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C:/Mdi/Css/120029723_CSS_035_003-004_R1.3d. Comunications in Soil ... University, South St., WA 6150, Australia; E-mail: [email protected]. edu.au.
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Comunications in Soil Science (CSS)

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COMMUNICATIONS IN SOIL SCIENCE AND PLANT ANALYSIS Vol. 35, Nos. 3 & 4, pp. 427–440, 2004 1 2 3 4 5 6 7 8 9 10

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

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L. Huang,1,* R. W. Bell,1 B. Dell,1 and J. Woodward2 1

Division of Science and Engineering, Murdoch University, WA, Australia 2 Marine and Freshwater Research Laboratory, Murdoch University, WA, Australia

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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.) result from the glass matrix or/and repeated use of digestion tubes.

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*Correspondence: L. Huang, Division of Science and Engineering, Murdoch University, South St., WA 6150, Australia; E-mail: [email protected]. edu.au.

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DOI: 10.1081/CSS-120029723 Copyright & 2004 by Marcel Dekker, Inc.

0010-3624 (Print); 1532-2416 (Online) www.dekker.com

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Huang et al. 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 capes 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 mg/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.

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INTRODUCTION

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Wet digestion of plant material in concentrated HNO3 with or without H2O2, in an electrical heating block, has been well established and widely used for the determination of nutrient concentrations in plant samples.[1,2] Elemental concentrations in the HNO3 digest solutions are

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routinely determined by means of inductively coupled plasma spectrometry (ICP). This procedure has been used for accurate quantification of P, K, S, Ca, Mg, aluminum (Al), Cu, Zn, Mn, cobalt (Co), molybdenum (Mo), and B in plant material while the accuracy of Fe and sodium (Na) determinations using this digestion is dependent on the type of plant materials.[2] However, in practice, conventional wet digestion procedures with test tubes or flasks heated by electrical heating blocks involve digest solution transfer and digest tube cleaning, relatively lengthy turn-around time per batch of digestion (usually >3 h),[2] inconsistent digestion from edge effects in heating blocks, and relatively high background levels of trace elements from glassware (particularly for the analysis of B at low concentrations in small amounts of sample).[2] It also requires constant monitoring of acid consumption to avoid sample drying and burning (which leads to the loss of some elements such as K and B[2]), especially in the last hour prior to completion. After the completion of digestion, the digest solution is usually transferred into a volumetric container, which introduces another source of error for the final quantification by ICP. In addition, from our laboratory experience (data not shown), the repeated use of the same set of glassware (even when thoroughly washed and rinsed) for HNO3 digestion may result in unacceptably high background, which may lead to un-reliable estimation of low levels of trace elements (such as Cu, Zn, and B) especially when plants are deficient in these elements or/and the amount of samples available is small. Since the 1990s, with the development of commercial microwave ovens (e.g., CEM Mars5) and increased availability of ICP instruments in analytical laboratories, microwave-digestion techniques have been increasingly adopted for the preparation of animal,[3–5] plant,[6–8] and soil[9] samples for the ICP quantification of nutrient elements (e.g., P, K, S, Ca, Mg, Fe, Cu, Zn, Mn, B) and contaminant elements [e.g., Al, arsenic (As), cadmium (Cd), nickel (Ni), lead (Pb)]. In general, microwave digestion has the advantage of time-saving, reduced errors caused by digest solution spillage, reduced acid consumption, and the minimal loss of volatile elements.[10] So far, closed vessels (Teflon PFA) that withstand pressure of varying degrees (depending on the vessel type), have been mostly used for microwave digestion. Sample digestion occurs within the sealed vessels pressurized by the gaseous phase of the acid mix while being heated. The microwave-digestion process usually requires less than 1 h. This closedvessel microwave digestion system has been proven to be an effective and accurate method for the analysis of trace elements in both plant and animal samples.[4–7]

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Huang et al.

However, the closed-vessel, microwave-digestion system has some disadvantages that limit its wide adoption for routine digestion of a large number of plant samples for elemental analysis. Teflon PFA closed vessels are often quite expensive (refer to CEM and Milestone websites for prices). Great care is required for assembling the caps and vessels to avoid acid leakage (which may occur under pressure after heating, especially when there are small sample particles on the edge of vessel rim).[11] For one microwave oven, only 12–14 vessels (depending on the microwave oven model) are processed in each batch[11] and, therefore, it is not suitable for routinely processing large numbers of samples unless several microwave ovens are purchased and more than one operator is available for assembling and disassembling the capped vessels (labor intensive). The sample digest from each vessel is transferred into a volumetric container for making up final volume, which itself is a possible source of error and is also labor-intensive. The aim of this study was to investigate an alternative microwave digestion method suitable for routine processing of plant samples for nutrient analysis by means of open vessels as recommended in the CEM microwave-oven operational manual. In the present study, the use of the open vessels-DV-50 vessels (50 mL polypropylene centrifuge tubes with volume calibration, melting point 160 C) was examined for routine HNO3 digestion of plant samples in a microwave oven (CEM Mars5, CEM corp., Matthews, NC, USA) fitted with a temperature control sensor. These open vessels are substantially cheaper than the closed Teflon PFA vessels. Standardized digestion conditions for the openvessel microwave digestion method are reported here, and the validity of the present digestion method is demonstrated by investigating the precision and accuracy of the proposed method.

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MATERIALS AND METHODS

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HNO3 (70% w/w) and H2O2 (30% w/w) used in the present study were of analytical grade. Triple deionized water was used for digest dilution. All lab-ware were thoroughly cleaned with successive soaking in: Decon 90–EDTA–10% HNO3–10% HCl. Items were thoroughly rinsed with deionized water at each transfer between soaking solutions and at the end of the process. Published closed vessel digestion techniques have used acid mixtures of (1) HNO3 þ H2O2 (in most cases),[3,9,12] (2) HNO3 þ H2O2 þ HF,[8] and (3) HNO3 þ HF.[13] For plant samples, these acid mixes produced equally accurate results of elemental analysis. The mix of HNO3 þ H2O2 is

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chosen for the proposed method, due to the hazardous nature of HF. Preliminary tests have shown that it only required about 1 h from the start to the completion of digestion and sample dilution. The turntable, vessels, and caps used were made from polypropylene (50-mL centrifuge tubes with volume calibration). The turntable supplied by CEM holds 52 vessels, which allows the processing of 51 samples at once (1 position is retained for the temperature sensor). Each vessel is covered with a cap in which a ventilation hole of 3.2 mm diameter is drilled in the center, to allow pressure relief while minimizing acid loss. These caps were thoroughly washed with dilute acid and rinsed with triple deionized (TDI) water and reused until deterioration was observed. Plant materials used for precision and accuracy tests include NIST1515 (apple leaves) and APSAC (Australian Plant and Soil Analysis Council) standard reference materials No. 61 (canola leaves), No. 62 (tobacco leaves), No. 63 (mixed herbage), No. 64 (wheat grain), and No. 65 (clover leaves), to represent a wide range of nutrient concentrations in different tissue types. Bulk eucalypt leaves (fine powdered) for in-house quality control were used for repeated precision tests across batches and at different dates. All nonreference samples were finely powdered (