Inorganic Nutrients
99
to absorb radiation of well-defined (resonant) wavelengths. When a liquid sample is
aspirated into a flame, only a small fraction (about one in 1010) of the atoms present
ever attain sufficient energy to emit light. The overwhelming majority are in a ground
or neutral state. As such, they are capable of absorbing resonant wavelengths of
light.
In the atomic absorption spectrophotometer, light from a hollow cathode lamp, the
filament of which is made from the element of interest, is passed through a flame and
into a spectrometer where a resonant wavelength is isolated. When a sample is
atomized into the flame, the atoms in ground state absorb this wavelength and reduce
its intensity. The degree of absorption is proportional to the concentration of absorbing
atoms in the flame. Specificity is assured since an atom will absorb only at its resonant
wavelength.
Interference is a common problem. For example, P0 4 ions will bind with calcium to
form Ca 3 (P0 4 }z, which is a very stable molecule. To overcome the bonding between
calcium and the phosphate, either a high temperature flame is required or some type of
preferential binding agent must be added. In the case of calcium, lanthanum (LaCI 3 )
commonly is added to the solution. In this case the P0 4 ions will be freed to bind
preferentially with the lanthanum and the chloride with calcium. CaCl z is readily
dissociated by a low energy flame, and the preponderance of ground state atoms
permits the analysis of calcium in minute concentrations. A high energy flame is not
satisfactory in this case because ionization of Ca will occur and reduce the sensitivity.
Atomic absorption is useful for the quantitative analysis of calcium, magnesium,
potassium, sodium, manganese, aluminum, and a number of trace metals. Sodium is
particularly well suited to analysis by flame emission. Several analytical instruments
have the capability to analyze by both atomic absorption and flame emission
spectroscopy. Both methods of analysis require a working (standard) curve in which
absorbance or emission is plotted against concentration for known values. Standards
should be prepared in the same solvent as the unknown [see Beaty (1978)].
Inductively Coupled Plasma Emission Spectroscopy (ICP). ICP is based on the
measurement of specific wavelengths of light from excited atoms by optical spectroscopy. "Samples are nebulized to produce an aerosol. The aerosol is transported by
an argon carrier stream to an inductively coupled argon plasma, which is produced by
a radio frequency generator. In the plasma (which is at a temperature of 6,000 to
1O,0000K), the analytes in the aerosol are atomized, ionized, and excited. The excited
ions and atoms emit light at their characteristic wavelengths. The spectra from all
analytes are dispersed by a grating spectrometer, and the intensities of the lines are
monitored by photomultiplier tubes" [from Shugar et al. (1981)]. As a result, numerous
analytes can be measured in a single sample.
The ICP is used commonly for measuring major and trace metal concentrations
(e.g., Ca, Mg, Fe, and Mn) as well as SiOz [e.g., Willard et al. (1988) and Shugar et al.
(1981)].
Ion Chromatography
Ion chromatography (IC) is an ionic separation technique that depends on the affinity
of an ion for an exchange site. Ions in a liquid passing through an exchange column will
have differnent rates of migration through these columns because of their different
affinites for exchange sites. Therefore each ion can be identified in the eluent by
conductimetric detection of its retention time.
99
to absorb radiation of well-defined (resonant) wavelengths. When a liquid sample is
aspirated into a flame, only a small fraction (about one in 1010) of the atoms present
ever attain sufficient energy to emit light. The overwhelming majority are in a ground
or neutral state. As such, they are capable of absorbing resonant wavelengths of
light.
In the atomic absorption spectrophotometer, light from a hollow cathode lamp, the
filament of which is made from the element of interest, is passed through a flame and
into a spectrometer where a resonant wavelength is isolated. When a sample is
atomized into the flame, the atoms in ground state absorb this wavelength and reduce
its intensity. The degree of absorption is proportional to the concentration of absorbing
atoms in the flame. Specificity is assured since an atom will absorb only at its resonant
wavelength.
Interference is a common problem. For example, P0 4 ions will bind with calcium to
form Ca 3 (P0 4 }z, which is a very stable molecule. To overcome the bonding between
calcium and the phosphate, either a high temperature flame is required or some type of
preferential binding agent must be added. In the case of calcium, lanthanum (LaCI 3 )
commonly is added to the solution. In this case the P0 4 ions will be freed to bind
preferentially with the lanthanum and the chloride with calcium. CaCl z is readily
dissociated by a low energy flame, and the preponderance of ground state atoms
permits the analysis of calcium in minute concentrations. A high energy flame is not
satisfactory in this case because ionization of Ca will occur and reduce the sensitivity.
Atomic absorption is useful for the quantitative analysis of calcium, magnesium,
potassium, sodium, manganese, aluminum, and a number of trace metals. Sodium is
particularly well suited to analysis by flame emission. Several analytical instruments
have the capability to analyze by both atomic absorption and flame emission
spectroscopy. Both methods of analysis require a working (standard) curve in which
absorbance or emission is plotted against concentration for known values. Standards
should be prepared in the same solvent as the unknown [see Beaty (1978)].
Inductively Coupled Plasma Emission Spectroscopy (ICP). ICP is based on the
measurement of specific wavelengths of light from excited atoms by optical spectroscopy. "Samples are nebulized to produce an aerosol. The aerosol is transported by
an argon carrier stream to an inductively coupled argon plasma, which is produced by
a radio frequency generator. In the plasma (which is at a temperature of 6,000 to
1O,0000K), the analytes in the aerosol are atomized, ionized, and excited. The excited
ions and atoms emit light at their characteristic wavelengths. The spectra from all
analytes are dispersed by a grating spectrometer, and the intensities of the lines are
monitored by photomultiplier tubes" [from Shugar et al. (1981)]. As a result, numerous
analytes can be measured in a single sample.
The ICP is used commonly for measuring major and trace metal concentrations
(e.g., Ca, Mg, Fe, and Mn) as well as SiOz [e.g., Willard et al. (1988) and Shugar et al.
(1981)].
Ion Chromatography
Ion chromatography (IC) is an ionic separation technique that depends on the affinity
of an ion for an exchange site. Ions in a liquid passing through an exchange column will
have differnent rates of migration through these columns because of their different
affinites for exchange sites. Therefore each ion can be identified in the eluent by
conductimetric detection of its retention time.