Further Reading
4.4 Sample Dissolution and Decomposition
4.4.2 Wet Ashing Techniques
4.4.2.2 Closed Wet Digestion
In this approach, the reactions for dissolution–decomposition take place in a closed vessel where the free expansion and evaporation of the contents is prevented. Closed digestion vessels, also often called as digestion bombs, are much safer than their open counterparts. The sample and the digestion reagents are placed in a thick-walled PTFE beaker with a snugly fitting lid. It is customary to wet a powdered sample with a few drops of pure water before it comes into contact with any other reagent. This container is placed in a jacket to assure safe sealing. First models of digestion bombs were heated in conventional ovens. In this case, the bomb jacket is mostly made from steel. The heat emanated by oven is transferred to steel jacket, then to PTFE vessel and finally to sample digestion mixture.
Digestion bombs can be heated above the boiling point of the liquid contained, because elevated pressure prevents boiling. Therefore, higher temperatures can be used as compared to an open digestion where the temperature is limited by boiling point as long as some solution exists. Because of this kinetic advantage, dissolu- tion–decomposition takes place faster in digestion bombs. On the other hand, the use of concentrated H2SO4in PTFE vessels requires a good temperature monitoring and control, as the boiling point of the liquid reagent, 338 °C, is higher than the melting point of PTFE, 327 °C. PTFE starts getting soft and will be deformed at 260°C.
Closed-digestion systems have the following additional advantages as compared to the open systems:
● Contamination from surroundings is minimized.
● Loss of volatile species is prevented.
● At elevated temperatures, HNO3becomes a better oxidant and thus can be used without the need of any other reagent in many cases. The resulting matrix is preferred for most of the techniques such as ETAAS, ICP-AES and ICP-MS.
● Since relatively lower amounts of reagents are used, contamination is mini- mized and resulting matrix is simpler.
Digestion bombs have one important disadvantage; the amount of sample is usu- ally limited to 0.5 g. Therefore, depending on the final volume of dissolved sample, resulting analyte concentration may be too low with respect to the limit of quantita- tion of the analytical technique to be applied. In such a case, the need for a more sen- sitive technique will directly elevate the cost of analysis.
The digestion bombs are designed to have the weakest part at the bottom, so that any accidental, undesired explosion will cause minimum damage. Some bombs have relief mechanisms to open if internal pressure exceeds a limit.
In the last 10 years, microwave heating has replaced conventional heating in most laboratories for sample digestion. In case of microwave heating, the jacket around the PTFE vessel should be transparent to the radiation; PTFE or other fluorinated polymers such as PFA, perfluoroalkoxy fluorocarbon are used. The bomb should not contain any metal parts since metals absorb and are thus not transparent to microwave radiation. Some manufacturers provide sample vessels made of TFM, a chemically modified form of PTFE, with a smoother and harder surface and lower permeability to gases as compared to ordinary PTFE.
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Some compounds including water absorb microwave radiation. Radiation energy is used by dipole rotation and ionic conductance; this causes rapid heating of the absorbing bodies. Since the jacket and vessel are transparent to microwave radiation, temperature rises much faster as compared to conventional oven where transfer from oven to jacket, vessel and sample takes some time. Therefore, the digestion requires a shorter period of time in microwave ovens.
In some microwave-heated bombs, there is a safety membrane that is ruptured if excessive pressures are formed. The membrane is placed in the upper part of the bomb, so that sample loss is minimized. In some other systems, there are tempera- ture and pressure sensors to control the power applied. One manufacturer produces bombs that will open briefly to release some gas in case of pressure build-up; a spe- cial ring and spring system then causes very rapid closing after release. However, the indicator ring changes its original position so that the user can be informed if there had been a gas release during digestion procedure (Figure 4.1). Although the time allowed for gas release is very short, the user then decides whether the contents should be used for subsequent analysis or not. If the decision is negative, the diges- tion may be repeated using a fresh sample and a programme of lower temperature.
Microwave ovens manufactured for digestions in a chemistry laboratory are more expensive, but also more sophisticated than the ones used in kitchen. Regarding this comparison, the following useful features should be mentioned:
● There are gas sensors with a feedback electronic system to cut off power in case of leakage from the bomb.
● Mechanical strength of walls to protect laboratory personnel in case of an explosion.
● Temperature and pressure sensors to monitor inside of the bombs to provide safety and control over the digestion procedures.
● Fume exhaust system.
● Rotation of vessels to homogenize the exposure to microwave radiation.
Figure 4.1 Operating principle of gas release and informing the user in microwave heated digestion bombs (Adapted from Ref. 10 with permission from Mr. Francesco Visinoni and Milestone S.r.l., Italy10)
Because of the advantages mentioned above, close digestion by microwave ovens has become the method of choice in many laboratories.
As a concluding remark for this section, one should be reminded that sample decomposition by acid attack is mostly based on trial and error; previous experience including relevant literature must be always considered.
Some digestion procedures using microwave heating for several food samples are given below:
Rice flour11
● Weigh around 1.0 g of powdered sample, place in TFM vessel.
● Add 5.0 mL of conc. HNO3, 1.0 mL of conc. H2O2and 5.0 mL of H2O; gently swirl the mixture to homogenize.
● Close the vessel, using 1000 W power; apply the following heating regime:
Time (min) Temperature (°C)
3 25–85 (ramp)
9 85–145 (ramp)
4 145–180 (ramp)
15 180
● Cool to room temperature before opening the vessels.
● The cooled solution is ready for further handling for analysis.
Wheat flour11
● Weigh around 0.5 g of powdered sample, place in TFM vessel.
● Add 6.0 mL of conc. HNO3and 2.0 mL of conc. H2O2; gently swirl the mixture to homogenize.
● Close the vessel, using up to 1000 W power; apply the following heating regime:
Time (min) Temperature (°C)
6 25–200 (ramp)
15 200
● Cool to room temperature before opening the vessels.
● The cooled solution is ready for further handling for analysis.
Milk powder11
● Weigh around 1.5 g of sample, place in TFM vessel.
● Add 12.0 mL of conc. HNO3; gently swirl the mixture to homogenize.
● Close the vessel, using up to1000 W power; apply the following heating regime:
Time (min) Temperature (°C)
10 25–180 (ramp)
10 180
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● Cool to room temperature before opening the vessels.
● The cooled solution is ready for further handling for analysis.
Mushrooms11
● Weigh around 0.5 g of sample, place in TFM vessel.
● Add 8.0 mL of conc. HNO3and 2.0 mL of H2O2; gently swirl the mixture to homogenize.
● Close the vessel, using 1000 W power; apply the following heating regime:
Time (min) Temperature (°C)
7 25–100 (ramp)
7 100–200 (ramp)
7 200 (ramp)
● Cool to room temperature before opening the vessels.
● The cooled solution is ready for further handling for analysis.
References
1. N.T. Crosby and I. Patel,General Principles of Good Sampling Practice, Royal Society of Chemistry, London, 1995.
2. FAO/WHO-EFP/83.53,Guidelines for the Study of Dietary intakes of Chemical Contaminants, World Health Organization, Geneva, 1983.
3. National Academy of Sciences,Recommended Dietary Allowances, 10th edn, Washington, DC, 1989.
4. OECD,Food Consumption Statistics1964–1978, Paris, 1981.
5. T.S.M. Van Schaik and L.M. Dalderup, A study in food consumption in eleven areas of the European Communities in connection with radioactive contamination, notably in Friesland,Voeding, 1968,28, 1–15.
6. P. Koivistoinen (ed), Mineral element composition of Finnish foods. Acta Agriculturae Scandinavica Suppl., 1980,22, 1–171.
7. T. Mumcu, I. Gokmen, A. Gokmen, R. Parr and N.K. Aras, Determination of minor and trace elements in Turkish diet by duplicate portion technique,J. Radioanal.
Nuc. Chem., 1988,24, 289.
8. M. Sinisalo, J. Kumpulainen, M. Paakki and R. Tahvonen, Content of major and minor mineral elements in weekly diets of eleven Finnish hospital,J. Hum. Nutr.
Diet., 1988,2, 43.
9. Analytical Methods Committee, Notes on perchloric acid and its handling in analytical work,Analyst, 1959,84, 214.
10. ETHOS PLUS User Manual, Revision 1/2000, Milestone Microwave Laboratory Systems, Italy, 2000.
11. ETHOS PLUS Application Notes, Milestone Microwave Laboratory Systems, Italy, 2000.
Further Reading
1. J. Dolezal, P. Povondra and Z. Sulcek, in Decomposition Techniques in Inorganic Analysis, D.O. Hughes, P.A. Floyd and M.S. Barratt (eds, English translation), London Iliffe Books Ltd., London, 1968.
2. R. Anderson, “Sample Pretreatment and Separation”,Analytical Chemistry by Open Learning, Wiley, Chichester, 1991.
3. R. Bock, in A Handbook of Decomposition Methods in Analytical Chemistry, I.L.
Marr (Translated and Revised), International Textbook Company, Glasgow, 1979.
4. E. Prichard, G.M. MacKay and J. Points (eds), Trace Analysis: A Structured Approach to Obtaining Reliable Results, The Royal Society of Chemistry, Cambridge, 1996.
5. D.A. Skoog, D.M. West and F.J. Holler,Fundamentals of Analytical Chemistry, Chapters 32 and 33, Saunders College Publishing, New York, 1996.
6. M. Zief and J.W. Mitchell, Contamination Control in Trace Element Analysis, Chemical Analysis series, Vol 47, Wiley, New York, 1976.
7. H.M. Kingston and L.B. Jassie,Introduction to Microwave Sample Preparation:
Theory and Practice, American Chemical Society, Washington, DC, 1988.
8. A.G. Howard and P.J. Statham, Inorganic Trace Analysis: Philosophy and Practice, Wiley, New York, 1993.
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