Solid State Nuclear Magnetic Resonance 26 (2004) 16–21

A simple, inexpensive, and precise magic angle spinningspeed controller

Eric Hughesa and Terry Gullionb,�

aNestle Research Center, P.O. Box 44, CH-1000 Lausanne 26, SwitzerlandbDepartment of Chemistry, West Virginia University, Clark Hall P.O. Box 6045, Morgantown, WV 26506, USA

Received June 20, 2003; revised June 20, 2003

Abstract

Certain magic-angle spinning heteronuclear dipolar recoupling experiments using rotor-synchronized pulse trains require very

precise control of the sample-spinning rate. An inexpensive spinning speed controller for use in magic-angle solid-state NMR

experiments is described which can control the spinning rate to within70.2Hz. The apparatus is based on a simple micro-controller

and is self-contained. Experimental results are presented that show the importance of good spinning speed control.

r 2003 Elsevier Inc. All rights reserved.

Keywords: MAS NMR; REDOR; REAPDOR; Controller

1. Introduction

Magic angle spinning (MAS) provides high-resolutionNMR spectra of rare spin-1/2 nuclei in solids. However,important structural information may be lost from thespectrum since MAS spatially averages the dipolarinteraction to zero [1]. Solid-state NMR experimentshave been developed to recouple the heteronuclear andhomonuclear dipolar interaction under magic anglespinning conditions to provide distance informationbetween specific nuclei. These NMR experiments havebeen applied to various systems ranging from inorganicglasses to membrane bound proteins and have providedunique structural information difficult to obtain byother analytical methods [2–9]. The majority of dipolarrecoupling experiments apply a pulse train synchronizedto the sample rotation. Heteronuclear dipolar couplingsbetween spin-1/2 nuclei can be measured by REDOR[10,11], and heteronuclear dipolar recoupling has beenextended to quadrupolar nuclei through experimentssuch as REAPDOR [12–14], TRAPDOR [15], andMQ-MAS REDOR [16].The successful implementation of REAPDOR and

some versions of REDOR [17] require precise controlof the sample-spinning rate. For example, in the

REAPDOR experiment the rotor-synchronized pulsetrain used to recouple the dipolar interaction betweenthe observed spin-1/2 nucleus and the quadrupolarnucleus does not refocus the chemical shift anisotropy(CSA) of the observed spins until the end of the dipolarevolution period [18]. Consequently, small variations inthe spinning speed may disrupt the refocusing of theCSA and produce phase distortions in the spinningsidebands and considerable reduction in signal intensity.In this paper, a simple stand-alone magic-angle

spinning rate controller is described that can keep awell-balanced sample rotor spinning to within 0.2Hz ofthe desired sample speed. At the heart of the apparatusis an inexpensive 8-bit micro-controller. A micro-controller was chosen to keep the design as simple aspossible and to leave the controller open to improve-ment through software modifications. 13C MAS NMRspectra obtained with this controller will be shown thatillustrates the substantial benefits of accurate spinningspeed control.

2. Experimental setup

It is routine and sufficient for most MAS NMRexperiments to stabilize the rotor spinning speed towithin 72Hz of the desired set point, and electronic

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�Corresponding author. Fax: +1-304-293-4904.

E-mail address: terry.gullion@mail.wvu.edu (T. Gullion).

0926-2040/$ - see front matter r 2003 Elsevier Inc. All rights reserved.

doi:10.1016/S0926-2040(03)00063-8

circuits for spinning speed controllers have beendescribed [19,20]. However, some MAS NMR experi-ments require more rigorous control of the spinningspeed. We measure the spinning rate in a simple fashion.Marks are painted on the sample rotor. An optical fibershines light on the rotor and the passing marks aredetected by another optical fiber and counted forone second to determine the spinning rate of the sample.The sample-spinning rate is then compared to the setvalue, and a feedback circuit actively controls anelectrically driven gas valve to maintain the desiredspinning rate. In typical operation of the systemdescribed below, five marks are used to provide controlof the spinning rate to within 70.2Hz. For the resultspresented in this paper, ten evenly spaced marks werepainted on the rotor to give a precision of70.1Hz. Theelectronic circuitry is provided below in detail, but first adescription of the modifications to the air supply thatreduce pressure surges affecting sample spinning isdescribed.A typical reciprocating compressor will maintain the

air pressure between two set points. When the high setpoint is reached, the compressor turns off. During thistime the air pressure slowly drops to the low set point.When the low set point is reached, the compressor turnson. This sudden switching on of the compressor sends asmall pressure wave down the air line, which in turncauses a sample rotor to increase its spinning rate by asmall amount (by about 1Hz or so). We have found thata series of ballast tanks connected in series, with eachstepped down in pressure with a regulator, eliminatesthe pressure fluctuations. Our air handling system uses arotary valve compressor equipped with an aftercooler(CompAir Model 707). A rotary compressor stays on atall times and keeps the pressure between the two setpoints by adjusting the air intake valve to thecompressor stage. The pressure in the 100 gallon tankattached to the compressor varies between 90 and110 psi. The 100 gallon tank delivers air to a dessicantdryer. After the dryer, the air is delivered to a 60 gallontank maintained at 80 psi, followed by a 60 gallon tankmaintained at 60 psi, and finally followed by a 40 gallontank regulated at 40 psi. This arrangement of ballasttanks provides a very stable source of compressed air forthe MAS probe.

3. Description of the controller

In day to day operation the sample-spinning rate isinitially set manually to within 75Hz of the desiredcontrolled value by manual adjustment of the bearingand drive gas pressures. Due to improvements in thestable spinning of the rotors provided by the air ballasttanks, the MAS controller needs only to correct for slowdrifts over time. Therefore, the controller requires only a

small number of functions to control the samplespinning speed. The functions include a reset button,which restarts the control algorithm and sets the drivegas control valve to a predetermined position. A coarseadjustment switch is available to bring the sample closeto the desired spinning rate by gradually opening orclosing the drive gas valve. A manual mode is availablewhere the controller acts as a simple frequency counter,and an automatic control mode allows the controller toactively keep the spinning rate of the sample equal to theset point value. The unit described below is stand-alonewith the set point value entered via thumb-wheels andthe spinning rate displayed using numeric LEDs. Ananalogue voltmeter is used to display the output voltageof the digital to analog controller (DAC) chip thatcontrols the position of the drive gas control valve.The controller was designed around the use of a small

8-bit micro-controller that can be easily programmed.This micro-controller simplifies the design of theelectronics and increases the flexibility of the controllerby keeping the control algorithm in software. Thechosen micro-controller is an 8-bit PIC16F84 devicefrom Microchip (Chandler, AZ). It is manufactured inan 18 pin DIP (dual inline package) format and is simpleto work with. Thirteen of the pins can be used for inputor output to control peripheral devices. The PIC16F84has 1024 words of electrically reprogrammable(FLASH) memory to store user code and 68 bytes ofdata RAM to hold program variables that reside on themicro-controller. Software to program the device isprovided free by the manufacturer and consists of agraphical integrated development environment that runsunder the Microsoft family of operating systems. Themicro-controllers are well established, well supported,readily available, and are inexpensive. Simple program-mers can be built to program the micro-controllerand commercial programmers are available at a reason-able cost (EPIC microEngineering Labs Inc., CO).The software is normally written using assemblerlanguage and there are only 32 instructions with whichto become familiar. If desired, high-level programminglanguages are also available. The device can run at clockspeeds between 0–10MHz, depending on the oscillatorsource used for the master clock. Each instruction usedto program the micro-controller takes a single clockcycle to execute except for jump and subroutine callcommands, which require two cycles. Therefore, accu-rate software time delays can be created when a suitablecrystal oscillator is employed. We used a 2.0MHzcrystal oscillator (Jameco Electronics, part number27924) as the input clock to the micro-controller. A 1 ssoftware delay, used when counting the spinning rate,was calibrated by measuring the frequency outputof a PTS frequency synthesizer (PTS160 ProgrammedTest Sources Inc, Littleton MA) over the range900Hz–50 kHz. The PTS synthesizer was used as

ARTICLE IN PRESSE. Hughes, T. Gullion / Solid State Nuclear Magnetic Resonance 26 (2004) 16–21 17

the standard oscillator input taking the place of thephoto-generated signal produced by the spinningsample. A frequency was dialed into the PTS synthesizerand measured by the spinning speed controller. Thesoftware loops defining the 1 s delay of the micro-controller were repeatedly reprogrammed until thereading from the controller exactly matched the knownfrequency from the PTS synthesizer. Once the correctparameters had been found, the software delay waschecked over the frequency range 900Hz–50 kHz. Nochange in precision was observed.A block diagram of the spinning speed controller is

shown in Fig. 1. The micro-controller lacks sufficientcontrol lines to control the DAC, counters, LED displayand thumb-wheels individually; therefore, the differentperipheral components are accessed using two serialstreams. The input stream utilizes 5 parallel input/serialoutput 74HC165 chips daisy-chained together throughtheir serial output. Using this method we can latch thevalue of the counters, thumb-wheel values, and opera-tion mode switches of the controller simultaneously andthen read the information into the micro-controllerserially bit by bit under software control. The values arereconstructed in software and stored in memory locationin the micro-controller. The output stream applies thesame principal only in reverse. Seven parallel output/serial input 74HC595 chips are linked together through

their serial input. Information corresponding to themeasured spinning rate value, the new voltage valueto the 12-bit DAC (Analog Devices AD565AD), andthe information to indicate the status and mode of thecontroller are sent to the peripherals in a stream of bits.When all the information has been sent, the data islatched into the 74HC595 chips and is output to thecorresponding peripheral’s input pins. These twostreams require a total of seven control lines. Thefrequency counter consists of two cascaded 8-bitcounters (74HC590) and has three separate control linesto start, stop, and clear the counters. Hence, only 10 ofthe 13 control lines on the micro-controller are used.The controller has been designed to interface withcommercial Chemagnetics (Fort Collins, CO) solid-stateNMR pencil probes and to a homebuilt probe utilizing aChemagnetics spinner head.The signal from the probe is detected optically and

converted to a voltage. In the homebuilt probe thisconversion is done in the controller box using aphototransistor circuit and a tuned bandpass filter(Frequency Devices: Model 780 RT-3 range 0–20 kHz)that is adjusted manually for the given speed from a dialon the front panel of the controller. The DAC outputvoltage controls a MKS airflow controller (MKS:Model 0248A-05008V, Andover, MA) that is positionedin the drive air line. This particular MKS valve has arange of 5000 SCCM and cannot be completely closed,which is beneficial since a power outage does not cause asudden loss of drive gas. The bearing gas is not activelycontrolled. With this setup the spinning of the rotor canalways be started and stopped manually. The flowcontroller is activated when the spinning rotor is close tothe set value.

4. Control algorithm

The control algorithm in the micro-controller isshown in Fig. 2. Powering up the controller, or uponreset, the DAC output to the solenoid of the MKS valveis set to 3V. A setting of 3V causes the MKS valve toopen sufficiently for easy spin-up of the rotor. Thevariable holding the spinning rate value is set to zero atstart-up. The algorithm then goes into a loop structure.At the start of each loop cycle, the binary value of themeasured spinning rate and the bit pattern correspond-ing to the new voltage applied to the electro-valve areoutput serially to the numeric frequency display and tothe DAC chip. Next, the frequency counter chips arecleared and the frequency count is started. After 1 s, thefrequency measurement is stopped and the measuredvalue is read into the micro-controller chip serially.Also, the rotor spinning rate set point value given by thesettings on the thumbwheels and the positions of thevarious mode switch values (auto, manual, increment,

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590 590

Counter Counter

MicrocontrollerPIC16F84

IN

OUT

595 595

DAC

595 595 595 595 595

NUMERIC LED DISPLAY

590 590 590 590 590

THUMBWHEEL INPUTS

- - - - -

+ + + + +

FILTER780 RT-3

590

595

LEDS

Flow ControllerMKS

Fig. 1. Block diagram of the spinning rate controller. The diagram

shows the micro-controller interacting with the peripherals by passing

data in and out serially.

E. Hughes, T. Gullion / Solid State Nuclear Magnetic Resonance 26 (2004) 16–2118

decrement) are read into the PIC16F84 chip. Once thesevalues are read into the micro-controller chip, thecurrent measured frequency value is updated, thethumbwheel values are converted to a binary numberfrom their original BCD values, and the controller modeflags are set corresponding to the switch settings.The control algorithm branches according to the

settings of the mode control flags. They are tested inthe following order. First, if the auto flag is set, thealgorithm attempts to actively control the spinningrate of the rotor. The algorithm subtracts the set pointvalue from the measured frequency value to determinewhether the spinning rate of the rotor is above or belowthe set point value. If the spinning rate is above the setpoint value, then the existing binary DAC value isdecreased by one unit. If the spinning rate of the rotoris below the set point, the current binary DAC value isincreased by one. If the two values are equal then thecurrent DAC value is left unchanged. The algorithmthen returns to the beginning of the loop cycle.If the auto flag is not set, then the controller is in

manual mode and the algorithm tests the increment and

decrement flags which will be set if the operator hadattempted to increase or decrease the spinning speed ofthe rotor manually. The algorithm tests the incrementflag first, if it is set, then the current DAC value isincreased by 10 in order to increase the sample speedand the algorithm returns to the start of the loop. If theincrement flag is not set, then the decrement flag istested. If this flag is set then the current DAC value isdecreased by 10 in order to slow the spinning rate andthe algorithm returns to the beginning of the loop. Ifneither flag is set, then the controller behaves as a simplefrequency counter and, in this case, the current DACvalue is left unchanged and the control algorithm jumpsto the beginning of the cycle loop to begin outputtingthe current DAC value to the DAC chip and themeasured frequency value to the numeric LED display.

5. Results and discussion

The controller has been extensively tested on a home-built triple-channel spectrometer using a Tecmag Librapulse programmer, a 3.55 T magnet, and a home-builttriple-channel transmission line probe that incorporatesa Chemagnetics 7.5mm pencil rotor spinning assembly.The spectrometer is used mainly for performingREDOR and REAPDOR experiments and a typicalexperimental spinning rate is 3.125 kHz. Normally, fiveevenly spaced black marks are drawn on the rotor withan indelible black marker using a stencil made fromflexible plastic that wraps around the rotor. Five markson the rotor and a 1 s sampling period lead to acontroller precision of 70.2Hz.In order to illustrate the performance of the

controller, experiments were performed using a rotorwith 10 black marks and a spinning rate set to 2 kHz.The pulse sequence shown in Fig. 3 was used to test thestability of the spinning rotor for a sample of alanine.This particular pulse sequence is part of REAPDOR

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Set DAC to 3VSet LEDs to 0

UpdateDAC & LEDs

Countfor 1 sec

Auto Mode?

ManualIncrease?

ManualDecrease?

IsSpeed Too

High?

IsSpeed Too

Low?

DecrementDAC

DecrementDAC

Increase DACby

10 Units

Decrease DACby

10 unitsYes

Yes

No

No

No

Yes Yes

Fig. 2. Control algorithm used in the spinning speed controller.

CP decouple

rotor

0 6Tr

x y x y φ φ x y x y

CP-φ

H1

C13

Fig. 3. The 13C-observe pulse sequence used to test the spinning rate

stability. The evolution period is shown for 6 rotor cycle. The 1H and13C cross-polarization pulses were applied with RF field strengths of

50 kHz for 1ms duration. The 1H decoupling RF field strength was

110 kHz. The 13C p pulses are phased according to the xy-4 phase

scheme, except for the two shown with the same phase f as used with

the 13C CP pulse.

E. Hughes, T. Gullion / Solid State Nuclear Magnetic Resonance 26 (2004) 16–21 19

and certain REDOR experiments. Protons are used forcross-polarization (CP) and are then decoupled subse-quently by a strong RF field. A train of rotor-synchronized xy-4 phased [21] 13C p pulses lasting forNc rotor cycles follows CP. Data acquisition follows thetrain of p pulses. The time between centers of adjacent ppulses is one-half of a rotor period. During the first halfof the evolution period the chemical shift anisotropy willdephase the 13C transverse magnetization. It is duringthe latter half of the evolution period that this dephasingis refocused to form an echo at the beginning of theacquisition period.Fig. 4 (bottom) shows the 13C spectrum of alanine

after an evolution period of 30 rotor cycles at a spinningspeed of 2000Hz. The spinning speed is under activecontrol and has been set for the correct spinning speed.The number of marks on the rotor is ten and thereforethe nominal accuracy of the controller is within 0.1Hz.The whole spectrum is phased correctly and the signalfrom the carboxyl carbon at 177 ppm is quite intensewhen compared to the methyl carbon at 19.7 ppm. Theother spectra of Fig. 4 show the results of the experimentwhen the pulse sequence timings are set for an ideal2000.0Hz sample spinning speed, but the actual samplespinning speeds are deliberately miss-set by 0.1Hzincrements. Distortions in the phase of the spinningsidebands of the carboxyl 13C resonance become morepronounced and unacceptable as the deviations in thespinning speed increases. The results show that spinningspeed deviations as small as 0.2Hz can have a noticeableeffect on the quality of the spectra under theseexperimental conditions.The spectra in Fig. 5, also generated with the same

pulse sequence as described in Fig. 3, show the effects ofplus and minus deviations from the ideal spinning rate.The pulse sequence timings were set for an ideal 2 kHz

spinning rate for all spectra shown in Fig. 5. The bottomspectrum (2000 scans) was acquired with the idealspinning rate of 2000.0Hz. The top two spectra of Fig. 5were acquired with spinning rates of 1999.0 and2001.0Hz, and 4000 transients were acquired for eachspectrum. The top two spectra show that the sense of thedispersive artifacts of the spinning sidebands is depen-dant on whether the spinning speed is too high or toolow when compared to the ideal spinning rate.Furthermore, it is clear from the relative intensities ofthe methyl and alpha-carbon 13C resonances that the13C magnetization of the carboxyl carbon suffersconsiderably due to imperfect refocusing of the CSA,which is especially evident when compared to thespectrum taken with the ideal spinning rate (Fig. 5,bottom). Fig. 5 also shows a spectrum obtained byadding the top two spectra. This summed spectrum isproperly phased due to the cancellation of the oppositedispersive character of the spinning side bands uponaddition. However, even with this addition a significantloss in signal intensity occurs, which is most notable inthe carboxyl 13C resonance.The experiments shown in Fig. 5 model the behavior

of a sample that can only be kept spinning at a spinningrate within 71Hz of the desired value. From theseresults it is clear that a properly phased spectrum doesnot imply excellent spinning rate control. Equalcontributions to the total acquired signal come fromlow and high spinning rate contributions to producecorrectly phased signals. Hence, it is necessary to use acontroller that is very stable. With large anisotropiesand long evolution times, poor spinning rate control canlead to the complete loss of the signal, especially for 13Cresonances characterized by large anisotropies. The

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050100150200250 ppm

-0.5 Hz

-0.4 Hz

-0.3 Hz

-0.2 Hz

-0.1 Hz

δC

Fig. 4. 13C spectra of natural abundance alanine taken with the pulse

sequence shown in Fig. 3 with Nc set to 30. The pulse timings in the

pulse sequence were set to values consistent with an ideal 2 kHz

spinning rate experiment for all spectra. The bottom spectrum was

taken with a spinning rate of 2 kHz. The other five spectra were taken

with spinning rates lower than 2 kHz by the amount indicated.

050100150200250 ppm

(x2)

(/2)

2000 Hz

sum

2001 Hz

1999 Hz

δC

Fig. 5. 13C spectra of natural abundance alanine obtained with the

pulse sequence shown in Fig. 3 using pulse timings set to an ideal

spinning rate of 2 kHz and Nc equal to 30. The actual spinning rate for

the top two spectra were purposely set 1Hz below and 1Hz above the

ideal spinning rate. The second from bottom spectrum is the simple

addition of the top two spectra. The bottom spectrum was acquired at

2 kHz, which is the correct spinning rate for the timings used in the

pulse sequence. All spectra have been scaled relative to one another to

take into account the number of transients acquired.

E. Hughes, T. Gullion / Solid State Nuclear Magnetic Resonance 26 (2004) 16–2120

spinning rate controller just described reduces the loss ofsignal considerably.In conclusion, a spinning rate controller that is

compact, simple to operate and simple in design hasbeen described and demonstrated. It can control thespinning rate with high accuracy and precision. The 13Cspectra show the necessity of precise spin rate controlfor certain types of MAS experiments.

Acknowledgments

We would like to thank Mr. Bob Smith of theDepartment of Chemistry at WVU for constructing thespeed controller. Financial support for this work wasprovided by the NSF grant CHE-0091663.

References

[1] C.P. Slichter, Principles of Magnetic Resonance, 3rd Edition,

Springer, New York, 1989.

[2] R. Tycko, Annu. Rev. Phys. Chem. 52 (2001) 575.

[3] R.G. Griffin, Nat. Struct. Biol. 5 (1998) 508.

[4] L.M. McDowell, J. Schaefer, Curr. Opin. Struct. Biol. 6 (1996)

624.

[5] S.O. Smith, K. Aschheim, M. Groesbeek, Rev. Biophys. 29 (1996)

395.

[6] J.R. Garbow, T. Gullion, in: N. Beckman (Ed.), Carbon-13 NMR

Spectroscopy of Biological Systems, Academic Press, New York,

1995.

[7] E. Robert, A. Whittington, F. Fayon, M. Pichavant, D. Massiot,

Chem. Geol. 174 (2001) 291.

[8] M. Jansen, H. Eckert, Solid State Ionics 136 (2000) 1007.

[9] M. Bertmer, L. Zuechner, J.C.C. Chan, H. Eckert, J. Phys. Chem.

B 104 (2000) 6541.

[10] T. Gullion, J. Schaefer, in: W. Warren (Ed.), Advances in

Magnetic Resonance, Vol. 13, Academic Press, New York, 1989.

[11] T. Gullion, J. Schaefer, J. Magn. Reson. 81 (1989) 196.

[12] T. Gullion, Chem. Phys. Lett. 246 (1995) 325.

[13] L. Chopin, S. Vega, T. Gullion, J. Am. Chem. Soc. 120 (1998)

4406.

[14] Y. Ba, H. Kao, C.P. Grey, L. Chopin, T. Gullion, J. Magn.

Reson. 133 (1998) 104.

[15] C.P. Grey, A.J. Vega, J. Am. Chem. Soc. 117 (1995) 8232.

[16] C. Fernandez, D.P. Lang, J.P. Amoureux, M. Pruski, J. Am.

Chem. Soc. 120 (1998) 2672.

[17] J.R. Garbow, T. Gullion, Chem. Phys. Lett. 192 (1992) 71.

[18] L. Chopin, R. Rosanske, T. Gullion, J. Magn. Reson. A 122

(1996) 237.

[19] J.N. Lee, D.W. Alderman, J.Y. Jin, K.W. Zilm, C.L. Mayne,

R.J. Pugmire, D.M. Grant, Rev. Sci. Instrum. 55 (1984) 516.

[20] T.G. Maier, T.H. Huang, J. Magn. Reson. 91 (1991) 165.

[21] T. Gullion, D.B. Baker, M.S. Conradi, J. Magn. Reson. 89 (1990)

479.

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