Other Instrumental Methods in Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 329

problem, more sample or a longer run-time is needed. Third, unlike 1 H-NMR, the
area under a 13 C-NMR is not proportional to the number of atoms giving rise to the
signal, implying that integration cannot be routinely used for 13 C-NMR unless
special techniques are used. Lastly, the range of the chemical shift in 13 C-NMR
spectra is much wider than 1 H-NMR (220 ppm vs. 12 ppm), so the potential signal
overlap is minimized. This is an advantage of 13 C-NMR.

12.2.3 Applications of NMR in Environmental
Analysis

Recall from Chapter 8 that, UV and IR spectroscopy can be used to identify the
presence of conjugation and functionalities in an organic molecule. The underlying
chemistry for UV spectroscopy is the electronic excitation upon a molecule's
exposure to UV. In IR spectroscopy, the energy is not sufficiently strong to cause the
excitation of electrons, but it is strong enough to cause atoms and group of atoms to
vibrate. In an NMR spectroscopy, a radiofrequency (rf) of even lower energy
radiation is used to cause nuclear spin. As we have described, NMR spectroscopy
identifies the carbon-hydrogen framework of an organic compound.
The overall utility in structural identification is in the increasing order: UV < IR

NMR. The combined use of 1 H and 13 C NMR provides the most useful
information. An added advantage of an NMR is that it does not need a standard for
structural confirmation. This is advantageous over molecular mass spectroscopic
techniques. In many fate and transport studies on the degradation of environmental
contaminants, standards of new compounds such as intermediates and products are
commonly not available.
Despite its superior power in structural identification, NMR has its major
limitation due to its low sensitivity for use in quantitative measurement of
contaminants' concentrations. The overall sensitivity is in the order: MS

< MS <

> UV > IR

13 C NMR. Research is under way to improve the sensitivity of the
NMR for environmental uses, such as the detection of pharmaceutical chemicals in
the environment using capillary LC-NMR. New NMR sample probes have also been
designed for the simultaneous analysis of multiple samples in NMR microcoils
which are capable of accommodating very small amount of samples (Borman, 2004;
Lens and Hemminga, 1998).

1 HNMR

> >

12.3 MISCELLANEOUS METHODS

12.3.1 Radiochemical Analysis

Sources and Properties of Several Important Radionuclides

Radiochemical analysis has become a commonplace in today's environmental labs.
An apparent reason is that both naturally occurring and artificial sources of
radioactive chemicals can pose significant hazards to human and living organisms.