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Figure 7.8. (A) Dual isolated-coil NMR probe, and representative 13C spectra of (B) 13CCl4 and (C) 13CH3OH obtained simultaneously using this probe. (Adapted with permission from J. Magn. Reson., 138: 160. Copyright Academic Press, 1999).
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tively. As the NMR spectra in Figure 7.8B and 8.4C show, there was no cross-talk between the two coils. The concept of using decoupled coils has been further developed by Li et al.36 In this case an RF switch was used for time-domain multiplexing of the signals into a single receiver channel. A four-sample system (shown in Figure 7.9A) was constructed using printed circuit boards for operation at 6 Tesla in a wide-bore (89-mm) magnet that incorporated microcoils with observe volumes of 28 nL. The microcoil-based resonant circuits were mounted one above another with a vertical spacing of 5 mm between adjacent coils. Alternate coils were rotated 90 with respect to each other to reduce the electromagnetic coupling. The matching networks were also placed at 90 to each other, again to reduce coupling. The whole system was surrounded by a container lled with FC-43, a magnetic susceptibility matching uid. Although the original implementation required four preampli ers in the receiver chain, this was due to a relatively high-loss (2.3 dB) switch being used. In subsequent realizations, a much lower loss (0.1 dB) switch has been substituted, which results in a simpli ed receiver design. The time required for shimming the four coils to 2 to 4 Hz was typically less than 30 minutes. Both one-dimensional and two-dimensional correlation spectroscopy (COSY) experiments were carried out at 250 MHz for protons, and the spectra obtained from four solutions of 250 mM fruc-
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Figure 7.9. (A) Photograph of the four-coil probe developed by Li et al. showing the con guration of the microcoils and matching networks (top), and the schematic of coil arrangement (bottom): each coil is oriented at 90 with respect to adjacent coils. (B) One-dimensional spectra of (top left) galactose, (bottom left) adenosine triphosphate, (top right) chloroquine, and (bottom right) fructose (all 250 mM in D2O) acquired using the four-coil probe. (C) COSY spectra of the same four samples as in (B) acquired using the four-coil probe. (Adapted with permission from Anal. Chem., 71: 4815. Copyright American Chemical Society, 1999).
tose, chloroquine, galactose, and adenosine triphosphate, all in D2O are shown in Figure 7.9B and 7.9C. No signal bleed-through was observed from one spectrum to another. Since the acquisition time was approximately an order of magnitude less than the recycle delay, a full factor of four improvement in throughput was achieved; in fact the number of coils could be increased to improve the temporal ef ciency further, assuming these additional coils could be shimmed. Recent developments have included the design of a two-coil probehead, with each coil having a much larger volume (15 mL), allowing the study of metabolites at concentrations below 10 mM.111 In this instance the solenoid coils were separated horizontally, and had electrical isolations greater than 30 dB. The switching of the two coils between the transmitter and receiver was realized by a network containing two RF switches, as shown in Figure
parallel nmr techniques
7.10A. Both of the coils were double-tuned to proton and nitrogen frequencies to allow multiple-nuclear and multiple-sample experiments to be carried out. The COSY and HMQD spectra (Figure 7.10B and 7.10C) obtained using this two-coil probe show no evidence of cross-talk between the two coils. 7.2.3. Multiple Coils Connected in Parallel
A third approach in parallel NMR detection is to connect multiple coils together in the same RF circuit (see Figure 7.4C). The rst implementation of this method, termed multiplex NMR, was achieved by MacNamara et al.,37 and a picture of the multiplex NMR probe head is shown in Figure 7.11. The NMR coils were of solenoid geometry constructed from ve turns of polyurethane-coated high-purity 42-gauge copper wire wrapped around glass capillaries. The coil had an i.d. of 0.8 mm and length of 0.5 mm, and the sample volume was 60 nL. The inter-coil (center-to-center) distance was 3.2 mm. The entire coil array was surrounded by Fluorinert FC-43, a susceptibility matching uid that has a very similar volume magnetic susceptibility to copper and has been shown to improve magnetic eld homogeneity by minimizing eld distortions induced by copper NMR coils.24 A single resonance was observed for the circuit, and the circuit had a tuning range of ~2 MHz with a quality factor (Q) value of 60. Te on tubes were attached to the capillaries to allow ow introduction of samples. A standard NMR spectrometer was used in these experiments. Because all the sample coils are wired into the same resonant circuit, there is no ill effect of the coupling between different coils in the multiplex NMR probe. However, since the detected signal is a combination of all the samples, it is necessary to differentiate the signals emanating from a particular sample. To carry out this task, magnetic eld gradients were applied across the different coils. To illustrate how the eld gradient works, a schematic diagram is shown in Figure 7.12 for the hypothetical case of a single analyte such as water in all four coils. Figure 7.12A shows the situation after the probe has been properly shimmed and no linear eld gradient is applied to the sample region. The four sample regions experience the same magnetic eld, B0, and consequently the NMR signals from all four coils coincide; thus the spectrum only contains one peak. However, when a eld gradient is applied to the sample region, each of the sample coils experiences a eld given by B0 + Gz zi, where Gz is the strength of the linear eld gradient and zi is the vertical position of the ith sample coil. The resultant spectrum contains four lines, since each sample experiences a different magnetic eld and each peak can be identi ed with a particular coil (see Figure 7.12B).