A purity monitoring system for the H1 liquid argon calorimeter by Barrelet, E, Andrieu, B, Babaev, A, Banas, E, Bederede, D, Biddulph, P, Borras, K, Brasse, F, Brisson, V, Burmeister, P, Buschhorn, G, Canton, B, Cornett, U, Cozzika, G, Cvach, J, Cyz, A, Darvill, D, David, M, Descamps, F, Drescher, A, Dretzler, U, Ernst, G, Falley, G, Felst, R, Feltesse, J