C. Jaeger et al. have very recently introduced a new technique to suppress probe background signals, spectral distortions due to deadtime ringdown effects and acoustic ringing artifacts . The method named EASY (Elimination of Artifacts in NMR SpectroscopY) does not require lengthy phase cycling, is rather easy to implement and is suitable for quantitative NMR studies. This application note demonstrates the background suppression with EASY. The pulse sequence is shown in Figure 1. It consists of acquiring two FIDs, FID 1 and FID 2, without any relaxation delay between them. While the first FID contains signals of the sample and the probe background, the second one represents the background signal only, because background experiences small flip angles and tends to have short T1 relaxation times. These two FIDs are subtracted to produce the spectrum of the sample. This basic unit can be followed by a relaxation delay and repeated to increase signal-to-noise ratio. The most appealing feature of the experiment is that background signals are subtracted immediately, in every unit, and virtually within the same experimental time.
Figure 1. EASY pulse sequence. FID 1 and FID 2 are recorded without any delay and subtracted to remove background signals. The pulse sequence can be repeated to accumulate spectra after a relaxation delay. Flip angles of 90º are preferable.
Figure 2 shows a comparison of 1H single pulse and EASY spectra recorded on a 500 MHz spectrometer (JNM-ECA500II) equipped with a 3.2 mm CPMAS probe. The spectra of adamantane were recorded at MAS 8 kHz with one scan. While the single pulse spectrum shown in brown contains a broad background signal due to construction materials of the probe, this signal is greatly eliminated by the EASY pulse sequence as shown in green.
Figure 2. 1H single pulse (brown) and EASY (green) spectra of adamantane recorded with one scan at MAS 8 kHz with a 3.2 mm HXMAS probe.
Figure 3. 1H saturation recovery (brown) and EASY saturation recovery (green) spectra of adamantane recorded with one scan and delay of 1 ms.
Figure 3 demonstrates that the EASY approach can be implemented successfully into other solid-state NMR experiments. The example given here is a saturation recovery experiment allowing one to measure T1 relaxation time. The spectra were recorded with a very short delay to saturate the peaks of adamantane. For this reason, the ordinary saturation recovery spectrum shows the very broad background signal, while the EASY version is free of any signal.