Nuclear Magnetic Resonance Measurements at High PressureJ. Jonas Citation: Review of Scientific Instruments 43, 643 (1972); doi: 10.1063/1.1685713 View online: http://dx.doi.org/10.1063/1.1685713 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/43/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Pressure-resisting cell for high-pressure, high-resolution nuclear magnetic resonance measurements at veryhigh magnetic fields Rev. Sci. Instrum. 72, 1463 (2001); 10.1063/1.1334630 High-pressure autoclave for multipurpose nuclear magnetic resonance measurements up to 10 MPa Rev. Sci. Instrum. 70, 2448 (1999); 10.1063/1.1149775 A new high pressure sapphire nuclear magnetic resonance cell Rev. Sci. Instrum. 67, 240 (1996); 10.1063/1.1146578 High pressure nuclear magnetic resonance measurement of spin–lattice relaxation and selfdiffusion in carbondioxide J. Chem. Phys. 97, 2022 (1992); 10.1063/1.463139 Nuclear Magnetic Resonance in Liquids under High Pressure J. Chem. Phys. 22, 2003 (1954); 10.1063/1.1739982

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PHOTOMULTIPLIER HOUSING 643

• R. J. Smith-Saville and S. Ness, J. Sci. Instrum. 44, 631 (1967). 6 C. W. Sherring, J. Phys. E 3, 1016 (1970). 6 G. W. Carriveau, J. Phys. E 3, 929 (1970). 7 R. W. Engstrom, Rev. Sci. Instrum. 18, 587 (1947). 8 A. R. Franklin, W. W. Holloway, Jr., and D. H. McMahon, Rev.

Sci. Instrum. 36, 232 (1965). 9 C. J. Bronco, R. M. St. John, and R. G. Fowler, Rev. Sci. Instrum.

29, 1145 (1958). 10 R. M. St. John, Rev. Sci. Instrum. 32, 370 (1961). 11 G. G. Harman, Rev. Sci. Instrum. 30, 742 (1959). 12 W. T. Whitney, Rev. Sci. Instrum. 36, 1656 (1965).

THE REVIEW OF SCIENTIFIC INSTRUMENTS

13 J. Gerber, Rev. Sci. Instrum. 41,916 (1970). 14 A. T. Young, App!. Opt. 2, 51 (1963). 16 J. Sharpe, Document No. R/P021, EMI Electronics Ltd., Hayes,

Middlesex, England, 1966. 16 M. R. Zatzick, Application Note 71021, SSR Instruments Co.,

Santa Monica, Calif. 17 A. R. Boileau and F. D. Miller, App!. Opt. 6, 1179 (1967). 18 A. T. Young, Rev. Sci. Instrum. 38, 1336 (1967). 19 J. P. Keene Rev. Sci. Instrum. 34, 1220 (1963). 20 R. B. Murr~y and J. J. Manning, IRE Trans. Nuc!. Sci. NS-7,

No. 2-3, 80 (1960).

VOLUME 43. NUMBER 4 APRIL 1972

Nuclear Magnetic Resonance Measurements at High Pressure*

J. JONAS

Department of Chemistry, School of Chemical Sciences and Materials Research Laboratory, University of IUinois, Urbana, Illinois 61801

(Received 26 October 1971; and in final form, 20 December 1971)

Instrumentation for the measurement of spin lattice relaxation times in liquids at pressures up to 5 kilobar and temperatures from -50 to 350°C is described. The experimental setup allows the use of the Fourier transform technique to obtain high resolution spectra and/or relaxation of chemically shifted nuclei under these extreme conditions.

INTRODUCTION

Nuclear magnetic resonance relaxation time measure­ments provide valuable information about molecular mo­tions and interactions in liquids. So far the great majority. of these experiments have been carried out either at room temperature or at variable temperatures at 1 atm. Since the change in temperature is also accompanied by a change in density of the liquid, measurements at elevated pressures can separate these effects. In such high pressure experi­ments one can keep the kinetic energy of a molecule constant and follow the effects of changing only the density of the fluid. In this work we describe the experi­mental setup which has successfully been used in a number of NMR high pressure studies1- s performed recently in our laboratory. In a previous communication6 we described a simple high pressure sample cell which was used for the NMR relaxation time measurements in liquids at room temperature and at pressures up to 2 kilobar.

Since the pioneering work by Benedek and PurcelF relatively few high pressure NMR studiesS- 12 have been performed. With the exception of the work of Powles and GoughlO and Hertz and Radle,ll all other measurements were carried out only at room temperature. It seems that most of the problems encountered in these studies were connected with the design of the high pressure sample cell in which the liquid must be kept oxygen-free during the pressure cycle. There were, of course, no difficulties with oxygen in the self-diffusion measurements carried out by McCall and coworkers13- 1S in a number of liquids at room

temperature and pressures up to 0.8 kilobar, and by Wade and Waugh16 in liquid ethane.

Our experimental setup has a number of very good features: (i) Relaxation time measurements can be carried out over a wide temperature (-SO-350°C) and pressure (0-5 kilobar) range; (ii) samples are oxygen-free even when repeating the pressure cycle many times; (iii) the sample cell is of very simple construction. There is also a high homogeneity of the magnetic field over the sample volume even under these extreme conditions of temperature and pressure. Therefore, it is possible to use the Fourier transform techniqueS to obtain high resolution spectra or measure spin lattice relaxation times of each individual line in a high resolution spectrum.

GENERAL EXPERIMENTAL SETUP

A special wide gap (9.5 cm) electromagnet Varian model V-3800-1 producing a magnetic field of 14092 G represents a very important part of our experimental system. The wide gap provided us with enough working space so that we could optimize the design of high pressure probes and sample cell. The electromagnet is equipped with field homogeneity coils and a solid state flux stabilizer in order to maintain the necessary homogeneity and stability of the magnetic field. A Varian model V-2708 current regulated 20 kW power supply and a Varian model V-3524 heat exchanger for temperature control of the magnet cooling water were used in our system. The homogeneity of the magnetic field will be discussed later when we deal with the

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644 J. JONAS

HEISE GAUGE 15000 BAR)

SEPARATOR ICS 2 )

HAND PUMP

/

(OIL)

TO HIGH PRESSURE PROBE

HEISE GAUGE 12000 BAR)

f PRESSURE INTENSIFIER

IOILl

FIG. 1. Schematic drawing of the high pressure generating system.

Fourier transform method. The short term stability of the magnetic field was better than one part in 107•

The high rf power pulsed NMR spectrometer operates at any frequency from 4 up to 60 MHz. The NMR Special­ties, Inc., model P-1l8 rf unit and the high power amplifier model P-103R were used as a transmitter. The receiver system consisted of a narrow band (O.S MHz) preamplifier (RHG Electronics Laboratory, Inc., model F-T), gain 30 dB; and narrow band (0.5 MHz) amplifiers (RHG Elec­tronics Laboratory, Inc., model E-V-T) variable gain 0-80 dB, tuned to various operating frequencies within the range of 4-60 MHz. A Hewlett-Packard model 10S34-A mixer was employed as a phase sensitive detector. A versatile pulse sequence generator which permits the use of the usual pulse sequences used in NMR relaxation measure­ments was built in the School of Chemical Sciences Electronics Shop. The rf signal, after phase detection, was stored in a FabriTek FT-I074 computer averager equipped

FIG. 2. Photograph of the high pressure equipment setup.

with numerical display. The amplitude of the free induc­tion decay was digitally displayed on an oscilloscope, or the memory content was transferred to a paper tape using a Tally model 420 paper tape punch. This was done only if Fourier transformation had to be carried out on the central computer Sigma-S of the Materials Research Laboratory. Spin lattice relaxation times Tl were measured using a 180o -r'-90° pulse sequence; the logarithm of the mag­netization was plotted as a function of r to obtain Tl to ±S%.

The high pressure generating system, depicted sche­matically in Fig. 1, consists of Enerpac hand pumps, pres­sure intensifier, liquid separator, Heise-Bourdon gauges, and necessary high pressure tubing and valves. Carbon disulfide was used as a pressure transmitting fluid for proton relaxation measurements at lower temperatures, and tetrachloroethylene at high temperatures. For other nuclei a mixture of SO% SAE lOW motor oil and SO% kerosene was used for temperatures up to ISO°C, at higher temperatures only motor oil was used. A number of additional pressure transmitting fluids, particularly for temperatures higher than 300°C, are now being tested. It proved advantageous to have all pressure generating equipment on a movable table which was attached to the probe. The high pressure probe could be moved out of the magnet without disconnecting the high pressure tubing

FIG. 3. Photograph of the high temperature, high pressure probe and probe support.

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MAGNETIC RESONANCE 645

which was made of nonmagnetic stainless steel SS 316 (American Instrument Co., Inc.). A photograph of the high pressure equipment table showing also the two shafts on which the probe support moves is given in Fig. 2. For the Fourier transform experiments it proved necessary to have good, reproducible vertical and horizontal positioning of the high pressure probe in the electromagnet gap. Due to the probe dimensions and its weight, we built a probe support which could be moved out of the electromagnet on two ground ball bushing shafts using ball bushings. This proved necessary when changing samples and/or probes. We also found it advantageous to use a three-point support for the probe base support plate since the load was off center. We used a chain drive and three ball nuts and ball sqews (Saginaw Steering Gear Division, GM) to achieve good reproducibility of vertical positioning of the high pressure probe. Details of the probe support are seen in the photograph in Fig. 3.

-------------, ,-- --------- -------- -- --- ---------L-.J----LJ--'\""'--'

OUTLET

FIG. 4. Schematic drawing of the beryllium-copper probe with thermostating jacket.

FIG. 5. Schematic drawing of closure plug.

HIGH PRESSURE PROBE

A material which is used to construct high pressure NMR probes must be nonmagnetic and of high mechanical strength. Paul, Benedek, and Warschauer17 have discussed the use of beryllium-copper and stainless steel in the construction of nonmagnetic pressure vessels. In our ex­periments we used both beryllium-copper (Berylco 25) and the relatively new titanium alloy IMI-680 (Imperial Metal Industries, England) which was fIrSt used for this specific purpose by Powles and Gough.lO

We built two high pressure NMR probes; one for the lower temperature range (-5Q-+80°C) made of beryllium­copper, and the other made of IMI-680 for the high

COPPER

FIG. 6. rf lead terminal.

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646 J. JONAS

, I I I I I I

--..l1.7o-­Icm' 42.0cm

n It

PLUG

TOP VIEW

-8 BOTTOM VIEW

FIG. 7. Titanium high pressure probe.

temperature range 25-350°C. According to our experience, Berylco 25 should not be used for temperatures above 100°C, and even then prolonged operation at SO-100°C shortens considerably the lifetime of this pressure vessel. Also, we found that Berylco 25 was rather difficult to test for defects. The probe is roughly machined from Berylco 25 (half-hard) and after heat treatment for 2 h at 315°C (tensile strength 14000 kg· cm-2) it is fine machined. There is no difference in machinability of the titanium alloy before and after heat treatment; therefore, all machining was done on hot rolled, solution heat treated and aged IMI-6S0 alloy (see Imperial Metal Industries specifica­tions). The tensile strength of titanium alloy is 12600 kg·cm-2 and, what is very important, it retains its tensile strength of 11 200 kg· cm-2 even over extended periods (1000 h) at 400°C.

A conventional design18 was used for the construction of the high pressure probes. A schematic drawing of the low temperature, high pressure beryllium-copper probe is given in Fig. 4. The Berylco 25 vessel is enclosed in a brass thermostating block through which the thermostating liquid was circulated by a constant temperature circulator (Lauda Model, Brinkman Instruments, Inc.). The tem­perature stability was better than ±0.2°C. In order to

prevent variation of temperature of the pole faces, thin cooling plates parallel to the pole faces of the magnet were used. Water at magnet temperature was circulated through them. Figure 5 gives a schematic drawing of the closure plug with holes for the rf and thermocouple terminals. The closure plug was of conventional design,18 the seal was obtained by using Everdur washers and lead washers. For pressures up to 2.5 kilobar buna rubber O-rings (Parker Seal Company, Cleveland, Ohio) could be used to provide hermetic closure. A schematic drawing of the rf lead terminal is shown in Fig. 6. The same design was used for the thermocouple terminal.

The great advantage of the titanium alloy is its excellent mechanical strength at high temperatures. Figure 7 shows schematically the high temperature titanium pressure vessel. This vessel was externally heated by a Nichrome wire noninductively wound on asbestos insulation. Only the middle part of the vessel was heated, as shown in Fig. S. On the contrary, the top of the vessel which con­tains the closure plug with rf and thermocouple terminals was cooled by a cooling jacket. The temperature on the surface of the titanium vessel near the top and near the bottom of the external heating coil was continuously monitored by a digital temperature measuring system using constantan/copper thermocouples. The fact that the closure plug was always maintained at temperatures below 100°C enabled us to use simple rf and thermocouple terminals (see Fig. 6) even when working at temperatures well above 200°C. Powles and GoughlO have reported serious difficulties with rf leads at higher temperatures. A

IN

COOLING WATER OUT

COOLING JACKET

OUT

COOLING WATER IN

Ti 1M1 680 VESSEL

THERMOCOUPLES

HEATING COIL

FIG. 8. High temperature probe assembly.

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MAGNETIC RESONANCE 647

FIG. 9. High pressure sample cell.

GLASS- METAL SEAL

55316-- -BELLOWS

5 316 TUBE

316 SPACER

COPPER

2 kW dc power supply was used for heating. It was surprising to find that even when operating at high temperatures, the heating coils did not seriously impair the magnetic field homogeneity over the sample volume. A readjustment of the field homogeneity controls was neces­sary for each setting of the power supply. There was no dissipation of heat to the magnet even at temperatures above 300°C since cooling plates were used around the high temperature vessel, as shown in Fig. 8.

FIG. 11. IH free induction decay of a mixture of TMS, chlorobenzene, and acetonitrile at 2.5 kilobar and 26°C. (Time in sec; intensity scale in arbi­trary units.)

o o o ~

o o d a>

g d "?

g

RF LEADS

"'-'_ ,-" THERMOCOUPLE

FIG. 10. Sample holder.

NI<-..,.II'I---- RF COIL

HIGH PRESSURE SAMPLE CELL

So far the only high pressure sample cell suitable for high temperature operation has been described by Powles and Gough.1° This cell was built entirely of glass using mercury as separating liquid, the displacement of which accommo­dated volume changes of the liquid sample under com­pression. Powles and GoughlO commented that no metal

o ~+------r------r----'----r------',------,,------,,-----,,------,,----~

0.00 .06 .12 .18 ~ ~ _ ~ M M .60 TIME

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648 J. JONAS

8 '" N

8

FIG. 12. Fourier transform of the free induction decay shown in Fig. 1i. (Frequency in Hz; intensity scale in arbitrary units.)

~+-----'-----'-----'-----'-----'------r-----r-----'-----r----~ 0.00 40.00 50..00 60.00

FREQ (X 101)

10..0.0 20.0.0. 30.00 70.00

should be in contact with the liquid in view of the danger of contamination especially at high temperatures. On the contrary, our experience with the all glass metal construc­tion of our high pressure sample cell was good in all respects. Figure 9 is a schematic drawing of the high pres­sure sample cell. A glass (Pyrex 7740) to metal (SS 316) seal (Quartz Scientific, Inc., Palo Alto, Calif.) connects the actual glass sample chamber to the SS 316 tubing (o.d. 6 mm). Stainless steel bellows (Mechanized Science Seals, Inc., Los Angeles, Calif.) accommodate the volume change of the liquid due to compression. Hermetic closure of the sample cell was attained by a (60° angle) copper cone in a similar (60° angle) seat made of stainless steel (see Fig. 9). In order to ensure good mechanical strength of the plug and upper bellows adaptor, these parts were made from SS 316 which were brazed to the copper ring cones. We did not encounter difficulties with these hermetic closures. The spacer in the sample cell serves only to reduce the total volume of the sample (1.5 ml). Either SS 316 or high temperature polyimide plastic (Vespel, Du Pont de Nemours & Co., Wilmington, Del.) was used to make this spacer. In order to permit easy cleaning of the cell, the upper glass part of the cell was easily removable. The sample was degassed using the usual freeze-pump-thaw technique and then sealed under vacuum in a glass ampOUle. This ampoule was transferred to an inert gas dry boxl9 and the sample was transferred to the sample cell. Repeated pressure cycles at high temperature had no detectable effect on the relaxation times of the liquid even when measuring T 1 longer than 30 sec. One detail about the design of the sample cell should be emphasized; the bulky

80.00 90.00 100.00.

metal parts with bellows should be at least 35 mm from the glass chamber. Otherwise, the homogeneity of the magnetic field over the sample volume is impaired.

The sample cell is simply inserted into the sample holder which is schematically depicted in Fig. 10. The material used for its construction was either polytetrafluoroethylene (Teflon), nylon, or the high temperature polyimide plastic Vespel. We found the mechanical stability and inertness of Vespel at high temperatures excellent for our purposes. The dimensions of the transmitter/receiver coil (No. 18 copper wire) were 20 mm length X 12 mm diam, the num­ber of turns depending on the operating frequency (10-20 turns). The actual sample volume was a cylinder of 12 mm length, 8 mm diam so that a good homogeneity of the rotating magnetic HI field was achieved. We also found that the copper/constantan thermocouple located ap­proximately 7 mm from the coil had no effect on the homogeneity of the magnetic field over the sample volume.

For the first time, it has been possible to achieve a high homogeneity of the magnetic field over the sample volume even under extreme conditions of temperature and pres­sure. Therefore, a Fourier transform technique5 is appli­cable and a typical linewidth is, e.g., 19F (at 56.4 MHz) ~1'!=4 Hz for an 8 mm diam sample. As an example of the resolution attained we give the free induction decay of a hydrogen signal measured at 60 MHz for a mixture of tetramethyl silane (TMS), chlorobenzene and acetonitrile at 26°C and 2.5 kilobar (Fig. 11). The corresponding Fourier transform of this free induction decay is given in Fig. 12. Several studies20 are now in progress to determine

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MAGNETIC RESONANCE 649

the effect of density on chemical shifts or on relaxation of chemically shifted nuclei.

There is enough additional space in the electromagnet gap to accommodate field gradient coils which are neces­sary for the measurement of self-diffusion by the spin echo method.21 So far, we have used a fixed field gradient, and measurements of D in liquids up to 5 kilobar have been performed successfully.

* Work partially supported by the Advanced Research Projects Agency, under Contract No. HC-15-67-C-0221 and by the National Science Foundation, under Grant No. GP 28268X.

1 J. Jonas, Ber. Bunsenges. Phys. Chern. 75, 257 (1971). 2 T. E. Bull, J. S. Barthel, and J. Jonas, J. Chern. Phys. 54, 3663

(1971). 3 H. J. Parkhurst, Jr., Y. Lee, and J. Jonas, J. Chern. Phys. 55,1368

(1971). • Y. Lee and J. Jonas, J. Magnetic Resonance 5, 267 (1971). 5 D. J. Wilbur and J. Jonas, J. Chern. Phys. 55, 5840 (1971).

THE REVIEW OF SCIENTIFIC INSTRUMENTS

6 J. Jonas, T. E. Bull, and C. A. Eckert, Rev. Sci. Instrum. 41, 1240 (970).

7 G. Benedek and E. M. Purcell, J. Chern. Phys. 22, 2003 (1954). 8 A. W. Nolle and P. P. Mahendroo, J. Chern. Phys. 33, 863 (1960). • A. A. Brooks, B. D. Boss, E. O. Stejskal, and V. W. Veiss, J. Chern.

Phys. 48, 3826 (1968). 10 J. G. Powles and M. C. Gough, Mol. Phys. 16,349 (1969). 11 H. G. Hertz and C. Radle, Z. Physik. Chern. (Frankfurt) 68,324

(1969). 12 D. E. Woessner and B. S. Snowden, Jr., J. Chern. Phys. 52, 1621

(1970). 13 D. W. McCall, D. C. Douglass, and E. W. Anderson, Phys.

Fluids 2, 87 (1959). 14 D. C. Douglass, D. W. McCall, and E. W. Anderson, J. Chern.

Phys. 4, 152 (1961). 15 D. W. McCall, D. C. Douglass, and E. W. Anderson, J. Chern.

Phys. 31, 1555 (1959). 16 C. G. Wade and J. S. Waugh, J. Chern. Phys. 43, 3555 (1965). 17 W. Paul, G. B. Benedek, and D. M. Warschauer, Rev. Sci.

Instrum. 30, 874 (1959). 18 D. M. Warschauer and W. Paul, Rev. Sci. Instrum. 29, 675

(1958). ,. T. L. Brown, D. W. Dickerhoof, D. A. Bafus, and G. L. Morgan,

Rev. Sci. Instrum. 33,491 (1962). 20 D. J. Wilbur and J. Jonas (unpublished). 21 H. Y. Carr and E. M. Purcell, Phys. Rev. 94, 630 (1954).

VOLUME 43, NUMBER 4 APRIL !972

Phase Transients in Pulsed NMR Spectrometers*

M. MEHRINGt AND J. S. WAUGH

Department oj Chemistry and Laboratory oj Electronics, Massachusetts Institute oj Technology, Cambridge, Massachusetts 02139

(Received 9 September 1971; and in final form, 11 November 1971)

When a square wave modulated rf voltage is applied to a tuned probe in an NMR spectrometer, the effective transverse magnetic field in the rotating reference frame changes direction as it grows and decays. The resulting deviations in the nutational behavior of the spins from naive expectations are typically small, but may lead to important cumulative errors in experiments employing long pulse trains. These errors can be made to vanish by very accurate tuning of the probe circuit to its free ringing frequency w, [not to WO= (tILC)l] and by providing suitable coherence between the rf carriers and the timing of pulses.

INTRODUCTION

In transient NMR experiments one frequently applies a pulse of oscillating voltage, of rf carrier frequency w near the Larmor frequency, to a coil aligned perpendicular to the Zeeman field and containing the sample. It is assumed that the resulting pulse of transverse rf magnetic field can be replaced in the reference frame rotating at frequency w

by a video magnetic field which grows, persists, and decays along a fixed direction defined by the rf carrier phase, with a time dependence corresponding to the envelope of the rf voltage pulse.

This replacement is an approximation in several senses, though a very good one for many purposes. First, if the rf transmitter contains nonlinear elements (such as class C power amplifiers) it is clear that the effect of a pulse on the spins depends not only on the width and amplitude of the pulse, but also on the carrier phase wIth which it begins and ends. Consider as an extreme case a pulse containing only one-half cycle of the rf carrier, applied to the grid of a

single-ended class C amplifier. If the starting phase is such as to admit the negative half-cycle, there is no output at all. This consideration has led some workers to advocate synchronizing the gate timing with the rf carrier frequency in pulse spectrometers. A different effect, related to the Bloch-Siegert shift! has been reported by MacLaughlin.2

The component of oscillating rf field rotating oppositely to the Larmor precession of the spins has a small effect which again depends on the tum-on and turn-off phases of the pulse. Again it is considered desirable to provide pulse timing which is coherent with the rf carrier when a strong rf field is used, although this effect could also be com­pensated for by exciting the spins with a rotating rather than an oscillating field.

In this paper we point out a third effect which calls, in principle and perhaps sometimes in practice, for rf coherent timing. This is the transient deviation in phase of the rf current (and magnetic field) during the rise and fall of a pulse when, as is always true, the sample coil is part of a

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