Pressure Ashing

Although the sample is protected from losses by volatilization, unwanted materials, especially carbon, are also not removed, and these can cause problems in some cases. For samples containing much organic material, the carbon remaining in the samples after this wet ashing can interfere with the determination of several metals especially arsenic and selenium by ICP-MS [92], 

---

Table 8.11 Main characteristics of high-pressure ashing...

Method development for high-pressure ashing and closed microwave digestion was reported for wet oxidation and extraction of Pb, Cd, Cr and Hg from various food packaging materials [57]. Use of HPA resulted in the highest median recoveries of the spiked elements (Pb and Cd, 92% Cr, 97% Hg, 83%). The use of In as an internal standard improved the accuracy... 

V. E. Negretti de Bratter, P. Bratter, A. Reinicke, G. Schulz, W. O. L. Alvarez, N. Alvarez, Determination of mineral and trace elements in total diet by inductively coupled plasma atomic emission spectrometry comparison of microwave-based digestion and pressurized ashing systems using different acid mixtures, J. Anal. Atom. Spectrom., 10 (1995), 487-491. 

Much of the research on biological sample digestion has included results from standard reference materials for validation purposes. Also, much of the development work has been conducted in parallel with a more traditional method such as conventional heating [157,158] and also with new methods such as pressurized ashing [117]. 

Two sets of bottles were kept at +6°C and +20°C during a period of 12 months and As, Hg and Se were determined to assess the possible losses of volatile compounds. Samples were digested by high-pressure ashing with a mixture of HNO3, HCl and H2SO4 at 320°C for 2 h. The final determination methods were HAAS (As, Se) and CVAAS (Hg). No instability could be demonstrated. 

High-Pressure Digestion, The introduction of a high-pressure ashing (HPA) technique by Knapp [45] has not only reduced the effective ashing time to ca. two hours but also opened the way to digestion of extremely resistant materials [46J. High-... 

A single ashing procedure is often insufficient for the complete decomposition of a complex matrix, leading some authors to recommend a combination of two or more techniques. One example will suffice to illustrate the principle pressure ashing followed by UV photolysis. Thus, it has been shown that analysis of olive leaves for heavy metals by voltametric methods leads to distorted results after pressure digestion alone. Reliable data can be obtained only by supple-menting the digestion with UV irradiation to ensure adequate decomposition of the matrix [41]. 

The nature of any safety devices employed can also play a role with respect to uncertainty in the case of a pressure ashing system. For example, serious losses may be incurred if sample constituents are entrained during a pressure blow-off. With an irreversible safety device like a rupture disc such a problem may be immediately apparent, but the activation of a reversible device like a valve generally cannot be documented even though major losses may be excluded by auto-... 

Figure 31. High-pressure ashing apparatus HPA-S (Courtesy of Perkin Elmer)...

The gasifier must interface with the pressurized oxidant delivery system (air or oxygen-blown gasifiers being feasible for coals) with the pressurized coal and sorbent preparation and feed systems with the raw fuel gas pressurized processing system and with the pressurized ash processing system. Coals may be... 

Haldipur, G., Schmidt, D and Smith, K. (1988) A 50-month gasifier mechanistic study and downstream unit process development program for the pressurized ash-agglomerating fluidized-bed gasification system, final report volume II. Technical Report, KRW Energy Systems Inc. 

A 50-Month Gasifier Mechanistic Study and Downstream Unit Process Development Program for the Pressurized Ash-Agglomerating Fluidized-Bed Gasification System, Final Report Volume II. Tech. Rep., KRW Energy Systems Inc. 

Zbinden, R, and Andrey, D. (1998). High pressure ashing—Determination of trace element contaminants in food matrices using a robust, routine analytical method for ICP-MS. Af. Spectrosc. 19(6), 214. 

---