Analyte transfer extractions

On-line SFE coupled to GC or SFC, according to the thermal stability of the analytes, are both very competitive with classical methods of analysis in terms of sensitivity and analysis time. Since all of the extracted analytes are transferred to the GC system, much higher method sensitivities can be obtained. Several modes of operation are possible utilising on-line SFE-GC, including quantitative extraction of all analytes from a sample matrix quantitative extraction and concentration of trace analytes selective extractions at various solvating powers to obtain specific fractions and periodic sampling (multiple-step extractions) of the effluent at various pressures for qualitative characterisation of the sample matrix. 

Soxhlet extraction (EPA SW-846 3540) is a very efficient extraction process that is commonly used for semivolatile petroleum constituents. In the method, the solvent is heated and refluxed (recirculated) through the soil sample continuously for 16 hours, or overnight. This method generates a relatively large volume of extract that needs to be concentrated. Thus, it is more appropriate for semivolatile constituents than for volatile constituents. Sonication extraction (EPA SW-846 3550) can also be used for semivolatile compounds, and as the name suggests, involves the use of sound waves to enhance analyte transfer from sample to solvent. Sonication is a faster technique than Soxhlet extraction and can require less solvent. 

Analytical-scale SFE can be divided into off-line and on-line techniques. Off-line SFE refers to any method where the analytes are extracted using SFE and collected in a device independent of the chromatograph or other measurement instrument. On-line SF techniques use direct transfer of the extracted analytes to the analytical instrument, most frequently a chromatograph. While the development of such on-line SFE methods of analysis has great potential for eventual automation and for enhancing method sensitivities [159-161], the great majority of analytical SFE systems described use some form of off-line SFE followed by conventional chromatographic or spectroscopic analysis. 

Liquid-liquid extraction is often quantified by the recovery R, i.e. the fraction of the total amount of analyte transferred from the aqueous phase into the organic phase R= Oor -... 

Solid-phase microextraction uses a 1-cm length of focused silica fiber, coated on the outer surface with a stationary phase and bonded to a stainless steel plunger holder that looks like a modified microliter syringe. The fused-silica fiber can be drawn into a hollow needle by using the plunger. In the first process, the coated fiber is exposed to the sample and the target analytes are extracted from the sample matrix into the coating. The fiber is then transferred to an instrument for desorption. The technique has been promoted by Pawliszyn (69). 

Analyte in aqueous samples extracted by purge and trap a measured volume of sample purged with helium volatile analytes transferred into the vapor phase and trapped on a sorbent trap analyte thermally desorbed and swept onto a GC column for separation from other volatile compounds detected by HECD, ECD, or MSD. 

For sample preparation, protein precipitation or liq-uid/liquid extraction can also be applied instead of solid phase extraction. Gluth et al. (1988) described for a toxicokinetic assay for Sotalol a threefold combination of these principles. A protein precipitation using 5 M perchloric acid was followed by a liquid/liquid extraction into a mixture of n-pentanol-chloroform 1/3 at pH 9. Thereafter, the organic phase was transferred to another glass tube and the analyte back extracted into 0.05 M sulfuric acid. 

Derivatization of target analytes has also been performed in acoustically levitated droplets for the determination of mono-, di-, tri- and tetrabutyltin [119]. The target analytes were extracted simultaneously from acetate buffer to hexane and derivatized using NaB(C2H5)4. Then, the organic phase was transferred for separation—determination by GC-AES. The results were comparable to those provided by conventional derivatization. 

A solid-phase microextraction process involves two steps, namely partitioning of the analytes between the coating and the sample, and desorption of the concentrated species into an analytical instrument. In the first step, the coated fibre is exposed and the target analytes are extracted from the sample matrix into the coating. In the second step, the fibre with the concentrated analytes is transferred to an instrument for desorption. A third, clean-up step can also be incorporated by using selective solvents, as in SPE. 

When on-column injection is used the end of the transfer capillary is inserted into the column inlet or retention gap where decompression of the supercritical fluid occurs. Carbon dioxide gas exits through the column and the seal made between the restrictor and septum (unless a closed injector is used). The analytes are focused by cold trapping in the stationary phase. The transfer line must be physically removed from the injector at the completion of the extraction to establish the normal carrier gas flow for the separation. Analyte transfer to the column is virtually quantitative but blockage of the restrictor is more conunon and involatile material accumulates in the injection zone eventually degrading chromatographic performance. The on-column interface is probably a better choice for trace analysis of relatively clean extracts with modest fluid flow rates than the split interface. When optimized both the on-column and split interfaces provide essentially identical peak shapes to those obtained using conventional solution injection. 

In most cases preconcentration of analytes occurs with simultaneous matrix accumulation. Matrix simplification can be achieved through the sampling process, the use of a rinse solvent prior to analyte transfer, and selective transfer of front-, heart- or end-cuts of the desorbed sample extract. Dual precolumn systems allow solvent exchange and separate trapping of analytes with a wide range of breakthrough volumes. The first column of a dual precolumn system can be used to retain strongly sorbed matrix components that would contaminate the second precolumn or interfere in the separation of the transferred extract. 

In headspace SPME, there are two processes involved the release of analytes from their matrix and the adsorption of analytes by the liber coating. The volatile organic analytes are extracted, concentrated in the coating and transferred to the analytical instrument for desorption and analysis. In comparison to well-established techniques, SPME is inexpensive, solvent free, and convenient. In addition, because relatively mild conditions can be used, i.e., systems at equilibrium and temperatures less than 50°C, SPME gives a better quantitative estimate of the flavor profile. ... 

Automated in-tube solid-phase microextraction (SPME) has recently been coupled with liquid chromatography/electrospray ionisation mass spectrometry (LC/ESI-MS), e. g. for the determination of drugs in urine [60, 62]. In-tube SPME is an extraction technique in which analytes are extracted from the sample directly into an open tubular capillary by repeated draw/eject cycles of sample solution. The analyte is then desorbed with methanol and transferred to an analytical HPLC-column. 

The techniques mentioned earlier are all characterized by liquid donor and acceptor phases. However, a gaseous acceptor phase is also possible, and that would be the most convenient and compatible arrangement for direct connection with GC. This is realized with the membrane extraction with a sorbent interface (MESI) technique. MESI can be used for either gaseous or aqueous samples, and the equipment employs a membrane module with a (usually) silicone rubber hollow fiber, into which the analytes are extracted from the surrounding liquid or gaseous sample. The carrier gas of a gas chromatograph flows inside the fiber and transports the analyte molecules as they are extracted from the membrane into a cooled sorbent trap where they are trapped. The analytes are subsequently desorbed from the sorbent trap by heating and are transferred to GC analysis. 

A known volume of air is drawn at 1.51/min through a glass sampling tube containing XAD-4 resin. A minimum sampling time of 1 h is recommended. The XAD-4 resin is transferred to a GC autosampler vial and quinoline (internal standard) is added. Internal standard and collected analytes are extracted into solvent (ethyl acetate containing 0.01% v/v triethylamine) and an aliquot is analyzed by gas chromatography with thermionic-specific (i.e., N-selective) detection. 

Figure 20 Separation of some xanthines in tea extract by HPLC. 1. theophylline 2, caffeine 3, theobromine each 1 pg/10 pi eluent flow rate, 0.75 cm3/min. (a) Using direct injection sample volume, 10 pi. (b) Using preseparation in a tank and analyte transfer by means of the OPLC interface sample volume 10 pi, applied on to the layer, developed twice with ethyl acetate prior to analysis. (Reproduced by permission from Ref. 71.)...

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