Atomic Absorption Spectrometry (AAS) and Emission Spectrometry (ES) There are similarities as well as distinct differences between Atomic Absorption Spectrometry (AAS) and Emission Spectrometry (ES). I will begin be describing the similarities, then we will look at the differences both in theoretical bases and practical operations for the analyst. The Similarities. In the older type of instruments, both methods relied on combustion flames or electrothermal methods to atomize and introduce the sample into the optical path and to free the analyte from other substances generally present in the solution. A solution of the sample was sprayed as an aerosol through a nebulizer into a flame. The nebulizer functions because the high velocity of the combustion gases of the fuel and oxidant rushing past a small orifice draws the liquid into the flow as small droplets. The design of the nebulizer is to limit the size of the atomized sample or droplets introduced to the flame to a very small size (~5 - 10m). Droplets larger than this are stopped by baffles or spoilers and end up flowing to waste. The variables that control the small of droplets are the 1) difference in the velocity gas and the liquid, 2) the density of the liquid, 3) the viscosity of the liquid, and 4) the volume flow rates of both the gas and the liquid.
Both methods use a monochromator to isolate the wavelength of interest to the analyst, and rely on photometric detectors for their measurements. Unlike other spectrometry methods in solutions or solids, both AAS and ES deal with extremely sharp spectra lines, with band widths in the range of 10 –3-10 –2 nm.
The Differences As the names suggest, in Emission Spectrometry (or atomic emission spectrometry), the radiation is the result of the emission of excited atoms. (In recent years the ICP (induced coupled plasma) method has been added; it is also an emission method,
using a plasma flame as its excitation source.) These atomic excitations are caused by thermal excitation of the atoms within the flame. The wavelengths of light emitted as the excited electron returns to the ground state are characteristic of the elements present. The intensity of an isolated wavelength is proportional to the number of excited atoms, and thus to concentration of the analyte in the solution. No lamp is required, the emission lines of the element is the only radiation source. The sample must be excited and emit characteristic wavelengths in order to be detected. In Atomic Absorption Spectrometry, the line spectrum of the element being analyzed is emitted from a hollow cathode lamp (HCL) and passes through the optics of the instrument. The atomized sample is introduced into the flame (also part of the optical path) and absorbs resonance lines from the line spectra of the element. The decrease in the light intensity of the resonance lines is related to the concentration of the analyte by the Beer-Lambert Law in a similar fashion to other absorption methods, that is, A = -log T = kc . The flame or (plasma in is essentially the ‘cell’ where the absorption measurement is made. This method does not depend on thermal excitation of the atoms within the sample for its analysis.
Instrumentation for both methods The following units are common to both AAS and ES LIGHT SOURCE In AAS the source is the hollow cathode lamp; in ES, the flame, plasma or electric arc. SAMPLE In both methods the sample is in the flame, plasma, or electric arc. MONOCHROMATOR In both methods a monochromator is used to separate the desired lines from all of the other spectral lines that may be present. Compared to other spectrometry methods, higher grade monochromators are generally required. DETECTOR In both methods the detector converts the intensity of radiation to an electrical signal. There are a variety of detector types used in both AAS and ES. READOUT In older ES instruments the readout was photographic films or plates. Modern instruments use meters, recorders, or direct logging to computers. Details of AAS Hollow Cathode Lamps As mentioned above the light source for AAS is the hollow cathode lamp. A diagram of a HCL is shown below.
As the name suggests the lamp consists of 2 parts (anode and cathode) and is filled with an inert gas (generally Ar or Ne). The HCL is connected to a regulated low voltage low current DC power supply. The flowing electrical current causes excitation of the element present on the cathode and generates the characteristic line spectra of that element. The shape and configuration of the HCL helps to focus the radiation into the optical path of the instrument. Provision is generally made in the instrument to further enhance the focus of the radiation. Although there are multiple element hollow cathode lamps, generally one uses the specific HCL for the element being analyzed. Each specific HCL has specified current settings, so be sure to adjust the DC power supply to be within the operational parameters of that lamp. The reason for using the line source of the element for the radiation source is its very narrow and well-defined lines. The bandwidth of a selected line may approach 0.005nm, something that most monochromators could not accomplish. The Atomizer The atomizer may be a flame or an electro thermal apparatus known as the graphite furnace.
Again, its function is to atomize the sample, not cause the excitation of the atoms to higher electronic states. The length of the burner (or plasma region of the graphite
furnace) is the approximate path length of the cell. The path length is generally not known exactly, but its value remains a constant during standardization and analysis. For the burner a variety of fuels and oxidants may be used as shown in your textbook, table 9.3, page 231 selected according to the required temperature. The burner can generally be adjusted vertically to select the oxidizing (Oxidant-rich) or reducing (Fuel-rich) portion of the flame. This is a parameter the analyst may change to obtain the best sensitivity for a given element. The most common fuel and oxidant is acetylene and air.