Solvent suppression presaturation

However, due to its simple setup and generally efficient solvent suppression, presaturation is still a useful method for acquiring an exploratory one-dimensional (ID) experiment in order to gauge properties of the NMR sample. 

MHz H NMR spectra were measured on solutions of ca 25 mg pectin/0.5 ml D2O on a Bruker AMX600 NMR spectrometer. The temperature was 350 K to diminish the viscosity of these solutions and 32 scans were measured. Solvent suppression was performed using presaturation during the recycle delay. 

The most commonly used solvent suppression technique is presaturation. Modern NMR spectrometers are normally equipped with at least two independent frequency channels. Using these, there are usually no problems in conducting experiments where two solvents are presatureated at the same time. However, the number of solvents which can be suppressed is limited by the available hardware. 

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... 

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). 

Figure 9.23. Solvent suppression schemes based on presaturation (a) presaturation alone, (b) ID NOESY and (c) FLIPSY, Sequence (b) makes use of the conventional NOESY phase cycle whereas FLIPSY uses EXORCYCLE on one (or both) of the 180° pulses (i.e. pulse = X, y, —X, —y, receiver = x, —x,...

The simplest, most robust and most widely used technique is presaturation of the solvent [62]. This is simple to implement, may be readily added to existing experiments and leaves (non-exchangeable) resonances away from the presaturation frequency unperturbed. It involves the application of continuous, weak rf irradiation at the solvent frequency prior to excitation and acquisition (Fig. 9.23a), rendering the solvent spins saturated and therefore unobservable (Fig. 9.24). Invariably resonances close to the solvent frequency also experience some loss in intensity, with weaker irradiation leading to less spillover but reduced saturation of the solvent. Longer presaturation periods improve the suppression at the expense of extended experiments so a compromise is required and typically 1-3 s are used trial and error usually represents the best approach to optimisation. Wherever possible the same rf channel should be used for both the presaturation and subsequent proton pulsing, with appropriate transmitter power switching. 

Solvent suppression is a particular problem in LC/NMR and has been a theme throughout its development. Early methods for suppression of the pro-tonated solvent signals which otherwise dominate the NMR spectrum made use of binomial pulse sequences [124-126]. Methods in use today either use fully deuterated solvents, or make use of solvent suppression schemes such as the NOESY presaturation technique [127], WATERGATE [28,128], WET [29,129], or excitation sculpting [30,130,131]. These methods have for some time made it possible to study relatively low-level (several %) impurities [132,133]. The need... 

PURGE Presaturation utilizing relaxation gradients and echoes (solvent suppression) 10.5.3... 

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... 

There has been a continued interest in developing new methods for solvent suppression in NMR spectroscopy of biomolecules and biofluids and many methods have been proposed. These fall into five categories (1) presaturation, (2) nonexcitation,... 

Figure 13 2D Correlation spectroscopy spectrum with presaturation pulse for solvent suppression of 2 mM sucrose at 25°C and 475 MPa pressure recorded at 500 MHz.

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