Acid-digestion plant materials

With very strong sulphuric acid, the plant material is dehydrated and most of the organic matter oxidised at rather high temperature. The digestion is completed by hydrogen peroxide at elevated temperature. 

A standard official method [6] has been used for the determination of cadmium in plant material. In this method the sample is digested with 1 4 v/v perchloric acidmitric acid at 200 °C and the residue dissolved in hydrochloric acid. Cadmium is then converted to the diethyldithiocarbamate. A chloroform extract of this solution is used for the determination of cadmium at the 228.8 nm emission line. See also Sects. 7.34.1 and 7.34.4. 

A standard official method has been published for the determination of cobalt in plant material [8 ]. The samples are digested with 1 4 v/v perchloric acidmitric acid and the residue dissolved in nitric acid. Cobalt is then extracted into chloroform as the diethyldithiocarbonate. The latter complex is decomposed by bromine and cobalt extracted into dilute hydrochloric acid. Following the addition of a borate buffer, cobalt is then extracted as the o-nitrocresol complex [9]. Excess coupling agent is removed by repeated extraction with copper acetate solution and cobalt determined spectrophotometrically at 360 nm. See Sects. 7.34.1, 7.34.3 and 7.34.4. 

Mortatti et al. [16] has described a method for the determination of total iron in extracts of perchloric acid-nitric acid digests of plant materials. Their method involves flow injection analysis of the 1 10 phenanthroline complex. 

An official method has been published for the determination of magnesium in plant material [27]. A hydrochloric acid digest of the sample is treated with strontium chloride perchloric acid releasing agent and magnesium is determined by AAS using the 285 nm emission line. See Sect. 7.34.1. 

This element has been determined in perchloric acid digests of plant materials by a spectrophotometric procedure as permanganate ion obtained by oxidation with periodic acid or by AAS using the 279.5 nm emission line [28]. 

In an official method [33] for the determination of mercury in plant material, the sample is digested at 150 °C with 70% m/v concentrated nitric acid in a pressure vessel. Potassium permanganate is added to prevent loss of mercury and remove nitrogen oxides, and the concentration of mercury was determined either by flameless AAS at 253 nm or by flameless atomic fluorescence. 

Recoveries of molybdenum obtained by the nitric acid-hydrogen peroxide digestion procedure ranged from 96.4 2.9% for lettuce to 100.9 5.4% for beet leaves compared to 101.1 5.2% for lettuce to 98.8 4.95 for beet leaves obtained by dry oxidation. Recoveries obtained for reference plant materials are listed in Table 7.3. 

In a standard official method [50], the plant material is prepared for analysis by either digestion with 60% wlw perchloric acid, 70% wlw nitric acid, 1 3 mlv, and digestion of the residue with 2 M hydrochloric acid, or by dry combustion at 500 °C followed by extraction of the residue with 6 M hydrochloric acid. The concentration of nickel in these extracts is determined by AAS at 232.0 nm employing either background correction, or by an AA spectrophotometric procedure involving formation of the nickel ammonium pyrrolidiniedithio-carbamate followed by chloroform extraction. 

The hydride generation ICP-mass spectrometric technique [75] had a sensitivity of 6.4 ng/g selenium in plant material and was applied to digests of corn, kale and rice. In the isotope dilution mass spectrometric technique [77], the samples were spiked with 76-selenium solution and digested on a heating block at 150 °C with a mixture of nitric acid and hydrogen peroxide. Solid-phase microextraction was used to extract selenium from plant material prior to the gas chromatographic techniques [76]. See also Sects. 7.34.1 and 7.34.2. 

Sodium has been determined in plant material by a standard official method [ 79 ]. In this method, the plant material is first digested with perchloric acid 60% m/m nitric acid 70% m/v (1 4 v/v), the residue is dissolved in hydrochloric acid, and then it is analysed by AAS at the 589.0 nm emission line. See Sect. 7.34.1. 

In an official method [83] for determining zinc in plant material, the sample is digested with perchloric acid 60% nitric acid 70%, m/v 1 4, followed by 2 M hydrochloric acid. Alternatively, the plant material is dry ashed and the residue dissolved in 6 M hydrochloric acid. The extract is evaluated by AAS at the 213.9 nm emission line. See also Sect. 7.34.1,7.34.4 and 7.34.7. 

A variety of plant species were used to determine the applicability of this method, including mosses (Sphagnum capillaceum, S.fallax), lichen (Caldonia spp.) and higher plants (Chamaedeephne calyculata). The plant material dissolved completely in the acid and no difficulty was observed in the digestion. It was observed however, that some plant species dissolved more readily than others. 

In this method, portions (2 g) of the plant material were digested in 30 ml of digestion acid until fumes of nitric acid are produced then the solution is cooled, and diluted to 50 ml in 15% hydrochloric acid-10% sulfuric acid vlv. This solution was used for analysis. Recovery studies were made by spiking the digestion acid or sample just prior to the digestion procedure with varying amounts of standard solution. Quantification of the six elements was made from linear calibration curves verified by the method of standard additions. 

Nitric Acid and Organic Matter. The use of the mixed acids to digest organic matter may result in explosions. Explosions with vegetable oil,19 milk, calcium oxalate precipitates from plants,21 rat carcasses,22 finely ground plant material,21 and animal tissues21 have been documented. 

A comparison of different mineral acids for wet digestion of plant material for AAS analysis was carried out by Dokiya et al. (1975), who concluded that nitric acid was best for the determination of Cu, Mn and Pb by flameless AAS, whereas a nitric sulphuric mixture was best for Fe, Mn, Cu and Zn by FAAS. Most literature reports dissolve plant material in nitric acid (Middleton and Stuckey, 1954) or in nitric and perchloric acids (Isaac and Johnson, 1975 Williams, 1978, Thompson and Wood, 1982). This method will not dissolve aluminium containing particles from soil or other contaminations (Pierson and Evenson, 1988 Ramsey et al., 1991) (see also Standard Reference Materials). 

For trace metals other than those described above, it is necessary to use larger samples of plant material. Often digestion with nitric acid, followed by perchloric acid is used to digest upto 2 g of plant tissue, with final dilution to 25 ml. This allows determination of elements such as nickel by flame AAS. However, great care is necessary when using perchloric acid because of the explosion risk. A suitable, wash-down fume hood should be available, or at least an efficient fume scrubbing system. 

Nickel is determined more often, and with better sensitivity, by flame AAS than by flame AFS or AES techniques, even when a nitrous oxide-acetylene flame is employed in AES. The AAS detection limit at 232.0 nm under carefully optimized conditions in an oxidizing air-acetylene flame is about 10 ng ml-1, which is adequate for those environmental applications where a low sol-ution-to-sample weight ratio may be used. For example, if 1 g of plant material is digested with a mixture of nitric acid plus perchloric acid, and the mixture diluted only to 10 ml, nickel may be determined directly by flame AAS. Similarly the... 

G. C. L. Araujo, M. H. Gonzalez, A. G. Ferreira, A. R. A. Noggueira, J. A. Nobrega, Effect of acid concentration on closed-vessel microwave-assisted digestion of plant materials, Spectrochim. Acta, 57B (2002), 2121-2132. 

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