Tips for the Teacher

Language of Chemistry Language of Chemistry

absorption spectrum plot of absorbance vs. wavelength of radiation absorbed for a given substance or mixture.

chromatography technique for separating mixtures using differences in solubility and absorptivity.

emission spectrum dispersion by a prism or diffraction grating of the wavelengths of electromagnetic radiation produced by an emitting source. For example, an atomic emission spectrum consists of those specific wavelengths corresponding to electronic transitions from higher to lower energy levels.

infrared spectroscopy detection and recording of the wavelengths of infrared radiation causing molecular vibrations (that is, vibrations of bonds between atoms) absorbed by molecules and polyatomic ions.

nuclear magnetic resonance spectroscopy measurement of the absorption of energy by nuclei in a strong magnetic field. The energy is supplied by high frequency radio waves.

spectrophotometer instrument that passes electromagnetic radiation through a substance at a given wavelength (or frequency) and measures the amount of radiation absorbed or transmitted.

Pattern Recognition

Infrared spectra provide the means to identify substances by comparing the spectrum of an unknown compound with spectra recorded in atlases of IR spectra. The absorption pattern is characteristic and allows relatively quick identification, as discussed in Pictures in the Mind. Similarly, substances can be identified from the NMR spectrum.

Problem Solving

  1. The Beer-Lambert Law gives students the opportunity to do some simple mathematical calculations involving logarithms. A scientific calculator with a logarithm function greatly simplifies the calculation. The law, as shown in Part II of Activity 1 , is straightforward in the sense that it may be solved in a chain operation using a calculator. The law states:

    A = log 10 100 - log 10 (%T) = 2 - log 10 (%T) = k'c

    Since the logarithm of 100 = 2, the calculation using a calculator then means entering 2, then a subtraction (-) sign, then (%T), and finally the log button. Pressing the equal (=) button will give the absorbance. Percent Transmittance (%T) to Absorbance (A) Conversions table in the Appendix, which gives the conversions between %T and absorbance for values from 1%T through 100%T, may be used as a student handout or as a transparency master.

  2. Determining Crystalline and Molecular Structure. Many naturally occurring substances have physiological activity, and quite a few of them have a beneficial therapeutic effect. Normally, it is difficult, if not impossible, to obtain large amounts of these beneficial compounds from natural sources since they exist only in minute amounts. To synthesize the compound and its close derivative to be used in medication, it is necessary to know the compounds' structures. Thus one reason for undertaking a structure determination is to be able to synthesize a compound of interest.

    One of the problems faced, and solved, is to use modern X-ray diffraction for this purpose coupled with mass spectrometry, and frequently nuclear magnetic resonance. For organic compounds, infrared spectra are also helpful.

    The breakthrough was development of instrumental techniques allowing useful results to be obtained from rather small quantities of substances. It is now possible to determine the crystalline and molecular structures for minute amounts of substances. Armed with this information, chemists may develop schemes for synthesizing the compounds. This was frequently not possible with classical chemical methods of structure determination since not enough of the active compound could be isolated for study.

  3. Other Problem-Solving Instruments. CAT-scanning and X-ray mammography are other examples of problem solving using instrumentation utilizing X-rays. It must be acknowledged that these techniques, although not primarily chemical in nature, have had a large impact upon the practice of medicine as diagnostic tools. Magnetic Resonance Imaging (MRI), discussed previously, is another such technique. It should be emphasized to students that something that results from basic research frequently is then applied in somewhat surprising ways to solve seemingly unrelated problems.
Decision Making

Instrumentation is used to monitor the environment: acid rain, radon testing, asbestos abatement, ozone depletion, etc. Do we place too much reliance on instrumentation? What about biochemical instrumentation? How reliable are automated blood tests? AIDS tests? Breathalyzer tests (see Chemistry in Medicine module)? These are some questions that may be assigned to students to research and report back to the class.

In January 1991, the Environmental Protection Agency issued final drinking water standards for 38 contaminants (Ember, 1991, Chemical and Engineering News). The total of standards for drinking water now federally enforced is 60. These standards include 17 pesticides, 10 volatile organic compounds, polychlorinated biphenyls (PCBs), eight inorganic compounds, and two substances used to treat drinking water. Although the standards are enforceable by the federal government, states have to administer them. This means that the water supplies in about 80,000 systems nationwide probably will be monitored primarily by instrumentation and/or colorimetric tests that require some treatment, perhaps separations. The monitoring cost is estimated at about $24,000,000 annually, adding from $10 to $800 to consumers' bills for drinking water per year. It is estimated that the rules for pesticides alone will force consideration of pesticide control in land use. This is an example of how monitoring pollutants brings about decisions about their control. Students might be asked to develop a scenario involving a new instrument that could detect PCBs to a level 1000 times lower than our present capability. How might this new instrument affect future regulations regarding PCBs? Is there ever a limit below which we cannot go?

Modern automated instrumental blood tests have been available for some time. One such test checks 27 different and related blood factors from serum glucose to the LDL/HDL ratio (analytes). To do this on a relatively small sample of blood requires a very sophisticated system of separation and instrumental analysis. How reliable is such a system? This question is very important since the results of such tests are used by physicians to prescribe a corrective regimen when an analyte is out of the reference range. Such tests also play a large role in a physician's diagnosis and prognosis. The tests are used extensively for preventive medicine and to detect an abnormality before other symptoms are manifested. Blood testing is now reasonably accessible to most of the country's population wherever doctors are available. Undoubtedly such testing is extremely beneficial to that part of the population willing to pay for it.

Instrumentation and separations are the basis for decision making in many respects, and the trend of reliance on such results will increase as new technology is developed. What kinds of public policies should be developed in light of these capabilities?


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