Among the most useful scientific instruments for chemical analysis are those that measure the radiation and absorption of energy by molecules from different parts of the electromagnetic spectrum. (The spectrum is the entire range of different types of electromagnetic waves.) By using different wavelengths of energy, it is possible to selectively excite electrons involved in molecular bonds. Electrons in various atomic orbits, or even the nuclei in the different atoms, can also be excited. The molecules and atoms of any single substance give the same characteristic patterns when absorbing radiation. These spectra can then be used as “fingerprints” to identify substances. Often, they also reveal how much of that substance is present Spectroscopes are instruments for producing and examining the spectrum of a ray of electromagnetic energy from any source.
Emission spectroscopy and atomic absorption
The region of the electromagnetic spectrum from the ultraviolet to the visible portion is used for identifying elements in materials and for quantitative measurements. When atoms absorb energy (atomic absorption) from this part of the spectrum, electrons “jump” from one orbit to an orbit of higher energy. A similar amount of energy is released when the electrons move back to their original, lower-energy orbits. Elements can thus be identified by the characteristic series of wavelengths that each produces when these energy changes occur.
The best-known series of emission lines appear in the visible spectrum. The atoms of the elements studied are excited by an electric spark or arc. They then emit light as they return to their original, unexcited state. The various wavelengths of emitted light are separated by a large prism. The separated light is used to expose a photographic plate. The elements present can then be identified by comparing the wavelengths of the lines with those produced from known pure substances.
Quantitative measurements using emission spectroscopy can be made by using a system involving an argon plasma torch. This system can easily measure the emission lines of a molecular sampling. However, quantitative measurements are generally easier to make using atomic absorption spectrometry. The extent of the energy absorption by a molecular sample is proportional to the number of atoms present. Thus, measuring the amount of energy absorbed also measures the concentration (number of atoms) of the element in the sample.
Organic compounds can also be analyzed using spectral regions. The emission lines, known as electronic spectra, occur as very broad bands—sometimes in a series of peaks and troughs in and around the visible region. These spectra arise due to the absorption of energy by the electrons forming the molecular bonds. Spectra are characteristic for each compound. Although they can assist in identification, they are of greatest value in the realm of quantitative analysis. Spectral measurement is useful for determining minute amounts of colored and unsaturated compounds.
Infrared and raman spectroscopy
Just as a tuning fork vibrates at specific frequencies, so the bonds between atoms absorb energy and vibrate—when absorbing radiation at infrared wavelengths. The actual wavelength at which vibration occurs is characteristic for the elements forming the bond. Thus, in any molecule formed from several different atoms, there are many different values at which vibrations occur. An infrared spectrum covering this region consists of a series of fairly sharp peaks of varying intensity. The infrared spectrum of a compound is specific to that compound and serves as a “fingerprint” for identification purposes.
Raman spectroscopy also studies the vibrations of interatomic bonds. It is used for structures that do not necessarily give straightforward infrared spectra. Unlike infrared, raman spectroscopy can be readily applied to samples dissolved in water. The information obtained is complementary to that from infrared and is especially useful for locating certain groups in molecules. It is also used for quantitative analysis of mixtures of complex organic compounds.
Several new advanced spectroscopic techniques subject molecules to strong magnetic or radio-frequency fields: nuclear magnetic resonance (NMR), electron spin resonance (ESR), and Mossbauer spectroscopy. NMR produces a spectrum of a series of absorption peaks corresponding to energy changes of the nuclei of one element in a compound. Hydrogen and carbon atoms are most frequently studied. The various elements in different chemical groups absorb at different magnetic field strengths. Compounds can be identified from the spectral patterns produced. In ESR, the energy levels of the electrons of atoms of a substance are studied while they are under the influence of magnetic fields. This is particularly useful with substances composed of rare earth elements and of transition metals. Mossbauer spectroscopy is a study of nuclear energy levels in substances.