Page 465 - Tandem Techniques

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peak assignments were obtained in the same way as those previously. A preliminary separation by
liquid chromatography allowed the spectra of each component to be identified, and then the spectra so
obtained could be used to assign the NMR peaks present in the spectrum of the mixture.

Sweatman et al. [8] examined the general performance of a directly coupled liquid chromatograph, with
a 600 MHz NMR spectrometer, and found that the H sensitivity was about 85 ng for ethyl benzene,
1
and the F sensitivity about 40 ng for 4-trifluorothymine. These levels of sensitivity are more than
19
adequate for the effective use of LC/NMR tandem instruments, but much still depends on the design of
the interface and, in particular, the volume of the flow cell. The tendency has been to concentrate on
micro-columns and packed capillary columns, which renders the interfacing procedure even more
difficult. In the majority of samples presented for analysis there is usually adequate material available
for the separation to be carried out satisfactorily on standard LC columns, although the extraction and
concentration of some samples may be necessary. It follows that small-bore columns, and, in particular,
packed capillary columns are not always necessary, and should only be employed when they are
essential to the satisfactory handling of the sample. In this way less strain is placed on the interface
design, and a far greater proportion of samples could be analyzed on LC/NMR tandem instruments. The
main advantage of the small-bore columns is their economic use of solvents, and thus deuterated
solvents can be used for the chromatographic separation without undue cost. Nevertheless, with a little
imagination a phase system can often be chosen that involves solvents that are acceptable to the NMR
spectrometer and, at the same time, provide the chromatographic selectivity that is required to achieve
the desired separation.

LC/NMR Detection of Peptides Employing Micro-Cells

Wu et al. [9] developed a microbore system interface for the analysis of peptides and amino acids. Their
radio frequency micro-coils were very simple in form and have been described previously by Wu et al.
[10]. The sensor was constructed from a fused silica tube, around which a tiny radio