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A high sensitivity toroid detector for 17O NMR
JOURNAL
OF MAGNETIC
RESONANCE
52, 5 18-522 ( 1983)
A High Sensitivity Toroid Detector for “0 NMR T. E. GLASS AND H. C. DORN Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received December 27, 1982
The low natural abundance (0.037%) and modest nuclear moment for the I70 nuclide have until recently severely restricted I70 NMR studies (I, 2). In addition, the quadrupole (I = 3) dominated relaxation mechanism and corresponding short T1 and T2 relaxation times typically yield I70 NMR spectra with linewidths of 10 Hz- 1 kHz. However, the advent of routine high field superconducting magnets (4.79.4 T) which provide dramatic increases in the number of I70 NMR spectral resolution elements (i.e., 20-4000) have led to a resurgence of interest in studies of this nuclide. For example, re&nt studies involving organometallic carbonyl compounds illustrate advantages in monitoring 170 instead of the 13C nuclide (3, 4). In spite of the routine availability of high field superconducting magnets, sensitivity remains a major limitation in I70 NMR which in many cases dictates the use of isotopically enriched oxygen-17 samples (3, 5). In a recent communication (6) we have presented data which suggest a potential S/N advantage of 3.9-4.5 for a toroid shaped cell and coil detector in comparison with a conventional Helmholtz coil detector. This study focused attention on narrow-line nuclides (i.e., ‘H) with relatively small sample volumes. The toroid design can also provide a dramatic sensitivity advantage for large sample volumes in comparison with the more commonly employed Helmholtz detector. However, of paramount concern in employing this detector for large volumes is B0 and BI homogeneity. This situation is relaxed to essentially a consideration of B, homogeneity for most cases involving broad-line NMR nuclides such as 170. To test the applicability of the toroid design for I70 NMR, four different toroid Pyrex cells and coils (cells l-4) were constructed for a narrow bore (54 mm) Jeol RX-200 spectrometer with an I70 operating frequency of 27.02 MHz. The toroid cells were symmetrically positioned in the xy plane of a homebuilt probe and tuned with a conventional tuning circuit (6). The dimensions and respective volumes for these toroid cells are presented in Table 1 and Fig. 1. The measured inductances and Q for these toroid coils are also presented in Table 1. In Table 2, signal-to-noise (\k) data is presented for the four toroid cells and comparative data for our commercial wide-band Jeol probe which has a conventional Helmholtz coil design (cell 5). The \k values were the average obtained for three independent measurements. In each case the same conditions (i.e., transmitter power, receiver preamplifier, acquisition time, and line broadening function) were employed as closely as possible. 518 0022-2364183 $3.00 Copyright 63 1983 by Academic PITS, Inc. All ri&ts of reproduction in any form reserved.
519
COMMUNICATIONS TABLE
1
CELL GEOMETRIES, SAMPLE VOLUMES, AND COIL PROPERTIES
TOROID
Coil’ rl (mm)
(Coil and sample cell’ I 2 3 4 !i
(Toroid) (Toroid) (Toroid) (z elongated toroid) (Helmholtz)
10 4.5 3.5 7.25
18.5d
r2
(mm)
Estimated sample volumeb (ml)
12
0.17
11.5 12.0 11.5 8.5d
1.70 2.70 2.70 2.86
Inductance W) 0.26 0.78
1.45 3.05
-
Q 113 163 244 256
-
“See Fig. 1. ’ Sample volumes for the toroid cells l-4 were obtained by filling toroid cells with a known volume. The volume for the Helmholtz case (5) was obtained by measuring the volume contained in a conventional 15 mm cylindrical tube hlled to a height of 18.5 mm. ’ The inductance and Q values were measured using a Hewlett-Packard Vector impedance meter. The toroid coils were close-wound with 23 (AWG) copper wire around the pyrex toroid cells in unifilar fashion. d The height and radius, respectively, for the conventional Jeol FX-200 multinuclear 15 mm Helmholtz coil.
As previously mentioned, the major consideration for the toroid design is B, homogeneity which has an r-l dependence. For the case of large volumes, it is difficult to achieve an r#, ratio approaching unity and still have a respectable volume for narrow bore supercon magnets. The rz/rl ratios for toroidal cells l-3 are 1.2, 2.6, and 3.4, respectively. Only the first has a reasonable ratio, but it has only a very modest volume (0.17 ml). Thus, optimum realization of the S/N advantage for the toroid cell on a unit volume basis is expected for only cell 1. In all cases, the sample volumes represent virtually the maximum possible sample volumes defined by the respective cell and coil geometries. This represents the situation where an unlimited quantity of sample is available to fill nearly the maximum volume defined by the ctoil geometries. On a unit volume basis, toroid cell 1, with good B1 homogeneity, has a 6.7 S/N advantage over the conventional Helmholtz detector, coil 5. In contrast,
A
FtG. 1. Cross-sectional view for toroid cells I-4: (A) design for toroid cells 1-3: (B) design for z elongated toroid 4.
520
COMMUNICATIONS TABLE 2 “0
NMR
SIGNAL-TO-NOISE (0) AND HELMHOLTZ
COMPARISONS BETWEEN CELL CONFIGURATIONS
Coil and sample cell” I (toroid) 2 (toroid) 3 (toroid) 4 (z elongated toroid) 5 (Helmholtz)
14.5 28.5 30.0 36.0 115.0
16 98 120 148 40
VARIOUS
TOROID
Per unit volume WJ”’
Toroid advantage Wm/Wd
94 58 44 55 14
6.1 4.1 3.2 3.9 -
a See Table 1. ‘The 90” pulse length (q,,.) was measured at a constant rf power level for all cases. In all cases, the +,,. was obtained from measured 180” pulses with the center band less than 1000 Hz from the line of interest (DrO). ‘The rms * values were measured for a sample of 25% deuterium oxide and 75% &acetone in all cases. The S/N was obtained for 1000 scans using r 90.and a delay of 139.2 msec. It was experimtxnally determined that this delay was sufficient for full recovery of the magnetization between pulses.
toroid cells 2 and 3 with poorer B, homogeneity still have S/N advantages of 4.1 and 3.2, respectively. In an attempt to provide greater sample volume without further increases in the rz/r, ratio, one could envision increasing the z dimension of the toroid cell. With this in mind, a z-elongated toroid cell was constructed (cell 4, see Fig. 1) providing the same volume as cell 3, but with a smaller r-*/r, ratio (r2/r1 = 1.6). A slight additional improvement in the 9 value was obtained in this case for the same volume. Although rwO was somewhat longer than expected, the coil Q is somewhat higher (Table 1) for cell 4 than for cell 3. In any case, cell 4 had the highest S/N of any of the cells examined. In Fig. 2 are the I70 spectra for a 25% deuterium oxide and 75% &-acetone solution utilizing toroid cell 4 and using the conventional Helmholtz coil. The superior performance of the toroid design is readily apparent in this figure even though the toroid cell has slightly lower volume (2.70 vs 2.86 ml). Hoult (7) has presented convincing data that the 90” pulse length (TV”) for a single coil experiment is directly proportional to the S/N for the same conditions (i.e., rf transmitter power, same sample volume, etc.). The appropriate relationship is given below where W is the transmitter power and l/@m is the irradiating field (7).
or
Although ideally comparisons of q should be made with the same sample volume (V,), it is interesting to note that the proportionality given by [2] is fairly accurate in predicting the S/N performance of toroid cells l-4 in comparison with the Helmholtz
521
COMMUNICATIONS
FIG. 2. “0 NMR S/N comparison for 25% DzO-toroid and Helmholtz configuration: (A) toroid cell and coil 4 (2.7 ml volume); (B) convention Helmholtz coil S (2.86 ml volume).
coil design. For example, Table 3 illustrates the rather good agreement between predicted \k JJ\k, ratios based on 90” pulse lengths and V, at constant W. These are compared with the experimentally observed \kdJ?lrsratios. In all cases the predicted and experimental results differ by -20% or less. It would be expected that the level of agreement would decrease for coils having poor Bi, homogeneity (i.e., poorer correspondence with Q,~). This could possibly be the case for toroid cells 3 and 4. It should also be mentioned that the transmitter power for determination of the 90” pulse lengths (Table 2) was lowered by - 10 dB because of rf arcing in some of the toroid cells. For example, the Jeol Helmholtz coil (cell 5) with full power typically exhibits 7900 = 30 @sec.The arcing problem for the toroids can probably be alleviated by using a uniform-space-wound toroid coil instead of the close-wound coils used in the present study. Other toroid coil designs are also contemplated to help alleviate this potential problem. TABLE 3 COMPAFUSON OF EXPERIMENTAL AND PREDICTED RATIOS BASED ON 90” PULSE LENGTHS
*,I*5 42P5
%I*5 *4l*5
0.47 2.40 3.61 3.02
q
0.40 2.45 4.0 3.15
’ Predicted (U Jq,) ratios based on rwe and V, (see Tables 1 and 2). b Experimental (9 J*‘,) ratios (see Table 2).
522
COMMUNICATIONS
It is also noteworthy that the decreasing \k per unit volume for toroid cells 1 to 3 appear to reflect the increasing r2/rl ratio. This is consistent with the expected decrease in 9 with poorer B,, homogeneity (8). It was somewhat surprising that all the toroid cells examined (cells l-4) exhibited reasonable signal nulls for 180” pulses. In each case, the residual magnetization after a 180” pulse was -5% of the equilibrium magnetization MO. A study of the B ixy spatial distribution for the toroid cell would certainly provide additional insight. Unfortunately, the closed sample volume of the toroid cells hampers simple B,, mapping experiments. However, a zeugmatography field gradient experiment (8) could be used for these B,, spatial mapping experiments. In conclusion, the results of the present study confirm the S/N advantage of the toroid configuration for I70 NMR studies in comparison with the commonly employed Helmholtz coil design. For the case of narrow bore magnets where the r2/rl ratio is much greater than unity (poor Z?, homogeneity), the S/N advantage is m-3.2-4.1 on a unit volume basis. In experiments requiring better B, homogeneity, (i.e., T, measurements) sample volume might have to be sacrificed. For the case of wide bore magnets, reasonable r2/r1 ratios can be readily obtained and a S/N advantage of 4-5 for the toroid configuration should be possible for relatively large sample volumes (i.e., lo-20 ml). Finally, it should be mentioned that the toroid cell design has distinct advantage for other low sensitivity NMR nuclides with broad lines, (such as 14N or 33S),where B0 homogeneity is not a severe constraint. The addition of shim coils which provide a homogeneous torus B0 field (6) could allow extension to narrowline nuclides (e.g., 13C). Presently, the major constraint in using the toroid design is user flexibility. That is, the probe must be removed from the magnet to change samples. However, for the present I70 NMR study we have found that this can be accomplished in -5 min. REFERENCES 1. W. G. KLEMPERER, Angew. Chem. Int. 2. C. RODGER AND M. SHEPPARD, “NMR 3.
4. 5.
6. 7. 8.
Ed. Engl. 17, 246 (1978). and the Periodic Table” (R. K. Harris and B. E. Mann, Ed%), Chap. 12, pp. 383-400, Academic Press, New York, 1978. S. AIME, D. O~ELLA, L. MILONE, G. E. HAWKES, AND E. W. RANDALL, J. Am. Chem. Sot. 103,592O (1981). S. AIME, Inorg. Chim. Actu 62, 5 I (1982). J. A. GERLT, P. C. DEMON, AND S. MEHDI, J. Am. Chem. Sot. 104, 2848 (1982). T. E. GLASS AND H. C. DORN, J. Magn. Reson. 51, 527 (1983). D. I. HOULT AND R. E. RICHARDS, J. Mugn. Reson. 34, 71 (1976). D. I. HOULT, Bog. NMR Spectrosc. 12, 4 (1978).
OF MAGNETIC
RESONANCE
52, 5 18-522 ( 1983)
A High Sensitivity Toroid Detector for “0 NMR T. E. GLASS AND H. C. DORN Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Received December 27, 1982
The low natural abundance (0.037%) and modest nuclear moment for the I70 nuclide have until recently severely restricted I70 NMR studies (I, 2). In addition, the quadrupole (I = 3) dominated relaxation mechanism and corresponding short T1 and T2 relaxation times typically yield I70 NMR spectra with linewidths of 10 Hz- 1 kHz. However, the advent of routine high field superconducting magnets (4.79.4 T) which provide dramatic increases in the number of I70 NMR spectral resolution elements (i.e., 20-4000) have led to a resurgence of interest in studies of this nuclide. For example, re&nt studies involving organometallic carbonyl compounds illustrate advantages in monitoring 170 instead of the 13C nuclide (3, 4). In spite of the routine availability of high field superconducting magnets, sensitivity remains a major limitation in I70 NMR which in many cases dictates the use of isotopically enriched oxygen-17 samples (3, 5). In a recent communication (6) we have presented data which suggest a potential S/N advantage of 3.9-4.5 for a toroid shaped cell and coil detector in comparison with a conventional Helmholtz coil detector. This study focused attention on narrow-line nuclides (i.e., ‘H) with relatively small sample volumes. The toroid design can also provide a dramatic sensitivity advantage for large sample volumes in comparison with the more commonly employed Helmholtz detector. However, of paramount concern in employing this detector for large volumes is B0 and BI homogeneity. This situation is relaxed to essentially a consideration of B, homogeneity for most cases involving broad-line NMR nuclides such as 170. To test the applicability of the toroid design for I70 NMR, four different toroid Pyrex cells and coils (cells l-4) were constructed for a narrow bore (54 mm) Jeol RX-200 spectrometer with an I70 operating frequency of 27.02 MHz. The toroid cells were symmetrically positioned in the xy plane of a homebuilt probe and tuned with a conventional tuning circuit (6). The dimensions and respective volumes for these toroid cells are presented in Table 1 and Fig. 1. The measured inductances and Q for these toroid coils are also presented in Table 1. In Table 2, signal-to-noise (\k) data is presented for the four toroid cells and comparative data for our commercial wide-band Jeol probe which has a conventional Helmholtz coil design (cell 5). The \k values were the average obtained for three independent measurements. In each case the same conditions (i.e., transmitter power, receiver preamplifier, acquisition time, and line broadening function) were employed as closely as possible. 518 0022-2364183 $3.00 Copyright 63 1983 by Academic PITS, Inc. All ri&ts of reproduction in any form reserved.
519
COMMUNICATIONS TABLE
1
CELL GEOMETRIES, SAMPLE VOLUMES, AND COIL PROPERTIES
TOROID
Coil’ rl (mm)
(Coil and sample cell’ I 2 3 4 !i
(Toroid) (Toroid) (Toroid) (z elongated toroid) (Helmholtz)
10 4.5 3.5 7.25
18.5d
r2
(mm)
Estimated sample volumeb (ml)
12
0.17
11.5 12.0 11.5 8.5d
1.70 2.70 2.70 2.86
Inductance W) 0.26 0.78
1.45 3.05
-
Q 113 163 244 256
-
“See Fig. 1. ’ Sample volumes for the toroid cells l-4 were obtained by filling toroid cells with a known volume. The volume for the Helmholtz case (5) was obtained by measuring the volume contained in a conventional 15 mm cylindrical tube hlled to a height of 18.5 mm. ’ The inductance and Q values were measured using a Hewlett-Packard Vector impedance meter. The toroid coils were close-wound with 23 (AWG) copper wire around the pyrex toroid cells in unifilar fashion. d The height and radius, respectively, for the conventional Jeol FX-200 multinuclear 15 mm Helmholtz coil.
As previously mentioned, the major consideration for the toroid design is B, homogeneity which has an r-l dependence. For the case of large volumes, it is difficult to achieve an r#, ratio approaching unity and still have a respectable volume for narrow bore supercon magnets. The rz/rl ratios for toroidal cells l-3 are 1.2, 2.6, and 3.4, respectively. Only the first has a reasonable ratio, but it has only a very modest volume (0.17 ml). Thus, optimum realization of the S/N advantage for the toroid cell on a unit volume basis is expected for only cell 1. In all cases, the sample volumes represent virtually the maximum possible sample volumes defined by the respective cell and coil geometries. This represents the situation where an unlimited quantity of sample is available to fill nearly the maximum volume defined by the ctoil geometries. On a unit volume basis, toroid cell 1, with good B1 homogeneity, has a 6.7 S/N advantage over the conventional Helmholtz detector, coil 5. In contrast,
A
FtG. 1. Cross-sectional view for toroid cells I-4: (A) design for toroid cells 1-3: (B) design for z elongated toroid 4.
520
COMMUNICATIONS TABLE 2 “0
NMR
SIGNAL-TO-NOISE (0) AND HELMHOLTZ
COMPARISONS BETWEEN CELL CONFIGURATIONS
Coil and sample cell” I (toroid) 2 (toroid) 3 (toroid) 4 (z elongated toroid) 5 (Helmholtz)
14.5 28.5 30.0 36.0 115.0
16 98 120 148 40
VARIOUS
TOROID
Per unit volume WJ”’
Toroid advantage Wm/Wd
94 58 44 55 14
6.1 4.1 3.2 3.9 -
a See Table 1. ‘The 90” pulse length (q,,.) was measured at a constant rf power level for all cases. In all cases, the +,,. was obtained from measured 180” pulses with the center band less than 1000 Hz from the line of interest (DrO). ‘The rms * values were measured for a sample of 25% deuterium oxide and 75% &acetone in all cases. The S/N was obtained for 1000 scans using r 90.and a delay of 139.2 msec. It was experimtxnally determined that this delay was sufficient for full recovery of the magnetization between pulses.
toroid cells 2 and 3 with poorer B, homogeneity still have S/N advantages of 4.1 and 3.2, respectively. In an attempt to provide greater sample volume without further increases in the rz/r, ratio, one could envision increasing the z dimension of the toroid cell. With this in mind, a z-elongated toroid cell was constructed (cell 4, see Fig. 1) providing the same volume as cell 3, but with a smaller r-*/r, ratio (r2/r1 = 1.6). A slight additional improvement in the 9 value was obtained in this case for the same volume. Although rwO was somewhat longer than expected, the coil Q is somewhat higher (Table 1) for cell 4 than for cell 3. In any case, cell 4 had the highest S/N of any of the cells examined. In Fig. 2 are the I70 spectra for a 25% deuterium oxide and 75% &-acetone solution utilizing toroid cell 4 and using the conventional Helmholtz coil. The superior performance of the toroid design is readily apparent in this figure even though the toroid cell has slightly lower volume (2.70 vs 2.86 ml). Hoult (7) has presented convincing data that the 90” pulse length (TV”) for a single coil experiment is directly proportional to the S/N for the same conditions (i.e., rf transmitter power, same sample volume, etc.). The appropriate relationship is given below where W is the transmitter power and l/@m is the irradiating field (7).
or
Although ideally comparisons of q should be made with the same sample volume (V,), it is interesting to note that the proportionality given by [2] is fairly accurate in predicting the S/N performance of toroid cells l-4 in comparison with the Helmholtz
521
COMMUNICATIONS
FIG. 2. “0 NMR S/N comparison for 25% DzO-toroid and Helmholtz configuration: (A) toroid cell and coil 4 (2.7 ml volume); (B) convention Helmholtz coil S (2.86 ml volume).
coil design. For example, Table 3 illustrates the rather good agreement between predicted \k JJ\k, ratios based on 90” pulse lengths and V, at constant W. These are compared with the experimentally observed \kdJ?lrsratios. In all cases the predicted and experimental results differ by -20% or less. It would be expected that the level of agreement would decrease for coils having poor Bi, homogeneity (i.e., poorer correspondence with Q,~). This could possibly be the case for toroid cells 3 and 4. It should also be mentioned that the transmitter power for determination of the 90” pulse lengths (Table 2) was lowered by - 10 dB because of rf arcing in some of the toroid cells. For example, the Jeol Helmholtz coil (cell 5) with full power typically exhibits 7900 = 30 @sec.The arcing problem for the toroids can probably be alleviated by using a uniform-space-wound toroid coil instead of the close-wound coils used in the present study. Other toroid coil designs are also contemplated to help alleviate this potential problem. TABLE 3 COMPAFUSON OF EXPERIMENTAL AND PREDICTED RATIOS BASED ON 90” PULSE LENGTHS
*,I*5 42P5
%I*5 *4l*5
0.47 2.40 3.61 3.02
q
0.40 2.45 4.0 3.15
’ Predicted (U Jq,) ratios based on rwe and V, (see Tables 1 and 2). b Experimental (9 J*‘,) ratios (see Table 2).
522
COMMUNICATIONS
It is also noteworthy that the decreasing \k per unit volume for toroid cells 1 to 3 appear to reflect the increasing r2/rl ratio. This is consistent with the expected decrease in 9 with poorer B,, homogeneity (8). It was somewhat surprising that all the toroid cells examined (cells l-4) exhibited reasonable signal nulls for 180” pulses. In each case, the residual magnetization after a 180” pulse was -5% of the equilibrium magnetization MO. A study of the B ixy spatial distribution for the toroid cell would certainly provide additional insight. Unfortunately, the closed sample volume of the toroid cells hampers simple B,, mapping experiments. However, a zeugmatography field gradient experiment (8) could be used for these B,, spatial mapping experiments. In conclusion, the results of the present study confirm the S/N advantage of the toroid configuration for I70 NMR studies in comparison with the commonly employed Helmholtz coil design. For the case of narrow bore magnets where the r2/rl ratio is much greater than unity (poor Z?, homogeneity), the S/N advantage is m-3.2-4.1 on a unit volume basis. In experiments requiring better B, homogeneity, (i.e., T, measurements) sample volume might have to be sacrificed. For the case of wide bore magnets, reasonable r2/r1 ratios can be readily obtained and a S/N advantage of 4-5 for the toroid configuration should be possible for relatively large sample volumes (i.e., lo-20 ml). Finally, it should be mentioned that the toroid cell design has distinct advantage for other low sensitivity NMR nuclides with broad lines, (such as 14N or 33S),where B0 homogeneity is not a severe constraint. The addition of shim coils which provide a homogeneous torus B0 field (6) could allow extension to narrowline nuclides (e.g., 13C). Presently, the major constraint in using the toroid design is user flexibility. That is, the probe must be removed from the magnet to change samples. However, for the present I70 NMR study we have found that this can be accomplished in -5 min. REFERENCES 1. W. G. KLEMPERER, Angew. Chem. Int. 2. C. RODGER AND M. SHEPPARD, “NMR 3.
4. 5.
6. 7. 8.
Ed. Engl. 17, 246 (1978). and the Periodic Table” (R. K. Harris and B. E. Mann, Ed%), Chap. 12, pp. 383-400, Academic Press, New York, 1978. S. AIME, D. O~ELLA, L. MILONE, G. E. HAWKES, AND E. W. RANDALL, J. Am. Chem. Sot. 103,592O (1981). S. AIME, Inorg. Chim. Actu 62, 5 I (1982). J. A. GERLT, P. C. DEMON, AND S. MEHDI, J. Am. Chem. Sot. 104, 2848 (1982). T. E. GLASS AND H. C. DORN, J. Magn. Reson. 51, 527 (1983). D. I. HOULT AND R. E. RICHARDS, J. Mugn. Reson. 34, 71 (1976). D. I. HOULT, Bog. NMR Spectrosc. 12, 4 (1978).