A method is described to quantitatively determine Aluminum (Al), Silicon (Si), and Iron (Fe) in biological materials by Inductively Coupled Plasma (ICP) spectrometry using Hydrofluoric Acid (HF) to modify the microwave digestion/dissolution method. This method was tested using five plant tissues: NIST 1547 (peach), UCD 155 (Avocado), UCD 176 (Citrus), UCD 124 (Barley Hulls), and UCD 190 (Rice Straw). Values ranged from 50–700 μg/g Fe, 400–2000 μg/g Al, and 700–70000 μg/g Si. The method uses 250 mg plant material, 0.5 mL cone. HNO3, 2.0 mL 30% H2O2, and 150 ul cone. HF in a two step program of 5 and 8 min. The sample was diluted to a final volume of 15 mL using a boron solution to neutralize excess HF. The solutions were analyzed by ICP spectrometry. A smaller sample size (100 mg) is needed to obtain complete recovery of Si from botanical material containing Si levels higher than 5000 μg/g.