Atomic absorption spectroscopy
Atomic absorption spectroscopy in analytical chemistry is a technique for determining the concentration of a particular metal element within a sample. Atomic Absorption Spectroscopy can be used to analyze the concentration of over 62 different metals in a solution.
Atomic Absorption Spectroscopy was first developed during the 1950s by a team of Australian chemists, lead by Alan Walsh, working at the CSIRO (Commonwealth Science and Industry Research Organization) Division of Chemical Physics, in Melbourne Australia. Typically, the technique makes use of a flame to atomize the sample, but other atomizers such as a graphite furnace are also used. Three steps are involved in turning a liquid sample into an atomic gas:
1. Desolvation – the liquid solvent is evaporated, and the dry sample remains
2. Vaporisation – the solid sample vaporizes to a gas
3. Volatilization – the compounds making up the sample are broken into free atoms.
The flame is arranged such that it is laterally long (usually 10cm) and not deep. The height of the flame must also be controlled by controlling the flow of the fuel mixture. A beam of light is focused through this flame at its longest axis (the lateral axis) onto a detector past the flame.
The light that is focused into the flame is produced by a hollow cathode lamp. Inside the lamp is a cylindrical metal cathode containing the metal for excitation, and an anode. When a high voltage is applied across the anode and cathode, the metal atoms in the cathode are excited into producing light with a certain emission spectra. The type of hollow cathode tube depends on the metal being analyzed. For analyzing the concentration of copper in an ore, a copper cathode tube would be used, and likewise for any other metal being analyzed. The electrons of the atoms in the flame can be promoted to higher orbitals for an instant by absorbing a set quantity of energy (a quantum). This amount of energy is specific to a particular electron transition in a particular element. As the quantity of energy put into the flame is known, and the quantity remaining at the other side (at the detector) can be measured, it is possible to calculate how many of these transitions took place, and thus get a signal that is proportional to the concentration of the element being measured.
Background correction methods
The narrow linewidths of hollow cathode lamps make spectral overlap rare. That is, it is unlikely that an absorption line from one element will overlap with another. Molecular emission is much broader, so it is more likely that some molecular absorption band with overlap with an atomic line. This can result in artificially high absorption and an improperly high calculation for the concentration in the solution. Three methods are typically used to correct for this:
· Zeeman correction - A magnetic field is used to split the atomic line into two sidebands (see Zeeman effect). These sidebands are close enough to the original wavelength to still overlap with molecular bands, but are far enough not to overlap with the atomic bands. The absorption in the presence and absence of a magnetic field can be compared, the difference being the atomic absorption of interest.
· Smith-Hieftje correction (invented by Stanley B. Smith and Gary M. Hieftje) - The hollow cathode lamp is pulsed with high current, causing a larger atom population and self-absorption during the pulses. This self-absorption causes a broadening of the line and a reduction of the line intensity at the original wavelength.
· Deuterium lamp correction - In this case, a separate source (a deuterium lamp) with broad emission is used to measure the background emission. The use of a separate lamp makes this method the least accurate, but its relative simplicity (and the fact that it is the oldest of the three) makes it the most commonly used method.
Hollow cathode lamp
A hollow cathode lamp (HCL) is type of lamp used in physics and chemistry as a spectral line source and as a frequency tuner for light sources such as lasers.
An HCL usually consists of a glass tube containing a cathode made of the material of interest, an anode, and a buffer gas (usually a noble gas). A large voltage across the anode and cathode will cause the buffer gas to ionize, creating a plasma. These ions will then be accelerated into the cathode, sputtering off atoms from the cathode. These atoms will in turn be excited by collisions with other atoms/particles in the plasma. As these excited atoms decay to lower states, they will emit photons, which can then be detected and a spectrum can be determined.
An HCL can also be used to tune light sources by making use of the opto-galvanic effect, which is a result of direct or indirect photoionization. By shining the light source into the HCL, one can excite or even eject electrons (directly photoionize) in the atoms inside the lamp, so long as the light source includes frequencies corresponding to the right atomic transitions. Indirect photoionization can then occur when electron collisions with the excited atom eject an atomic electron.
The newly created ions cause an increase in the current across the cathode/anode and a resulting change in the voltage, which can then be measured. By tweeking your light source, you can then tune it to a specific transition frequency by looking for a resonance on the data plot of your signal vs. source tweaking parameter.