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of an element, or the light absorbed by an element in vapor form. This technique is particularly useful
for trace elements analysis and for obtaining the element ratios of a substance. The characteristic
emission of an element is usually obtained by passing the sample through a plasma and monitoring the
spectrum produced. Atomic absorption spectra are usually obtained by passing light, having exclusively
the wavelengths that will be absorbed by the element, through the vaporized sample and measuring the
intensity of the light transmitted. Chiral substances rotate the plane of polarized light to the right, or to
the left, depending on the molecular structure of the molecules through which the light is passing.
Chiral spectroscopy will determine the extent of this rotation, and this information can be used either to
identify the presence of the optical isomer, or help confirm its identity. The technique is relatively
insensitive, but its use in tandem instruments is becoming more important because of the selective
biological activity of the different optical isomers of many drugs and biological materials. Nuclear
magnetic resonance spectroscopy is the most powerful technique for structure elucidation, unfortunately
it is also one of the least sensitive. Certain spinning nuclei have asymmetrical charges and thus
constitute nuclear magnets having associated magnetic fields. Because of the conservation of angular
momentum, when such nuclei are situated in a strong magnetic field, they precess at a frequency that
depends on the strength of the field. If exposed to electromagnetic waves of the precessing frequency,
the nuclei can absorb energy and this energy absorption can be detected. The precise frequency that the
nuclei precess will depend on the their magnetic environment which, in turn, will depend on the
adjacent atoms, and the protons associated with the adjacent atoms. Thus the proton NMR spectra (the
position of the absorption peaks and the fine structure) will provide information on the elements present
in the molecule, the distribution of the protons in the molecule, and from the areas of the peaks in the
spectrum, the relative proportion of the protons throughout the molecule. Mass spectrometry has a
much higher sensitivity than NMR, and the mass spectrum is useful for both confirming the identity of
a substance and for elucidating the details of an unknown molecular structure. The sample is ionized by
appropriate procedures, to produce molecular ions, molecular ion fragments, or both. The ions are then
subjected to either electrostatic or magnetic forces, in a manner that