Analytical methods and quality assurance

Abstract

Microwave‐oven digestion of plant materials for the determination of plant‐nutrient concentrations is an aggressive and effective dissolution procedure. Commercial ovens are expensive, however, and ovens designed for home use may provide an inexpensive alternative. We evaluated three microwave ovens: Panasonic NN5510, Sharp Carousel II, and a commercial unit CEM MDS‐2000; and two Teflon vessels: Savillex 571R2 and CEM 327011 for their ability to digest batches of six or twelve plant samples and to compare their analyses with those from conventional block‐heater digestions. For the microwave‐oven digestions, we placed in each vessel 0.1 g National Institute of Standards and Technology (NIST) standard reference material (SRM) 1547 peach leaves, 1.0 mL conc. HNO3, and 0.6 mL 30% w/v H2O2. The digestates were stirred, soaked for 15 min., heated in a rotating carousel as described below, cooled 15 min, added 8.4 mL deionized‐distilled water to make 10 mL volume, filtered, and analyzed with ICP‐AES. The block‐heater method used a nitric/peroxide digestion, unsealed glass tubes, 0.5 g samples and a 25 mL dilution. There were no differences between Ca, K, S, P, Zn, Cu, and Fe concentrations of NIST SRM 1547 when samples were digested using commercial or domestic microwave ovens (P>0.05) and recoveries ranged from 84 to 95% of NIST certified values. Recoveries were greater from the block heater digestions than with the microwave digestions from four of the seven elements analyzed (P<0.05). Variability was greater from the microwave oven digestions than the block heater. We recommend home‐use microwave ovens used for the digestion of plant materials be set to 300 to 350 watts and heated for ≥30 min. for six‐vessel digestions and 45 min. for twelve‐vessel batches. Our procedure produces low vessel pressures (≥410 kPa) with little chance of relief valves rupturing and the loss of samples. The domestic ovens may be useful for those requiring occasional analyses or those who analyze samples of similar nature negating the need for frequent method modification. Laboratory personnel must adhere to our recommended sample sizes and reagent volumes to prevent explosions.