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7.3 Graphite Tube Type Selection in Furnace Method
AA-6800/6650 Instruction
7.3.2 When Using a Pyrolitic Coating Graphite Tube
Generally, the pyro-coated graphite tube is effective for elements which are easily combined with carbon or the
major component of graphite tube (carbide is easily formed). Ni, Ca, Ti, Si, V and Mo are the typical elements.
In the high density tube, a sample easily permeates into the graphite, resulting in a larger contact area between
the element and carbon. In the pyro-coated tube, however, a smaller contact area can suppress the carbide
formation, and as the result, a higher sensitivity is obtained.
When the peak profiles are compared, a sharp peak is observed in the case of pyro-coated tube, while a broad
peak is observed in the case of high density tube.
Figure 7.1 shows the example in Cu. In the case of pyro-
coated tube, however, the result is easily affected by an acid concentration in sample
and a sample spreading
in the tube varies widely. As the result, the reproducibility is tend to be inferior to that of the high density tube.
The effect of acid concentration on sensitivity is shown in
Figure 7.3. The sensitivity varies more drastically as
the ac
id concentration changes, when compared with those of platform tube or high density tube.
7.3.3 When Using the Platform Type Tube
As a feature of the platform tube, the sample solution is injected onto the platform and heated as shown in
Figure 7.4. In the furnace method, the graphite tube is heated firstly from the wall, therefore, in normal tube, the
sample is heat
ed and atomized as the wall is heated. In platform tube, however, the sample is atomized after
the temperature of entire tube reaches the atomization temperature (see
Figure 7.5). In the measurement with
platform tube, the atomiza
tion peak becomes more broad in spite that the platform material is pyrolitic graphite
(see Figure 7.1). Also, it is more effective to set the temperature a little higher, compared with normal pyro-
coated tube. With this difference of heating sequence, however, using the platform tube can minimize the
influence of the
sample liquid nature. This is because a recombination of atoms after atomization does not
easily occur in the platform tube, while it easily occurs with a low temperature of tube center in the case of wall
heating. As shown in Figure 7.3, the sensitivity change is small when the acid concentration changes largely.
Also, when a background signal is generated, a timing of atomization signal can be shifted from it. Therefo
re,
this is effective for samples with complicated matrix, such as biological sample, waste water and sea water.
This feature of platform tube often makes the latter part of peak profile not get down to baseline level within the
atomization stage. Set the time of atomization stage a little bit longer in such case.
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