production of selected secondary metabolites in transformed ...

The gaseous hydrides were directed towards the center of the bent part of the molybdenum
foil via a wide bore quartz GC–capillary. See Ref. [5] for more details.
RESULTS
The influence of the molybdenum foil temperature and concentration of hydrogen in the
gaseous phase on trapping behavior of bismuth and antimony were investigated for a bare
molybdenum surface and argon-hydrogen atmosphere of the flame supporting gas mixture.
Maximum trapping efficiencies were found for antimony and bismuth in the temperature
ranges of 650-750 °C and 500-600 °C, respectively. These optimum temperatures are
approximately 300-500 °C lower than those found for arsenic and selenium (1100-1200°C).
Capability of trapping antimony and bismuth hydrides on a modified surface of the
molybdenum trap was also investigated. Rhodium, platinum and iridium were chosen as
permanent modifiers and were introduced stepwise on the surface in amounts of 10, 30, 100
and 200 μg. Significant depletion of signals of collected antimony and bismuth was observed
when the amount of any modifier used exceeded 30 μg. Evidently, all modifiers inhibit the
interaction of both analytes with active sites on the molybdenum surface. In contrary, these
modifiers do not significantly affect trapping of arsenic and selenium on the molybdenum
surface. The maximum trapping efficiency of arsenic and selenium was independent of the
modifier amount applied in the range from 0 to 200 μg. Their signal profiles were higher,
more reproducible and symmetrical when increasing modifier amount.
The overall efficiency of generation of hydrides and their transport into the trapping
chamber is independent on the injection gas flow rate between the minimum and the
maximum achievable rates of 40 ml min -1 and 260 ml min -1 , respectively. Maximum trapping
efficiency was reached at a flow rate close to 70 ml min -1 , and at a distance of 2 mm between
the tip of the introduction capillary and the foil surface. Obviously, aerodynamic conditions
prevailing near the capillary orifice and the molybdenum foil during the trapping step play the
same role in trapping of all analytes studied.
Vaporization experiments showed that antimony, arsenic and selenium are strongly bonded
to the molybdenum surface. Collected antimony is completely released at temperatures above
2200 °C and arsenic and selenium at temperatures above 2400 °C. To the contrary, bismuth
exhibits a different behavior. A relative low temperature of 1200 °C is sufficient for complete
vaporization of trapped Bi. The heating vaporization pulse should be very short to prevent
losses of analyte on the inner quartz wall of the trap chamber and to perform an efficient
transport of the analyte into the diffusion flame. In the present experimental arrangement, the
optimum heating pulse in duration of 0.4 s was found.
ACKNOWLEDGEMENT
This work was supported by The Grant Agency of the Czech Republic (Project
No. 203/06/1441) and by Ministry of Education, Youth and Sports of the CZ
(FRVS 1054/2006).
REFERENCES
[1] J. Dedina, D. L. Tsalev: Hydride Generation Atomic Absorption Spectrometry,
Wiley & Sons, Inc., Chichester (1995).
[2] H. Matusiewicz and R. E. Sturgeon, Spectrochim. Acta, Part B, 51 (1996) 377-397.
[3] F. Barbosa Jr., S. Simiao de Souza, F.J. Krug, J. Anal. At. Spectrom., 17 (2002) 382-388.
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