Pressure digestion bombs

As to the analysis of trace elements in paper, cardboard and raw materials for the production of paper, high concentration elements such as Cu, Fe or Ti can easily be determined by FAAS Cd and Pb are frequently analysed by GFAAS. Cadmium in pulp and paper was determined by AAS after pressurised digestion with nitric acid [145]. An interlaboratory comparison of Cd in wrapping paper was reported, mainly based on pressure digestion in FIFE bombs with sub-boiled nitric acid, followed by ETAAS [59]. For wrapping paper used for foodstuffs, next to the total content of toxic heavy metals, the soluble or leachable fraction is of particular interest. 

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. 

Pressure dissolution and digestion bombs have been used to dissolve samples for which wet digestion is unsuitable. In this technique the sample is placed in a pressure dissolution vessel with a suitable mixture of acids and the combination of temperature and pressure effects dissolution of the sample. This technique is particularly useful for the analysis of volatile elements which may be lost in an open digestion [24]. 

Decomposition in closed Teflon vessels at high pressure (up to 85 bar) with microwave heating. (Microwave Acid Digestion Bomb, Parr Instrument Company, USA.)... 

Hydrothermal synthesis does not require the water to be above its critical point. Huan, et al. published a synthesis of VOC6H5PO3XH2O prepared from phenylphosphonic acid, CeH5PO(OH)2 and vanadium(III) oxide, V2O3 (Huan et al., 1990). The two reagents were added to water, sealed in a Teflon acid digestion bomb, and heated to 200°C. Pure water has a vapor pressure of 225 PSI at 200°C, well within the bursting pressure of the bomb (1800 PSI). Unlike the quartz example, in this case, the solvent became incorporated into the final product. 

Digestion of samples using high pressure oxygen bomb combustion is an excellent technique for sample preparation, particularly trace metal analysis. This technique can be applied to most plastics provided that small sample ( 0.25 g) of fine grain sizes of plastics are used. The solutions obtained are clean and easily analysed for metal content against standards prepared in the same solution added to bomb. 

Table 4.10 Results of analysis of selected plastic materials for listed metals by ICP-AES after microwave acid digestion (A), dry eishing to 650°C with PTSA (B) or high pressure oxygen bomb combustion (C)...

The next level of containment is probably the fluoropolymer-lined digestion bomb. These autoclaves are sold by a number of vendors and are inexpensive (ca. 300 each), easy to handle, and require no special auxiliary equipment [31]. They are the vessels of choice for a number of workers doing some remarkable chemistry [32]. They also have the important advantage that they are inert to aqueous base and fluorides, making them a workhorse for the zeolite industry. However, they have the limitation that the fluoropolymer liner will deform and weaken above about 240 °C. Also, the autoclaves have simple screw top seats meaning they can rarely contain pressures exceeding 140 bar. Thus they cannot contain true supercritical aqueous fluids. Nonetheless their convenience makes them the vessel of choice for the large amount of hydro-thermal work done at 200 °C and below. 

From a safety perspective, it is critical that no more than 0.1 g of sample be used for this technique. The sample is weighed into the Teflon digestion vessel. Approximately 4 mL of nitric acid are added. The vessel is capped and placed into the microwave oven. Four vessels are simultaneously processed. The microwave is set at 125 W for 15 min. The oven then ramps up to 190 W for another 15 min. Care must be taken not to keep internal temperature and pressure within the capability of the vessels. Excessive heat and pressure will cause the digestion bombs to deform and potentially leak. After the cycle is finished, the vessels are placed into an ice bath for at least one hour to cool. The dissolved sample is washed into a 25-ml volumetric flask and brought to volume with water. 

The use of a hydrothermal bomb for preparation of a metal organic framework is a well-known technique in inorganic chemistry [31], However, the use of a hydro-thermal bomb for the preparation of scaffold is very rare. The final mixmre with the appropriate composition for scaffold preparation is sealed in a PTFE-lined acid digestion bomb and heated at 40°C for 8 h under autogeneous pressure. After that, the bomb is kept at room temperature to cool the product, which is then frozen at —20°C, Finally, the product is vacuum dried to obtain the desired scaffolds [32-34] (Dutta PK et al, unpublished results). 

Figure 2-18 Microwave digestion bomb lined with Teflon. [Parr Instrument Co., Moline, IL] A typical 23-mL vessel can be used to digest as much as 1 g of inorganic material (or 0.1 g of organic material, which releases a great deal of gaseous CO2) in as much as 15 ml of concentrated acid. The outer container maintains its strength up to 150°C, but rarely rises above 50°C. If the internal pressure exceeds 80 bar, the cap deforms and releases the excess pressure.

The acid digestion bomb is another powerful technique for solubilization of plastic samples, in which the digestive process is carried out in a sealed pressurized vessel called a digestion bomb and heated in a muffle furnace. The advantage of this approach is that the temperature of the process may reach well above the boiling point of the mixtures in the normal process, thus making possible the complete solubilization of some slow or incompletely dissolving components under open-vessel conditions. Volatilization of some of the analytes as well as sample contamination from airborne particulates can be also minimized. 

This approach has, however, certain limitation in terms of the amount of organic sample (most plastics materials) which can be digested while avoiding significant pressure buildup. Acid digestion bomb loading limits for some typical bomb sizes are shown in Table 11. 

Equipment for Sample Digestions 1.1.12.6.1 Pressure Dissolution Acid Digestion Bombs... 

An improved procedure has been described for atmospheric-pressure combustion of samples from high-purity metals [74]. High-pressure digestion with nitric and hydrofluoric acids 75], [76] and combustion techniques have both been shown to be suitable for the decomposition of samples of silicate-containing materials and fuels. Combustion in this case is carried out either in a stream of oxygen [77] or, to provide a closed system, in an oxygen bomb [78]. Mercury can be satisfactorily determined after prior Wickbold ashing [79]. 

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