Sample matrix chemical changes

Standard analytical techniques for sampling and pretreatment and analytical requirements for sediment studies are less available than for water and soil studies. To obtain meaningful results from laboratory experiments, the sediment samples should be kept in the original aqueous matrix, and analyses should be carried out immediately to minimize changes to the sample matrix due to chemical and biological processes that could occur during storage. 

It is important for all sample custodians to maintain the sample in its original physical and chemical condition so that it remains representative of the bulk system in terms of the analyte identity and concentration. Possible changes to avoid are 1) loss of sample matrix or solvent through evaporation or other means, 2) loss of analyte through evaporation, chemical reaction, temperature effects, bacterial... 

A problem related to the previous one is the separation of a group of similar compounds from a complex sample matrix. A group of chemically similar compounds, like a family of drugs or pesticides, may include widely dissimilar members with respect to the properties on which typical separations are based. For example, the hydrophobicity of a drug and its metabolites often differ substantially despite relatively small changes in the overall structure of the molecule. In such situations the inherent shape selectivity of MIPs may be quite useful to achieve a group separation. 

No change of the sample matrix, and in consequence elimination of potential risk of contamination due to added chemicals... 

Compensating for Chemical Matrix Effects Using Internal Standards or Isotope Dilution. If the analyte and internal standard signals are affected similarly by the sample matrix, internal standardization can effectively compensate for changes in sensitivity. For analytes that are efficiently ionized in the ICP the matrix-induced change in sensitivity depends on analyte mass [174]. Therefore, the internal standard must have a similar mass to the analyte ion [189,190],... 

Drying liquid samples. (2) Melting solid samples. (3) Removing or partially removing the matrix. (4) Catalysing chemical changes to enhance... 

The first step in the analysis of antioxidant phytochemicals is extraction of the compounds Ifom the plant matrix. The means by which the samples are prepared for extraction is very important because, if it is not carefully performed, compounds can be chemically altered or degraded during the process. In general, sample preparation conditions should avoid oxidation, excessive exposure to high temperatures, enzymatic reactions, and other chemical changes to the target compounds. The first step is to select a representative sample of plant material that has been collected as recently as possible. If extraction cannot be performed immediately, it is recommended that samples be stored at refrigeration... 

The electronic transitions which give rise to X-ray emission spectra involve core electrons and are therefore relatively insensitive to the chemical and physical form of the determinant (Bertin, 1978). As a result, analyses can be performed with a minimum of sample preparation directly on materials in the condensed phase. This insensitivity of sample matrix applies to the wavelength of the emitted X-rays, not to their intensities and as quantitation is based on intensity measurement, closely matched standards are required. X-ray emission spectra can be excited by primary X-rays in a fluorescence experiment or by changed particles via collisional excitation. The cross sections for excitation of X-ray emission are rather low and this is combined with the low efficiency of collection, collimation, diffraction and detection of the emitted X-rays. This low overall efficiency leads to a relatively low sensitivity in some cases and is compounded by high backgrounds either from scattered primary radiation in a fluorescence experiment or due to bremsstrahlung in the charged-particle-excitation methods. Methods based on X-ray spectrometry do not provide isotopic information about the sample. Nonetheless, there are a number of radio analytical problems which can be solved by methods based on X-ray spectrometry. 

The authors stated (8) that the middle lamella and the matrix of the cell wall is effected by chemical changes such as oxidation and hydrolysis to a higher extent than the cellulose microfibrils. Ultrastructural changes would then be expected to occur in areas rich in hemicelluloses, lignin, or pectic substances. Later, Borgin et al. (10) found that the lignin in the ancient wood samples had been oxidized. A decrease in lignin content was evident when the samples were analyzed by a hydrochloric acid procedure. 

Comparisons of modeled concentrations and the chemistry of water (e.g., pH, Si02(aq), Na, K ) and gas samples (CO2) collected over 5 years show that the THC model captures the trends and magnitude of chemical changes. CO2 concentrations in gas samples increase from an initial value of about 1000 ppmv to greater than 25,000 ppmv as a result of exsolution from matrix pore water and transport into fractures. The CO2 dissolves into condensate waters at lower temperatures, resulting in a drop in pH and also dissolution of calcite. 

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