SFE/SFC

Figure 6.7 shows a schematic diagram of an on-line SFE-SFC coupled system (42), with details of the interface being shown in Figure 6.8 (42). 

Figure 6.7 Schematic diagi am of an on-line SFE-SFC system (from ref. 42) 1, carbon dioxide 2, pump 3, oven 4, exti action cell 5, interface 6, SFC unit.

In 1994, Nam and King (68) developed a SFE/SFC/GC instrumentation system for the quantitative analysis of organochlorine and organophosphorus pesticide residues in fatty food samples (chicken fat, ground beef and lard). In this way, SFC was used as an on-line clean-up step to remove extracted material. The fraction containing pesticide residues is then diverted and analysed by GC. 

In on-line extraction the process is coupled directly ( hyphenated ) to the analytical technique used for further analysis of the extract (either spectroscopy or, more frequently, chromatography, because of the limited selectivity of extraction). Common examples include SFE-GC, SFE-SFC, SFE-HPLC, SFE-FTIR,... 

SFE and SFC require a high-purity feedstock of liquid C02 (electron capture impurities below 100 ppt, and mass responsiveness impurities below lOppb). Impurities can be detrimental to the use of SFE in trace analysis. Hinz and Wenclawiak [323] have investigated SFE/SFC grade C02 by means of GC with FID, ECD and MS detection. Quantification of the impurities, using FID or ECD, was achieved introducing an internal standard into the C02 flow. 

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. 

Wieboldt et al. [560] have described SFE-SFC-FTIR analysis of hindered phenol primary antioxidants and phosphite secondary antioxidants in PE. SFE is more selective for the lower-range low-MW polymer than Soxhlet-type extraction. This yields a chromatogram with less interference from low-MW polymer peaks in the region where the additive components elute. As a result, SFE appears to be a better choice than Soxhlet-type extraction for the selective removal of additives from flaked polymer. SFE and dissolution/precipitation methods were compared for a PVC/stabiliser system [366]. 

Many SFC applications in the literature concern the analysis of UV-active additives in polymers and coatings after extraction [319-335], On-line SFE-SFC has also been used [319,321-325,333-335] (cf. Section 7.1.1.4). cSFC is often a matter of choice because every kind of polymer additive can be detected with FID [326,328,330,331,335-337], Some publications show interesting examples of pSFC with UV detection [329,332,337-339]. Figure 4.5 shows the pSFC separation developed for routine formulation control of... 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

SFE-SFC operation consists of extraction, trapping, and transfer to the column for chromatography. Concentration of the solutes of interest before sample introduction to SFC may be achieved by ... 

Most of the SFE-SFC devices developed are designed to obtain qualitative results. However, various quantitative analyses of polymer additives have been reported [82,87,91-93], The ability to remove the SCF is particularly important when SFE is coupled on-line... 

On-line SFE-SFC modes present several distinct advantages that are beyond reach of either technique when used separately (Table 7.13). An obvious advantage of SFE is that it is an ideal way to introduce a sample into an SFC system. Because the injection-solvent is the same as the mobile phase, in this respect the criteria for a successful coupling of different techniques are fulfilled [94], i.e. the output characteristics from the first instrument and the input characteristics of the second instrument are compatible. Supercritical fluid techniques can separate high-MW compounds are significantly faster than classical Soxhlet extractions and require less heat and solvent. SFE-SFC techniques are versatile,... 

In the mid-to-late 1990s, SFC became an established technique, although only holding a niche position in the analytical laboratory. The lack of robust instruments and the inflexibility of such systems has led to the gradual decline of SFE-SFC. Only a small group of industrial SFE-SFC practitioners is still active. Also the application area for SFC is not as clearly defined as for GC or HPLC. Nevertheless, polymer additives represent a group of compounds which has met most success in SFE-SFC. The major drawbacks of SFE-SFC are the need for an optimisation procedure for analyte recovery by SFE (Section 3.4.2), and the fair chance of incompatibility with the requirements of the chromatographic column. The mutual interference of SFE and SFC denotes non-ideal hyphenation. 

Many reviews have dealt with SFE-SFC coupling [23,24,95-101], some specifically with SFE-cSFC [84], and others with SFE-pSFC [90], See also Section 4.3. 

Applications SFE-SFC solves problems in such diverse areas as polymers/monomers, oils/lubricants, foods, pharmaceuticals, natural products, specialty chemicals, coatings, surfactants and others. Off-line SFE-SFC survives alongside on-line determinations of additives, because of the need for representative sample sizes. Off-line SFE-SFC was used for extraction of AOs from PP [102]. In cases where the analyst wishes to perform further analysis on the extracted species, it is useful to be able to isolate the extract from the solvent. The ability to remove the solvent easily is particularly important when SFE is coupled on-line to chromatographic techniques, but is equally important for trace analysis when it is useful to concentrate... 

Figure 7.3 SFE-SFC-FID chromatogram of an HDPE/(Irgafos 168, Irganox 1010) extract. After Tikuisis and Dang [112]. Reprinted with permission from T. Tikuisis and V. Dang, in Plastics Additives, An A-Z Reference (G. Pritchard, ed.), Chapman Hall, London, pp. 80-94. (1998) Copyright CRC Press, Boca Raton, Florida...

Flame retardants in polyurethane foams were determined by SFE-SFC [117]. Off-line SFE-SFC-FID was used for the analysis of additives in polyurethanes [52], and on-line SFE-SFC for extraction of additives from isocyanate formulations [107]. 

On-line SFE-SFC-ELSD analysis of the textile and fibre finish components, butyl stearate/palmitate/myri-state, was reported [118]. Similarly, Kirschner et al. [119] used on-line SFE-pSFC for fibre finish analysis on-line SFE-SFC was also instrumental in the analysis of the total composite finish on a commercial textile thread [120]. 

Various on-line SFE-SFC applications for the analysis of polymer additives and oligomers have been reported, such as additives in PE [121] polymer additives and oligomers in PP nylon pellets and PEEK granules [15] and oligomers in PET films [103], PS [122] and PMMA [123], sometimes in quantitative fashion [15,103]. SFE-SFC has identified the cyclic trimer tris-(ethylene terephthalate) in commercially available PET film [103]. Also, SFE-SFC extraction and... 

Table 7.14 Experimental SFE-SFC concentrations of different additives from various polymers...

On-line SFE-SFC has also been used for the quantification of erucamide and antioxidants in PE [110]. Cotton et al. [15] have reported quantitative extraction of additives from PP at five different extraction pressures, at a constant flow-rate and temperature. Below 50 atm, extraction was negligible between 50 and 200 atm, Tin-uvin 326 and 770 were extracted, along with small quantities of oligomers. Higher pressures lead to the extraction of all the additives present, with the integrated peak areas conforming well to the actual concentrations. 

On-line SFE-SFC method development for validated quantitative analysis of PP/(Irganox 1010/1076, Tinuvin 327) has been reported [93]. SFE conditions required optimisation of extraction time and pressure, matrix type (particle or film) and matrix parameters (particle size, film thickness, sample weight). About 30% of extracts were lost during collection. Very poor recoveries (20-25 %) were reported from ground samples (particle size 100 p,m dependent recoveries of 45-70% for 30-p.m-thick films. Biicherl... 

The minute sample sizes allowed in SFE-SFC analysis (typically 0.5 mg cf. the approximate weight of 30 mg for a single pellet), which is several orders of magnitude smaller than the sample weights used in GC, HPLC or IR analysis (5-10g), allows us to perform additive dispersion studies on a pellet-to-pellet basis [106]. 

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