Sample modification, chemical

How do we decide whether to separate a mixture by gc or hplc In gc, mixtures are examined in the vapour phase, so that we have to be able to form a stable vapour from our mixture, or convert the substances in it to derivatives that are thermally stable. Only about 20% of chemical compounds are suitable for gc without some form of sample modification the remainder are thermally unstable or involatile. In addition, substances with highly polar or ionisable functional groups often show poor chromatographic behaviour by gc, being very prone to tailing. Thus hplc is the better technique for macromolecules, inorganic or other ionic species, labile natural products, pharmaceutical compounds and biochemicals. 

Shcherbakova, Petrova, and co-workers (258-263) studied the chemical modification of silica for applications in gas chromatography (for example, in the adsorption separation of hydrocarbons by gas chromatographic methods). The adsorption properties are investigated as a function of the degree of surface modification with ClSi(CH3)3. A number of silica samples were chemically modified so that they would have the desired adsorption properties (17). Chemical modification is an effective means of changing the shape of adsorption isotherms. 

The concept of matrix modification, from the sample prep perspective, is to add to the sample a chemical reagent that will cause a desirable chemical reaction or inhibit an undesirable reaction (94). For metals that tend to volatilize, one can add a modifier that reduces analyte volatility by increasing the volatility of the matrix. Consider the determination of Pb in highly salted aqueous samples such as seawater. Seawater contains appreciably elevated levels of chloride salts. Adding an ammonium ion to the seawater followed by heating the sample to a high temperature causes the following reaction to occur ... 

NMR spectroscopy is a useful analytical tool to perform analyses on water soluble compounds extracted during soaking. The fast preparation of samples avoids chemical modifications of the samples and possible bias due to necessary preparation (such as derivatization). Using ID and 2D NMR studies, 3 sugars (and assumption on 2 fiucto-oligosassharides), 14 amino acids and 6 organic acids were identified. A complete quantification of the main sugars was achieved with an internal standard (TSP). Other... 

Plasma treatment is a method of modifying the chemistry and often the topography of a surface. It uses a highly ionized, activated gas to react with the molecules of a surface. The plasma gas can vary from an inert gas, such as argon or helium, which would be expected to cause the species at the surface to react with one another, or a polymeric monomer, which could polymerize on the surface and create a thin plasma-treated layer. Plasmas can also be employed to clean surfeces before modification. Chemical vapor deposition is a technique in which the sample is exposed to a vapor that reacts with the surface to modify it. 

The wrinkle recovery angle provides a measure of the degree of chemical modification. This is calculated by blending a small sample and measuring the recovery to the flat configuration (180°). Whereas the untreated cotton recovers approximately 90°, the cross-linked cotton sample recovers 120—140°. If this is measured on dry fabric, it is termed conditional wrinkle recovery angle if on wet fabric, it is termed wet wrinkle recovery. At one point, wet wrinkle recovery was important, particularly in Europe. In the United States, the widespread use of clothes dryers has made conditional wrinkle recovery important. 

Chemically modified polymers have been used to determine polar compounds in water samples (37, 71). Chemical modification involves introducing a polar group into polymeric resins. These give higher recoveries than their unmodified analogues for polar analytes. This is due to an increase in surface polarity which enables the aqueous sample to make better contact with the surface of the resin (35). 

The PES measurements arc performed with reference to the Fermi level of the photoclectron spectrometer, in solid specimens, as dealt with here, by the way the spectroscopy works. Thus, in cases when the Fermi level shifts due to some chemical modifications of the sample, i.e., in the intercalation of graphite or other layered compound [16] or in the doping of conjugated polymers 1171, il is necessary to account for the change in the Fermi energy level before interpreting spec-... 

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