Graphite furnace atomic absorption spectroscopy is a type of spectrometry that uses a graphite-coated furnace to vaporize the sample. Briefly, the technique is based on the fact that free atoms will absorb light at frequencies or wavelengths characteristic of the element of interest. Within certain limits, the amount of light absorbed can be linearly correlated to the concentration of analyte present. Free atoms of most elements can be produced from samples by the application of high temperatures. In GFAAS, samples are deposited in a small graphite or pyrolytic carbon coated graphite tube, which can then be heated to vaporize and atomize the analyte. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. Applying the Beer-Lambert law directly in AA spectroscopy is difficult due to variations in the atomization efficiency from the sample matrix, and nonuniformity of concentration and path length of analyte atoms. Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. The main advantages of the graphite furnace comparing to aspiration atomic absorption are the following:
The detection limits for the graphite furnace fall in the ppb range for most elements
Interference problems are minimized with the development of improved instrumentation
The graphite furnace can determine most elements measurable by aspiration atomic absorption in a wide variety of matrices.
System Components
GFAA spectrometry instruments have the following basic features: 1. a source of light that emits resonance line radiation; 2. an atomization chamber in which the sample is vaporized; 3. a monochromator for selecting only one of the characteristic wavelengths of the element of interest; 4. a detector, generally a photomultiplier tube, that measures the amount of absorption; 5. a signal processor-computer system.
Mode of Operation
Most currently available GFAAs are fully controlled from a personal computer that has Windows-compatible software. The software easily optimizes run parameters, such as ramping cycles or calibration dilutions. Aqueous samples should be acidified to a pH of 2.0 or less. GFAAs are more sensitive than flame atomic absorption spectrometers, and have a smaller dynamic range. This makes it necessary to dilute aqueous samples into the dynamic range of the specific analyte. GFAAS with automatic software can also pre-dilute samples before analysis. After the instrument has warmed up and been calibrated, a small aliquot is placed, either manually or through an automated sampler, into the opening in the graphite tube. The sample is vaporized in the heated graphite tube; the amount of light energy absorbed in the vapor is proportional to atomic concentrations. Analysis of each sample takes from 1 to 5 minutes, and the results for a sample is the average of triplicate analysis.
