1. LAW, L.M., and GOERLITZ, D.F.: J. Ass. Offic. Anal. Chem.53, 1276–1285 (1970).
2. MILLS, P.A.: J. Ass. Offic. Anal. Chem.51, 29–32 (1968).
3. HORWITZ, W. (Ed.):Official Methods ofAnalysis, 11th Ed., Association of Official Analytical Chemists, Washington, D.C., 1970, secs. 29.001–29.027.