So far, all methods have dealt with probing molecular properties. we discussed the general theory of electronic transitions and said that molecules give rise to band spectra, but atoms yield clearly defined line spectra. In atomic emission spectroscopy (AES), these lines can be observed as light of a particular wavelength (colour). Conversely, black lines can be observed against a bright background in atomic absorption spectroscopy (AAS). The wavelengths emitted from excited atoms may be identified using a spectroscope with the human eye as the ‘detector’ or with a spectrophotometer.
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 atomic orbitals (which correspond to statistical and spatial localisation of electrons around the atom nucleus) 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 3 s to the 3 p orbital and its return ( Figure 1 ).
Fig1. Energy levels of atomic orbitals in the sodium atom. Each atomic orbital can be occupied by electrons following the rules of quantum chemistry until the total number of electrons for that element is reached (in the case of sodium: 11 electrons). The energy gap between the 3 s and the 3 p orbitals in the sodium atom is such that it can be overcome by absorption of orange light.
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
• The spins of electrons in one orbital need to be paired in an anti-parallel fashion (Pauli principle).
Together, these limitations mean that emission and absorption lines are characteristic for an individual element.
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 using inductively 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.
The presence of sodium results in 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 of samples and standards, polyethylene bottles are used, since glass can absorb and release metal ions, and thus impact the accuracy of this sensitive technique.
Cyclic analysis may be performed that involves the estimation of each interfering substance in a mixture. Subsequently, the standards for each component in the mixture are doped with each interfering substance. This process is repeated two or three times with refined estimates of interfering substance, until self-consistent values are obtained for each component.
Flame instability requires experimental protocols where determination of an unknown sample is bracketed by measurements of the appropriate standard, in order to achieve the highest possible accuracy.
Biological samples are usually converted to ash prior to determination of metals. Wet ashing in solution is often used, employing an oxidative digestion similar to the Kjeldahl method.
Applications
Atomic Emission and Atomic Absorption Spectrophotometry
The contemporary instrumentation found in analytical laboratories consists of ICP AES instruments that can detect less than 1 ppm of each of the common elements with the exception of alkali metals. The relative precision is about 1% in a working range of 20–200 times the detection limit of an element. In cases where special attention is applied, precision may be improved to 0.2%. Sodium and potassium are assayed at concentrations of a few ppm using simple filter photometers. The modern emission spectrophotometers allow determination of about 20 elements in biological samples, the most common being calcium, magnesium and manganese.
AES and AAS have been widely used in analytical chemistry, such as environmental and clinical laboratories. Frequently, however, these techniques are replaced by the use of ion-selective electrodes.
Atomic Fluorescence Spectroscopy
Despite being limited to only a few metals, the main importance of atomic fluorescence spectroscopy (AFS) lies in the extreme sensitivity. For example, zinc and cadmium can be detected at levels as low as 0.1–0.2 ppb. AFS uses the same basic set-up as AES and AAS. The atoms are required to be vapourised by one of three methods (flame, electric, ICP). The atoms are excited using electromagnetic radiation by directing a light beam into the vapourised sample. This beam must be intense, but is not required to be monochromatic, since only the resonant wavelengths will be absorbed, leading to fluorescence.
