Digestion vessels

Organic selenium compounds and siUceous materials (rock, ore, concentrates) are fused with mixtures of sodium carbonate and various oxidants, eg, sodium peroxide, potassium nitrate, or potassium persulfate. For volatile compounds, this fusion is performed in a bomb or a closed system microwave digestion vessel. An oxidizing fusion usually converts selenium into Se(VI) rather than Se(IV). 

Attention is drawn to the extremely inert character of Teflon, which is so lacking in reactivity that it is used as the liner in pressure digestion vessels in which substances are decomposed by heating with hydrofluoric acid, or with concentrated nitric acid (see Section 3.31). 

It is further important to note that if only solid samples are taken, the calibration curve passes through the origin as is obvious in Figure 4.4. An intercept usually caused by blank values from reagents and digestion vessels in wet analysis can be excluded except in the case of matrix modification. 

In the Wickbold method, solid samples are vaporised in an oxygen stream and fed into an oxyhydrogen flame, which bums in a cooled quartz tube. The combustion products are condensed here, or are captured in an absorption solution as gaseous materials. Although combustion in a Wickbold apparatus is a quick and effective method for destroying organic material of all types, incomplete destruction may occur [19]. In special digestion vessels, known as cold-plasma ashers (CPA),... 

Samples of digest were prepared using either the maxibomb digester shown in Figure 3 or a smaller digestion vessel (minibomb) described elsewhere (12). The maxibomb consisted of a length of 25 mm o.d. stainless steel tube closed at one end... 

Organic materials, Sulfuric acid Analytical Methods Committee, Analyst, 1976, 101, 62-66 Advantages and potential hazards in the use of mixtures of 50% hydrogen peroxide solution and cone, sulfuric acid to destroy various types of organic materials prior to analysis are discussed in detail. The method is appreciably safer than those using perchloric and/or nitric acids, but the use of an adequate proportion of sulfuric acid with a minimum of peroxide is necessary to avoid the risk of explosive decomposition. The method is not suitable for use in pressure-digestion vessels (PTFE lined steel bombs), in which an explosion occurred at 80° C. 

Reactions were performed in sealed thick-walled glass tubes or in Teflon acid-digestion vessels, in domestic microwave ovens [13]. Teflon vessels can be used at pressures up to 14 atm, at temperatures below 250 °C, and are resistant to most commonly used chemicals, although they deform at temperatures >250 °C. 

Fig. 14.8 Simplified schematic diagram of a high-pressure digestion vessel with an EDL. A. plug and seal, B. quartz pressure reaction vessel with a sample solution, C. EDL with an antenna, D. vessel jacket with a screw cap, E. airflow. Adapted from Ref. [44],...

A 1-g air-dry, sieved ( 10 sieve <2.00 mm) soil is added to a polytetrafluo-roethylene (PTFE) digestion vessel. To this is added 10 mL of 70% nitric acid (HN03), and the mixture is microwaved at 15% power for 10 minutes. At this point, the extract can be filtered and analyzed or more extensive microwave digestion carried out as outlined in Sastre et al. (adapted from Reference 16). 

More recently, microwave ovens have been used for sample dissolution. The sample is sealed in a Teflon bottle or a specially designed microwave digestion vessel with a mixture of suitable acids. The high-frequency microwave, temperature (ca. 100-250°C) and increased pressure have a role to play in the success of this technique. An added advantage is the significant reduction in sample dissolution time [25, 26],... 

Selection of digestion vessels the size of the digestion vessels depends on the sample volume. Standard vessels are 25, 40, SO, 100 and ISO ml glass. For extreme trace analysis, quartz and Teflon vessels are available. Vessels can also be supplied with dust covers and reflux extensions. 

References to sample contaimination in the literature are numerous. Abercrombie, ef. oJ.(54) have noted that the information obtainable from ICP emission analysis warrants additional care because of the increased number of elements monitored. For example, if one were looking for boron and sodium, the use of glassware should be avoided. Certain digestion vessels that are presumed to be cleaned may not be acceptable. Polyethylene bottles (acid-soaked), for example, have been shown to continue releasing zinc from the matrix for as long as seven days(92). A number of trace elements may be leached from commercial disposable test tubes. Some of the more important contaminants include Ba, Sr, Zr, Na, K, Al, Ca and Mg. 

The acid addition and heating protocol were designed to avoid excessive foaming and boiling within the microwave-digestion vessel. 

The mildest solution that will digest the sample is preferred, as stronger acids are more likely to add to the blank, attack digestion vessels, and generally require more care in the laboratory. Concentrated acids may be used individually, in mixtures, or in sequence. Hot concentrated acids will dissolve many metals and alloys. Nitric acid oxidizes the sample and should be used before a stronger oxidizer such as perchloric acid, to remove the more readily oxidized material. 

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