1 Report No.

 2 Government Accession No.

4. Title and Subtitle

13

Development of LC- C NMR

7 Ajino'is)

H. C. Dorn*, 0. S. Wang, T. E. Glass

9. Performing Org»m*»'>on Name and AdO'ess

Virginia Polytechnic Institute and State University

Department of Chemistry

Blacksburg, VA 24061-0699

12. Sponsoring Agency Name and Address

Dr. Gary Seng

NASA Lewis Research Center

21000 Brookpark Road, M.S. 500-303

Cleveland, OH 44135

ATTFNTION* rnmmprrial Arrnunt1; Section

5 Report Date

March 17, 1986

6 Perlormme Organitation Cooe

B Pertormmg OreannaliOn Report No

10 Work Unit No

11 Contract or Grant No.

NAG3-517

13. Type o< Report and Period Covered

Fina 1 1 1 1 Ul Renor 1 I\V»- | ' V/t 1 \*

1 /in/84 to 1/9/85

14. Sponsoring Agency Code

15. Supplementary Notes

Annual Report

if Aaatraci

-

This study involves the development of.^3C nuclear resonance as an online detector for liquid chromatography (LC- C NMR) for the chemical

characterization of aviation fuels. The.initial focus of this-study was the

development of a high sensitivity flow

 1JC NMR probe. Since C NMR

sensitivity is of paramount concern, considerable effort during the first year

was directed at new NMR probe designs. In particular, various toroid coil

designs were examined. In addition, corresponding shim coils for correcting

the main magnetic field (B ) homogeneity were examined. Based on these

initial probe design studies, an LC- C NMR probe was built and flow

 13C NMR

data was obtained for a limited number of samples.

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Aviation Fuels,

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Liquid Chromatography ^Gb/>, ^\\]S^

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TABLE OF CONTENTS

Page

SUMMARY !

INTRODUCTION 2

EXPERIMENTAL METHODS *»

RESULTS AND DISCUSSION 15

REFERENCES . 17

FIGURE CAPTIONS 19
SUMMARY

The first year of this study focused attention on ways of improving

13 sensitivity in the LC- C NMR experiment. In particular, various studies

explored the development of a toroid-shaped sample and coil system applicable

to the LC- C NMR approach. Although the toroid cell provides a dramatic

improvement in signal-to-noise (S/N) in comparison with the more common

Helmholtz design, main magnetic field inhomogeneity (B ) remains as a major

problem in the use of this detector. Therefore, considerable effort was

directed at developing shim coils which would correct this B inhomogeneity

problem. Although considerable improvements were obtained, a linewidth of

only -10 Hz was achieved for

 1H NMR at 200 MHz for a torus volume of (-0.25

ml). The corresponding linewidth for C NMR would still be marginally

acceptable (-3 Hz) for a 0.25 ml volume. However, a flow C NMR probe was

constructed using conventional probe technology (Helmholtz coil) in order to

13 initiate flow C NMR studies during the first phase of this study. Finally,

flow C NMR spectra were obtained on several typical fuel components

utilizing this conventional probe design.
INTRODUCTION

In 1978, Watanabe reported results for direct coupling of high

performance liquid chromatography to H nuclear magnetic resonance utilizing a

stop-flow mode^ '. The first continuous flow LC- H NMR experiments were

reported in 1979^ '. In the last few years, several papers have been

published which clearly indicate the evolution of a new analytical tool (LC- H

NMR) for rapid structural elucidation of components present in complex

mixtures'3"

15'. Initial LC- H NMR studies were hampered by signal-to-noise

(S/N) limitations of the electromagnetic based NMR systems. However, more

recent studies employing high field superconducting NMR magnets (e.g., 4.7-

9.4T) have amply demonstrated that sensitivity considerations are greatly

alleviated with these new "state-of-the-art" NMR systems' '. Presently,

continuous flow detection limits for a single compound in a complex mixture is

10-20 ug for molecules having molecular weights of 100-300 daltons (200 MHz *H

NMR). In addition, the superconducting NMR systems have dramatically

increased the number of resolution elements in a given LC-NMR spectral

profile. This is not only because of increased chemical shift dispersion

(e.g., 100 MHz vs. 400 MHz *H NMR spectra), but better main magnetic field

(BQ) homogeneity of the superconducting magnet and technical improvements in

flow cell design.

A major problem originally envisioned for the LC- H NMR approach was the

limited choices of chromatographically and/or NMR acceptable solvent systems

which could be employed without extensive background signals in the H NMR

spectra. This problem has been largely overcome by: 1) the use of deuterated

solvents, chlorinated and/or fluorinated solvents, and 2) the use of solvent

suppression homodecoupling sequences for hydrogen containing solvents^ '.
The progression to smaller analytical scale columns and lower injection

volumes (i.e., 25-100 ml) has dramatically decreased the total solvent (-20-30

ml) necessary for a given LC-NMR experiment. This has helped alleviate costs

when relatively expensive deuterated solvents must be employed. Demonstration

of LC-*H NMR as a quantitative analytical tool was first reported in 1982^10'.

Methods for obtaining average molecular properties (e.g., average molecular

weights) for a given liquid chromataographic fraction (e.g., monocyclic

aromatic, dicyclic aromatics, etc.) of fuels was also reported in 1982' ' .

These NMR derived molecular parameters are very useful for predicting and

understanding physical properties (e.g., smoke points and frreezing points) of

various fuels^ '. The results of the latter study were in excellent

agreement with GC-MS data obtained in an independent laboratory.

All common liquid chromatographic separation modes including sizeexclusion* ' and reversed-phase* ' have been demonstrated with the LC- H NMR

technique. To date, applications of the LC- H NMR approach have been

demonstrated for fuel samples^ ' >9»

13', samples of biological interest^ ' ',

(\A\

and organic chemistry reaction mixture analysisv '. However, widespread

application of this technique for characterization of complex mixtures has

been limited by the expense of the instrumentation and access.

Development of C NMR as a detector for liquid chromatography (LC- C

NMR) has not been reported; however, flow C NMR studies have recently been

reported' '. A major impediment in the use of the C nucl ide is the much

lower sensitivity to NMR observation. This is because of the lower natural

abundance of C (1.11%) and lower nuclear moment in comparison with the H

nucl ide. However, it should be noted that for studies of aviation fuels,

sample size is not a limitation. Thus, the chromatographic column size and/or

sample injection volume can be increased.
EXPERIMENTAL METHODS

The toroid coll has potential advantages for both static and flowing NMR

studies in superconducting solenoids* ' . For example, we have reported

data indicating the toroid coil has a (S/N) advantages of 4-6 in comparison

with the more commonly employed Helmholtz coin '. Also NQR results reported

by Zussmanv ' suggested a factor of 2 improvement for the toroid in

comparison with a solenoid configuration at 4.6 MHz. In addition, the more

efficient and linear (r) dependence for the B, field generated in a toroid

coil dramatically reduces 90° pulse lengths^ '. Unfortunately, the poor

Bo homogeneity usually encountered for the torus region has limited the

\

utility of this detector. Furthermore, most commercial shim coil systems are

specifically designed for spherical or cylindrical sample volumes. For

example, commercial shim systems provide an improvement of only 10-20% for

typical toroid geometries^ ' . To improve the Bo homogeneity specifically for

a given torus region, we have constructed various experimental shim coil

systems. An improved shim coil system for the toroid-shaped sample volumes is

described (vide infra).

The present design is based on the classic Anderson shim coil

approach* . Two types of the Anderson shims are appropriate for the toroidshaped sample volume. One of them is the quadratic shim which consist of four

loops above and below the toroid sample.

ct2Bz 2 a2Bz 2 a2Bz (1)

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The configuration is sketched in Figure la. The quadratic gradient can be

produced with current loops centered on the Z which we designate as a "C"
c

Figure 1. Sketch of four current loops for the gradient shim (la) and

opposed linear gradients (lb), respectively. The torus sample

volume is also indicated for reference purposes.
shim. Another is the linear shim which consists of two opposed current loops.

This configuration is sketched in Figure Ib. The linear field gradient is

produced by these two current loops centered on the Z axis and is similar to a

coil system reported by Cooper^ '. In this coil configuration (designated

"Z"), there exists a region in the plane perpendicular to the axis between

these opposed coils where the field is homogeneous^ '.

The structure of the H toroid probe (Probe A) with the toroid-shaped

sample and shim systems is illustrated in Figure 2. There are three groups of

shim "C" coils (Cj, C2, and C3) and two groups of shim "Z" coils. The group

of C, shims, consist of four planar coils, however, the groups of shims ^2»

Cg, Zj and 1^ consist of cylindrical colls. The coils are mounted on an outer

glass tube (-30 mm) and held in plane with "super glue." The homogeneity of

the field within the toroid-shaped sample volume is improved by adjusting the

current in these coils.

In an attempt to improve the homogeneity of the field within a specific

portion of the toroid-shaped volume a point shim system was also utilized.

The structure of this shims is illustrated in Figure 3. There are eight

groups of shims P,, P« ?„. Each group is a "Z" shim consisting of two

coils placed above and below the probe.

The method for adjusting the position of this probe in the superconducting solenoid is shown in Figure 4. The support C is made from plastic.

The probe position can be changed (screws a and b) along two axes of freedom

in the plane perpendicular to the Z axis. The photographs for the completed

probe is shown in Figure 5.

A second

 13C/1H probe (Probe B) was built using a conventional flow LC- H

to g\

NMR probe design similar to a design previously publishedv ;. Pictures of

this probe are presented in Figures 7 and 8. This probe consists of a tilted
Figure 2. Sketch of toroid shim system. The inner two pairs of C2 and

C_ shims were wound on a rod (d=A mm) symmetrically placed

3.5 and 8.5 mm, respectively, above and below the torus plane.

The outer two pairs of C_C, shims were wound on a glass cylinder

(d=22 mm) at the same position as indicated for the inner pairs

. of C? and C. shims. The outer Z, and 2 shims were wound on a 30 mm

glass cylinde r in simila r fashion. The planar shims C. are 14 mm

above and below the torus plane. The glass toroid sample container

has a volume (CHC1,) of 225 pi with an inner and outer diameter of

6 and 7.5 mm, respectively. The receiving and transmitting coil was

wound about thi s glass container (not indicated in sketch).
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9

Figure 4. Sketch of mechanical linkage for positioning toroid shim

system in the xy plane of B .
10

ORIGINAL PAGE IS

POOR QUALITY

Figure 5. Photograph of assembled H NMR flow probe and components.
11

ORIGINAL PAGE IS

OF POOR QUALITY

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 3C NMR flow probe B
13

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Figu re8.

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of c.ose-up v - of ^ con

•mcluding Helnholtz H co,l am
solenoid coil tuned to 50 MHz (13C) and a Helmholtz coil for *H decoupling 200

MHz. Although not clearly indicated, a pyrex glass cell with volume of -250

Ml was used for the C NMR flow experiments.

 :
15

RESULTS AND DISCUSSION

The results for experimental probe A and shim coil system provide a

significant improvement in the BQ homogeneity for the toroid-shaped sample

volume. Using only shim coil groups C, and C2 the static H NMR linewidths

can be reduced from -50 Hz to -7.5 Hz (see Figure 6). The resolution of (-.05

ppm) obtained with this shim system is certainly adequate for certain NMR

nuclides (e.g., 0, N, etc.) which are broadened by quadropolar

interactions *'. However, this is unsatisfactory for high resolution H NMR

and marginally acceptable for C studies. The resolution appears to be

limited by magnetic susceptibility distortions and/or discontinuities between

the glass, sample, copper, and surrounding medium of the torus region.

Initial results for probe B are presented in Figure 9 . The flow C

NMR spectrum for neat isooctane was obtained in 16 scans at a flow rate of 2

ml/min. Other conditions for this spectrum are indicated in Figure 9. In

addition, the linewidths are artificially broadened by experimental weighing

of the F.I.D. to enhance the (S/N).

The next step in this study would involve direct coupling of a high

performance liquid chromatography (HPLC) to the C NMR flow detector.

Unfortunately, further work on this study was terminated after the first year

of this project.
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References

1. Watanabe, N. and Niki, E., Proc. Jpn. Acad. 1978, 54, 194.

2. Bayer, E., Albert, K., Nieder, M., Grom, E. and Keller, T., Advan.

Chromatogr.,1979, 14, 525.

3. Haw, J. P., Glass, T. E., Hausler, D. W., Motel 1, E. and Dorn, H. C.,

Anal. Chem. 1980, 52, 1135.

4. Bayer, E., Albert, E., Nieder, M., Grom, E. and Keller, T., J.

Chromatogr., 1979, 186. 497.

5. Buddrus, J. and Herzog, H., Org. Magn. Reson., 1980, 13, 153.

6. Bayer, E., Albert, K., Nieder, M., Grom, E. and Zhu, An Fresenius1 Z.,

Anal. Chem. 1980, 304, 111.

7. Buddrus, J., Herzog, H. and Cooper, J. W., J. Magn. Reson., 1981, 42,

453.

8. Haw, J. F., Glass, T. E. and Dorn, H. C., Anal. Chem. 1981, 5,3, 2327.

9. Haw, J. F., Glass, T. E. and Dorn, H. C., Anal. Chem. 1981, 53, 2332.

10. Haw, J. F., Glass, T. E. and Dorn, H. C., J. Magn. Reson. 1982, 49, 22.

11. Bayer, E., Albert, K., Nieder, M., Grom, E., Wolff, G. and Rindlisbacher,

M., Anal. Chem., 1982, 54, 1747.

12. Haw, J. F., Glass, T. E. and Dorn, H. C., Anal. Chem. 1983, j>5,22

13. Dorn, H.C.,

 lflH and 13C Fourier Transform NMR Characterization of Jet

Fuels Derived from Alternative Energy Sources", Contract #N00173-78-0424,

Final Progress Report, Naval Research Laboratory, Washington, DC and Air

Force Wright Aeronautical Laboratories, Dayton, OH (1978-1981).

14. Dorn, H. C., Glass, T. E. and Roy, J. R., unpublished results (1982).

15. Laude, D.A., Jr., Lee, R. W. K. and Wilkins, C. L., J. Magn. Reson., 60,

453 (1984).

16. (a) D. W. Alderman and D. M. Grant, 21st Experimental NMR Conference,

Tallahassee, Florida (1980); (b) S. B. W. Roeder, A. A. V. Gibson and E.

Fukushima, 23rd Experimental NMR Conference, Madison, Wisconsin (1982).

17. T. E. Glass and H. C. Dorn, J. Magn. Reson., _51, 527 (1983).

18. T. E. Glass and H. C. Dorn, J. Magn. Reson.. 5>2, 518 (1983).

19. A. Zussman, M. Oron and E. Rapoport, Rev. Sci. Instrum., 47, 388 (1976).

20. R. K. Cooper and J. A. Jackson, J. Magn. Reson.. 41, 400 (1980).
18

21. W. A. Anderson, Rev. Scl. Instrum., .32, 241 (1961).

22. R. Freeman and A. Bax J. Maqn. Reson., ,37, 177 (1980).

23. D. I. Hoult, Prog. NMR Spectro.. 12 4 (1978).