Education
Graphite Furnace Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy
Atomic Absorption Spectroscopy is a widely used technique for the analysis of trace amount of elements in a sample. Several Atomic Absorption Spectroscopy techniques are available and Flame Atomic Absorption (FAAS) and Graphite Furnace Atomic Absorption Spectroscopy (GFAAS) are two main techniques that are used worldwide. GFAAS is more sensitive than FAAS.
FAAS | GFAAS | ||
Temp (K) | 1500–2500 | 2000 | |
gas | air/acetylene | argon | |
LOD (conc) | 10ppb | 0.1 ppb | |
LOD (mass) | 20 ng | 5 ng | |
Heating Method | combustion | Voltage across graphite tube | |
Common Application | ppm of 1 metal (high volume) | Single element low volume (ppb, pg) | |
Price | $35,000 | $55,000 | |
SME | SE | SE |
Table 1: Comparison of FAAS with GFAAS
LOD = The limit of detection for iron by concentration
SME = Single or multi element analysis possible on a single sample
Theory Behind
Atoms of the ground state absorb radiation energy and the excited atoms transit to an excited state. Beer-Lambert law is used to determine the quantity of the analyte.
I0 = incident radiation It = transmitted radiation |
Where
N0 = the concentration of atoms
L = path length
K = a constant related to the absorption coefficient
History
Walsh is the first person who practically used atomic absorption spectroscopy as a quantitative analytical tool where he used flame to produce atoms from the sample. L’vov is the first person who used a graphite furnace as the atomizer, where he obtained improved sensitivities with LODs compared to the flame technique.
Over View
Figure 1: Schematic diagram of the instrument
Commercial tubes are typically ca. 2.5 cm long, 0.6 cm in diameter, and composed of graphite with a pyrographite coating. This pyrographite coating minimizes the analyte loss from vapor diffusion through graphite. A tantalum liner, which was used in the original L’vov’s graphite furnace atomizer was later replaced by pyrographite coating. In early days there were different atomizer designs such as rods, braids and tubular. Due to the ability of providing higher temperature and semi-enclosed environment, tubular designs were favored. Some of the reasons to choose graphite are high purity, readily apparent, high sublimation temperature ( ca. 3400 K), and oxidize to form surface oxides which are removed as CO(g) and CO2(g) with simple heating.
Temperature profile should be optimized accordingly to obtain a fine analytical signal and to increase accuracy. The figure given bellow shows the three main heating cycles: dry, char, and atomization. The analytical signal is given at the atomization cycle.
Figure 2: Temperature profile and the analytical absorbance signal
