1. Waltking, A., J. Ass. Off. Anal. Chem. 54:533 (1971).
2. Coon, F.B., F.J. Baur, and L.R.L. Symmes, Ibid. 55:315 (1972).
3. Association of Official Analytical Chemists, “Official Methods of the Association of Official Analytical Chemists,” Washington, D.C., 1975, Method 26.015-020.