Solid-phase trapping

Miller and Hawthorne [416] have developed a chromatographic method that allows subcritical (hot/liquid) water to be used as a mobile phase for packed-column RPLC with solute detection by FID, UV or F also PHWE-LC-GC-FTD couplings are used. Before LC elution the extract is dried in a solid-phase trap to remove the water. In analogy to SFE-SFC, on-line coupled superheated water extraction-superheated water chromatography (SWE-SWC) has been proposed [417]. On-line sample extraction, clean-up and fractionation increases sensitivity, avoids contamination and minimises sources of error. 

Also, subcritical (hot/liquid) water can be used as a mobile phase for packed-column RPLC with solute detection by means of FID [942]. In the multidimensional on-line PHWE-LC-GC-FTD/MS scheme, the solid sample is extracted with hot pressurised water (without the need for sample pretreatment), and the analytes are trapped in a solid-phase trap [943]. The trap is eluted with a nitrogen flow, and the analytes are carried on to a LC column for cleanup, and separated on a GC column using the on-column interface. The closed PHWE-LC-GC system is suitable for many kinds of sample matrices and analytes. The main benefit of the system is that the concentration step is highly efficient, so that the sensitivity is about 800 times better than that obtained with traditional methods [944]. Because small sample amounts are required (10 mg), special attention has to be paid to the homogeneity of the sample. The system is... 

Hartonen K, Bowadt S, Hawthorne SB, et al. 1997. Supercritical fluid extraction with solid-phase trapping of chlorinated and brominated pollutants from sediment samples. J Chromatogr A 774(l-2) 229-242. 

The feasibility of extracting substituted phenols from an aqueous solution with supercritical CO2 is reported A special extraction vessel was used in order to overcome the mechanical difficulty in retaining the liquid matrix in the extraction vessel. Solid phase trapping was utilized with a diol silica bonded phase. Methanol was used to rinse the trap. Below 300 atm extraction recovery paralleled CO2 pressure at fixed temperature. Phenol was least extractable while, 2,4-dichlorophenol yielded the greatest percent recovery. Above 300 atm extraction yield declined with pressure. It is theorized that at high CO2 density there is less mixing with the aqueous phase because of increased fluid-fluid interaction. 

Bjorklund, E., L. Mathiasson, P. Persson, et al. 2001. Collection capacity of a solid phase trap in supercritical fluid extraction for the extraction of lipids from a model fat sample. J. liquid Chromatogr. Rel. Technol. 24 2133-2143. 

Almost all MAE applications involve off-line procedures, and on-line systems have not been utilized in food and agricultural analyses. Only a few approaches involving an on-line system have been published, for example in the determination of organophosphorus compounds in air particulates (21). In this MAE-GC system, an interface based on solid-phase trapping was used. Methanol was as the extraction solvent, and before the solid-phase trap the extract was diluted with water to enable efficient trapping to a polymeric sorbent. The sorbent was then dried with a nitrogen flow and the trapped analytes were eluted with organic solvent to the GC equipped with a PTV injector. A similar system should be applicable to the analysis of other types of solid samples as well. 

In on-line coupling, PHWE is performed in dynamic mode, i.e., the water is continuously flushed through the extraction vessel. After the extraction, the water is cooled and the extracted analytes are collected either on a solid-phase trap (for LC or GC) or a membrane extraction unit (for GC) 35-39). PHWE-LC... 

In SFE-LC, the most common interfrce is based on solid-phase trapping 2.15,44,58-63,66-68,70-80), although a few other types of interfrices such as impactor interfrce 69) open-tubular trapping 64) and the sample loop interface (65) have been developed as well. Direct trapping into a conventional packed LC column is not possible because of the high back pressure that analytical columns create. Because of the back pressure, the fluid cannot be efficiently decompressed and thus it will retain (partially) its solvation properties and efficient trapping will not be achieved, especially if modifiers are used in the... 

Method development in SFE is not so simple since several parameters have to be optimized, including temperature and pressure of die SF, extraction time, flow rate, addition of cosolvent (type of solvent and amount) and finally collection mode (e.g. in a solvent, in an empty vessel or on a solid-phase trap). Furthermore, the methods are generally matrix-dependent, i.e. a method developed for a particular target-molecule(s) cannot be directly applied to other types of samples than the one(s) it was optimized for. 

Syringe pump and liquid trapping (high temperature) vs. reciprocating pump and solid-phase trapping (medium temperature)... 

Mannila, M., Koistinen, J., and Vartiainen, T., Development of SFE with a solid-phase trapping for fast estimation of toxic load of polychlorinated dibenzo-p-dioxins-dibenzofurans in sawmill soil, J. Chromatogr. A, 975, 189-198, 2002. 

It also minimizes sample handling, provides fairly clean extracts, expedites sample preparation, and reduces the use of environmentally toxic sol vents.SFE has been applied to the extraction of carcinogenic AAs from soil and sand. The paper studies the possibilities of using MAE and SFE in determination of AAs by HPLC after reduction of the azo colorants. Two SFE pieces of equipment differing in the trapping step (solid-phase trap or solvent collection) were utilized for the extractions. The MAE experiments were then performed with a vessel system with temperature and pressure control. 

Increasing the temperature of water extractions also increases the amount of matrix materials that are coextracted, especially for samples high in organic matter. Coextractants can reduce trapping efficiency and reproducibility on solid-phase traps. Using microporous membrane hquid-liquid extraction (MMLLE) in place of a sohd trap can more selectively trap PAHs from hot water extracts and minimize or eliminate sample cleanup. " The extraction solvent, cyclohexane, is immobilized in the pores of a polypropylene membrane where hquid-liquid mass transfer occurs. Limits of quantitation of about 1 for very small samples (5 to 10 mg) with an average... 

An alternative to liquid-solid extraction is supercritical fluid extraction (SEE) which allows the extraction of analytes from solid samples, i.e., marine sediments, to be performed faster and more efficiently since these have a lower viscosity and higher diffusivity than liquid solvents." CO2 is the most widely used supercritical fluid with or without a modifier, e.g., methanol and toluene. SFE can be combined with solid-phase trapping.Compared with Soxhlet extraction, SFE gave similar yields, but the extracts were much cleaner and it was not necessary to clean the extracts before GC analysis."" ... 

Supercritical fluid extraction (SFE) with solid-phase trapping has been used for the extraction of DBDE and PBBs together with PCBs and chlorinated benzenes from sediment samples, with CO2 as the supercritical fluid. ° Before the extraction, the sediment sample can be mixed with copper powder and sodium sulphate for the removal of moisture and sulphur. Usually, the extraction combines static and dynamic extraction. The time required for the extraction ranges from 40 to 60 min, the extraction temperature is around 120°C and the pressure 374 bar. Compared with Soxhlet extraction, SFE gives similar yields, but the extracts are generally much cleaner and it might not be necessary to clean the extracts before GC analysis. 

Pressurized hot water extraction (PHWE) has also been used for the analysis of several brominated analytes in sediment. The extracted analytes were trapped into a solid-phase trap (Tenax TA), from which these were eluted with pentane ethyl acetate mixture after drying the trap with nitrogen. No further cleanup of the extract was required. Best results were obtained at 325°C, using a pressure of 118 bar and an extraction time of 40 min. Compared with Soxhlet extraction, extraction yields were clearly better, and the extract was much cleaner. 

Purge of headspace gases onto solid-phase trap. 

Lehotay, S. J., and Valverdcgarcia, A, (1997). Evaluation of different solid-phase traps for automated collection and clean-up in the analysis of multiple pesticides in fruits tind vegetables after supercritical fluid extraction. J. Chromatogr, A 765, 69-84. 

Fritz, J. S. and Macka, M., Solid-phase trapping of solutes for further chromatographic or electrophoretic analysis, Journal of Chromatography A, 902, 137, 2000. 

The SFE instrument consists of a solvent supply, a pump to maintain the pressure and the flow rate, an extraction cell mounted in a heated oven in order to maintain the temperature at the specified value, a restrictor connected to the outlet of the cell, and a collection device. There are different ways of collecting analytes as discussed in the section Collection Devices . In a representative experiment, the cell has a volume of 10ml and is loaded with 1-5 g of sample matrix. Carbon dioxide is pumped at a rate of 0.5-4 ml min measured as liquid at the pump. The pressure is 150-450 bar and the temperature 40-150°C, with a dynamic extraction time of 10-60 min. The extracting fluid is transported through a restrictor. At the restrictor, the pressure of the extracting fluid drops as the extraction fluid expands, releasing the extracted compounds. The gaseous extraction fluid is vented and the analytes are collected in a small volume of solvent (5-20 ml) or onto a solid-phase trap (commonly octadecyl-silica), which is rinsed with solvent in a subsequent step. The extract is often ready for direct analysis. An internal standard is added to the vial to correct for differences in final extract volumes. 

A coaxial heated restrictor consisting of a syringe with a movable piston position controls the flow, or alternatively a small metal disk is pushed toward an orifice in order to regulate the flow rate. The former is suitable for collection in organic solvents, while the latter has been utilized for solid-phase trapping. 

Figure 2 Two types of restrictors, combined with organic solvent collection or solid-phase trapping. (A) Solvent collection (B) solid-phase trapping.

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