Volatile sorbent selection

In 1985, Ruzicka and Hansen established the principles behind flow injection optosensing [13-15], which has subsequently been used for making reaction-rate measurements [16], pH measurements by means of immobilized indicators [17,18], enzyme assays [19], solid-phase analyte preconcentration by sorbent extraction [20] and even anion determinations by catalysed reduction of a solid phase [21] —all these applications are discussed in Chapters 3 and 4. Incorporation of a gas-diffusion membrane in this type of sensor results in substantially improved sensitivity (through preconcentration) and selectivity (through removal of non-volatile interferents). The first model sensor of this type was developed for the determination of ammonium [13] and later refined by Hansen et al. [22,23] for successful application to clinical samples. 

The most common analytical methods used were gas chromatography, HPLC, AA spectrophotometry, polarography, colorimetry, and potentiometry with ion-selective electrodes. In this study GC/MS and other more expensive instrumentation were avoided. If sorbent tubes could not be used for gaseous substances, then the less desirable miniature bubblers or impingers were considered. Although these devices are inconvenient they were often used because no better alternatives were available. Bags were used in a few cases where the analyte could not be retained on a sorbent because of volatility and a small tendency to sorb. Filters were used for particulates. Combinations of collection devices were used if we felt that both particulates and vapor might be present in the analyte. 

The extract is collected by depressurization on a column packed with a solid sorbent, in a vessel containing the appropriate solvent, in a collection device connected to a chromatograph, or on combined solid phase-solvent traps [92]. For extraction of volatile compounds, such solvents as acetone, CH2CI2, methanol, or liquid nitrogen are used. Silica gel columns are the most popular way of trapping solids. In this case, the selectivity of the process can be improved by selective elution of the sorbent [88, 92]. SFE can be conducted in a static mode in which sample and solvent are mixed and kept for a user-specified time at a constant pressure and temperature, or in a dynamic mode where the solvent flows through the sample in a continuous manner [56]. The extracted analytes can be collected into an off-line device or transferred to an on-line chromatographic system for direct analysis. 

The above principles of the method are indicative of its great potential and flexibility (see, e.g., Beroza [141] and Tumlinson and Heath [142]). Rcker and Sievers [146,147] proposed to apply selective complexation by a europium(lll) coordination polymer sorbent for the pre-fractionation of volatile compounds (e.g., ketones, aldehydes, alcohols and carboxylic acids). 

Cartoni et al. [88] studied perspective of the use as stationary phases of n-nonyl- -diketonates of metals such as beryllium (m.p. 53°C), aluminium (m.p. 40°C), nickel (m.p. 48°C) and zinc (liquid at room temperature). These stationary phases show selective retention of alcohols. The retention increases from tertiary to primary alcohols. Alcohols are retained strongly on the beryllium and zinc chelates, but the greatest retention occurs on the nickel chelate. The high retention is due to the fact that the alcohols produce complexes with jS-diketonates of the above metals. Similar results were obtained with the use of di-2-ethylhexyl phosphates with zirconium, cobalt and thorium as stationary phases [89]. 6i et al. [153] used optically active copper(II) complexes as stationary phases for the separation of a-hydroxycarboxylic acid ester enantiomers. Schurig and Weber [158] used manganese(ll)—bis (3-heptafiuorobutyryl-li -camphorate) as a selective stationary phase for the resolution of racemic cycUc ethers by complexation GC. Picker and Sievers [157] proposed lanthanide metal chelates as selective complexing sorbents for GC. Suspensions of complexes in the liquid phase can also be used as stationary phases. Pecsok and Vary [90], for example, showed that suspensions of metal phthalocyanines (e.g., of iron) in a silicone fluid are able to react with volatile ligands. They were used for the separation of hexane-cyclohexane-pentanone and pentane-water-methanol mixtures. 

Beiner et al. tested the collecting capacity of several metal compounds for some volatile sulfur substances. They noted high enrichment rates and good selectivity for silver sulfide. It was used in combination with membrane extraction, thermodesorption, and GC-MS to analyze sulfides, thiols, and tetrahydrothiophene from water samples. Detection limits down to the lower ng/1 range were achieved. The disadvantages of the method are the experimental equipment, the long analysis times, and displacement reactions between the matrix and analytes on the sorbent s surface. 

III) coordination polymer sorbent to trap selectively various oxygenated compounds from a complex mixture of urinary volatile metabolites. 

The sorbates utilized in this study were chosen by consulting the literature on the odor/flavor characteristics of soybean oil and meal (19,20). Specific components, such as 2,4-decadienal, 2-pentylfuran, and ethyl esters have been identified as major contributors to the flavor chemistry of soybean oil by Frankel (22) and other investigators (23). These compounds and selected solutes comprising homologous series of 2-methyl ketones, aliphatic aldehydes, and n-alcohols were chosen as sorbates based upon their probable occurrence in trace quantities during the supercritical fluid process. It should be noted that many of the sorbates used in this study exhibit appreciable volatility and would be expected to have small breakthrough volvimes on the sorbents cited previously. 

Solid phase extraction is used primarily to prepare liquid samples and extracts of semi-volatile or non-volatile analytes, but may also be used for solids pre-extracted into solvents. The choice of sorbent is the key factor in SPE, because this can control parameters such as selectivity, affinity, and capacity. This choice depends primarily on the analytes and their physicochemical properties, which should define the interactions with the chosen sorbent. However, results also depend on the kind of sample matrix and interactions with both the sorbent and the analyte. SPE sorbents range from chemically bonded silicas, such as with the C8 and C18 organic groups, to graphitized carbon. 

Compounds with high affinity from pesticides with no or lower affinity to the sorbent=chemical filtration or selective elution of analytes Compounds with no or lower affinity from pesticides with higher affinity to the sorbent = separation of matrix components during sorption or washing steps Compounds less volatile than pesticides that undergo thermal cracking while pesticides are carried to a trap by a stream of nitrogen (mainly separation of lipids from thermally stable pesticides)... 

Cleanup The concentration and/or isolation technique applied has a great influence on the cleanup procedure. HSA techniques such as static headspace and purge-and-trap with thermal desorption have the advantage that no further cleanup is necessary. These techniques are restricted to the analysis of volatile compounds. A high selectivity of the concentration and/or isolation technique decreases the necessity of a further cleanup procedure. By a proper selection of the extraction solvent or the sorbent, the determinands can be extracted and separated simultaneously from the majority of coextractives. Also, a derivatization procedure can increase the selectivity of the method. The following procedures can be used for cleanup. 

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