Sample dissolution partial

Elemental analysis of organie and inorganie samples requires partial or total dissolution of the sample prior to speetrometrie analysis. Mierowave-assisted extraetion (MAE) provides an alternative to traditional digestion methods and it is reeognized as one of the most effieient methods for the dissolution of soil 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 ... 

As noted earlier, some of the steps that precede the insertion of the treated sample into the instrument for measurement (e.g. dissolution, clean-up, preconcentration, individual separation, derivatization) can have a critical influence on accuracy and precision depending on the particular step. All analytical processes include a sample preparation step which is a function of a number of factors such as the physical state of the sample, the nature of the sample matrix and analytes or the type of detector, for example. The first distinction therefore refers to the nature of the sample solid, liquid or gas. Solid samples are the most difficult to process as most analytical instruments cannot handle them. Therefore, the first operation in solid sample preparation involves transferring the target analytes to a liquid phase. This can be carried out in various ways including total dissolution of the test sample or partial dissolution or separation of a portion thereof. The different choices, which can be assisted by ultrasound, are depicted in Fig. 2.2, and discussed in the following sections. 

Suitable inlets commonly used for liquids or solutions can be separated into three major classes, two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile impurities in the liquid will have been mixed with the sample itself, and these impurities will appear in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the separate examination of levels of residues left by solvents in the absence of any sample. 

The polymer/additive system in combination with the proposed extraction technique determines the preferred solvent. In ASE the solvent must swell but not dissolve the polymer, whereas MAE requires a high dielectric solvent or solvent component. This makes solvent selection for MAE more problematical than for ASE . Therefore, MAE may be the preferred method for a plant laboratory analysing large numbers of similar samples (e.g. nonpolar or polar additives in polyolefins [210]). At variance to ASE , in MAE dissolution of the polymer will not block any transfer lines. Complete dissolution of the sample leads to rapid extractions, the polymer precipitating when the solvent cools. However, partial dissolution and softening of the polymer will result in agglomeration of particles and a reduction in extraction rate. 

For PMMA/additive dissolutions, it was not possible to identify any additive characteristic mass peaks, either by direct laser desorption or with matrix-assistance (dithranol, DHBA or sinapinic acid, 4-hydroxy-3,5-dimethoxy-cinnamic acid). This has again been ascribed to very strong interaction between PMMA and additives, which suppresses desorption of additive molecules. Also, partial depolymerisation of pho-tolytically labile PMMA by laser irradiation may play a role, which leads to saturation of the detector by PMMA fragment-ions and disappearance of additive mass peaks below noise level. Meyer-Dulheuer [55] has also reported MALDI-TOFMS analysis of a coating/2-ethylhexyldiphenylphosphate sample. Quantitative determination of the additives by means of MALDI-ToFMS proved impossible. Possibly the development of reproducible (automated) sample handling procedures or thin films might overcome this problem. 

Obviously, the technology exists for obtaining analytical results without special preparation and analysis in a laboratory. However, at the present time there is no acceptable substitute for direct laboratory examination of samples if we want the kind of accuracy and confidence we have come to expect. All conventional methods for analysis of solid materials require one or more of the following preparation activities before an analytical method can be properly executed 1) particle size reduction, 2) homogenization and division, 3) partial dissolution, and 4) total dissolution. Let us briefly discuss each of these individually. 

The geometry of the pore tips is pyramidal, with facets formed by (111) planes, for Jap < JPS. This is the case when the current density is limited by the applied bias, as is the case for the samples shown in Fig. 9.13 b and c. If the bias is increased, dissolution at the pore tip occurs partially in the isotropic electropolishing regime (/tip=/ps)- This reduces the tendency to form facets and the tip geometry becomes almost hemispherical, as shown in Fig. 9.13d. 

Figure 10. Comparison of isotopic fractionations determined between Fe(II)aq and Fe carbonates relative to mole fraction of Fe from predictions based on spectroscopic data (Polyakov and Mineev 2000 Schauble et al. 2001), natural samples (Johnson et al. 2003), DIR (Johnson et al. 2004a), and abiotic formation of siderite under equilibrium conditions (Wiesli et al. 2004). Fe(II)aq exists as the hexaquo complex in the study of Wiesli et al. (2004) hexaquo Fe(II) is assumed for the other studies. Total cations normalized to unity, so that end-member siderite is plotted at Xpe = 1.0. Error bars shown reflect reported uncertainties analytical errors for data reported by Johnson et al. (2004a) and Wiesli et al. (2004) are smaller than the size of the symbol. Fractionations measured on bulk carbonate produced by DIR are interpreted to reflect kinetic isotope fractionations, whereas those estimated from partial dissolutions are interpreted to lie closer to those of equilibrium values because they reflect the outer layers of the crystals. Also shown are data for a Ca-bearing DIR experiment, where the bulk solid has a composition of q)proximately Cao.i5Feo.85C03, high-Ca and low-Ca refer to the range measured during partial dissolution studies (Johnson et al. 2004a). Adapted from Johnson et al. (2004a).

Rock samples collected from archived core, mine workings and outcrop were pulverized, homogenized, then analyzed using four-acid dissolution (SGS Lab, Toronto) to determine the near-total lithogeochemical composition (cassiterite, rutile, monazite, zircon, sphene, gahnite, chromite and barite are partially dissolved). Gold analyses were done by Fire Assay with Atomic Absorption finish on 30g samples and have a detection limit of 5 ppb. 

The total content of residual solvent is determined by partially dissolving a weighed sample of the powder in a solvent (a mixture of alcohol and ether) and adding water so as to precipitate the nitrocellulose from the solution in llocculant form. The weight of this nitrocellulose is determined by evaporation to dryness, repeated dissolution and precipitation with water, and by final drying. The difference between the weight of the powder sample and that of the nitrocellulose is the weight of the residual solvent. 

Figures 4-b and 4-d depict the pore size distribution curves of the SBA samples after these different treatments. For the sample SBA-A treated in acidic medium, the BET surface area (869 m2g" ), the mean pore diameter (6.4 nm) and the pore size distribution curve are similar to those from the pure parent silica SBA. For neutral treatment, the surface area (667 m2 g 1) decreases slightly. This can be related to the reduction of the microporous phase of the sample as shown in the pore size distribution curve. However, the mean pore diameter remains unchanged. Conversely, the structural properties of SBA-B are modified after treatment in basic solution. In this case, we observe a strong decreasing of the specific surface (454 m2 g 1) accompanied by a total loss of the microporous phase and an increasing of the mean mesoporous diameter (7.2 nm). It seems that in basic medium, a leaching phenomenon inside the mesoporous channels does occur, leading to a partial dissolution of the wall and resulting in smaller wall thickness (4.3 nm). Compared with the results on MCM-41, which show that the mesoporous structure collapses in basic solution [9,10], we can say that the stability of SBA materials in this medium is much higher.

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