Shaped Pulses

Shaped rf pulses are used frequently in modern NMR experiments for selective excitation and more efficient inversion. The figure below shows some of the pulse shapes in the Bruker shape library measured with an oscilloscope on an AVANCE III console. Each 50.3 MHz rf pulse was 1 msec in duration and was measured at the output of the signal generation unit.

"Absolute" Water Suppression

Collecting ¹H NMR spectra of aqueous samples is complicated by the presence of the enormous, broad water signal which is often many orders of magnitude more intense than the signals from the solute of interest. The water signal can be suppressed by presaturation or multiple pulse techniques employing gradients (such as WATERGATE). These techniques are compared here and do a very good job, but neither is able to completely suppress the water signal in very dilute challenging samples. Recently, a technique introduced by Buuan Lam and Andre Simpson* which uses both presaturation and W5-WATERGATE has been used to suppress the water signal. In this case, the presaturation consists of many low power shaped 180° pulses. The combination of these two techniques provides incredible water suppression. The authors are able to measure the ¹H NMR spectra of dissolved organic matter in natural waters without any preconcentration of the samples. Their spectra are completely free of the water signal! At the 2011 ENC the same group presented a poster** highlighting the water-free ¹H NMR spectrum of the dissolved organic matter in melted glacial ice. The water suppression is so remarkable that this technique should be called "The Simpson Sledgehammer". The figure below illustrates further examples of the implementation of this suppression method. The bottom trace shows the 32 scan ¹H NMR spectrum of a dilute unlabelled protein (which includes the NH resonances, albeit attenuated due to chemical exchange). The middle trace shows the 32 scan ¹H NMR spectrum of supernatant human saliva. The upper trace shows the ¹H NMR spectrum of rain water collected from an asphalt tile roof. The spectrum was collected overnight and shows the presence of long chain hydrocarbons from the asphalt as well as formaldehyde. In all cases the water signal is completely absent in the spectrum.

* Buuan Lam and Andre J. Simpson. Direct ¹H NMR Spectroscopy of Dissolved Organic Matter in Natural Waters. The Analyst 113 263 (2008).

** Brent Pautler, Andre Simpson, Li-Hong Tseng, Manfred Spraul, Ashley Dubnick, Martin Sharp and Myrna Simpson. Trace Level Analysis of Dissolved Organic Matter in Glacial Ice Using SPR-W5-WATERGATE. POSTER 264, ENC (2011).