Dry ashing of plant material and dissolution of the ash in HF for the colorimetric determination of silicon

Abstract

In the past, determination of silicates in plant material was laborious and/or difficult. In several research fields outside plant analysis, digestion of material was obtained with the use of HF, followed by a boric‐acid treatment to eliminate excess HF.

In the present study, this procedure was applied to plant material. Before digestion with HF, the plant sample is dry‐ashed. After suitable dilution, Si can be determined colorimetrically. The proposed method proved to be rapid and accurate.     

Open‐vessel microwave digestion of animal waste samples for multi‐element analysis

Abstract

Wet acid digestion on a heat block or in a closed‐vessel microwave digestion system normally use half a gram to 1 gram of organic samples, but it is difficult to take a small quantity of representative sample from animal waste materials due to their heterogeneity. This study investigated an alternative microwave digestion system to prepare animal manure samples for plant nutrient analysis. Two types of solid animal manure and two reference plant samples were digested with an open‐vessel microwave digestion system and with the conventional nitric/perchloric acid block digestion, and analyzed for macro‐and micronutrients. Results of the open‐vessel microwave digested samples were comparable to those digested by conventional block digester and in high agreement with the certified values of reference materials. The open‐vessel microwave can reduce digestion time from about 4 hours to about 30 minutes. Fresh samples up to 10 grams can be completely digested directly by the open‐vessel microwave without sample drying and grinding. Sample representation of bulk manure should be better when sample size increased from 1 to 10 grams. Eliminating drying and grinding before digestion improves lab efficiency.     

Copper,zinc, and cadmium accumulation in two prairie soils and crops as influenced by repeated applications of manure

A study was conducted to determine the effect of repeated (5–7 y) annual application of liquid swine or solid cattle manure on the plant availability of copper (Cu), zinc (Zn), and cadmium (Cd) at two field sites in the W‐central and E‐central agricultural regions of Saskatchewan, Canada. Soil samples, plant‐straw and grain samples from the 2003 growing season were collected and analyzed for total Cu, Zn, and Cd concentrations using nitric acid microwave digestion followed by atomic‐absorption spectroscopy. An ammonium bicarbonate diethylenetriaminepentaacetic acid (AB‐DTPA) extraction was performed on the soil samples as a measure of the plant‐available fraction. Crop plants that were tested included wheat (Triticum aestivum L.) and barley (Hordeum vulgare L.). The results of this study indicated that long‐term repeated applications of manure fertilizer sometimes resulted in increased plant availability of Cu, Zn, and Cd, as reflected in increased concentrations of the plant‐available metal observed both in the soil and plant tissue. In the case of Cu and Zn, these increases were related to the rate of application, as the manure is a source of Cu and Zn. Changes in soil conditions from repeated manure application, including a decrease in pH and stimulated plant‐root growth can explain the effect that both manure and urea‐fertilizer application had on increasing the Cd concentration in the plant. Overall, there does not appear to be any concern about soil metal loading and plant accumulation in these soils after 5–7 y of manure application, as soil and plant concentrations were well below the allowable limits.     

An application of a modified microwave total dissolution technique for soils

Abstract

Application of a microwave dissolution technique to soil materials aided by acid digestion procedures has many advantages, but the incomplete dissolution of sandy soils has been noted. The objectives of this study were to modify a microwave oven digestion technique to facilitate the determination of the total elemental content of soils and geologic materials containing greater than 45% sand and apply these data to determine natural separations in parent materials. A microwave dissolution technique using aqua regia (HNO₃ and HCl) and hydrofluoric acid (HF) was modified such that the HF is added to the solid 16 hours prior to aqua‐regia addition and subsequent microwave heating. Elemental recoveries were validated using a NBS coal fly ash standard. This procedure was applied to soil core samples representing a wide range of geological and pedological weathering sequences and soil textural classes. Separations in the parent material were determined by observing natural breaks in the elemental concentrations and comparing them to soil morphology data.     

Trace Element Analysis via Open‐Vessel or Microwave‐Assisted Digestion in Calcareous Mediterranean Soils

Abstract

Different digestion methods can be applied for analyzing trace elements in soils. Selection of the most useful and convenient method is critical for the adequate determination of their concentrations in each type of soil to appropriately detect potential environmental pollution. Open‐vessel digestion using a digester block (method 1) and microwave‐assisted digestion (method 2) were compared using a certified reference material (CRM) with similar properties and characteristics to calcareous Mediterranean soils. Both methods were contrasted in terms of accuracy by calculating the recovery of cobalt (Co), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), and zinc (Zn) in the CRM and precision of replicate analysis through the relative standard deviation (RSD). Recoveries from the digester block method were slightly higher than recoveries from microwave digestion because the use of perchloric acid (HClO₄), whereas RSD values for microwave digestion were generally lower but with higher measured precision than values obtained by open‐vessel digestion. The digestion methods were compared for analyzing trace elements in calcareous agricultural soils devoted to vegetable crops in the Alicante province (southeast Spain), as a representative area of the European Mediterranean region. Results of the paired‐sample t‐test showed significant differences between both methods for Co, Cr, Cu, and Pb in calcareous Mediterranean soils, whereas regression analysis indicated a good correlation between both methods for Cu, Ni, Pb, and Zn. Consequently, the choice of the digestion method is more relevant for minor elements such as Co and Cr. Microwave‐assisted digestion seems to be the best option to determine most of the trace elements in calcareous Mediterranean soils, because closed‐vessel digestion exhibited lower variability according to the RSD values obtained in the study area, particularly for Co and Cr. Trace element concentrations were in agreement with background levels, except for Cu and Pb in some soils, which seem to be related to anthropic activities.     

Microwave Digestion for Colorimetric Determination of Total Si in Plant and Mineral Samples

A new method for determining the soluble silicon (Si) concentrations in nonliquid plant and mineral samples was developed using sodium hydroxide (NaOH)–hydrogen peroxide (H₂O₂) matrix for microwave digestion (MWD-Na) followed by colorimetric Si testing. MWD-Na was compared with autoclave-induced digestion and alkali fusion. The data have shown no significant difference between these three methods. HNO₃–H₂O₂ or HNO₃?–H₂O₂?–HF microwave technique results in Si loss. The reduction of the total Si can be realized via low solubility of Si in the acid solutions or silicon tetrafluoride formation. The Mullen and Riley method for colorimetric testing of Si in a solution that uses ascorbic acid to reduce the silicomolybdate complex formed under acid conditions to an intensely blue complex was modified using iron sulfate. The combination of MWD-Na and modified colorimetric blue method provides cheap and valid data on determining the total Si in both plant and mineral samples.     

High‐Throughput Microwave‐Assisted Digestion and Extraction Procedures for Agricultural Materials

Abstract

Despite the advances in microwave‐assisted procedures, the sample throughput still remains a critical problem that requires the development of reactor vessels, rotors, and procedures able to deal with a large number of samples simultaneously. In this work, a 36‐vessel rotor was combined with diluted nitric acid solution for digestion of agricultural materials. Accurate results were obtained for aluminum (Al), calcium (Ca), copper (Cu), iron (Fe), potassium (K), magnesium (Mg), manganese (Mn), phosphorus (P), and zinc (Zn) in bovine liver and spinach leaves standard reference materials. The EPA‐3051 method was also evaluated using this system. The procedure led to quantitative recoveries of chromium (Cr), Cu, Mn, nickel (Ni), and Zn in soil. All measurements were carried out using inductively coupled plasma—optical emission spectrometry (ICP‐OES). The main advantages of the proposed procedures are the increase of sample throughput in microwave‐assisted digestions or extractions and the lower dilution required before pneumatic nebulization of digests obtained by using dilute nitric acid.     

Comparison of a microwave digestion system to other digestion methods for plant tissue analysis

Abstract

Most digestion procedures for the preparation for plant tissue for elemental analysis are often very laborious. Elemental contents of plant tissue prepared by microwave digestion were compared to those obtained by conventional dry ashing and sulfuric acid‐hydrogen peroxide wet digestion. All three digestion methods produced comparable quantitative values for the elements P, K, Ca, Mg, Fe, Mn, Zn, and Cu for all plant materials used. Thus, the microwave digestion technique was shown to be a viable rapid method for digesting relatively small numbers of samples prior to elemental analysis.     

Effectiveness of six digestion procedures to evaluate the status of major elements (Ca,K, Mg,and Na) in citrus leaves

Abstract

Plant physiologists resort to plant tissue digestion to evaluate the nutrient status of crops, and to study ionic kinetic absorption and antagonism. This paper describes the effectiveness of six extraction procedures (2 non‐digestion, 3 wet‐digestion and the conventional dry‐ashing technique), in evaluating the content of 4 major cations (Ca²⁺, K⁺, Mg²⁺ and Na⁺) in sour orange leaves (Citrus aurantium L.). Analytical results, experimental advantages and limitations have been compared and discussed among all six procedures. The statistical studies carried out recommend the wet‐digestion as the optimum procedure to evaluate the mineral status of citrus leaves, and as a method with numerous technical advantages.     

Modified nitric acid plant tissue digest method

Abstract

The Colorado State University biosolids research program has routinely used concentrated nitric acid for digesting plant tissue for subsequent inductively coupled plasma‐atomic emission spectrometry analyses. The original method outlined by Havlin and Soltanpour calls for samples to be heated on an aluminum block at 125°C until the liquid volume is reduced to 2–3 mL. However, some plant species require >17 hours to reduce to 3 mL, while other plant species boil out of the digestion tubes at 125°C; some do both. The objective of this research was to determine if a timed digestion period could yield similar results to the original volume reduction method for plant species routinely analyzed in our laboratory. We performed digests on winter wheat grain and straw (Triticum aestivum L., ‘TAM 107'), fringed sage (Artemisia frigida Willd.), and an NIST wheat flour standard (NIST Standard Reference Material 1567a). We predigested 1.00 g plant material in 10 mL of concentrated nitric acid (69% by weight) at room temperature for 24, 48, or 72 hours, and we predigested plants for 24 hours followed by sample placement on an aluminum block digester. All samples were vortexed at 6, 24, 48, or 72 hours, accordingly. Block samples were initially digested at 80°C until fuming ceased, followed by an increase in temperature to 120°C and digested for 2, 4, 6, or 8 hours, or volume reduction to 3 mL. We used four replications and two blanks for all digests. We analyzed all samples using ICP‐AES and compared all results to the original digestion method. Overall comparison of all plants showed the block digestion to be superior to all predigestion periods. Winter wheat grain digested the quickest on the block, with all digests being complete within ～4 hours. The 4 hour block digestion compared well with the normal digest for our wheat grain. The 8 hour block digestion period for the other plant species all compared well with the normal digest. We suggest the following procedural modification: predigest all samples for 24 hours, with subsequent vortexing at 6 and 24 hours; place samples on an aluminum block at 80°C until fuming ceases; increase the temperature to 120°C and digest for 8 hours or until 3 mL remain, whichever occurs first.     

Total elemental analysis digestion method evaluation on soils and clays

Abstract

The standard digestion method for total elemental analysis of soil material by the Soil Survey Laboratory (USDA Natural Resources Conservation Service) uses a 2‐mL clay suspension with hydrofluoric acid (HF) in a closed Teflon digestion vessel (method HF‐SUS). The accuracy and efficiency of elemental recovery by method HF‐SUS was compared to: (a) modification of HF‐SUS by use of a dried sample (method HF‐DRI); (b) modification of HF‐SUS by use of a dried sample and HF+aqua regia (method HF+AR); (c) sample digestion by Li metaborate fusion (method FUS); and (d) microwave digestion of samples with HF+aqua regia in Teflon bombs (method MICRO). Three replications of three standard reference materials (SRMs), fine‐earth (<2 mm) from 12 soils, and the clay (<2 urn) from 10 of those soils were analyzed. Method HF+AR shows the most consistent statistical agreement with the certified SRM values. Analysis of variance (ANOVA) indicates significant effects (α=0.05) for method of digestion, nonsignificant effects for method times SRM and method times clay, but significant effects for method times fine‐earth. Composition and/or variability of material are significant factors in the method of digestion. Method HF+AR yields significantly higher experimental means of A1₂O₃, Fe₂O₃, and K₂O contents and oxide recovery (summation of experimental means for oxides of all reported elements) than all other methods.     

Improved recovery of fluoride in plant material using a low temperature sealed chamber digestion technique in conjunction with a fluoride ion‐specific electrode

Abstract

Seven different sample preparation methods (five ashing and fusion methods, an acid digestion method, an oxygen flask combustion method) and the AOAC extraction method (975.04) were compared with a newly developed sealed chamber digestion method for the determination of fluoride (F) in vegetation. Subterranean clover tissue was the test material. Fluoride concentration in digests was measured using a F ion‐selective electrode. The sealed chamber digestion carried out with nitric acid in Teflon chambers at 120°C for 6 hours gave the highest F concentration in the plant material and the highest recovery of added F of all methods. Tests on several types of plant material confirmed the superiority of the closed chamber digestion. The superiority of the method is attributed to the complete solubilisation of F in the sample and elimination of losses of volatile F.     

Limitations of acid digestion techniques for the determination of fluoride in plant material

Abstract

Procedures for determination of fluoride (F) in plant material employing acid digestion and solution analysis by ion specific electrode (ISE) were compared to alkali fusion using a range of plant materials. The efficiency of the methods were assessed using standard reference plant material (SRM) not previously available for plant F analysis. All acid digestion procedures tested failed to obtain the certified value for F in the SRM. This was due to failure of the acids to liberate F bound strongly within silicate minerals found in the plant materials. Acid digestion is therefore not recommended for determination of total F, but could be used to determine labile F in plant materials. During investigation of the acid digestion procedures, it was also found that F concentrations determined in solution using the ISE are sensitive to solution pH, even at solution pH values where complexation of F with hydrogen ions (H⁺) can be discounted. It is therefore recommended that both ionic strength and pH of sample and standard solutions be matched when determining F concentrations in solution using the F‐ISE.     

Boron analysis in soil extracts and plant tissue by plasma emission spectroscopy

Abstract

Nine Colorado soils were treated with sodium borate and were subjected to 3 wetting and drying cycles. These soils were extracted with hot water for boron analysis. Plant samples, including NBS standard reference materials (SRM) 1571, 1570, 1573, were dry ashed and wet digested using nitric acid. All soil extracts and plant digests were analyzed for boron using ICP‐AES and colorimetrically using the Azomethine‐H method.

A high degree of correlation (r² = .99) was found between boron determination by ICP and the Azomethine‐H method for soil extracts and plant digests. The Azomethine‐H method gave B values 9% higher than ICP‐AES on the average.

Boron levels determined by ICP were similar to NBS boron values for both the dry ashed and wet digested SRM plant samples. Boron levels determined colorimetrically were comparable to the NBS values for dry ashed SRM plant samples. Plant samples digested in nitric acid could not be analyzed for boron by the Azomethine‐H colorimetric method due to interferences resulting from nitrate complexes in the wet digest.     

Evaluation of Commercial Fe(III)‐Chelates Using Different Methods

Abstract

A collaborative assay among three laboratories was made in order to compare both the ion (CEN. EN 13368‐2:2001 E. Determination of chelating agents in fertilizers by ion chromatography. Part 2: EDDHA and EDDHMA, 2001a) and the ion‐pair (Lucena, J.J.; Barak, P.; Hernandez‐Apaolaza, L. Isocratic ion‐pair high‐performance liquid chromatographic method for the determination of various iron(III) chelates. J. Chromatogr. A 1996, 727, 253–264) high performance liquid chromatography (HPLC) methods as well as the soluble and complexed Fe (CEN. EN 13366:2001 E. Treatment with a cation exchange resin for the determination of the chelated micronutrient content and of the chelated fraction of micronutrients, 2001b) methods. Fifteen and ten samples of commercial fertilizers of Fe‐EDDHA, Fe‐EDDHMA, respectively were analysed by three laboratories using these methods. No significant differences were observed between the results obtained for the Fe‐EDDHA content using the Lucena et al. or CEN method. The first method makes it possible to distinguish between the meso and DL‐racemic diasteroisomers of Fe‐o, o‐EDDHA. For the Fe‐EDDHMA formulations, the CEN method gives higher values than the ion‐pair method, since in the first one Fe‐EDDH4,6MA coelutes with FeEDDHMA. Also the CEN method does not makes it possible to distinguish between Fe‐EDDHMA and Fe‐EDDH5MA products. The variability among laboratories was larger for the CEN method than for the Lucena et al. method.     

Nitric acid digestion and multi‐element analysis of plant material by inductively coupled plasma spectrometry

Abstract

Inductively coupled plasma spectrometry (ICPS) was used for the simultaneous determination of P, K, S, Ca, Mg, Na, Al, Cu, Zn, Mn, Fe, Co, Mo and B in the nitric acid soluble portion of a variety of plant materials. Conditions for pre‐digestion, digestion and the requirement to grind cereal grain were investigated.

Digestion with nitric and perchloric acids caused loss of K (due to the low solubility of potassium perchlorate) and B (due to volatilization). The accuracy of Fe and Na determinations using nitric acid digestion was dependent upon the type of plant material.

The accuracy and precision of the proposed digestion and analytical procedure was confirmed by co‐operation in an interlaboratory quality assurance program using a variety of standard reference plant materials, and the analysis of National Bureau of Standards, Standard Reference Material 1571 (orchard leaves).     

Determination of low concentrations of organic phosphorus in soil solution

Abstract

Phosphorus (P) export from agricultural land is an important source of water‐quality deterioration in many areas of the world. Part of the total phosphorus in the soil solution is represented by dissolved organic P at concentrations that can be as low as 1x10^‐6 M of P. The suitability of four digestion methods for the destruction of organic P and determination of orthophosphate at low concentrations and small volumes using the malachite green method has been evaluated. The acid digestion procedures evaluated were 1) sulfuric and perchloric acid, 2) sulfuric acid and potassium persulphate, 3) nitric acid, and 4) nitric and perchloric acid. As inositol hexaphosphate (IHP) represents one of the more resistant molecules to acid hydrolysis in soils, this compound has been chosen to assess the recovery assay of the recommended procedure. The digestion procedures were adapted for the malachite green spectrophotometric method, in order to obtain lower analytical limits for P determination. The sulfuric‐perchloric acid digestion gave excellent recovery and reproducibility, and can therefore be used for determining organic P in solution at concentrations as low as 6.45x10^‐7 M.     

A comparison of two modified Kjeldahl digestion techniques for multi‐element plant analysis with conventional wet and dry ashing methods

Abstract

The efficiency of two modified Kjeldahl procedures (H₂SO₄‐H₂‐O₂‐Li₂SO₄‐Se and H₂SO₄‐Na₂SO₄) for digestion of plant tissue for analysis of P, K, Ca and Mg contents was compared with a conventional wet (HNO₃‐HClO₄) and a dry ashing procedure. Six plant tissues were chosen as test material: leaves of Malus pumila Mill., Medicago sativa L., Dactylis glomerata L., fruit and wood tissue of M. pumila and Nothofagus mensiesii Oerst. leaf litter. Apart from low P contents of M. pumila wood tissue by dry ashing, the mean P, K, Ca and Mg contents as determined after the four digestion procedures were in good agreement. Furthermore, the precision of the data for each element was generally quite similar for each of the digestion methods. The N contents determined by the two modified and a conventional Kjeldahl procedure (H₂SO₄‐K₂SO₄‐CuSO₄‐Se) also agreed closely.     

Major Extractable Organic Compounds in the Biologically Degradable Fraction of Fresh,Composted and Anaerobically Digested Household Waste

The recirculation of plant nutrients from urban areas to agriculture and horticulture and a need to reduce waste disposal by landfilling and incineration are the main reasons for recycling the biologically degradable fraction of household waste. However, before using the recycled material in agricultural and horticultural production it is necessary to make sure that the material does not contain any unwanted contaminants. In general, there is a lack of knowledge about organic contaminants in the degradable fraction of household waste and its compost and anaerobic digestion products. As a first step in a study of organic contaminants in household waste, and as part of the characterization of the degradation processes during composting and anaerobic digestion, the major extractable organic compounds were identified in the biologically degradable fraction of fresh, composted and anaerobically digested household waste. The organic compounds were isolated by supercritical fluid extraction using carbon dioxide as extraction medium. Gas chromatography-mass spectrometry was used for qualitative analyses. Natural products, such as fatty acids, fatty acid esters, n-alkanes, aliphatic alcohols, monoterpenes and triterpenes constituted the major organic components in the waste samples. In addition bis-(2-ethylhexyl) phthalate, frequently used as a plasticizer in polymers, was found in the extracts. In the chromatograms of the extracts of the fresh and composted household waste, fatty acids and fatty acid esters dominated with respect to peak heights and the number of peaks identified. In the anaerobically digested household waste n-alkanes were the dominating compound class. The pattern of the n-alkanes, also found in the fresh waste, possibly indicates a petrogenic origin of these compounds, perhaps caused by contamination during the collection of the household waste.