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The GFAA experiment is one that follows the laws of spectrophotometry, thus your knowledge of Beer’s law
should be applicable in this experiment. In this approach, microliter volumes of a sample are pipetted into a
small graphite tube (the furnace) that can be heated electrically. A series of three heating steps are usually
then applied to the sample contained in the graphite furnace. The first of these is the drying stage, designed
to gently evaporate the solvent from the liquid sample (usually water) without splattering the sample.
Following the drying stage, the temperature of the furnace is increased, often in the range of 400-800 ° C
for a period of time called the ashing (or charring) stage. During this stage organic components (fats and
oils) in the matrix can be charred, the composition of the sample can be changed chemically, or high boiling
volatile components can be removed. The impact of this stage is one which further simplifies the sample
matrix to help reduce interferences and make calibration easier. Finally, the atomization stage is applied. In
this stage, the temperature of the furnace is very rapidly raised to temperatures often as high as 2700 ° C,
effectively volatilizing the remaining components on the rod wall (mostly inorganic/metallic substances).
Many of the volatilized metals will come off of the walls in the atomic form, creating a cloud of atomic vapor,
trapped in a very small tube. The actual absorbance measurement that occurs during this stage by using a
hollow cathode lamp shined through the furnace, creating the actual transmittance measurement that is
common to all forms of spectrophotometry. A signal trace of absorbance versus time is the measurement
that is used for quantitative purposes. The trace characteristically shows a transient absorbance signal as a
function of time (similar to that of the Gaussian-like peaks observed in gas chromatography). For best
consistency, the peak areas are integrated and these areas are directly proportional to the actual mass of
the element detissueed to the rod (or to the actual concentration if consistent volumes of sample are used).
Calibrations are performed by measuring the areas of a number of standards.
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