Solvent Suppression Pulses

Special pulses containing one or several notches in their excitation profiles have been designed for purposes of solvent suppression. Obviously such pulses could be used also for suppression of parent lines in applications involving isotopi-cally diluted nuclei. One of the simplest experiments of this kind is the jump and return experiment [24] and corresponding higher order binomial pulses with alternating phases. Similarly to binomial excitation pulses these constant amplitude solvent suppression pulses create sidebands and excitation sidelobes. Corresponding amplitude modulated pulses [25,26] provide a better alternative. 

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Wolfender et ah, and this coupled technique is treated more comprehensively in Chapter 1. The LC-NMR technique is by nature rather insensitive however, high-field magnets and recent improvements in solvent suppression, pulse field gradients, and probe technology have made it possible to achieve useful results on various flavonoid struc-tures. The detection limit with a 60 p.1 cell in a 500 MHz instrument for a compound with a molecular weight of around 400 amu may typically be around 20 p.g, and the information provided is hitherto mainly based on NMR spectra or correlation experiments. 

Direct on-line coupling of an NMR spectrometer as a detector for chromatographic separation, analogous to the use of MS for such applications, has required the development of technical features such as flow-probe hardware, efficient NMR solvent suppression pulse sequences and new software. 

V. Smith, J. Kurhanewicz, T.L. James, Solvent-suppression, pulses. III. design using simulated-annealing optimization with in vitro and in vivo testing. Journal of Magnetic Resonance, 96 (1992) 345. 

Finally, a comparison of the solvent suppression pulse sequences was performed with purposeful errors in pulse width (—10%) or carrier position (—4 Hz). 

The choice of suitable solvent suppression pulse sequences is not trivial. A variety of solvent suppression techniques yield excellent solvent-reduced spectra under qualitative considerations, but some of these techniques may lead to substantial quantification errors. Generally, cautious reduction of the solvent signal in combination with data analysis (e.g., Lorentzian-Gaussian curve fitting) should always be preferred to its complete suppression. Commonly used techniques such as presaturation are not recommended for quantitative studies because they are not sufficiently... 

Dalvit C, Shapiro G, Bohlen J-M, and Patella T (1999) Technical aspects of an efficient multiple solvent suppression pulse sequence. Magnetic Resonance in Chemistry 37 7-14. 

Figure 5 Solvent suppression pulse sequences based on filtering methods. Method (A) uses a 7, filter to discriminate resonances of solvent and solutes. The difference in molecular diffusion coefficients is used in method (B). is the spin-echo time. (C) Double-quantum filtering COSY, which uses the fact that there is no J-coupling between the two equivalent protons in water molecules and thus it cannot be excited to higher quantum coherence. The PFG pulses in (A) and (B) are used to attenuate radiation damping effects and dephase any transverse magnetization. They are used for the desired coherence selection in (C).

In this chapter we illustrate a direct method of characterisation of polymer/additive dissolutions by means of (500 MHz) NMR, which takes advantage of selective signal suppression allowing elimination of unwanted signals, such as the ca. 105 x more intensive PE signal. The most effective approach to solvent suppression is the destruction of the net solvent magnetisation by pulsed... 

The use of actively shielded magnetic field gradients has made the use of pulsed field gradients possible. The use of pulsed field gradients reduces experiment time, minimizes artifacts, and allows for further solvent suppression. 

Figure 1.16 Representation of the WET pulse sequence for multiple solvent suppression...

The NOESY sequence proved to be very effective for the reduction of one particular signal such as the methyl group of acetonitrile. However, very often the mobile phase has a composition of several solvents, together with up to six solvent signals. Here, the application of the soft pulse multiple solvent suppression technique is advisable. 

The H LC-NMR spectra were obtained on peaks stored in the BPSU-36 storage loops. Data were acquired with WET [41] solvent suppression on the residual water and acetonitrile signals. A composite 90° observe pulse, (tt/2)y—(tt/2) x—(tt/2) y—(tt/2)x, was employed. Spectra were collected into 32K data points over a width of 12 019 Hz, giving an acquisition time of 1.36 s, with an additional relaxation delay of 1.5 s. The data were multiplied by a line-broadening function of 1 Hz to improve the signal-to-noise ratio and zero-filled by a factor of two before Fourier transformation. 

This main difficulty in coupling HPLC to NMR spectroscopy is faced by methods known as solvent suppression techniques, where the large solvent signals are reduced by special pulse sequences, switched prior to the information-selecting and acquisition pulses. Therefore, many efforts have been made to develop effective and minor-disturbing pulse sequences, such as presaturation, zero excitation and PFG-pulse sequences (WET) (see Chapter 1 and the following chapters). Despite the possibility of also suppressing several of the... 

Selective probe heads are used for optimal sensitivity for a particular nucleus. Sensitivity of a selective H probe head is normally greater than that of a switchable probe head with indirect observation. With a selective X-nucleus (a nucleus other than proton) probe head, decoupling of protons is normally possible. Because of their limited usefulness, selective probe heads are rare in NMR laboratories. Other probe heads are also available, for example, those for triple resonance experiments and experiments utilizing pulsed-field gradients. In addition to their suitability for 2-D experiments, the gradients are particularly suitable for solvent suppression (20). 

H and 19F NMR spectra are recorded with a normal one-pulse sequence or, alternatively, the XH spectra are recorded with a sequence that allows simultaneous solvent suppression with presaturation (31) or a sequence that includes some other method of suppression 13C 1H and 1P 1H spectra are recorded with proton broadband (composite pulse) decoupling (32), and 31P spectra with gated proton decoupling (33). 

Dalvit C, Bohlen IM, Multiple-Solvent Suppression in Double-Quantum NMR Experiments with Magic Angle Pulsed Field Gradients, Magn. Reson. Chem., 34 829-833, 1996. 

Figure 14.4. Pulse sequence of a solvent suppression experiment.

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