Sample Dissolution Techniques

The importance of the total sample dissolution technique mentioned earlier becomes apparent because the detrital phase contains Th and U that is both lattice-bound and adsorbed but in unknown proportions. Bischoff and Fitzpatrick (1991) demonstrated that it is not possible to quantitatively separate these two components by selective leaching. 

Because of the risk of lead poisoning, the exposure of children to lead-based paint is a significant public health concern. The first step in the quantitative analysis of lead in dried paint chips is to dissolve the sample. Corl evaluated several dissolution techniques. " In this study, samples of paint were collected and pulverized with a Pyrex mortar and pestle. Replicate portions of the powdered paint were then taken for analysis. Results for an unknown paint sample and for a standard reference material, in which dissolution was accomplished by a 4-6-h digestion with HNO3 on a hot plate, are shown in the following table. 

Investigated is the influence of the purity degree and concentration of sulfuric acid used for samples dissolution, on the analysis precision. Chosen are optimum conditions of sample preparation for the analysis excluding loss of Ce(IV) due to its interaction with organic impurities-reducers present in sulfuric acid. The photometric technique for Ce(IV) 0.002 - 0.1 % determination in alkaline and rare-earth borates is worked out. The technique based on o-tolidine oxidation by Ce(IV). The relative standard deviation is 0.02-0.1. 

Totland M, Jarvis 1, Jarvis KE (1992) An assessment of dissolution techniques for the analysis of geological samples by plasma spectrometry. Chem Geol 95 35-62... 

Bischoff JL, Fitzpatrick JA (1991) U-series dating of impure carbonates An isochron technique using total-sample dissolution. Geochim Cosmochim Acta 55 543-554 Broecker WS, Olson EA, Orr PC (1960) Radiocarbon measurements and annual rings in cave formations. Nature 185 93-94... 

Table 3.4 summarises the main characteristics of a variety of sample preparation modes for in-polymer additive analysis. Table 3.5 is a short literature evaluation of various extraction techniques. Majors [91] has recently reviewed the changing role of extraction in preparation of solid samples. Vandenburg and Clifford [4] and others [6,91-95] have reviewed several sample preparation techniques, including polymer dissolution, LSE and SEE, microwave dissolution, ultra-sonication and accelerated solvent extraction. 

G.J. DeMenna and W.J. Edison, Novel Sample Preparation Techniques for Chemical Analysis - Microwave and Pressure, Dissolution, Chemical Analysis of Metals, ASTM STP 994 (F.T. Coyle, ed.), American Society for Testing and Materials, Philadelphia, PA (1987), p. 45. 

Table 7.90 shows the main characteristics of the polymer/additive analysis procedure, consisting of dissolution followed by p. SEC-GC. An advantage of this approach is the minimisation of the extraction losses inherent in conventional sample preparation techniques. The determination of higher-molecular-weight additives is restricted, due to the upper interface temperature attainable in the GC-MS system. The usefulness of the tool for quantitative analysis has been ascertained [948,976],... 

Application to solid polymer/additive formulations is restricted, for obvious reasons. SS-ETV-ICP-MS (cup-in-tube) has been used for the simultaneous determination of four elements (Co, Mn, P and Ti) with very different furnace characteristics in mg-size PET samples [413]. The results were compared to ICP-AES (after sample dissolution) and XRF. Table 8.66 shows the very good agreement between the various analytical approaches. The advantage of directly introducing the solid sample in an ETV device is also clearly shown by the fact that the detection limit is even better than that reported for ICP-HRMS. The technique also enables speciation of Sb in PET, and the determination of various sulfur species in aramide fibres. ETV offers some advantages over the well-established specific sulfur analysers very low sample consumption the possibility of using an aqueous standard for calibration and the flexibility to carry out the determination of other analytes. The method cannot be considered as very economic. 

Quite obviously, a disadvantage of the classical sample preparation technique, consisting of dissolving a sample in a solvent, which may eventually lead to volatilisation and degradation of the additives, is not totally eliminated (see Section 3.7). Actually, the solvent choice is more restrictive (Table 9.5). In fact, NMR for polymer/additive dissolutions is feasible only in cases of a common solvent for polymer and additives, compatible... 

Pressure dissolution and digestion bombs have been used to dissolve samples for which wet digestion is unsuitable. In this technique the sample is placed in a pressure dissolution vessel with a suitable mixture of acids and the combination of temperature and pressure effects dissolution of the sample. This technique is particularly useful for the analysis of volatile elements which may be lost in an open digestion [24]. 

More recently, microwave ovens have been used for sample dissolution. The sample is sealed in a Teflon bottle or a specially designed microwave digestion vessel with a mixture of suitable acids. The high-frequency microwave, temperature (ca. 100-250°C) and increased pressure have a role to play in the success of this technique. An added advantage is the significant reduction in sample dissolution time [25, 26],... 

Flow injection analysis is based on the injection of a liquid sample into a continuously flowing liquid carrier stream, where it is usually made to react to give reaction products that may be detected. FIA offers the possibility in an on-line manifold of sample handling including separation, preconcentration, masking and color reaction, and even microwave dissolution, all of which can be readily automated. The most common advantages of FIA include reduced manpower cost of laboratory operations, increased sample throughput, improved precision of results, reduced sample volumes, and the elimination of many interferences. Fully automated flow injection analysers are based on spectrophotometric detection but are readily adapted as sample preparation units for atomic spectrometric techniques. Flow injection as a sample introduction technique has been discussed previously, whereas here its full potential is briefly surveyed. In addition to a few books on FIA [168,169], several critical reviews of FIA methods for FAAS, GF AAS, and ICP-AES methods have been published [170,171]. 

The earliest applications for quantitative analysis of liquid samples and solid preparations entailed sample dissolution in an appropriate solvent. A number of moisture determinations in APIs and pharmaceutical preparations based on both reflectance and transmission measurements have been reported. Their results are comparable to those of the KF method. The high sensitivity provided by the NIR technique has fostered its use in the determination of moisture in freeze-dried pharmaceuticals. ° The noninvasive nature of NIR has been exploited in determination of moisture in sealed glass vials. " " ... 

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