DPHSE

As can be seen from Fig. 6.9, dynamic pressurized hot solvent extraction (DPHSE) has evolved similarly to ASE however, as noted earlier, DPHSE has been the subject of many fewer reports, primarily as a result of the lack of commercially available equipment for implementation. In any case, the relatively scant reported applications of DPHSE are of especial interest as regards automation of the analytical process in fact, the dynamic nature of the system facilitates its coupling to other dynamic systems with a view to accomplishing preconcentration [39,42,45,145], filtration [42,45], chromatographic separation [145,146], derivatization [46,57] and detection [44,147], among others, and the partial or total automation of the analytical process. 

Fig. 6.9. Growth in the number of publications on ASE and DPHSE over the period 1995-2000. Empty bars correspond to ASE and shaded ones to DPHSE,...

Because of the above-stated lack of commercially available DPHSE equipment, applications of the dynamic extraction mode have all been developed using laboratory-built configurations that comprise the following basic elements ... 

The ovens used in DPHSE range from those for supercritical fluids [39,150,151] to gas chromatographic models [42,82] to specially designed units [75,146,148,152]. 

Fig. 6.10. Typical experimental set-up for dynamic pressurized hot solvent extraction (DPHSE). SR solvent reservoir, HPP high-pressure pump, IV inlet valve, OV outlet valve, PH pre-heater, EC extraction cell, C cooler, R restrictor.

The extraction cells used in DPHSE include those typically employed in SFE [109,151], empty HPLC columns [149] and specially designed stainless steel chambers [57,75,148]. The use of an SFE chamber as extraction cell is not recommended because the polyimide seals in it can fail at high temperatures [6]. On the other hand, the problem with empty HPLC columns is that they usually have small diameters and hence low volumes — this can result in excessive pressure build-up and bursting of the column. As a rule, DPHSE extraction cells consist of a stainless steel cylinder closed with screws at either end that permit the circulation of the leaching solvent through them. The screw caps also contain stainless steel filter plates to ensure that the sample remains in the extraction chamber. 

Fig. 6,12. Miniaturized device for DPHSE of solid and semi-solid samples. (Reproduced with permission of Elsevier.)...

The steps involved in a dynamic pressurized hot solvent extraction process are similar to those of ASE, with only a few, slight differences — particularly at the last stages. Thus, the DPHSE process involves the following five steps (see Fig. 6.13) ... 

The most analytically unfavourable feature of DPHSE is that analytes are diluted in the liquid extract, which requires concentration (usually by static liquid-liquid or solid-phase extraction). However, the increased flexibility of DPHSE relative to ASE has been used to develop various approaches to the partial or complete automation of methods based on the use of hyphenated techniques. Such approaches not only allow the dilution problem to be overcome but also enable automation and/or facilitate the development of other steps of the analytical process such as filtration, detection or chromatographic separation. The principal approaches to automation and improved implementation of analytical steps subsequent to DPHSE are discussed below. 

One very common, undesirable occurrence in DPHSE is the presence of solid particles in the liquid extract due to oversaturation during cooling. This shortcoming can be circumvented by fitting a filtration unit in-line with the extractor the unit can be placed... 

The potential of the DPHSE technique for coupling to subsequent operations of the analytical process is only limited by the analyst s ingenuity and material resources. The above-described systems can be combined by altering the sequence of steps and introducing appropriate modifications to develop fully automated systems for specific purposes. Thus, as many as four different steps have been coupled for the determination of pesticides in soil [42] and food [45] (DPHSE, filtration, SPE and HPLC) and that of Hg in soil [42] (DPHSE, SPE, derivatization and detection) in both, the analytical process was conducted in a completely automated manner. 

Like ASE, pressurized hot solvent extraction has been virtually exclusively applied to solid samples. There is only a single reported use with liquid samples that involved altering the extractor and is dealt with separately at the end of this section on account of its innovative character. As in ASE, most applications are concerned with environmental samples there are however, several interesting uses in the biological field. This section discusses the more interesting applications of DPHSE in terms of the matrix types and analytes involved. 

One of the few DPHSE applications using an extractant other than water is that involving the extraction of spice red pepper oil with subcritical propane for the determination of the carotenoid and tocopherol contents [177]. 

Like ASE, DPHSE has been used to extract additives from various types of polymers including nylon [18,178], poly(l,4-butyleneterephthalate) [18] and polypropylene [34,52, 178], with good results in most instances. As in ASE, choosing an appropriate solvent and temperature is crucial with a view to ensuring quantitative extraction of the additives without dissolving or degrading the polymers. The solvents used to date for this purpose include hexane [ 18,178], propane [52] and 2-propanol [34], but not water. 

The DPHSE technique has also been used for the determination of organic pollutants and metals in fly ash and coal, respectively. The extraction of dioxins [48,179] and PAHs [180] from fly ash was accomplished with toluene [48,180] or a toluene-methanol mixture [179], with results as good as or even better than those provided by Soxhiet extraction for 24 h. On the other hand, the extraction of major ash-forming elements (Fe, AI, Ca, Mg, Na and K) [148] and minor inorganic pollutants (As, Se and Hg) [46] from coal was done with acidified water. In the latter case, a combination of static and dynamic extraction was found to provide quantitative recoveries within a shorter time and with less dilution of the extracts than dynamic extraction alone. Acidified water is more corrosive than pure water, so the high temperatures required for extraction (150-200°C) call for the use of an extractor made of a material more corrosion-resistant than steel hastel-loid. However, in proportions above 4%, nitric acid — the acidulant most frequently added to the water — has been found to result in clogging of the system and the restrictor, so the recommended acid concentration is much lower than that. 

Comparison of DPHSE with other leaching techniques... 

As in ASE, the efficiency of DPHSE for extracting compounds from soils has been compared with that of other techniques such as Soxhlet, microwave-assisted (MAE) and, especially, supercritical fluid extraction (SEE). 

The advantages of DPHSE over Soxhlet extraction include not only decreased solvent consumption and operation times [25,158,179], but also the avoidance of organic solvents as extractants (water is the usual leaching agent) and the amenability to coupling to other operations of the analytical process, as described in the previous section. 

As with ASE, the advantages and disadvantages of DPHSE with respect to MAE and SEE are not so clear and depend on the particular application. 

In fact, DPHSE has rarely been compared with MAE. The former has been found to provide results similar to [167] or better than [52] those of the latter in the extraction of Irganox from polypropylene, where prolonged microwave exposure resulted in degradation of the polymer. 

SEE has so far been the technique most frequently used to validate DPSE methods such as those for the extraction of dioxins from high- and low-carbon fly ash [48], triazolo-pyrimidine sulphonanilide herbicides, trichloropyridinol and PCBs from soil [150,152, 168], and carotenoids and tocopherol from spice red pepper [177]. As noted earlier, neither technique can be said to be better than the other it depends on the characteristics of the analytes to be extracted (e.g. on their polarity and high-temperature stability). Thus, in the extraction of cloransulam-methyl from soil, while the use of subcritical water provided higher recoveries than SEE, the analyte was not hydrolytically stable above 150°C, which entailed using a lower temperature and hence an increased extraction time [152]. In the extraction of PAHs from bituminous coal fly ash [180], extraction with supercritical CO, yielded better recoveries than DPHSE using toluene and methylene... 

Fig. 6.16. Scheme of a DPHSE-SFE system. SR solvent reservoir, SP solvent pump, SV solvent valve, CO,P CO, pump, CO,V CO, valve, IV inlet valve, OV outlet valve. (Reproduced with permission of Elsevier.)... 

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