Sample solubility

Oxides (Ln Oj), fluorides (LnF ), sulfides (Ln S, LnS), sulfofluorides (LnSF) of lanthanides are bases of different functional materials. Analytical control of such materials must include non-destructive methods for the identification of compound s chemical forms and quantitative detenuination methods which does not require analytical standards. The main difficulties of this analysis by chemical methods are that it is necessary to transform weakly soluble samples in solution. 

J. A. Apffel, T. V. Alfredson and R. E. Majors, Automated on-line multi-dimensional high performance liquid chromatographic techniques for the clean-up and analysis of water-soluble samples , J. Chromatogr. 206 43-57 (1981). 

In the case of soluble samples such as fish, mussels, etc. the speciation analysis has been achieved most successfully (e.g. Quevauviller et al. 1996a). 

The instrumental requirements for supercritical fluid extraction are quite simple. A pump is essential to generate the extraction pressure in a themostated extraction vessel. The soluble sample components are then swept from the vessel through a flow restrictor into a collection device that is normally at ambient pressure. The fluid used for supercritical fluid... 

Advantages and disadvantages of HS-GC over regular GC are summarised in Table. 4.26. HS-GC fingerprinting chromatograms obviously include only the volatile components present and do not provide a complete picture of sample composition on the other hand, when solvent extraction is used, all the soluble sample constituents are removed, including also those having no appreciable vapour pressure at the equilibration temperature. Headspace analysis enhances the peaks of volatile trace components. 

Preparation of Soluble Sample for Decarboxylation. The photolyzed fabric swatch was placed in 20 ml of distilled DMAc at 100°C for 5 minutes. The DMAc solution was filtered to remove any insoluble fibrous material from the decarboxylation flask (a round 30 ml two-neck Pyrex flask with a protruded bottom well) and dried in vacuo with the temperature kept below 50°C leaving a residual film on one side of the flask. 

This new and novel method to study the photochemical degradation of Kevlar-29 fabric in air divides into four steps (1) fabric cleaning, (2) photolysis at specified temperature and time in 0.2 atm - 02, (3) preparation of the degraded (DMAc-soluble) sample surface for decarboxylation at 25° and 196°C in the concentrated sulfuric acid, and (4) the total carbon dioxide analyses by gas chromatography and the isotopic carbon dioxide ( °C02 and 48co2) ratios by GC-mass spectrometer. 

Most methods of pKa measurement were developed using water-soluble samples. However, many drugs are poorly soluble in water alone, and require the presence of a water-miscible co-solvent to keep them in aqueous solution. The solvent affects the pKa in two ways (i) it causes the pH scale to shift and (ii) it causes the pKa to shift. The consequence is that apparent pKa values measured in the presence of solvent are different from aqueous values. 

A water-soluble sample of a solid containing some barium nitrate is to be analyzed for barium. The barium is to be precipitated as barium sulfate. Answer the following questions about this experiment. 

Military and ammunition sites contaminated with explosives can cover substantial areas (Gerth et al. 2005). Soil contamination in these sites is often heterogeneous. Explosives are relatively non-volatile, and have low aqueous solubility. Sampling from sites within a few decimetres of one another can result in concentration differences of up to one hundredfold (Jenkins et al. 1996). For example, the coefficients of variation across samples taken from 11 abandoned sites in the USA were 248% for TNT and 137% for Hexogen (Crockett et al. 1998). As a result, sampling error greatly exceeds measurement error. Thus to obtain representative results... 

Serum, plasma, urine, cerebrospinal fluid (CSF), and aqueous extracts of cells and tissues are encountered as soluble samples and often require no pretreatment. They can be directly analyzed by 2-DE following a solubilization step with a suitable buffer (e.g., mostly phosphate buffered saline, or PBS). 

Urea, ranging from 5 to 9 M, is the most common choice of chaotropic agent, and disruption of hydrogen and hydrophobic bonding between proteins is the main mode of action. When used in conjunction with thiourea (from 0 to 2 M), the two can aid solubilization of poorly soluble samples. The use of urea or urea-thiourea mixtures in SD buffers has no effect on the charge of the proteins within the sample. This is ideal for use in separating proteins according to their p in the first... 

There are several possible ways to improve the saturation rate. One reason for the delay in the attainment of equilibrium is the decrease in effective surface area during the dissolution process. This can be overcome by using a substantial excess of solid in the solubility sample (Higuchi et al., 1979). The surface area of the solid can also be increased by preprocessing the solubility samples. Both votexing after adding a smallLton ball and sonication are very effective techniques for this purpose. 

Some sample matrices are inherently difficult to ash. Foodstuffs with high sugar content are an example. Dry ashing must be done slowly and requires over 30 hours. Soluble samples such as sugar can be aspirated in solution directly into the AAS, but the solution must be quite dilute. This leads to high detection limits, and the recovery of analytes tends to be low. 

The use of NMR spectroscopy as an analytical technique is well established ( 1 8). In order to quantitate our spin-echo height to the number of protons present, we performed an independent calibration using standard solutions of naphthalene in carbon tetrachloride. Concentrations for the standards were chosen to correspond to the anticipated supercritical C02 solubilities, and all calibration measurements were performed using a sample cell of the same dimensions as the solubility sample cell previously described. The response of our spectrometer to the standard solutions was linear over the concentration range. The reproducibility for independent measurements of the calibration curve was 3 . Throughout the experiment, all spectrometer conditions (pulse lengths, phases, receiver amplifier gain, etc.) were closely monitored, and frequent checks on the calibration of the spectrometer were performed. In this way we were able to obtain the molar solubility of solid naphthalene in supercritical carbon dioxide to an estimated experimental accuracy of 6%. 

Irradiated PET has heen measured by IR spectroscopy (at 3290 cm M and by non-aqueous titration on soluble samples ). ... 

Another method for separating low-solubility samples, which is well known in the downstream processing of biopolymers, starts with a large volume of sample dissolved in a chromatographically weak solvent. This solution is fed to the column. As the solute has a relatively strong affinity to the stationary phase, e.g. silica, it is adsorbed and concentrated at the column inlet. This has the effect of introducing the sample as a concentrated plug. 

A solubility sample is typically prepared by adding an excess amount of solid to the solubility medium in a stoppered flask or vial. The amount added does not need to be accurately measured. While it is important to ensure that enough material is added so the sample is a suspension, it is also important not to add too much material to significantly alter the properties of the solubility medium including its pH. 

Centrifugation or ultracentrifugation may be preferable for certain samples that are difficult to filter. Solubility samples in co-solvent systems with high viscosity are such examples. If the solute is less dense than the solubility medium, it will float on the surface, making it difficult to sample the solution. This may be particularly problematic for compounds with low solubility where a single particle carried over to the solution may cause significant overestimation of the true solubility. 

Examination of the residual solid from solubility samples is one of the most important but often overlooked steps in solubility determinations. Powder X-ray diffraction (PXRD) is the most reliable method to determine whether any solid state form change has occurred during equilibration. The sample should be studied both wet and dry to determine if any hydrate or solvate exists. Thermal analysis techniques such as differential scanning calorimetry (DSC) can also be used to identify any solid-state transformations, although they may not provide as definitive an answer as the PXRD method. Other methods useful in identifying any solid-state changes include microscopy, Raman and infrared spectroscopy, and solid-state NMR (Brittain, 1999). When changes in solid-state properties are identified in solubility studies, it is important to link the new properties to the properties of known crystal forms so the solubility result can be associated with the appropriate crystal form. 

---