High resolution NMR probes for liquids may contain parts near the coil consiting of the nuclei being observed. The parts give rise to background signals which can severely affect the NMR data. When observing11B, there is a background signal from boron containing parts near the coil and also the borosilicate glass in the NMR tube containing the sample.
Cory and Ritchey* introduced a very simple, clever method to suppress background signals in 1988. Their method uses a composite pulse, consisting of a 90° and two 180° pulses with appropriate phase cycling, in place of a conventional 90° pulse. The phase cycled composite pulse is essentially a 90° pulse for all spins inside the coil and 0° for all spins outside of the coil. An example of its implementation is shown in the figure below. The bottom traces show the 11B [1H] NMR spectra for a dilute sample of NaBH4 and a "real" synthetic sample on the left and right, respectively. One can see an enormous background signal from both the NMR probe and the NMR tube. In the case of the "real" synthetic sample, the information from the spectrum is difficult or impossible to recover. The top traces show similar spectra acquired using the composite pulse. The only background signal remaining is that from the portion of the NMR tube inside the coil. This pulse sequence (without proton decoupling) is in the Bruker pulse program library called "zgbs". It is not exclusive to 11B.
D.G. Cory and W.M. Ritchey. Journal of Magnetic Resonance, 80, 128 (1988).
Monday, April 12, 2010
Solids NMR probes often contain boron rich parts near the coil in which the sample resides. Boron nitride, in particular, is a material very commonly used. This can be very problematic if one wishes to collect 11B NMR data, in that a strong background signal may be observed. Even though these parts are not directly inside the coil with the sample, they do experience a small amount of rf from the coil and the coil does detect a 11B NMR signal from them. One simple way to avoid this problem is to use a Hahn echo to observe the 11B spectrum. Unlike the sample inside the coil, which experiences the 90° and 180° pulses required for the Hahn echo, the boron rich parts outside of the coil experience pulses of much smaller flip angles and therefore the echo signal from them is much reduced. This is illustrated in the figure below, which shows 11B NMR spectra collected in a 4 mm MAS probe without magic angle spinning. In the lower trace a simple one-pulse measurement was made with high power 1H decoupling. The spectrum contains a very large background signal and it is very difficult to extract any useful information from the data. The spectrum in the upper trace was collected with a Hahn echo with high power 1H decoupling during the acquisition. The background signal is completely removed revealing a beautiful, information-rich line shape for the central transition resulting from the second order quadrupolar and chemical shielding anisotropy interactions.
Wednesday, April 7, 2010
Dummy scans (DS, Bruker) or steady state scans (SS, Varian) are scans taken in an NMR acquisition before the receiver is turned on and data are collected. Each dummy scan contains all of the rf pulses, delays and gradients used in the pulse program; the only difference is that the receiver is not turned on to collect data. Dummy scans are typically used to ensure that a spin system is in a steady state before data are collected. For example, One may collect a spectrum using a relaxation delay short with respect to the T1's of some of the resonances in the spectrum. The first scan may find the system at equilibrium, as the sample may have sat in the magnet for a few minutes while the probe was being tuned and matched or the magnet was being shimmed. The second and subsequent scans will find the system not at equilibrium, as the system has been perturbed by the rf pulses of the preceding scan(s) and not allowed to fully relax. The data from the first and seccond scan are therefore not the same. After several scans, although not at equilibrium, the system will be in a steady state before each additional scan and therefore each subsequent scan will collect similar data. An example of the use of dummy scans is shown in the figure below. The bottom trace shows a single scan 13C NMR spectrum of a concentrated solution of menthol in CDCl3 after the sample had sat in the magnet for a minute or so. The top trace shows a similar single scan acquisition preceded by 16 dummy scans. The relaxation delay was set to 1 second and the acquisition time for the FID was 1 second. 90° pulses were used in both spectra. One can see that some of the resonances in the top spectrum are attenuated in comparison to the bottom spectrum. The 13C signal from the CDCl3 (which has a long T1) is missing in the spectrum acquired with dummy scans. The dummy scans have selectively presaturated the solvent.