to band spectra, but atoms yield clearly defined line spectra. Inatomic emission spec-
troscopy(AES), these lines can be observed as light of a particular wavelength (colour).
Conversely, black lines can be observed against a bright background inatomic absorp-
tion spectroscopy(AAS). The wavelengths emitted from excited atoms may be identified
using a spectroscope with the human eye as the ‘detector’ or a spectrophotometer.12.7.1 Principles
In a spectrum of an element, the absorption or emission wavelengths are associated
with transitions that require a minimum of energy change. In order for energy
changes to be minimal, transitions tend to occur between orbitals close together in
energy terms. For example, excitation of a sodium atom and its subsequent relaxation
gives rise to emission of orange light (‘D-line’) due to the transition of an electron
from the 3sto the 3porbital and return (Fig. 12.19).
Electron transitions in an atom are limited by the availability of empty orbitals.
Filling orbitals with electrons is subject to two major rules:- one orbital can be occupied with a maximum of two electrons; and
- the spins of electrons in one orbital need to be paired in an antiparallel fashion (Pauli
principle).
Together, these limitations mean that emission and absorption lines are character-
istic for an individual element.12.7.2 Instrumentation
In general, atomic spectroscopy is not carried out in solution. In order for atoms to emit
or absorb monochromatic radiation, they need to be volatilised by exposing them to
high thermal energy. Usually, nebulisers are used to spray the sample solution into a
flame or an oven. Alternatively, the gaseous form can be generated by usinginductively
coupled plasma(ICP). The variations in temperature and composition of a flame make
standard conditions difficult to achieve. Most modern instruments thus use an ICP.
Atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) are
generally used to identify specific elements present in the sample and to determine
their concentrations. The energy absorbed or emitted is proportional to the number of
atoms in the optical path. Strictly speaking, in the case of emission, it is the number of
excited atoms that is proportional to the emitted energy. Concentration determination
with AES or AAS is carried out by comparison with calibration standards.
Sodium gives high backgrounds and is usually measured first. Then, a similar
amount of sodium is added to all other standards. Excess hydrochloric acid is
commonly added, because chloride compounds are often the most volatile salts.
Calcium and magnesium emission can be enhanced by the addition of alkali metals
and suppressed by addition of phosphate, silicate and aluminate, as these form non-
dissociable salts. The suppression effect can be relieved by the addition of lanthanum
and strontium salts. Lithium is frequently used as an internal standard. For storage of517 12.7 Atomic spectroscopy