Intoduction It is often important to identify the substances in a sample. This is true in fields as diverse forensic science, polymer chemistry, biology and biochemistry, natural medicine analysis, and even astronomy. In biology, scientists strive to understand the way that the folding of large macromolecules leads to specific chemical behavior, but they first need to figure out the shape of a molecule that may weigh 10,000 atomic mass units or more. In medicine, new and apparently pure substance may be isolated from a plant, and show excellent medicinal properties (such as Taxol), but next its molecular structure must be identified so that efforts can be made to produce it synthetically. In astronomy, space ships are sent into orbit and onto other planets in efforts to identify the molecules drifting in outer space, and to compare them to those on earth and on other planets in the solar system. In this way, one can try to understand how planets form, how they evolve, and why they differ, but first one needs to know not just what molecular are present, but (if possible) the relative amounts of the molecules. Chemists have been successfully identifying molecules since well before 1900. But, older methods of identification required large amounts of sample (>10 mg), very high purity of sample, and a lot of time. Older methods were destructive of the samples, and involved burning them to find out relative amounts of various elements (elemental analysis) or conversion of the samples into something different and more readily identified (derivative formation). Sometimes it is not possible to get so much sample to be so pure, and at times one cannot wait for 1-7 days for an analysis, especially if a sample is unstable, or data is required with extreme urgency for health reasons. An ideal method would have as many as possible of the following properties:
The ideal method described above is available through modern technology, and is called spectroscopy. Actually, there are various types of spectroscopy that tend to be specific for analysis of different features and properties of molecules, but all types of spectroscopy fit the general criteria described in the previous paragraph. In general, spectroscopy involves passing energy through a sample, or bouncing it off of a sample. The emergent energy is then analyzed to see what (if any) wavelengths have been absorbed by the sample. Contrary to "common sense" notions, when light or other types of energy strike a substance, not all wavelengths are absorbed with equal facility. Due to considerations of quantum mechanics that were discovered in the early part of the twentieth century, only certain wavelengths of energy will be absorbed, while others will simply pass through the sample. In a few types of spectroscopy, a molecule may absorb energy and then re-emit it at a different wavelength, such that the newly emitted energy can be measured. But, in all cases, spectroscopists analyze changes between the spectrum of the energy that enters the sample, and that which leaves the sample. Spectroscopic analysis is the science of correlating specific wavelengths of absorption with specific features of molecular structure. |