1. British Standard, BS 3762: Part 3, vol. 21, 1986, 4 pp
2. US EPA Method 8100, US Government Printing Office, Washington, DC, USA, 1986
3. US EPA Method 3540, US Government Printing Office, Washington, DC, USA, 1995
4. AOAC Method 963.15, Association of Official Analytical Chemist, USA, 1990