The second cassette layout (Fig. ) allowed for 'reverse elution' of the radiotracer from the MCX cartridges on which it was trapped and washed. By not passing the trapped radiotracer through the two MCX cartridges, a smaller volume of eluent was required for elution and a more concentrated fraction was collected. Although standard elution in large volumes of eluent may be sufficient for clinical studies using higher starting activities, the production of a highly concentrated prep may facilitate transportation to multiple sites distant from the original site of synthesis. Under these circumstances, however, care should be taken to prevent any potential radiolysis that might occur.