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Our coil area length is 16 mm. Should we be making samples exactly 16 mm long in the Shigemi tubes for the best sensitivity (assuming that we are limited on amount of material)? For example, we use D2O matched shigemi tubes for samples in 90/10 H2O/D2O. Is susceptibility matching affected by switch from D2O to H2O? Thanks! |
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We work on vendors recommendations that the solvent length should be slight longer than the coil length, by 2mm on each side. So for a 16mm coil, the optimal solvent length would be 20mm. I can see some slight degradation of solvent suppression quality when going shorter, but not too severe. So we do go shorter if sample amount/concentration is at a premium. |
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There is a difference of 0.22 ppm in magnetic susceptibility between D2O and H2O (eg. Kuchel et al), but taking into account that the susceptibility change is going to be outside the active region, the increment in linewidth should be small [assuming a perfectly mixed sample, otherwise the lineshape will change while D2O and H2O are getting mixed]. At the edges of the coil, pulses and gradients start deviating from the ideal behaviour. If the susceptibility matching were perfect, it may be better to have the sample in a smaller region (12-14 mm?). But susceptibility matching is never perfect, so filling 16mm is probably the best compromise between SNR and resolution. [I haven't used Shigemi tubes, so these are just my assumptions] Another option (although not so efficient) would be to use smaller tubes, eg. 3 mm tubes in a 5mm probe, or even use a 1.7 mm probe if available. |
