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B1 and B0 homogeneity considerations for a toroid-shaped sample and detector
JOURNAL
OF MAGNETIC
RESONANCE
51,
527-530
(1983)
COMMUNICATIONS B1 and B0 Homogeneity Considerationsfor a Toroid-ShapedSample and Detector T. E. GLASS AND H. C. I&RN* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received
October
19, 1982
The Helmholtz rf coil is undoubtedly the most commonly employed method for detection of NMR signals in superconducting magnet systems where the main magnetic field (BO) is normally parallel to the spinning z axis of the sample (orthogonal to the B1 field). The main reason for the widespread use of Helmholtz detection coils is user flexibility (i.e., ready removal of cylindrical samples). However, it is well established that the Helmholtz coil represents a rather poor compromise between user flexibility and optimum signal-to-noise (S/N). Hoult (I) was the first to point out the potential loss in S/N by as much as a factor of three utilizing Helmholtz coils in comparison with solenoid coils. This loss in S/N can be largely recovered by employing “sideways spinning” cylindrical samples and solenoid detection coils, but usually requires a wide bore supercon system for moderate to large sample volumes (2). Alternative methods for optimizing S/N are potentially possible in cases where cylindrical spinning samples can be avoided entirely. For example, flow NMR systems devoted to studies of transient intermediates (3-5), liquid chromatography detectors (69), and other flow applications (10, II) do not suffer from this constraint. The utility of toroid-shaped resonators for NMR has been suggested by other workers (12). A potential advantage of the toroid coil is the c,onfinement of the B1 field to the torus region. As part of our continuing effort to improve S/N for flow LC-‘H NMR, we have recently examined a toroid shaped sample cell and single coil detection system. Three different sealed toroid sample cells containing chloroform were constructed from Pyrex glass. The cell and coils are described in Table 1 and Fig. 1. The toroid coils were tuned to 200 MHz by a conventional tuning circuit (Fig. 1) and symmetrically placed in the xy plane of a homebuilt probe for an FX-200 JEOL Instrument. One concern in utilizing a toroid sample cell and coil for high resolution NMR studies is the spatial homogeneity of the rf (B,) field. Since the rf field has an r-’ dependence, a configuration which minimizes the ratio t-Jr, should allow a relatively homogeneous B, field. However, for reasonable sample volumes, a compromise between B, and &, homogeneity requirements must be made. For the toroid cells described above good nulls for 180” pulses were observed for coils 1 and 2. However, * To whom
correspondence
should
be addressed. 527
0022-2364/83/030527-04$03.00/O Copyright Q 1983 by Academic Press, Inc. All rigbu of reproduction in any fom reserved.
528
COMMUNICATIONS TABLE 1 DIMENSIONSAND 90” PULSE LENGTH COMPARISONSFOR VARIOUS TOROID AND HELMHOLTZ RECEIVERCON~GURATIONS ’
Coil and sample cell 1 2 3 4 5
Type
*I (mm)
r2 (mm)
Estimated sample volume &l)
90” pulse. length (P=d
Toroid Toroid Toroid HelmholtF Helmholtz’
5.5 5.3 4.4 15d 13d
6.5 7.3 7.0 5d 4.56
30 95 180
1.8 2.2 3-4b 7.0
40 120
10.0
’ All measurements were performed at 200 MHz with a single coil configuration using the same transmitter power level. b In this case an accurate measurement (+lO%) for the 90” pulse length was not possible. ’ The cells used in cases4 and 5 have been previously described (7, 8) and consist of 2 mm i.d. and 4 mm i.d. cylindrical flow tubes (parallel to &). The volumes estimated from geometry considerations (40 and 120 al) are in good agreement with values estimated from flow studies. d The height and radius, respectively, for the Helmholtz coils used in these cases.
the results for coil 3 indicate that B, homogene:ity could be compromised for rz/rl 2 1.4. Hoult (I) has previously suggested that the S/N is proportional to the 90” pulse length for single coil systems with the same conditions (i.e., same sample volume, rf
I+---r2--4-64 FIG. 1. Toroid coil and Pyrex cells 1-3. Dimensions for rr and r, are presented in Table 1. The toroid coils were close wound with 30 (AWG) copper wire with 3, 3, and 4 coils in parallel for cells 1, 2, and 3, respectively. The inductance ranged from 0.15-0.24 gh for these three coils. Ci and C, were Johanson nonmagnetic variable capacitors, 0.8-10 and 0.4-3.5 pf, mspectively.
COMMUNICATIONS
529
FIG. 2. ‘H spectra for chloroform (I pulse) with toroid sample cell 2 without torus field shims (a), one shim (b), and two shims (c). The half linewidths are 60, 28, and 15 Hz, respectively. In each case, the homogeneity was maximized using the normal JEOL room temperature shims.
power, etc.). In Table 1, we have compared the 90” pulse length for the three toroid coils with other Helmholtz coils commonly utilized in our laboratory (cells 4 and 5). In the cases where B, homogeneity appears to be reasonably uniform over nearly the same sample volume (i.e., cell 1 compared with cell 4, or 2 with 5), one can estimate an improvement in S/N for the toroid by a factor of 3.9-4.5 in comparison with the Helmholtz coil design. However, realization of this improvement for actual flow NMR applications requires further improvements in & homogeneity. The most severe constraint in utilizing the toroid configuration is the problem of 8, spatial homogeneity over the torus region. Obviously, most commercial shim coil systems are specifically designed for spherical or cylindrical sample volumes. To illustrate the problem, toroid cells l-3 exhibit half linewidths of 45-70 Hz with our present JEOL FX-200 spectrometer without the normal high resolution JEOL room temperature shim coils connected. If these shims are connected and maximized, an improvement of only lo-20% is obtained. To improve the & homogeneity specifically for a given torus region, experimental shim coils were constructed. Basically, these consist of four separate coils above and below the torus region of interest and centered along the: z axis providing two pairwise shim controls. The fields of these coils are opposed creating a torus region of high homogeneity orthogonal to the z axis (23). The z axis position of the homogeneous torus region was adjustable by allowing unequal dc current flow in a given shim coil pair. The results obtained with this shim system are presented in Fig. 2. Without our home built shim system, the JEOL FX-200 shims provided a half linewidth for chloroform of 60 Hz (see Fig. 2a) for cell 2. A significant improvement was obtained with our experimental shim system. Specifically, each shim coil pair improves the homogeneity by a factor of -2 (see Fig. 2b and c). To date, the best linewidth achieved
530
COMMUNICATIONS
for toroid cell 2 is 11.7 Hz utilizing two shim coil pairs. Similar results were obtained with the other toroid cells, 1 and 3. Obviously, further improvements can be anticipated by incorporating more shims with the basic design described above. We anticipate that linewidths on the order of l-4 Hz can ultimately be obtained, which is adequate for most flow NMR studies. Since the symmetry of the present toroid cell (and copper coil) are not ideal, closer adherence to the torus symmetry during construction could help minimize local magnetic susceptibility variations across the torus sample region (lb). In conclusion, the results indicate that the toroid sample cell and coil with further improvements could provide a significant improvement in S/N without major sacrifices in B, and B0 homogeneity for flow NMR studies. However, a necessary prerequisite in most cases will be shim coils which minimize the B0 inhomogeneity of the torus region. Although the toroid cells described in this paper are for relatively small sample volumes, large toroid cells could also be applicable for low sensitivity NMR nuclides with inherently broad lines (e.g., “0). In these cases the B0 homogeneity requirement is obviously not a severe limitation. ACKNOWLEDGMENTS We thank
Mr.
F. Van Damme
and Mr.
A. Mollick
for constructing
the Pyrex
glass toroid
cells.
REFERENCES 1. (a) D. I. HOULT AND R. E. RICHARDS, J. Mugn. Reson. 34, 7 1 (1976); (b) D. I. HOULT, Prog. Nh4R Spectrosc. 12, 4 (1978). 2. J. T. BAILEY, R. C. ROSANSKE, AND G. C. LEW, Rev. Sci. Instrum. 54, 548 (198 1). 3. C. A. Fyp~, S. W. DANJI, AND A. KOLL, J. Am. Chem. Sot. 101,951 (1979). 4. C. A. FIFE, S. W. DANJI, AND A. KOLL, J. Am. Chem. Sot. 101,956 (1979). 5. R. KUHNE, T. S~HAFFHAUSER, A. WOKA~N, AND R. ERNST, J. Mugn. Reson. 35, 39 (1979). 6. J. BUDDRUS, H. HERZOG, AND J. W. COOPER, J. Mugn. Reson. 42,453 (1981). 7. E. BAYER, K. ALBERT, M. NIEDER, AND E. GROM, Anal. Chem. 54, 1747 ( 1982). 8. J. F. HAW, T. E. GLASS, AND H. C. DORN, Anal. Chem. 53,2327 (1981). 9. J. F. HAW, T. E. GLASS, AND H. C. DARN, J. Magn. Reson. 49, 22 (1982). 10. G. K. RADDA, P. STYLES, K. R. THULBORN, AND J. C. WATERTON, J. Mugn. Reson. 42,488 (1981). 11. M. A. HEMINCA AND P. A. DEJAGER, J. Mugn. Reson. 37, 1 (1980). 12. (Q) D. W. ALDERMAN AND D. M. GRANT, 21st Experimental NMR Conference, Tallahassee, Ra. (1980); (b) S. B. W. ROEDER, A. A. V. GIBSON, ANI) E. F~KUSHIMA, 23rd Experimental NMR Conference, Madison, Wis. (1982). 13. R. K. CARPER AND J. A. JACKSON, J. Mugn. Reson. 41, 400 (1980).
OF MAGNETIC
RESONANCE
51,
527-530
(1983)
COMMUNICATIONS B1 and B0 Homogeneity Considerationsfor a Toroid-ShapedSample and Detector T. E. GLASS AND H. C. I&RN* Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received
October
19, 1982
The Helmholtz rf coil is undoubtedly the most commonly employed method for detection of NMR signals in superconducting magnet systems where the main magnetic field (BO) is normally parallel to the spinning z axis of the sample (orthogonal to the B1 field). The main reason for the widespread use of Helmholtz detection coils is user flexibility (i.e., ready removal of cylindrical samples). However, it is well established that the Helmholtz coil represents a rather poor compromise between user flexibility and optimum signal-to-noise (S/N). Hoult (I) was the first to point out the potential loss in S/N by as much as a factor of three utilizing Helmholtz coils in comparison with solenoid coils. This loss in S/N can be largely recovered by employing “sideways spinning” cylindrical samples and solenoid detection coils, but usually requires a wide bore supercon system for moderate to large sample volumes (2). Alternative methods for optimizing S/N are potentially possible in cases where cylindrical spinning samples can be avoided entirely. For example, flow NMR systems devoted to studies of transient intermediates (3-5), liquid chromatography detectors (69), and other flow applications (10, II) do not suffer from this constraint. The utility of toroid-shaped resonators for NMR has been suggested by other workers (12). A potential advantage of the toroid coil is the c,onfinement of the B1 field to the torus region. As part of our continuing effort to improve S/N for flow LC-‘H NMR, we have recently examined a toroid shaped sample cell and single coil detection system. Three different sealed toroid sample cells containing chloroform were constructed from Pyrex glass. The cell and coils are described in Table 1 and Fig. 1. The toroid coils were tuned to 200 MHz by a conventional tuning circuit (Fig. 1) and symmetrically placed in the xy plane of a homebuilt probe for an FX-200 JEOL Instrument. One concern in utilizing a toroid sample cell and coil for high resolution NMR studies is the spatial homogeneity of the rf (B,) field. Since the rf field has an r-’ dependence, a configuration which minimizes the ratio t-Jr, should allow a relatively homogeneous B, field. However, for reasonable sample volumes, a compromise between B, and &, homogeneity requirements must be made. For the toroid cells described above good nulls for 180” pulses were observed for coils 1 and 2. However, * To whom
correspondence
should
be addressed. 527
0022-2364/83/030527-04$03.00/O Copyright Q 1983 by Academic Press, Inc. All rigbu of reproduction in any fom reserved.
528
COMMUNICATIONS TABLE 1 DIMENSIONSAND 90” PULSE LENGTH COMPARISONSFOR VARIOUS TOROID AND HELMHOLTZ RECEIVERCON~GURATIONS ’
Coil and sample cell 1 2 3 4 5
Type
*I (mm)
r2 (mm)
Estimated sample volume &l)
90” pulse. length (P=d
Toroid Toroid Toroid HelmholtF Helmholtz’
5.5 5.3 4.4 15d 13d
6.5 7.3 7.0 5d 4.56
30 95 180
1.8 2.2 3-4b 7.0
40 120
10.0
’ All measurements were performed at 200 MHz with a single coil configuration using the same transmitter power level. b In this case an accurate measurement (+lO%) for the 90” pulse length was not possible. ’ The cells used in cases4 and 5 have been previously described (7, 8) and consist of 2 mm i.d. and 4 mm i.d. cylindrical flow tubes (parallel to &). The volumes estimated from geometry considerations (40 and 120 al) are in good agreement with values estimated from flow studies. d The height and radius, respectively, for the Helmholtz coils used in these cases.
the results for coil 3 indicate that B, homogene:ity could be compromised for rz/rl 2 1.4. Hoult (I) has previously suggested that the S/N is proportional to the 90” pulse length for single coil systems with the same conditions (i.e., same sample volume, rf
I+---r2--4-64 FIG. 1. Toroid coil and Pyrex cells 1-3. Dimensions for rr and r, are presented in Table 1. The toroid coils were close wound with 30 (AWG) copper wire with 3, 3, and 4 coils in parallel for cells 1, 2, and 3, respectively. The inductance ranged from 0.15-0.24 gh for these three coils. Ci and C, were Johanson nonmagnetic variable capacitors, 0.8-10 and 0.4-3.5 pf, mspectively.
COMMUNICATIONS
529
FIG. 2. ‘H spectra for chloroform (I pulse) with toroid sample cell 2 without torus field shims (a), one shim (b), and two shims (c). The half linewidths are 60, 28, and 15 Hz, respectively. In each case, the homogeneity was maximized using the normal JEOL room temperature shims.
power, etc.). In Table 1, we have compared the 90” pulse length for the three toroid coils with other Helmholtz coils commonly utilized in our laboratory (cells 4 and 5). In the cases where B, homogeneity appears to be reasonably uniform over nearly the same sample volume (i.e., cell 1 compared with cell 4, or 2 with 5), one can estimate an improvement in S/N for the toroid by a factor of 3.9-4.5 in comparison with the Helmholtz coil design. However, realization of this improvement for actual flow NMR applications requires further improvements in & homogeneity. The most severe constraint in utilizing the toroid configuration is the problem of 8, spatial homogeneity over the torus region. Obviously, most commercial shim coil systems are specifically designed for spherical or cylindrical sample volumes. To illustrate the problem, toroid cells l-3 exhibit half linewidths of 45-70 Hz with our present JEOL FX-200 spectrometer without the normal high resolution JEOL room temperature shim coils connected. If these shims are connected and maximized, an improvement of only lo-20% is obtained. To improve the & homogeneity specifically for a given torus region, experimental shim coils were constructed. Basically, these consist of four separate coils above and below the torus region of interest and centered along the: z axis providing two pairwise shim controls. The fields of these coils are opposed creating a torus region of high homogeneity orthogonal to the z axis (23). The z axis position of the homogeneous torus region was adjustable by allowing unequal dc current flow in a given shim coil pair. The results obtained with this shim system are presented in Fig. 2. Without our home built shim system, the JEOL FX-200 shims provided a half linewidth for chloroform of 60 Hz (see Fig. 2a) for cell 2. A significant improvement was obtained with our experimental shim system. Specifically, each shim coil pair improves the homogeneity by a factor of -2 (see Fig. 2b and c). To date, the best linewidth achieved
530
COMMUNICATIONS
for toroid cell 2 is 11.7 Hz utilizing two shim coil pairs. Similar results were obtained with the other toroid cells, 1 and 3. Obviously, further improvements can be anticipated by incorporating more shims with the basic design described above. We anticipate that linewidths on the order of l-4 Hz can ultimately be obtained, which is adequate for most flow NMR studies. Since the symmetry of the present toroid cell (and copper coil) are not ideal, closer adherence to the torus symmetry during construction could help minimize local magnetic susceptibility variations across the torus sample region (lb). In conclusion, the results indicate that the toroid sample cell and coil with further improvements could provide a significant improvement in S/N without major sacrifices in B, and B0 homogeneity for flow NMR studies. However, a necessary prerequisite in most cases will be shim coils which minimize the B0 inhomogeneity of the torus region. Although the toroid cells described in this paper are for relatively small sample volumes, large toroid cells could also be applicable for low sensitivity NMR nuclides with inherently broad lines (e.g., “0). In these cases the B0 homogeneity requirement is obviously not a severe limitation. ACKNOWLEDGMENTS We thank
Mr.
F. Van Damme
and Mr.
A. Mollick
for constructing
the Pyrex
glass toroid
cells.
REFERENCES 1. (a) D. I. HOULT AND R. E. RICHARDS, J. Mugn. Reson. 34, 7 1 (1976); (b) D. I. HOULT, Prog. Nh4R Spectrosc. 12, 4 (1978). 2. J. T. BAILEY, R. C. ROSANSKE, AND G. C. LEW, Rev. Sci. Instrum. 54, 548 (198 1). 3. C. A. Fyp~, S. W. DANJI, AND A. KOLL, J. Am. Chem. Sot. 101,951 (1979). 4. C. A. FIFE, S. W. DANJI, AND A. KOLL, J. Am. Chem. Sot. 101,956 (1979). 5. R. KUHNE, T. S~HAFFHAUSER, A. WOKA~N, AND R. ERNST, J. Mugn. Reson. 35, 39 (1979). 6. J. BUDDRUS, H. HERZOG, AND J. W. COOPER, J. Mugn. Reson. 42,453 (1981). 7. E. BAYER, K. ALBERT, M. NIEDER, AND E. GROM, Anal. Chem. 54, 1747 ( 1982). 8. J. F. HAW, T. E. GLASS, AND H. C. DORN, Anal. Chem. 53,2327 (1981). 9. J. F. HAW, T. E. GLASS, AND H. C. DARN, J. Magn. Reson. 49, 22 (1982). 10. G. K. RADDA, P. STYLES, K. R. THULBORN, AND J. C. WATERTON, J. Mugn. Reson. 42,488 (1981). 11. M. A. HEMINCA AND P. A. DEJAGER, J. Mugn. Reson. 37, 1 (1980). 12. (Q) D. W. ALDERMAN AND D. M. GRANT, 21st Experimental NMR Conference, Tallahassee, Ra. (1980); (b) S. B. W. ROEDER, A. A. V. GIBSON, ANI) E. F~KUSHIMA, 23rd Experimental NMR Conference, Madison, Wis. (1982). 13. R. K. CARPER AND J. A. JACKSON, J. Mugn. Reson. 41, 400 (1980).