Pressure digestion

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). 

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. 

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],...

If tungsten is recovered from the wolframite group mineral, the wolframite concentrate is boiled or pressure-digested with 50% caustic soda solution. Alternatively, they may be fused or sintered with caustic soda, caustic potash or sodium carbonate and the fused mass then leached with water. The solution is filtered to separate sodium tungstate solution. The fdtrate is subjected to various treatments to remove molybdenum, phosphorus, and arsenic impurities. The filtrate at this point is essentially a solution of sodium tungstate and is treated in the same way as that obtained from the scheehte concentrate discussed above. 

H. Matusiewicz, R.E. Sturgeon, S.S. Berman, Trace element analysis of biological material following pressure digestion with nitric acid-hydrogen peroxide and micro-wave heating, J. Anal. At. Spectrom. 4 (1989) 323-327. 

Work Item B determination of mercury by CVAAS after pressure digestion. Work Item C determination of lead and cadmium by ETAAS after dry ashing. Work Item D performance criteria and general considerations. 

Work Item F determination of lead, cadmium, chromium and molybdenum by ETAAS after pressure digestion. 

The sample is subjected to pressure digestion with HN03, or dry ashing, boron in the digest is chelated with 2-ethyl-l,3-hexanediol, extracted with MIBK and its concentration determined by FAAS using a nitrous oxide-acetylene flame. 

The inspiration for pressure digestion studies came from a US Bureau of Mines report [76], which described how rapid dissolution of some mineral samples could be achieved using a microwave oven to heat samples of an acid mixture contained... 

A single digestion procedure is often insufficient for the complete decomposition of a complex matrix, leading some authors to recommend a combination of two or more techniques. Two examples suffice to illustrate the principle [95,133]. The first example is pressure digestion followed by UV photolysis. It has been shown that... 

Dynamic systems for high-pressure microwave treatment were developed much later than open-vessel systems. Operating under a high pressure reduces the flexibility afforded by working at atmospheric pressure. However, some recently developed devices allow microwave-assisted high-pressure digestion and extraction in a dynamic manner [33,34]. 

Microwave digestion systems have become very popular for decomposing samples. The photo shown is a closed-vessel microwave digestion system for high-pressure digestions. A microwave oven with a built-in fume exhaust system is shown along with sample trays that contain up to 1 2 samples. Teflon sample vessels can be operated at temperatures up to 2300 C and 625 psi. 

---