Instrumentation plays an important role in the modern practice of chemistry. It can provide useful information about the structure, identity, and quantity of a substance (see Chemical Bonding, Molecular Geometry, and Organic Chemistry modules). Very sensitive quantitative instrumental techniques routinely used by chemists have added to our wealth of chemical knowledge. This module will stress the use of spectrophotometry as applied to quantitative analysis.
Since this module probably does not stand alone in most chemistry courses, it may be partitioned into pieces to fit a particular need. Parts of the material may be used with qualitative analysis, atomic structure, descriptive chemistry, or as enrichment for honor classes.
- An instrument is a tool that allows for physical or chemical characterization of substances by utilizing a variety of probes. The most common probe is electromagnetic radiation.
- Atomic and molecular species absorb energy in the form of electromagnetic radiation that allows them to undergo certain types of transitions depending upon the energy range of the radiation. Ultraviolet and visible radiation induce electronic transitions, infrared radiation induces vibrational transitions and microwave radiation induces rotational transitions. There is some overlap between the types of transitions induced by the radiation ranges.
- The behavior of a covalent chemical bond has been likened to the behavior of a spring. Both are capable of stretching and bending. When induced by radiation of specific wavelength, a chemical bond will stretch and/or bend at a frequency corresponding to that of the incident radiation. The absorbed radiation brings about molecular vibrations. Instruments can measure the absorption frequencies and intensities that, when interpreted, can yield information about molecular structure.
- The relationship between the quantity of radiation absorbed by molecules, the concentration of the molecules, and the path length through which the radiation passes is known as the Beer-Lambert Law. This relationship may be used to determine the concentration of molecules in a solution.
- Several instrumental methods for determining structures of molecules in solids and gases, ionic solids, network solids (e.g., diamond and graphite), and biologically important substances (e.g., penicillin and insulin) are available and widely used. The more common of these include:
a. X-ray diffraction uses electromagnetic radiation with frequencies near 10 18 Hz (wavelength 10 -10 m). When applied to crystalline solids, the method gives conclusive evidence for atomic arrangement, bond angles, and bond distances.
b. Electron diffraction uses the wave characteristics of electrons to provide information about gaseous substances similar to X-ray diffraction information.
c. Infrared spectroscopy is used to obtain structural data mainly for noncrystalline molecular species. Infrared radiation causes molecules to vibrate in modes characteristic of their structures.
d. Microwave spectroscopy is similar to infrared spectroscopy except that it causes molecules to rotate rather than vibrate, and the method is applicable to a limited number of gaseous molecules.
e. Nuclear magnetic resonance spectroscopy utilizes the magnetic properties of atomic nuclei by causing an energy change (change in the direction of nuclear spin) at a characteristic electromagnetic frequency in the radiowave region. This frequency is partly dependent upon the strength of the magnetic field, the nature of the nuclear "magnet," and the arrangement of other nuclei and electrons in the region of the nucleus being studied. The frequencies absorbed reveal the type of nucleus and its chemical environment.
f. Mass spectroscopy measures mass to charge ratio for positive ions. Knowing the charge allows the determination of the mass, which, in turn, allows the identification of the substance from the molecular fragments produced.
- When atoms or molecules lose or gain electrons, they become electrically charged particles called ions. Positive ions (cations) result when electrons are lost, and negative ions (anions) result when electrons are gained.
- Electrically charged particles follow a circular path when passing through a magnetic field that is perpendicular to the particle's path. The radius of the circular path is directly proportional to the square root of the particle's mass and inversely proportional to the square root of the particle's charge. Measuring the radius and knowing the charge allows determination of the mass.
- Molecules can absorb electromagnetic radiation and undergo radiation induced rotational, vibrational, or electronic transitions.
Manipulative laboratory skills involving the use of common glassware and hardware are needed to perform the activities.
After completing their study of instrumentation, students should be able to:
- describe the fundamental concepts underlying common instrumentation, including spectrophotometry, X-ray diffraction, and nuclear magnetic resonance.
- make simple spectrophotometric measurements in the laboratory.
- describe some of the applications of instrumental techniques, e.g., quality control, consumer protection, pollution monitoring.
- describe some problem solving applications of instrumentation in research and investigative work.
- describe some uses of instrumentation in their communities, e.g., water quality testing, sewage treatment, medical applications, etc.
- state some of the vocational and career opportunities available that involve using instrumentation.