Rapid large-volume liquid sampling

There are basically three methods of liquid sampling in GC direct sampling, solid-phase extraction and liquid extraction. The traditional method of treating liquid samples prior to GC injection is liquid-liquid extraction (LLE), but several alternative methods, which reduce or eliminate the use of solvents, are preferred nowadays, such as static and dynamic headspace (DHS) for volatile compounds and supercritical fluid extraction (SFE) and solid-phase extraction (SPE) for semivolatiles. The method chosen depends on concentration and nature of the substances of interest that are present in the liquid. Direct sampling is used when the substances to be assayed are major components of the liquid. The other two extraction procedures are used when the pertinent solutes are present in very low concentration. Modem automated on-line SPE-GC-MS is configured either for at-column conditions or rapid large-volume injection (RLVI). 

Figure 13.10 LC-LC chromatogram of a surface water sample spiked at 2 p.g 1 with ati azine, and its metabolites (registered at 220 nm). Conditions volume of sample injected, 2 ml clean-up time, 2.60 min ti ansfer time, 4.2 min The blank was subtracted. Peak identification is as follows 1, DIA 2, HA 3, DEA 4, atrazine. Reprinted from Journal of Chromatography, A 778, F. Hernandez et al, New method for the rapid detemiination of triazine herbicides and some of thek main metabolites in water by using coupled-column liquid cliromatography and large volume injection , pp. 171-181, copyright 1997, with permission from Elsevier Science.

Benzyl cyanide (40 g. = 0 33 mole) is heated in a round-bottomed flask (capacity 0-5 1.), with a mixture of 50 c.c. of concentrated sulphuric acid and 30 c.c. of water. The flask is provided with an upright air condenser, and is placed in a conical (Babo) air bath. The heating is continued until the appearance of small bubbles of vapour indicates that a reaction, which rapidly becomes violent, has set in the liquid boils up, and white fumes are emitted. It is allowed to cool and then two volumes of water are added. After some time the phenylacetic acid which has crystallised out is filtered off with suction. If a sample of the material does not form a clear solution with sodium carbonate in water (presence of phenylacetamide), the whole of the crude material is shaken with sodium carbonate solution and the mixture is filtered. From the clear filtrate phenylacetic acid is reprecipitated with sulphuric acid, and can be recrystallised directly from a rather large volume of hot water or, after drying, from petrol ether. Because of its low melting point (76°) it often separates at first as an oil, but it can also be conveniently purified by distillation in vacuo from a sausage flask.2... 

Solvent mixing, less relevant commercially, is widely used in scientific studies to determine the natures of blends. By using dilute solutions of the components the polymers, miscible or immiscible in bulk, can be combined homogeneously. Slow removal of solvent from inherently immiscible polymer mixtures allows Hquid-liquid phase separation to proceed and the polymers to segregate. However, rapid solvent removal or co-precipitation into a large volume of non-solvent can result in intimate mixtures of even immiscible polymers results may depend on the solvent used. Thus, non-equilibrium, unstable mixtures of inherently immiscible polymers can be produced. Such mixtures may segregate when heated above the TgS of the samples when molecular mobility permits. This situation is encoimtered many times in studies of PCL blends. 

This section describes the equipment used for the analysis by SPME. The mechanical application of SPME is analogous to liquid injection by hand. The startup costs are small and a vast amount of information can be gained rather quickly. Typically, for a laboratory already performing GC analysis, all that is needed are SPME fibers and an SPME holder. A pack of fibers and a holder can be obtained for several hundred dollars. No modifications to a GC are needed. It is recommended that a reduced inlet liner (0.7 mm I.D.) be used in the injection port, although SPME can be successfully performed with a standard inlet liner. The injection of 1 ql of liquid sample will rapidly expand to till the 1 ml volume of a typical inlet liner. Because there is no liquid solvent being injected with SPME, there is no need for the relatively large inlet volume. The reduced volumes in the injection port produce narrow injection bandwidths resulting in sharper GC peaks. 

A selective, direct and relatively rapid method has been developed for the determination of thiodiglycol (TDG) in aqueous samples by using microcolumn liquid chromatography coupled on-line with sulfur flame photometric detection using large-volume injections and peak compression [32]. Figure 4.7 reports the separation improvement with the addition of -propanol to a TDG sample. The combined effect of peak compression by displacement with -propanol... 

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