Digestion and Sample Preparation

Digestion of soil samples for analytical purposes can be carried out in closed vessels (usually assisted by a microwave oven) or open crucibles (either as acid digestion or with a flux—mixture of salts—to open out the minerals). The details vary considerably and depend on the type of soil, the availability of proper equipment, and the analytical technique that will subsequently be used for the measurement, as demonstrated by the examples discussed later. After discussing some specific examples, we will try to derive the general guidelines and insights for the treatment of soil samples for the determination of the uranium content. 

FIGURE 3.3 Schematic flow chart of the borate fusion method for the determination of uranium and plutonium in soils samples. (Adapted from Croudace, I. et al.. Anal. Chem. Acta, 

Another variation on the acid digestion procedure used a mixture of hydrofluoric acid, nitric acid, and perchloric add in a sealed Teflon bomb heated at 150°C for 4 h 

Comparison of the Three Methods Aqua Regia Leaching, Fluoride-Pyrosulfate Fusion, and Borate Fusion for the Determination of Uranium [Bq kg ] in NIST CRM Soil Samples 

NIST 4353 (Rocky Hat soil) NIST 4353 (Rocky Hat soil) NIST 4357 (marine sediment) NIST 4357 (marine sediment) 

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Technicon Instruments Corp. (1977) Digestion and Sample Preparation for the Analysis of Total Kjeldahl Nitrogen and/or Total Phosphorus in Food and Agricultural Products using the Technicon BD-20/40 Block Digestor, Technicon Industrial Method No.369-75A, Technicon Instruments Corporation, New York, USA. 

One method of sample preparation proposed for MS/MS analysis of proteins is by performing enzymatic hydrolysis of gel pieces from SDS-PAGE (see paragraph 7.3.1) using trypsin, proteins are hydrolyzed at the carboxyl side of lysine and arginine residues. Protocol of enzymatic digestion and sample preparation is reported in Table 7.8. 

Table 7.8 Protocol of enzymatic digestion and sample preparation for MS/MS analysis of proteins from gel pieces after SDS-PAGE (Kwon, 2004).

TABLE 10.5. The SDS-PAGE, Enzymatic Digestion, and Sample Preparation Protocols for MS/MS Analysis of Wine Proteins2... 

Enzymatic Digestion and Sample Preparation Gel pieces dehydrated with acetonitrile and dried... 

Ammonium pyrrolidine dithiocarbamate (APDC) chelate coprecipitation coupled with flameless atomic absorption provides a simple and precise method for the determination of nanomol kg 1 levels of copper, nickel, and cadmium in seawater. With practice, the method is not overly time-consuming. It is reasonable to expect to complete sample concentration in less than 20 min, digestion in about 4 h, and sample preparation in another hour. Atomic absorption time should average about 5 min per element. Excellent results have been obtained on the distribution of nickel and cadmium in the ocean by this technique. 

Kotz etal. (1972, Decomposition of biological materials for the determination of extremely low contents of trace elements in limited amounts with nitric acid under pressure in a Teflon tube) Hartstein et al. (1973, Novel wet-digestion procedure for trace-metal analysis of coal by atomic absorption) Jackson etal. (1978), Automated digestion and extraction apparatus for use in the determination of trace metals in foodstuffs) Campos etal. (1990, Combustion and volatilization of solid samples for direct atomic absorption spectrometry using silica or nickel tube furnace atomizers) Erber et al. (1994, The Wickbold combustion method for the determination of mercury under statistical aspects) and Woit-tiez and Sloof (1994, Sampling and sample preparation). 

Microwave digestion has been proven to help speed up and improve the productivity and quality of the sample preparation in our laboratory. It has become the method of choice for many unknown and/or complicated matrix samples. Obtaining a representative sample can be a problem when smaller quantities (<0.2 g) are used for microwave digestion. Other applications, such as speciation, organic extraction, and sample preparation for wet chemistry are some areas to which we can apply this powerful tool in order to further improve our laboratory practice. 

The standard addition method [35] represents a combination of calibration with the aid of both external and internal standards. In ion chromatography, it is used predominantly for the analysis of samples with difficult matrices. Matrix problems may lead to an increase in nonprecision and/or express themselves as constant or proportional systematic deviations of the analytical results. Matrix influence can be identified via calculation of the recovery function. In constant systematic deviation, the error is independent of the analyte component. Such a deviation will cause a parallel shift of the calibration line. A possible origin for this deviation might be a codetection of a matrix component. In proportional systematic deviations, the error depends on the concentration of the analyte component. This type of deviation results in a change of the slope of the calibration line. Deviations of this kind can be caused by individual sample preparation steps such as sample digestion and sample extraction, and also by matrix effects. Systematic deviations can be identified by standard addition and/or calculation of the recovery function. 

Digestion A sample preparation technique that attacks and destroys most of e matrix, leaving analytes behind primarily for inorganics and metals. 

Fadda, S., Rivoldini, A., and Cau, I. (1995). ICP-MS determination of 45 trace elements in whole coal using microwave oven acid digestion for sample preparation. Geostand. Newsl. 19(1), 41. 

A web-site dedicated to sample preparation, which contains useft information about acid digestion and microwave digestion, is found at... 

Destructive solid sample preparation methods, such as digestion and mineralisation, are well known as they have been around for some time they are relatively cheap and well documented [13-15]. Decomposition of a substance or a mixture of substances does not refer so much to the dissolution, but rather to the conversion of slightly soluble substances into acid- or water-soluble (ionogenic) compounds (chemical dissolution). 

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