Solvent suppression technique

The development of simple, multiple-frequency solvent suppression techniques has greatly improved the quality of data that can be obtained using LC-NMR. One of the most useful methods for multiple resonance solvent suppression in LC-NMR is... 

PRACTICAL CONSIDERATIONS, SOLVENT SUPPRESSION TECHNIQUES, GRADIENT ELUTION AND PURITY OF HPLC SOLVENTS... 

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. 

Special solvent suppression techniques are necessary to handle the large background signals arising from such solvents. 

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... 

In principle, it would be possible to use fully deuterated solvents for HPLC-NMR coupling, and then solvent suppression techniques would be unnecessary however, due to the high costs of these solvents, only the use of D2O is economically acceptable. Solutions to this problem can be seen in the development of hyphenating capillary HPLC to NMR spectroscopy, and there, because of the reduced solvent consumption, the use of fully deuterated solvents would be reasonable. However, this approach is difficult and the development is still an on-going process (see Chapter 7.3 below). From the viewpoint of NMR spectroscopy, there could be a completely new situation, when separation techniques could be found which use proton-free solvents as the mobile phases. 

The most common solvent used in SFC is carbon dioxide (CO2), which is a proton-free molecule, so it represents an ideal solvent for H NMR spectroscopy. The necessity of difficult solvent suppression techniques can be discarded... 

Supercritical Fluid Chromatography, as well as Supercritical Fluid Extraction (SFE), are very often considered as niche detection techniques. However, it is obvious from Chapter 7.2 that SFC-NMR and SFE-NMR show the inherent advantage that no solvent suppression technique is needed and the whole proton chemical shift range can be used without any distortions of solvent signals and impurities. 

ID XH NMR N/A Quantitative overview as to the distribution of protons in a sample In the case of NOM, often 2D NMR is central to the interpretation of the ID NMR, which often contains considerable overlap. XH NMR, care must be taken to avoid water contamination in the sample. This is especially important for samples run in DMSO-d6, which is very hygroscopic. In aqueous solvents a solvent suppression technique is often required. 

Watanabe and Niki introduced the coupling of NMR to LC as an on-line detector [20]. After these initial stopped-flow experiments, Bayer et al. [21] reported the first continuous-flow LC-NMR experiment. However, a number of impediments associated with LC-NMR hindered routine analytical application for a number of years. Since then, new instrumentation and analytical methodologies for LC-NMR have been developed and commercialized. The development of high-field-strength magnets, better solvent-suppression techniques, more sensitive small-diameter transmitter/receiver coils, on-column sample preconcentration, and expanded flow cells have improved the sensitivity of LC-NMR. 

The BIS HMQC pulse sequence may give relatively intensive residual parent lines resulting from imj>erfections of the 180° pulse. These can be removed using the post-acquisition solvent suppression techniques. 

It is a natural consequence of the interest in SCF chromatography that there would be a desire to have on-line monitoring by NMR. Allen et al. [10] have published their design for such a system, and the group of Albert have also been active in this area [11,12]. The low concentrations and small sample volume mean that these systems are restricted to study by H NMR. As Albert et al. point out [11], use of CO2 as mobile phase, obviates the need for any solvent suppression techniques. However, small amounts of polar modifier, such as methanol, are often used in SCF chromatography, which could obscure part of the spectrum, though still to a considerably lesser extent than in HPLC-NMR. 

LC-NMR is typically limited to the last two options because of limited sample quantity. Keep in mind that LC-NMR may require incorporating single or double solvent-suppression techniques into these experiments if protonated solvent systems are used. Having selected and acquired a standard set of experiments, the next step is data interpretation as a first pass for structural elucidation. 

Detection schemes will also continue to evolve. While NMR is covered in another chapter in this volume, advances in NMR used as an LC detector have also been reported (25-28) that may eventually increase the utility of this tool for combinatorial chemistry. Used in both stopped flow and on line modes, LC/NMR can be extremely useful for structure elucidation provided the proper mobile phases or solvent suppression techniques are used (27). 

---