Sample partitioning

Liquid-liquid partitioning cleanup is sometimes directed solely to the removal of fat from the sample extract. This has been accomplished through use of either hexane for the removal of fat from acetonitrile (451, 434) or aqueous methanol sample extracts (452), or isooctane from acetonitrile sample extracts (454). When hexane was used for the initial extraction of 30 anabolics from fatty tissue samples, partitioning against methanol-acetate buffer, pH 5.2 proved to be an efficient procedure for fat removal because the analytes were preferentially partitioned into the aqueous buffer layer (433) further extract purification could be readily accomplished by back extraction of the analytes into dichloromethane. 

Yates, K., I. Davies, L. Webster, P. Pollard, L. Lawton, and C. Moffat. 2007. Passive sampling Partition coefficients for a silicone rubber reference phase. J. Environ. Monit. 9 1116-1121. 

Outputs from the LSC must be corrected for background radiation and counting efficiency. The parts-per-million (ppm) equivalent to the radioactive chemical in a biological, soil or water sample can then be determined by accounting for aliquot size, specific activity, and other correction factors specific for the sample. Partitioning characteristics and chromatographic data can be treated in a like manner. Although many... 

As noted earlier for the extraction of BTX compounds and shown in Fig. 4.28, coating/sample partition coefficients depend largely on the type of coating for a given analyte. Derivatization and temperature also have some influenee on selectivity and must be adjusted as a funetion of the analytes, matrix and state of the sample. 

Liquid-liquid extraction Isolation from liquid sample Partitioning Commonly available equipment and reagents... 

Figure 3 Microextraction with SPME. Vf, volume of fiber coating Kfs, fiber/sample partition coefficient I4. volume of sample Co, initial concentration of analyte in the sample.

In summary, a gas chromatograph functions as follows. An inert carrier gas (like helium) flows continuously from a large gas cylinder through the injection port, the column, and the detector. The flow rate of the carrier gas is carefully controlled to ensure reproducible retention times and to minimize detector drift and noise. The sample is injected (usually with a microsyringe) into the heated injection port where it is vaporized and carried into the column, typically a capillary column 15 to 30 m long, coated on the inside with a thin (0.2 fim) film of high boiling liquid (the stationary phase). The sample partitions between the mobile and stationary phases, and is separated into individual components based on relative solubility in the liquid phase and relative vapor pressures. 

Headspace sampling is probably the simplest and easiest technique. A brief introduction to the topic has been published by Hinshaw [16], and a complete coverage of the theory and practice has just appeared [17]. The sample (liquid or solid) is placed in a sealed vial and heated to a predetermined temperature for a fixed period of time. Volatile components of the sample partition between the gas and sample phases, usually reaching equilibrium. Residual monomers diffuse only slowly from some highly cross-linked polymers, so sufficient time must be allowed for the vaporization from these samples. 

Chromatography is a separation method in which the components of a sample partition between two phases one of these phases is a stationary bed with a large surface area, and the other is a gas which percolates through the stationary bed. The sample is vaporized and carried by the mobile gas phase (the carrier gas) through the column. Samples partition (equilibrate) into the stationary liquid phase, based on their solubilities at the given temperature. The components of the sample (called solutes or analytes) separate from one another based on their relative vapor pressures and affinities for the stationary bed. This type of chromatographic process is called elution. 

Calculate the fiber-sample partition coefficient (Afs) from the following equation ... 

Following the procedure listed in Section 29.2.8, determine fiber-sample partition coefficients (A )... 

AHjs can be calculated from the thermodynamic relationship between temperature and fiber-sample partition coefficients (Ku),... 

Hie determination of fiber-sample partition coefficients (Kis) is important to estimate the sensitivity of SPME extraction. 

Chromatography is a general term applied to a variety of separation techniques based on the sample partitioning between a moving phase, which can be a gas or a liquid, and a stationary phase, which may be either liquid or solid. 

Any gangliosides present in the sample partition into the upper layer, together with varying amounts of oligoglycosphingolipids. They can be recovered from this layer by dialysing out most of the impurities of low molecular weight, and then lyophilising the residue [459]. 

All fibers, even bleached ones, seem to have some resinous, hydrophobic material on their surfaces. This material is usually referred to as pitch. We have investigated a bleached sulphite pulp. To remove some of the pitch the pulp can be extracted with ethanol. If, on the other hand, the pulp is heat treated (105 °C 3 h) the pitch is believed to spread over the surface. We made partition experiments for fines fractions from these pulps. A bleached sulphite pulp is of course hydrophilic and all samples partition towards the dextran phase in the basal system. On addition of 0.2 ml 5% P-PEG the hydrophobicity index is 0 for the untreated fiber, for the ethanol extracted fines it is -9 and for the heat treated +5. These results are thus entirely in accordance with our expectations. 

In headspace SPME a three-phase equilibrium system—the liquid, the headspace, and the fiber coating—is present in the sampling vial. Each equilibrium is governed by a partition coefficient the coating-headspace partition coefficient [Kf, headspace-sample partition coefficient (/Tfc) and coating-sample partition coefficient (/Q. The total mass of an analyte after equilibrium is distributed among the three phases ... 

Dilute with 25 mL of n-pentane solvent and mix well to ensure a uniform solution of the sample. The sample should not display precipitate or flocculate at this point If a precipitate is present, asphaltenes may be removed by the proc ure of Appendix X.I, however, the precision statement no longer applies. It is important that the polar content result obtained be not greater than that for the sample size as specified above, since the capacity of the clay for retaining polar constituents becomes limit at these concentrations. If results exceed this specification, repeat the test using a smaller sample. Partitioning between aromatics and polar compounds is affected by sample size. Results using different sample size may not be equivalent. 

SPME has proven to be a simple, rapid, and sensitive method for collecting the volatile compounds from the headspace of a sample (4,5). In theory, under ideal conditions, the volatile compounds in a liquid sample partition between the liquid and gas phase and between the gas phase and the SPME fiber. After sufficient time, equilibria are established between the sample and the headspace and the headspace and the SPME fiber. In liquid samples, the equilibria can be shifted by adjusting the temperature, pH, using mechanical mixing, by the addition of salts such as NaCl, or a combination of these treatments. 

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