ARCA (Automated Rapid Chemistry Apparatus) is an automatic reaction apparatus. With ARCA, it is possible to perform many HPLC experiments in short intervalls. The time needed for 1 separation might be less than 1 minute. The dead volume of such a separation is only 0.035 ml. ARCA is built from chemical inert materials like Teflon. Hence absorption effects of transactinide atoms are minimized.
The production rates for heavy elements are so low that only single atoms are formed. These are transported to the apparatus by a helium gas jet system. The helium jet contains aerosols (mostly potassium chloride) as a carrier. The arriving atoms are "collected" for a certain time (30-60s) on a slider (That means, if there is an atom, it is on the slider at the end of the process). The slider contains a 0.5 mm pit in which the KCl particles are collected together with the adsorbed atoms. From there, the atoms are dissolved and fed onto a HPLC column. From this column it is possible to elute different fractions. During the elution, the material for the next separation is already collected in a second pit on the slider.
The changing between the two pits and between different eluents is done by microprocessor controlled pneumatic cylinders. Also the HPLC columns are changed automatically after each separation.
At the end of the elution, the atoms are assayed for their characteristic emission. Therefor the samples are evaporated on tantalum disks or titanium foils. This evaporating is a critical moment of the process because the controlled evaporating even of little amounts of water (<0.2 ml) takes a certain time. At the present, this takes about 20 seconds, nearly half the time of the single experiment.
Picture of ARCA -
Interactive flow diagram of ARCA
(still in German)
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October 1998 by Erik Strub.