JOURNAL OF MAGNETIC RESONANCE, Series A 123, 116–118 (1996)ARTICLE NO. 0221

Z Filtering in MQMAS NMR

JEAN-PAUL AMOUREUX,* CHRISTIAN FERNANDEZ,* AND STEFAN STEUERNAGEL†

*Laboratoire de Dynamique et Structure des Materiaux Moleculaires, CNRS URA801, F-59655 Villeneuve d’ascq, France; and†Bruker Analytische Messtechnik GmbH, Silberstreifen, D-76287 Rheinstetten, Germany

Received July 29, 1996

Two desired improvements to MQMAS experiments are general, MQMAS signal-to-noise (S /N) ratios are rathersmall, especially for high-order multiple-quantum coher-currently the decrease of the dispersion signal in order to

enhance the resolution and a high efficiency. Both goals can ences. Therefore, among the desired developments ofMQMAS techniques, it is important to obtain a good effi-be reached simultaneously by adapting z-filter principles to

MQMAS. ciency and to decrease dispersion signals in order to enhancethe resolution. These two goals can be achieved using z-NMR is a very powerful tool for structural analyses of

solids. Indeed, this method allows a detailed observation of filter principles (10) . Using the usual two-pulse sequence,most of the individual nuclei in a sample. When nuclei have pure absorption spectra can be observed if both symmetricala spin value I Å 1

2, the MAS technique allows an easy high- coherences ({pQ) are simultaneously selected and if theirtransfers toward the observable signal (01Q) have the sameresolution analysis of all different species. However, nucleiefficiencies. However, both these coherence-transfer path-with higher spin values are subjected to the quadrupole inter-ways (0, {p , 01) involve different jumps between the co-action resulting from the coupling between their quadrupole

moment and the electric field gradients. This interaction is herence levels. Therefore, generally they do not have thesame efficiencies. In order to approximate this requirement,very sensitive to the local atomic surroundings and can be

used as an effective microscopic probe. For example, it has and then to produce an amplitude-modulated (sine) signalduring t1 , one must carefully optimize the flip angle and RF-often been used in single crystals to analyze the changes

resulting from second-order or even incommensurate phase field amplitude of the second pulse.We have recently shown that equal efficiencies can betransitions (1) .

However, in powdered samples the quadrupolar interac- obtained at the first order in 3QMAS, either simultaneouslyfor all crystallites in the case of I Å 3

2 nuclei or only as ation often results in severe line broadening which prohibitsthe observation of chemically inequivalent sites. In most powder average for all other nuclei (11) . This implies thatcases, only the {1

2 central transition remains observable in dispersion components and rephasing problems remain gen-erally small in 3QMAS. The equalization of efficiencies can-the case of half-integer quadrupolar spins. Although the

width of the powder spectrum for the central transition is not be reached for higher-order MQ coherences so that dis-persion components always appear for 5, 7, and 9Q MASinversely proportional to the magnetic field, application of

high-field spectrometers rarely offers sufficient improvement experiments. A solution to this problem is possible by thesymmetrization of both coherence pathways using z-filterof resolution, even when MAS is applied to reduce the broad-

ening. principles. A similar approach has been used for severalyears in DAS experiments (12) . Since the spin system ini-Two methods (DOR and DAS) have been proposed for

the observation of high-resolution spectra of these nuclei tially lies along Oz, it must return there before observation.To achieve this, we can use a sequence of three RF pulses(2, 3) . Unfortunately, both methods suffer from crucial tech-

nological limitations. For this reason, Frydman and co-work- in order to select symmetrical coherence-transfer pathways:(0, {p , 0, 01). Another z-filter sequence has very recentlyers have proposed (4, 5) an elegant two-dimensional

MQMAS method allowing an easy correlation between iso- been proposed for MQMAS (13) . However, this sequencehas two drawbacks: ( i) it uses four pulses instead of threetropic and anisotropic parts of interactions. The initial

MQMAS method was later improved (6) by designing a and (ii) the RF pulses were not optimized, leading to a poorS /N ratio. In our method, the first pulse specifications aresimpler but more efficient two-pulse sequence, and it has

also been extended to higher MQ orders. Moreover, it has similar to those of the two-pulse MQMAS method (11) asthey are designed to create the maximum amount of {p-been shown that the simultaneous use of both mirror-coher-

ence-transfer pathways and an optimal setting of the second quantum symmetrical coherences. Both coherences evolveduring t1 and then must be optimally transferred along Oz bypulse facilitates a decrease of dispersion signals (7–9) . In

1161064-1858/96 $18.00Copyright q 1996 by Academic Press, Inc.All rights of reproduction in any form reserved.

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FIG. 1. The t1 rotor-synchronized z-filter 3QMAS 27Al spectrum of AlPO-14. Rotor speed Å 14.7 kHz, Larmor frequency Å 104.2 MHz, delaybetween the two last pulses Å 10 ms. The pulse specifications are as follows: first pulse: nRF Å 200 kHz, T Å 2 ms; second pulse: nRF Å 200 kHz, T Å0.7 ms; third pulse: nRF Å 11 kHz, T Å 7.5 ms. Number of scans for every t1 step Å 72. Dt1 Å 68 ms. Number of t1 increments Å 256. Recycling delayÅ 0.6 s. Full acquisition time Å 3 h.

the second pulse. The first two pulses must use the strongest sulting efficiencies are similar to that of the classical two-pulse MQMAS method. As expected, however, the z-filteravailable RF field (typically 100–200 kHz) in order to max-

imize excitation and conversion efficiencies (11) . By MQMAS method yields better results concerning the disper-sion components, because both symmetrical pathways (0,applying conventional phase cycling (or using an additional

delay in order to kill transverse magnetizations) , only popu- {p , 0, 01) now have exactly the same efficiencies regard-less of the spin and the order of the selected coherences.lations of {1

2 Zeeman levels are not negligible in the densityAlthough second-order quadrupole interactions may intro-matrix after the second pulse and the z filter. The last pulseduce additional dispersive signals, they are generally verytransforms both these populations into an observable signal.small. Moreover, the pulse optimization is easier than withTo be efficient, this last pulse must be a selective 907 pulsethe previous method, especially for high-order MQ coher-using a moderate RF field.ences. As the signal is amplitude modulated (cosine) as aWe have calculated the optimal efficiency of this methodfunction of t1 , it is possible to use the first spectrum ( t1 Åas a function of the quadrupole interaction and RF-field ratio.

For nearly all spin and multiple-quantum values, the re- 0) to optimize independently the lengths of the three pulses.

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118 COMMUNICATIONS

In addition, any missetting of these pulses is much less criti- MQMAS method has the advantage that the dispersive com-ponents in the 2D spectrum are completely canceled withoutcal than with the two-pulse method since efficiencies of both

pathways always remain equal. any pulse setting. This z-filter method is therefore more ro-bust and more efficient than the two-pulse method. AlthoughTo check the experimental validity of this method, we

have analyzed the 27Al 3QMAS spectrum of AlPO-14, a the z-filter method can always be recommended to avoidmixed-phase spectra, it should be used especially in twomicroporous aluminophosphate. This compound has already

been studied by two-pulse 3Q and 5QMAS experiments particular cases where dispersion signals are important:when the RF field is small or moderate (about õ100 kHz)(14) . Five aluminum species were observed: two tetrahedral

(S1, S2), one pentavalent (S3), one hexa-coordinated (S5), as in narrow-bore magnets and/or when using high-order(ú3) multiple-quantum methods. Another future applicationand an amorphous impurity (S4). A general characteristic

of MQMAS experiments is the sideband multiplicity along of this z-filter method could be in chemical-exchange analy-ses, using the delay between the last two pulses to obtainthe MQ dimension F1 . On the contrary, their presence along

F2 is rare. An important problem related to these numerous mixing processes. Nevertheless, it must be kept in mind thatthis method, as all other MQMAS methods, is not directlysidebands is the corresponding long experiment time. In-

deed, one cannot use a digital filter in the F1 dimension in quantitative. Experimental results must be analyzed usingsimulation software, taking into account all experimentalorder to avoid foldings. Therefore one must use a large F1

spectral width and hence many small t1 steps. When the specifications to give correct species concentrations (14) .resonance spread along F1 of all species is smaller than the

ACKNOWLEDGMENTSspinning speed, the use of a t1 step equal to the rotor periodallows one to avoid this problem (15, 16) .

We thank Professor M. Pruski and Dr. J. Hanna for fruitful discussionsThis synchronization has three advantages: ( i) the re-and Bruker-Spectrospin for technical assistance.

cording time of the 2D spectrum is decreased as the t1 dwelltime is longer due to a very small spectral width, ( ii ) the REFERENCESspectrum is composed of only a few centerbands whose

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