Emission spectroscopy and atomic absorption

The region of the electromagnetic spectrum from part of the ultraviolet through all of the visible light to part of the infrared portion of the spectrum is particularly useful. This range can be used for identifying elements present in materials and for quantitative measure­ments (determining the number of those ele­ments). When atoms absorb energy (atomic absorption) from this part of the spectrum, electrons "jump" from one orbit to another of higher energy level. A similar amount of en­ergy is released when the electrons move back to their original, lower-energy orbits. As a result, elements can be identified by the characteristic series of wavelengths that each produces when these energy changes occur.

The best-known series of emission lines are those in the visible spectrum. The atoms of the elements studied are excited by an electric spark or arc. The various wavelengths of emit­ted light are separated by a large prism. The separated light is used to expose a photo­graphic plate. Identification of the elements present is carried out by comparing the wave-

An infrared spectrome­ter(above) records the characteristic wavelengths at which various groupings in a molecule absorb radia­tion. The pattern of the wavelengths can then be used to identify the mole­cule. The absorption is usu­ally expressed in terms of percentage transmission (the vertical rise on the graph). Low transmission corresponds to strong ab­sorption. Thus, the spec­trum of aminopentane fright) has strong absorption bands for NH₂, the C-H bond, and the C—N bond.

Infrared spectrum of CH₃CH₂CH₂CH₂CH₂NH 2,500

4,000 100-

80-

60-

40-

20-

-1 r~

5 5

1,500

1,300
__ 1

1,100 1,000
J___

,1

Wavenumber (cm^-1)

800 700

_L

12 13 14 1 Wavelength (urn)

Analytical chemistry: Spectroscopic analysis 131

lengths of the lines with those produced from known pure substances.

Quantitative measurements using emission spectroscopy can be carried out by using a system involving an argon plasma torch. This system can easily measure the emission lines of a molecular sampling. However, most quan­titative measurements may be more readily carried out using atomic absorption spectrom­etry. The extent of the energy absorption by a molecular sampling 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.

NMR produces a spectrum of a series of absorption peaks corresponding to energy changes of the nuclei of one element in a com­pound. Hydrogen 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 substances are studied when under the influ­ence of magnetic fields. This is particularly useful with substances such as rare earths and transition metal ions. Mossbauer spectroscopy is a study of nuclear energy levels in sub­stances.

Electronic spectra

Organic compounds can also be measured in the ultraviolet, visible light, and infrared re­gions. The emission lines (known as electronic spectra) occur as very broad bands. They are sometimes in a series of peaks and troughs in and around the visible region. These spectra arise due to the absorption of energy by elec­trons forming the molecular bonds. Spectra are characteristic for each compound. Al­though they can be used to assist in identifica­tion, it is in the realm of quantitative analysis that they are of greatest value. As such, meas­uring spectra is useful for determining minute amounts of colored and unsaturated com­pounds.

Date: 2015-12-11; view: 1126