Digestion of solid samples

The digestion of solid samples to produce a solution is discussed in Section 13.2. For solution-based ICP MS analysis, the liquid is taken up through a thin tube via a peristaltic pump. This feeds directly into the instrument nebulizer, where argon gas is introduced into the liquid and a fine mist of droplets is expelled from the tip of the nebulizer. This sample aerosol is sprayed into the condenser to reduce the size of the droplets, ensuring an even sample loading and preventing cooling of the plasma. About 1% of the sample solution uptake is transported to the plasma torch, and any unused solution is drained away and may be recycled. 

Digestion of solid samples preparatory to chemical analysis... 

An advantage of ICP-MS compared to all other atomic mass spectrometric techniques including TIMS is that usually only simple sample preparation (e.g., by microwave induced digestion of solid samples) is necessary. Sample preparation steps for ICP-MS analyses are similar to those of ICP-OES. Concentrated solutions are analyzed after dilution with high purity water only. In order to correct mass drifts of the instrument, an internal standard element like In or Ir with known concentration (e.g., I Op.g 1) is added. The solution is then acidified with HN03 to stabilize the metal ions in aqueous solution. 

Digestion of solid samples with acid or solvent may be performed in small crucibles heated on a metal hot plate, or in an air bath (Fig. 11.38), or in the glass apparatus illustrated in Fig. 11.46. The last-named is heated over a micro burner, and then rotated so that supernatant liquid or solution may be poured off drop-wise without danger or loss. 

Incubation is the most widely used prooedure to accomplish the enzymatio digestion of solid samples. A mixture of sample and enzyme is stirred or shaken in a buffered medium at 37°C. The incubation time oan be (a) short (4-6 h) (b) medium (10 h) or (o) long (24 h) the last is the most frequent ohoioe [70]. 

An alternative to the acid digestion of solid samples is fusion under alkaline conditions. The procedure is usually very labor intensive, takes a long time, and involves a high risk of eiTOTS from contamination and loss of analyte. This method can be used for the determination of fluoride and chloride in geological materials after fusion with sodium carbonate and subsequent injection into an ion chromatography column. 

The use of microwave radiation provides an alternative heating source for acid digestion of solid samples. It has distinctive advantages over conventional heating systems (hot plate or muffle furnace), including ... 

Apparatus and reagents for digestion of solid samples (see also Section 12.4.2.6)... 

Surprisingly, FI on-line microwave digestion of solid samples coupled to an ICP-MS spectrometer has not been explored as much with ICP-MS as with optical emission. The complex, high dissolved solids matrix resulting from the sample digestion may be one of the reasons, as it is difficult to subject the sample on-line to the subsequent treatment processes (e.g. dilution, pH adjustment, matrix separation) required to enable the sample to be transported into the ICP-MS. Nonetheless, as has been shown throughout this section, there are FI methods for efficient matrix removal, so there are opportunities for future developments in this area. 

Elemental analysis of sample digests Non-destructive analysis of solid samples... 

Sample Dissolution — Sample introduction into most ICP systems, is by liquid nebuli-zation. This constraint partially limits the quality of the emission analysis to be dependent on the digestion, in the case of solid samples. The fact that several elements are easily monitored simultaneously places a greater demand on the care and choice of sample preparation. Also there are both advantages and disadvantages to the use of dissolved samples in analysis. Some disadvantages are ... 

The introduction of inductively coupled plasma (ICP) in inorganic mass spectrometry means that there is an effective ion source operating at atmospheric pressure. Whereas solid mass spectrometric techniques allow direct analysis of solid samples in ICP-MS, the determination of trace impurities or isotope ratios in solid samples is often carried out after digestion and dissolution of the material. For the determination of trace impurities and isotope ratios in liquids, an additional nebulization... 

Slurries An alternative to dry and wet decompositions is the preparation of slurries, which have been shown to provide a convenient way to introduce solid material into the plasma torch. The solid sample is not digested rather, it is finely ground and suspended in a liquid to be then introduced as an aerosol of fine, hydrated, solid particles. Thus, the direct analysis of solid samples as slurries reduces both the risk of sample contamination and the time required for sample preparation. 

Carbonell, V., de la Guardia, M., Salvador, A., Burguera, J.L., Burguera, M. On-line microwave oven digestion flame atomic absorption analysis of solid samples. Anal. Chim. Acta 238, 417 21 (1990)... 

Ultrasonic energy is frequently used to accelerate the dissolution of solid samples under soft conditions of temperature, pressure and chemical reagents. Similar to direct dissolution by agitation, US-assisted soft digestion is not used to the same extent as other operations of the analytical process such as leaching, derivatization or detection. The simplicity of this operation with some types of samples and the operator s lack of awareness of its error contribution are responsible for the absence of optimization studies for this process. Inappropriately conducted soft digestion can result in major errors and affect the quality of the results. 

This chapter describes the principal applications of microwaves to the pretreatment of solid samples, with special emphasis on digestion and extraction, which are their two main uses in analytical chemistry. The description is preceded by a discussion of the fundamentals of microwave energy and its interaction with matter, and also of the equipment used by analytical laboratories, which can be of the open or closed type depending on whether they operate at atmospheric pressure or a higher level and whether they use multi-mode or focused microwaves. Selected designs developed for specific purposes are also commented on. 

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