency region of 0.1-500 Hz, for this region contributes the most to the total observed noise production. As in work on slurry nebulization of ceramic powders, it was found that the analytical precision obtained was slightly inferior to that of ICP-OES of the samples subsequent to dissolution (12). We decided to evaluate the noise pattern of ICP-OES using slurry nebulization with the aid of a Babington nebulizer in detail. For this aim a measurement system was developed and commercial software for rapid Fourier transforms was used. The study included the use of an ICP torch according to Fassel et al. (13) as well as a torch according to Greenfield et al. (14). Also, the influence of the working conditions on the noise power spectra (NPS) and its spectral features has been studied.
EXPERIMENTAL SECTION ICP System. In this work, a free-running generator (FS-10 Linn, supplied by Kontron GmbH) operating at 27.12 MHz was used to generate an ICP discharge in Greenfield and Fassel torches. In the Greenfield type torch, the ICP was operated at an output power of 2.2 kW, using Ar as outer gas (25 L/min) and as intermediate gas (8.2 L/min). The smaller torch according to Fassel was operated at 1.8 kW and with Ar as outer gas (18.1 L/min) only. For the measurements of the line intensities a dual-channel 0.9-m Czerny-Turner monochromator was used, which is EPROM-controlled by a SBC 88/25 Intel microcomputer and linked to a standard PC-XT using communication software (15). Signals could therefore be directed either to the PC-XT or to a chart recorder (Linseis LS44) with a response time of about 0.2 s. An overview of the experimental setup is shown in Figure 1.
Sample Introduction Systems. Aerosols were produced from slurries and solutions by using a commercially available Babington nebulizer (G.M.K., Labtest). A peristaltic pump (Gilson Minipuls 2) is required for sample uptake. The nebulizer was operated with argon gas at a backpressure of 3 bar. When an a e m l gas capillary with an internal diameter of about 280 pm was used, an aerosol gas flow rate of 0.78 L/min was obtained. The sample uptake rate was selected at 1 mL/min. Slurries were prepared from an 0 1990 American Chemical Society
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powder (AKP30. Sumpowder analysis as obtained wim automated electron probe microanalysis (EPMA) (16). Rom. Japan). used lor slurry and dry
AI,03 powder (AKP3O. Sumitomo, Japan), which was suspended in distilled water to a concentration of 10000 pg/mL AI. The latter is normally the analyte concentration used for trace element analysis of ceramic powders hy ICP-OES. The particle size distribution of the powder was determined hy automated electron prohe microanalysis (EPMA) (16).It was found that the mean particle diameter (on a number basis) equaled 0.3 pm and that no particles larger than 16 pm were carried into the plasma. Accordingly, for the AI,03 matrix, incomplete evaporation at this particle size was not to be expected as known from calculations published earlier (2). The resulting number and mass size distributions as they were determined hy EPMA of analyte particles collected on a Nucleopore filter are displayed in Figure 2. Solutions containing the same ancentrations of AI as the AI,O, slurries were prepared either by dissolving AICI, in a 0.1 M HCI solution or from a 10000 pg/mL AI Titrisol standard (Merck, Darmstadt). Dry powder aerosols were produced with the aid of a rotating brush aerosol generator (Palas GmbH, Karlsruhe, FRG). Here, a constant stream of fine particulates, suspended in an argon gas flow. is generated. The device consists of a brush and a stainless steel hollow cylinder in which the compacted powder is contained. A pressurized piston under computer control slowly pushes the powder column upward in the cylinder. At the upper end of the column, small amounts of powder are continuously rubbed off by the rotating brush, transported upward, and dispersed in an argon gas flow, which is then fed intn the torch (Figure 3). The device is commercially available and its usage, which is well mainly lies in inhalation experiments, the dedocumented (f7), termination of filter efficiencies, and the calibration of mass
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(Babington nebulizer) and dry powders (rotating brush generator). concentration measuring instruments. To comply with the low argon flow tolerated in analytical use of the ICP. the gas flow rate was selected a t 1.3 L/min. The rate of powder removal is thus proportional to the product of the surface area of the powder column and the feed rate of the piston and is rather independent of the properties of the powder. In our experiments however, the Also3 powder used had to he mixed with Aerosil 380 (Degussa, FRG), being a finely divided SiO, powder (mean particle diameter = 7 nm) so as to prevent agglomeration. Noise Analysis System. In the normal operating mode of the ICP spectrometer, the anode current of the photomultiplier tuhe (EM1 9789 QB)is fed into the preamplification and integration circuit with a maximum integration frequency of 18 Hz,
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Flgure 5. Series of strong 100-Hz harmonics, produced by a modulation of the rf power: sample, 10 000 pg/mL AI solution; power, 2.2 kW; outer gas, 25 L/min Ai; intermediate gas, 8.2 L/min Ar; nebulizer pressure, 3 bar Ar; sample uptake rate, 1 mL/min.
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-60 0 6 12 ” f[Hz1 24 Flgure 4. Noise power spectra obtained with a Greenfield type torch: (a) (0-500 Hz) and (b) (0-24 Hz); sample 10 000 pglmL AI solution; power, 2.2 kW; outer gas, 25 L/min Ar; intermediate gas, 8.2 L/min Ar; nebulizer pressure, 3 bar Ar; sample uptake rate, 1 mL/min.
tolerating a 4% standard deviation. With minor modifications of the RC circuitry, incoming signals could be converted into bipolar signals ranging from -5 to +5 V with a -3 dB roll-off at 5 kHz (where dB = 20 log,, ( V / V,,)). The resulting signal was subsequently fed into a fast Fourier transform (FFT) board (STAC, Diisseldorf), mounted in a PC-XT. The board is equipped with a 12-bit analog/digital (A/D) converter with a 25-ps conversion time and an 8-MHz NEC uPD 7720 signal processor. By use of 1024 sample points, a maximum signal to noise (S/N) ratio of 80 dB can be achieved. Adapted software enabled the on-line display of the noise power spectra (NF’S). The processor sampling frequency could be varied from 48 Hz to 25.6 kHz, but was, for our purposes, set at 1500 Hz and 150 Hz for high- and low-frequency spectra, respectively. These settings provide a useful frequency range of 0 to 500 Hz and 0 to 24 Hz without aliasing problems. All spectra reported were averaged over 10 individual spectra. Regarding the frequency range aimed at, the time constant of the measurement circuit was decreased to 0.2 ms, for which R values of 100 MR and capacitancesof 47 pF are required. In this case, it was found that the signal-to-noise ratios obtained allowed measurement of the analyte line intensities which gave rise to anode currents in the range 0.005-5 pA, but not, however, the background intensities of an analytical ICP, which are in the case of the spectrometer used at the order of 0.5 nA. In all NPS described below the intensity of the AI I 396.1-nmline was selected as the analytical signal. Precisions stated were determined independent of the recording of the noise spectra, from six repetitive measurements of the line intensity for the same sample; the uncertainty on the precision was estimated to be 0.1%.
RESULTS AND DISCUSSION Comparison of Noise Generated in Torches According to Greenfield and Fassel. First, the occurrence of specific audio frequency (af)noise in the 200-Hz region, as known from the literature, was studied. When a Greenfield type torch was used, however, the high-frequency NPS showed a relative flat, smooth background of white noise in the 125-500 Hz region (Figure 4a). The absence of af noise in this region had previously only be reported for extended torches according to Fassel of both a laminar and tangential type and was therefore most surprising. The discrete peaks at 100 and 300
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Figure 6. Noise power spectra obtained with a Fassel type torch: (a) (0-500 Hz) and (b) (0-24 Hz); sample, 10000 pg/mL AI solution; power, 1.8 kW; outer gas, 18.1 L/min Ar; nebulizer pressure, 3 bar Ar; sample uptake rate, 1 mL/min.
Hz are typical for the generator used. They stem from a modulation (about 15%) of the anode current and the corresponding rf power. Indeed, it has been found that a strong modulation produces a series of 100-Hz harmonics with decreasing intensity (Figure 5). They can be generated by detuning the thyristor box of the generator. Apart from additional coupling phenomena that could enhance other possible noise bands, this type of spurious noise decreases the precision and the power of detection. Similar observations were made by Winge et al. (9),which stresses the importance of using Fourier noise analysis so as to detect instrumental noise sources. A broad-band type of noise occurred in the lower part of the spectrum with a maximum of 33 Hz. Also, a number of discrete noise peaks are found in the low frequency spectrum (Figure 4b). Their origin could not be traced within the experiments performed. The high-frequency NPS obtained with a torch accordulg to Fassel showed the expected broad af noise band in the 200-300 Hz region, as observed and reported in the literature (Figure 6a). On the other hand, no discrete noise peaks were observed in the lower portion of the spectrum (Figure 6b). This could be confirmed by the
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Figure 7. Pumping noise, in the 0-2 Hz region: GreenfieM type torch with a 10 000 pg/mL AI solution; power, 2.2 kW; outer gas, 25 Llmin Ar; intermediate gas, 8.2 L/mln Ar; nebuilzer pressure, 3 bar Ar; sample uptake rate, (a) 1 mL/min and (b) 2 mL/min. visual observation of the plasma generated in a Fassel torch, which seems to sustain a more stable plasma than a Greenfield torch, also without any sample being introduced. Noise produced by the peristaltic pump is confined to the 0-2 Hz region (Figure 7). It could be univocally identified by varying the pump rotation speed. Indeed, noise peaks at 0.2 and 0.4 Hz, observed by a sample uptake rate of 1 mL/min, doubled when doubling the pumping rate. The 0.2-Hz peak corresponds to one revolution of the main roller of the pump, the 0.4 Hz peak to half a revolution. The pulsation of the peristaltic pump is induced by the rotation of 10 smaller rollers, which gives rise to the noise peak observed at 2 Hz. The discrete peaks at higher frequencies thus are not directly related to pump rotations. Noise, induced by the pump, could be minimized by carefully stretching the sample tube against the rollers. The pumping noise was visible on a chart recorder as its recording speed is higher than the pump noise frequency (see insert in Figure 7 ) . af Noise Band Features. af noise frequencies are believed to stem from axially symmetrical plasma oscillations (topdown) of the intermediate gas as the intermediate gases flow from the torch into the surrounding static atmosphere (9) or from a rotating asymmetrical plasma, where the rotation is induced by the tangential flow of the intermediate gas in the torch (4). In both cases, a marked influence of the outer gas flow rate and rf power on the af noise band should be observed. When the outer gas flow rate is raised from 11.4 to 23 L/min, the af noise band subsequently shifts to higher frequencies, passes through a maximum at about 239 Hz for a flow rate of 18 L/min, and then shifts back to lower frequencies (Figure 8). The same behavior was already observed by Belchamber et al. (4). However, a closer look reveals some other interesting features. The white noise intensity also increases with increasing outer gas flow rate until it passes through a maximum a t 18 L/min. Due to the various noise contributions, the 50-Hz line frequency is only observable at low flow rates and disappears into the increasing white noise background at higher flow rates. The discrete 150-Hz peak can be due to coupling between the 100-Hz rf ripple and the line frequency as its intensity behaves much as this of the line
Figure 9. Effect of rf power on the NPS spectra: Fassel type torch: sample, 10000 pg/mL AI solution; outer gas, 18.1 L/min Ar; nebulizer pressure, 3 bar Ar; sample uptake rate, 1 mL/min. frequency. In several NPS, broad af noise bands a t lower frequencies could be observed. These frequency bands are in the same manner influenced by changes of the outer gas flow as the main af-noise band. This is illustrated for NPS recorded at 15.8, 18.1, and 20.5 L/min outer gas flow rates, respectively. Additional coupling of the 100-Hz peak may obscure the latter observations. Winge et al. (9) have explained the presence of such secondary af-noise peaks by the assumption that the plasma fluctuations need not to be restricted to a simple sinusoidal waveform and by the fact that especially a t higher gas flow rates and rf powers, they could involve more vorticity and therefore behave less sinusoidal. Irrespective of the type of plasma fluctuations, our measurements seem to confirm these hypothesis, as multiple bands were often observed with our ICP-OES system. The effect of the power on the intensity and frequency of the af-noise band is illustrated in Figure 9. For the main noise band an almost linear relationship between power and frequency can be observed. However, at certain powers a second band at a lower frequency was observed, e.g. the 236-Hz band a t 2.0 kW and the 158-Hz band a t 2.4 kW. Curious is also that the intensity of the 300-Hz peak, which is assumed to have an instrumental origin, increases with power, whereas the intensities of the 50- and 100-Hz frequencies remain constant. A strong coupling was observed at 2.2 kW when the af-noise band coincided with the intense 300-Hz peak, resulting in the appearance of a series of 5 0 - H ~harmonics. The influence of the observation height on the noise characteristics is illustrated in Figure 10. Although the afnoise band does not shift when changing the observation height, the amount of white noise increases about 10 dB. Also, a growth in amplitude of the harmonics, relative to the
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Flgure 10. Effect of observation height on the NPS spectra: Fassel type torch: sample. 10000 pglmL AI solution: power. 1.8 kW: outer gas, 18.1 Llmin Ar: nebulizer pressure, 3 bar Ar: sample uptake rate, 1 mLlmin.
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Coolant gas rate (Ipm) Flgure 13. Effectof outer gas rate on the audio frequency band for different sample types: Fassei type t u c k solution, 10000 pglmL Al: suspension, 10000 pglg Ai (AKPSO. Sumitomo. Japan): power, 1.8 kW; nebulizer pressure. 3 bar Ar: sample uptake rate, 1 mLlmin.
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Flgure 11. Nolse characteristics and precision of the ICP signal for the introduction of (a) a 10 000 pglmL Ai solution, (b) a 10 000 pglg Ai suspension. and (c) the dry powder (Ai,O,, AKP30. Sumitomo. Japan). Fassel type torch with outer gas 18.1 Llmin Ar: (a) power 1.75 kW. nebulizer pressure 3 bar Ar, sample uptake rate 1 mL1min: (b) power 1.70 kW. nebulizer pressure 3 bar Ar, sample uptake rate 1 mL1min: (c) power 1.80 kW. aerosol gas flow 1.3 Llmin. background, is observed. The same influence of the parameters mentioned was observed, irrespective of the sample type. Comparison of the Different Types of Sampling. For these measurements a tangential flow torch according to Fassel was used. Both the direct A1 signal and relative standard deviations were measured. The analyte was introduced either
as solution, as slurry, or as a dry powder. For all three cases nearly the same NPS were obtained, i.e. with the typical af noise hand in the 2W400 Hz region and the peaks a t 100 and 300 Hz, due to the ripple in the rf power (Figure 11). The slightly inferior precision for slurry nebulization could well stem from a somewhat higher l/f noise, which was attributed to a frequency smaller than 5 Hz, by which the precision decreased from 1.4% to 1.9%. The excess noise was most clearly visible when the signal was routed to the chart recorder (see inserts in Figure 11). The pump noise was found to he not significantly different. The introduction of the powder without water matrix led to RSDs of 7.5%, being attributed to low frequency (5 Hz) was observed as low as for solutions or suspensions. The precision of this method, however, appears favorable when compared to that obtained with other direct powder introduction systems, which range from 12 to 15% (18-20). The type of sampling apparently also influences the hehavior of the af noise hand. As small changes in the instrumental settings markedly influenced the af noise frequency, spectra for solutions and slurries were recorded as pairs, e.g. immediately after each other. As such, it was noted that with slurries the af noise hand often shifted hy 5-15 Hz to higher frequencies as the rf power was raised (Figure 12). A consistent behavior was seen when the outer gas rate was increased (Figure 13). Thus i t appears that the sample introduction system influences the gas flow dynamics of the plasma a t the c h m e n observation height. To find out whether
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Anal. Chem. 1990,62,2532-2536
this shift was due to the 1% lower water mass loading or to the presence of solid particles in the sample, an experiment was devised in which both the dust generator, operating at 1.3 L/min argon gas, and the Babington nebulizer, operating at 0.78 L/min argon gas, were connected over a Y-joint to the aerosol injector tube, thus obtaining about 2 L/min of total argon gas flow. The introduction of the dry powder led to a NPS with a af noise band at 260 Hz. Upon addition of a water aerosol (about 13 pL/L, assuming a 1% transport efficiency) via the nebulizer to the powder aerosol the peak did not shift significantly; however, when an A1 solution was nebulized, the af noise band was observed to have shifted about 10 Hz to lower frequencies. Thus, apparently, only the introduction of particles in the plasma induces a small increase in the af noise frequency. A further explanation of this phenomenon, however, could not be found.
CONCLUSIONS The use of slurries in ICP-OES is not hampered by major noise sources, different from those occurring when solutions are nebulized. Though small differences in the noise level have been observed when slurries are nebulized and a precision was obtained that is slightly smaller than that for solutions, it can be concluded that, for powders containing submicrometer particles, the slurry ICP-OES technique offers a promising approach for the analysis of powders.
ACKNOWLEDGMENT The authors thank R. Niessner of the Technical University of Munich for the loan of the rotating brush aerosol generator.
LITERATURE CITED (1) Broekaert, J. A. C.; Graule, T.; Jenett, H.; Tolg, G.; Tschopel, P. Fresenius' 2.Anal. Chem. 1989,332,825-838.
Raeymaekers, B.; Graule. T.; Broekaert, J. A. C.; Adams. F.; Tschopel, P. Specfrochlm. Acta, Part 8 1988, 438, 923-940. Walden, G. L.; Bower, J. N.; Nkdel, S.; W o n , D. 1.; Wlnefordner, J. D. Specfrochhn. Acta, Part 8 1980, 358, 535-546. Belchamber, R. M.; Horllck, G. Specfrochlm. Acta, Part8 1982,378, 17-27.
Benettl, P.; Bonelll, A.; Cambiaghl, M.; Frigleri, P. Spectrochim. Acta, Part 8 1982,378,1047-1053. Davies, J.; Snook, R. D. J. Anal. At. Spectrom. 1987, 2, 27-31. Montaser, A.; Clifford, A. H.; Sinex, S. A,; Capar, S. G. J. Anal. At. Spectrom. 1989, 4 , 499-503. Ingle, J. D., Jr.; Crouch, S. R. Specfrochemical Analysis; PrenticeHall: London, 1988; Chapter 5. Winge, R. K.; Eckels, D. E.; DeKalb, E. L.; Fassel, V. A. J. Anal. At. Spectrom. 1988, 3,849-855. Olesik. J. W.; Smith, L. J.; Williamsen, E. J. Anal. Chem. 1989, 67, 2002-2008.
Montaser, A.; Ishil, I.; Tan, H.; Clifford, R. H.; Golightly, D. W. Spectrochim. Acta, Part8 1989, 4 4 8 , 1163-1169. Graule, T.; von Bohlen, A.; Broekaert, J. A. C.; Grallath, E.; Klockenkamper, R.; Tschopel, P.; Tolg, G. Fresenlus' Z . Anal. Chem.
1989,335,637-642. Wendt, R. H.; Fassel, V. A. Anal. Chem. 1965, 37,920-922. Oteenfield, S.;Jones, I.LI.; Berry, C. T. Analyst 1964, 89, 713-720. Broekaert, J. A. C.; Hagenah, W.4.; Laqua. K.; Leis, F.; Stuwer, D. Specfrochim. Acta, Part 8 1986, 4 7 8 , 1357-1365. Van Bot", W. A.; Adams, F. C. Anal. Chim. Acta 1989, 278, 185-2 15.
Vereln Deutscher Ingenieure Genefatlng of test aerosols wlth a rotating brush generator, VDI-Richtlinlen 3491, part 9; Beuth Verlag GmbH: Berlin, 1989. Salin, E. D.; Horllck, G. Anal. Chem. 1979, 57, 2284-2285. Scott, R. H. Specfrochim. Acta, Part 8 1978,338,123-124. Ng, K. C.; Zerezghl, M.; Caruso, J. A. Anal. Chem. 1984, 56, 417-421,
RECEIVED for review April 19,1990. Accepted August 17,1990. This work has been supported by the "Ministerium fur Wissenschaft und Forschung des Landes Nordrhein Westfalen", by the "Bundesministerium fur Forschung und Technologie", Bonn, and by the "Deutsche Forschungsgemeinschaft (DFG)".
Effects of Paramagnetic Lanthanides on the Study of Carbonaceous Deposits on Zeolite Catalysts by Carbon- 13 Solid-state Nuclear Magnetic Resonance Spectroscopy Eric J. Munson and James F.Haw*
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Relaxation and rpln-countlng measurements have been used to probe the extent to which lac NMR spectroscopy wlth crow polarization and maglc-angk sph#ting is a useful technique for characterizing carbonaceous (coke) deposits In zeoilte catalysts containing rare-earth cations. Such ions were antldpated to complicate this analysis as a result of paramagnetk shifts and/or rdaxatlon effects. Contrary to expectatlono, no paramagnetic shHts were observed for carbonaceous deposits in the rare-earth-exchanged Y-type zeoHtes examined. Even more surprtdng was thal the mixture of rare earths used in InduWal catalysts has neg4gWe effects upon the relaxation processes central to cross pdarlzatlon. Small concentrationsof Ianthankles wlth long electron spinlattice relaxation times (in particular gadolinium), however, have a deleterious effect on characterizing carbonaceous deposlts by decreasing the proton rotating-frame spin-iattlce relaxation time.
* To whom correspondence should be addressed. 0003-2700/90/0362-2532$02.50/0
INTRODUCTION Zeolite catalysts are used in the petroleum industry for the catalytic cracking of crude oil into gasoline-range hydrocarbons (1,Z). A widely used industrial cracking catalyst is the synthetic zeolite Y, but a problem with this zeolite is structural stability a t high temperatures ( 3 , 4 ) . To improve stability, rare-earth (lanthanide) cations are often ion-exchanged for sodium or ammonium originally present in the zeolite. Such rare-earth-exchanged zeolites are much more stable at high temperatures. A significant problem in the operation of any type of cracking plant is the formation of carbonaceous deposits (termed coke) on the catalyat (5,6). These deposits eventually deactivate the catalyst and require removal of the coke before the catalyst can be reused. One approach to the study of coke deposits is the use of 13C NMR spectroscopy in conjunction with various line-narrowing techniques. Derouane and coworkers ( 7 ) used solid-state NMR spectroscopy to investigate the carbonaceous deposits formed on zeolites HZSM-5and H-Mordenite by small olefins and alcohols. 0 1990 American Chemical Soclety
