1. Jensen, S., New Sci., 32, 612 (1966).
2. Widmark, G., J. Ass. Off. Anal. Chem., 50, 1069 (1967).
3. Holden, A. V., and Marsden, K., Nature, 216, 1274 (1967).
4. Holmes, D. C., Simmons, J. H., and Tatton, J. O'G., Nature, 216, 227 (1967).
5. Koeman, J. H., Oskamp, A. A. G., Veen, J., Brouwer, E., Rooth, J., Zwart, P., v. d. Broek, E., and van Genderen, H., Med. Rijksfaculteit Landbouwwetenschappen Gent, 32, 3/4 (1967).