Rapid Nitric Acid Digestion of Plant Material with an Open-Vessel Microwave System

Abstract

Digestion of plant materials in hot (130–140°C) concentrated nitric acid (HNO3) is a common procedure for assessing their nutrient contents. In the conventional HNO3 digestion, desired temperatures are achieved through controlled electrical heating, and digestion occurs within Pyrex test tubes. The main limitations associated with the conventional digestion method may include (1) high labor requirement for monitoring acid levels in the tubes and digest solution transfer at the end of digestion and (2) relatively high background levels, in particular, of trace elements (e.g., Cu, B, Mn, etc.) resulting from the glass matrix or/and repeated use of digestion tubes. The availability of industrial microwave technology provides opportunities for developing improved digestion systems that overcome the above constraints while routinely processing large batches of plant samples. The present article describes a simple, reliable, and rapid digestion procedure for HNO3 with hydrogen peroxide (H2O2) digestion of plant material by using an open-vessel (50 mL polypropylene tubes with caps in which a 3.2 mm diameter ventilation hole is drilled in the center), microwave-digestion system (CEM Mars 5, manufactured by CEM Corp., USA), followed by elemental quantification using an ICP-AES. The proposed method consists of two stages: (1) the predigested (overnight) sample and HNO3 mix is heated at 75°C for 10 min, followed by 109°C for 15 min; (2) after cooling for 10 min, 1 mL of H2O2 is added to each vessel through the ventilation hole and the sample mix is heated at 109°C for a further 15 min. The analytical results were statistically analyzed by using linear regression, linear correlation, and two independent means tests to determine analytical precision and accuracy of the proposed digestion method. The results have demonstrated that this method is suitable for precise and accurate determination of macronutrients calcium (Ca), potassium (K), magnesium (Mg), phosphorus (P), sulfur (S), and micronutrients boron (B), copper (Cu), manganese (Mn), and zinc (Zn) in plant materials. The analytical variability (coefficient of variation) was mostly less than 5%, apart from that of iron (Fe) (9%). There were no significant (P ≤ 0.05) differences between the measured and certified concentrations of both macro- and micronutrients in the ASPAC and NIST1515 standard reference materials (SRM), except for Fe in NIST1515 SRM. The recovery rate of Fe in the digest solution varies with plant types, for cereal samples, higher than 90%, but for dicot species (e.g., NIST apple leaves) the recovery rate was as low as 70%. One of the important advantages of this method was the consistently (across samples and different batches) low background reading (mostly under detection limits of the ICP-AES used, for example, the concentrations of B in the blank digests were consistently less than 5 µg/L). The adoption of the present digestion method may result in time saving due to short turn-around time (less than 60 min per 50 samples) and cost saving due to low labor requirement, low acid consumption, and low-cost digestion vessels.