Mohamad, Sharifah, and M. Radzi Abas. 2005. “Enrichment of Cu, Zn and Cd by Solid-Phase Extraction Using Silica Gel Chemically Modified With Hydroxamic Acid Prior to Their Determination by Flame Atomic Absorption Spectrometry”. Malaysian Journal of Science 24 (2):65-70. https://juku.um.edu.my/index.php/MJS/article/view/8330.