Page 388 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN0011A-541 July 25, 2001 17:27

Organic Chemistry, Compound Detection 491

Enantiomeric separation and spectral simplification of must be found between the optional flow rate and the de-
1
overlapped resonances in high resolution H NMR spectra tector volume. Moreover, the HPLC–FTNMR operation
can be obtained with optically active paramagnetic shift is limited to certain types of solvents. Since most com-
reagents such as Eu(tfc) 3 . Detection of optical isomers monly used HPLC solvents contain hydrogen, the mem-
(±) 2,2-dimethyl-1-phenyl-1-propanol and (±) carvone, ory of the minicomputer used for data acquisition will be
as well as optically active terpenes, can rapidly be deter- saturated with the solvent signal, thus overshadowing the
mined by the use of optically active shift reagents. weak sample signal. This has necessitated the use of spe-
cial suppression techniques for solvent signals, such as
filtering, selective presaturation, selective excitation, and
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2. C-CP/MAS NMR
spin-echo techniques, or increasing the dynamic range of
It has been observed that line broadening of NMR spec- the minicomputer. It is possible to use deuterated solvents
1
tra of solid powders arises jointly from 13 C– H static as the HPLC solvents but this involves considerable ex-
dipolar interactions and from chemical shift anisotropy. pense. During the past decade FTNMR has been applied
These effects are eliminated or greatly reduced by dipo- to solid states especially to elucidate microstructures of
lar decoupling (dephasing) of protons and by spinning polymers. The term microstructure pertains to the level of
◦
the sample at an angle of 54.7 (magic angle) to the ap- isomerism of a polymer chain. For example oxidized sites
plied field. For solids a greater rate of spectral accumu- in polymers (carboxylic acids, ketones or alcohols) can be
lation is required, this method is called cross polarization detected and quantified at levels as low as two sites per
13
(CP). thousand residues from the C NMR spectrum. Although
13
High resolution C NMR which utilizes both cross po- carbon is ubiquitous in synthetic polymers, other nuclei
larization with dipolar dephasing and magic angle spin- have been used to advantage for polymer structure de-
ning (MAS) is called 13 C-CP/ MAS NMR. This tech- termination. For example, 15 N NMR for nylon polymers,
19
nique directly measures the organic carbon distribution 31 P NMR for poly(phosphazenes), and F NMR for fluo-
1
in terms of aliphatic and aromatic carbon structures in oil ropolymers. For some time H FTNMR has been regarded
shales, coals, humic substances, melanoidins, and other as less informative for synthetic polymers than the carbon
polymeric substances. resonance; it is now the focus of renewed interest.

4. Two-Dimensional NMR Spectroscopy
3. Applications of FTNMR
(2D-NMR)
Because of the known utility of NMR as a fingerprinting
technique,thereisrecentinterestinthecombinationofthis The term 2D-NMR is a misnomer because the NMR spec-
method with HPLC. Following the initial work on NMR tra that have been previously described are two dimen-
spectroscopy of static HPLC fractions, considerable ef- sional in the sense that they are plots of signal intensity
forts have been focused on the development of analyses of versus frequency (or its Fourier equivalent, signal inten-
flowing liquid chromatographic fractions in the presence sity against time). By contrast, 2D-NMR refers to spectral
of hydrogen containing solvents. The coupling of these data (usually several hundred) that are collected as a func-
two techniques may offer some advantages compared with tion of two time scales, t 1 (evolution) and t 2 (detection).
the HPLC–MS systems since there are no volatility (or 2D-NMR is, therefore, three dimensional: signal intensity
molecular weight) requirements, and the HPLC–FTNMR versus two frequencies F 1 (the Fourier transform in the t 1
technique is more informative and nondestructive than MS time domain) and F 2 (the Fourier transform in the t 2 time
(especially for studies of molecular stereochemistry). Fur- domain). The resulting data are then Fourier transformed
thermore, both HPLC and NMR measurements are car- in both the t 1 and t 2 time domains. The resulting 2D-NMR
ried out in solution and, thus, no phase transformations spectrum is in fact a map containing two frequency axes,
are required. Spectra can be obtained at low tempera- the F 1 axis which contains chemical shifts and the F 2 axis
tures for nonstable compounds such as the 19-hydroxy which may contain chemical shifts or coupling informa-
prostaglandins that were previously mentioned. The de- tion or even both. The 2D-NMR spectrum can be dis-
velopment of suitable interfaces has been hampered by played either as a stacked plot (Fig. 15), which portrays
several problems. Primarily, the intensity of the NMR all three dimensions in a perspective view, or a contour
signal is dependent on the flow rate; the extent of line plot (Fig. 16), which represents a horizontal cross-section
broadening and, consequently, the spectral resolution, de- of the stacked plot at a selected value of signal intensity.
pend on solvent viscosity and detector volume. Because Heteronuclear shift correlation (HETCOR) is a
of the relatively low sensitivity of NMR a compromise 2D-NMR technique where a two-dimensional map is