a tension sufficient to maintain the magnetic tape 12 against the guides 33 30 20 31 and 34 without the development of slack the entire spool of magnetic tape from tape cartridge 14 is thus submerged in water bath 27 to extract the ionic materials into the water bath and the wiping arrangement 40 for example comprising opposed brushes 42 and 43 ensures that the extracted ionic materials remain in the water bath concentrating the ionic materials in the small amount of water comprising the water bath the concentration allows a subsequent analysis of the ionic materials once substantially the entire length of magnetic tape has passed through the water bath 27 the magnetic tape is withdrawn from enclosure 15 and is rewound into the tape cartridge 14 for example by the drive system 60 referring additionally to fig3 the method for extracting the ionic materials comprises in step 70 threading the magnetic tape 12 into the guide system 18 and through the wiping arrangement 40 and drier 50 in the illustrated example the magnetic tape is also threaded along guides 33 30 and along guide 20 of guide system and threaded along guide 31 and through the brushes 42 43 of wiping arrangement or wiper 40 and along guide 34 and through the drier 50 to drive system 60 water is placed in the container 15 to form the water bath 27 and in one example in step 75.