Huang, L; Bell, RW; Dell, B; Woodward, J
Digestion of plant materials in hot (130-140degreesC)
concentrated nitric acid (HNO3) is a common procedure for asses-sing 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 75degreesC for 10 min, followed by 109degreesC 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 109degreesC 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 less than or equal to 0.05) differences between the measured and certified
concentrations of both macro- and micronutrients in the ASPAC and NIST1515 standard reference
materials (SIRM), 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 mug/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.