Off-line extraction

In off-line extraction the extracted analytes are collected and isolated independently from any subsequent analytical technique, which is to be employed next. For example, the extracted analyte can be collected in a solvent or on a solid sorbent. The choice of the collection method affects the possibilities for further analysis. The extracts may be used for final direct measurements (i.e. without further separation), e.g. UV and IR analysis. More usually, however, extraction is a pre-separation technique for chromatography, either off-line (the most common mode of SEE) or on-line (e.g. SFE-GC, SFE-LC-FTTR, etc.). The solvents used in extraction may affect subsequent chromatography. 

Off-line SFE is inherently simpler for the novice to perform, since only the SFE (and analyte collection) step needs to be understood. In off-line SFE further cleanup or a pretreatment step can be employed to eliminate interferences. With off-line SFE, sensitivities are limited by the fact that only about 1 p,L of the collection solvent is generally injected into the GC. The daily sample throughput can be higher using offline SFE, since SFE-GC requires that the GC be used for a sample collection device (rather than performing chromatographic separations) during the SFE extraction, whereas several off-line extracts can be loaded into an autosampler for unattended GC analysis. 

In general, supercritical fluid extractions can be performed in either an on-line extraction mode or an off-line extraction mode. Off-line supercritical fluid extraction is the most common mode and involves extracting the analytes from the matrix and collecting them in either a sorbent trap or a collection solvent [11]. Following the collection step, the analytes are determined on a separate instrument (for example, on a chromatograph or an infrared spectrometer). In the on-line supercritical fluid extraction experiment, the outlet of the supercritical fluid extraction system is connected to a second analytical... 

An analytical method for ultra-trace determination (down to 0.1 4g/L) of 15 fluorinated aromatic carboxylic acids, used as water tracers, was described [290]. The method comprised off-line extraction of the acids with Isolute ENV-f at pH 1. 5, elution with acetonitrile, and GC-MS quantification. The examination of the behavior of the fluorinated benzoic acids on the hypercrosslinked extracting material showed that with increasing acidity the breakthrough volume of the acid decreases, while increasing its molecular size increases the breakthrough volume because of more effective dispersion interactions with sorbent material. 

FIGURE 27.3 Off-line extraction and back extraction with back extraction solution management. 

Therefore, the online approach is generally preferred to the off-line mode. Online SPE has been applied to the determination of phenols in water samples using small precol-mnns with different adsorbents such as octadecyl-bonded silica [50,133-135], styrene-divinylbenzene copolymers PLRP-S or PRP-1 [133-145], and graphite carbon [134,146]. Recently, Cis and PSDVB extraction disks have been applied to the off-line extraction of phenols from water samples [147-149]. 

SPE cartridges filled with Gig, N-vinyl-pyrrolidone polymer, or PS-DVB material analysis have been adopted for the off-line extraction of some fimgicides [53-56] and herbicides [50,57-67] from various t)q)es of water. 

Recovery efficiencies of several drags and metabolites for off-line extraction methods are generally higher than 50%, except for the more polar substances (morphine, ecgonine methyl ester), for which values as low as 5% are reported. 

An extraction plant should operate at steady state in accordance with the flow-sheet design for the process. However, fluctuation in feed streams can cause changes in product quaUty unless a sophisticated system of feed-forward control is used (103). Upsets of operation caused by flooding in the column always force shutdowns. Therefore, interface control could be of utmost importance. The plant design should be based on (/) process control (qv) decisions made by trained technical personnel, (2) off-line analysis or limited on-line automatic analysis, and (J) control panels equipped with manual and automatic control for motor speed, flow, interface level, pressure, temperature, etc. 

Application of rotating coiled columns has become attractive for preparative-scale separations of various substances from different samples (natural products, food and environmental samples) due to advantages over traditional liquid-liquid extraction methods and other chromatographic techniques. The studies mainly made during the last fifteen years have shown that using rotating coiled columns is also promising for analytical chemistry, particularly for the extraction, separation and pre-concentration of substances to be determined (analytes) before their on-line or off-line analysis by different determination techniques. 

Although SFE and SFC share several common features, including the use of a superaitical fluid as the solvent and similar instrumentation, their goals are quite distinct. While SFE is used mainly for the sample preparation step (extraction), SFC is employed to isolate (chr-omatography) individual compounds present in complex samples (11 -15). Both techniques can be used in two different approaches off-line, in which the analytes and the solvent are either vented after analysis (SFC) or collected (SFE), or on-line coupled with a second technique, thus providing a multidimensional approach. Off-line methods are slow and susceptible to solute losses and contamination the on-line coupled system makes possible a deaease in the detection limits, with an improvement in quantification, while the use of valves for automation results in faster and more reproducible analyses (16). The off-line... 

Supercritical fluid extraction (SFE) has been extensively used for the extraction of volatile components such as essential oils, flavours and aromas from plant materials on an industrial as well as an analytical scale (61). The extract thus obtained is usually analysed by GC. Off-line SFE-GC is frequently employed, but on-line SEE-GC has also been used. The direct coupling of SEE with supercritical fluid chromatography (SEC) has also been successfully caried out. Coupling SEE with SEC provides several advantages for the separation and detection of organic substances low temperatures can be used for both SEE and SEC, so they are well suited for the analysis of natural materials that contain compounds which are temperature-sensitive, such as flavours and fragrances. 

The use of SPME for CE has not (yet) been studied widely. Li and Weber (170) reported an off-line SPME-CE approach for the determination of barbiturates in urine and serum, utilizing a sorbent of plasticized PVC coated around a stainless steel rod. Eor extraction, the coated rod was inserted for 4 min in a Teflon tube containing 50 p.1 of sample, and next the rod was repeatedly desorbed in another Teflon tube which each time contained 5 p.1 of desorption solution. This solution was transferred to an injection vial and an aliquot was injected into the CE system (Eigure 11.19). The extraction procedure appeared to be selective and effectively allowed the handling of very small samples. 

Figure 13.11 Column-switcliing RPLC trace of a surface water sample spiked with eight chlorophenoxyacid herbicides at the 0.5 p-g 1 level 1, 2,4-dichlorophenoxyacetic acid 2, 4-chloro-2-methylphenoxyacetic acid 3, 2-(2,4-diclilorophenoxy) propanoic acid 4, 2-(4-cliloro-2-methylphenoxy) propanoic acid 5, 2,4,5-trichlorophenoxyacetic acid 6, 4-(2,4-dichlorophenoxy) butanoic acid 7, 4-(4-chloro-2-methylphenoxy) butanoic acid 8, 2-(2,4,5-tiichlorophenoxy) propionic acid. Reprinted from Analytica Chimica Acta, 283, J. V. Sancho-Llopis et al., Rapid method for the determination of eight chlorophenoxy acid residues in environmental water samples using off-line solid-phase extraction and on-line selective precolumn switcliing , pp. 287-296, copyright 1993, with permission from Elsevier Science.

C. Aguilar, P. BottuII and R. M. Marce, On-line and off-line solid-phase extraction with styrene-divinylbenzene-membrane extr action disks for determining pesticides in... 

In order to reduce or eliminate off-line sample preparation, multidimensional chromatographic techniques have been employed in these difficult analyses. LC-GC has been employed in numerous applications that involve the analysis of poisonous compounds or metabolites from biological matrices such as fats and tissues, while GC-GC has been employed for complex samples, such as arson propellants and for samples in which special selectivity, such as chiral recognition, is required. Other techniques include on-line sample preparation methods, such as supercritical fluid extraction (SFE)-GC and LC-GC-GC. In many of these applications, the chromatographic method is coupled to mass spectrometry or another spectrometiic detector for final confirmation of the analyte identity, as required by many courts of law. 

The main error sources are noise in the wavefront sensor measurement, imperfect wavefront correction due to the finite number of actuators and bandwidth error due to the finite time required to measure and correct the wavefront error. Other errors include errors in the telescope optics which are not corrected by the AO system (e.g. high frequency vibrations, high spatial frequency errors), scintillation and non-common path errors. The latter are wavefront errors introduced in the corrected beam after light has been extracted to the wavefront sensor. Since the wavefront sensor does not sense these errors they will not be corrected. Since the non-common path errors are usually static, they can be measured off-line and taken into account in the wavefront correction. 

Castillo M, Domingues R, Alpendurada MF, et al. 1997. Persistence of selected pesticides and their phenolic transformation products in natural waters using off-line liquid solid extraction followed by liquid chromatographic techniques. Anal Chim Acta 353 133-142. 

The most widely employed techniques for the extraction of water samples for triazine compounds include liquid-liquid extraction (LLE), solid-phase extraction (SPE), and liquid-solid extraction (LSE). Although most reports involving SPE are off-line procedures, there is increasing interest and subsequently increasing numbers of reports regarding on-line SPE, the goal of which is to improve overall productivity and safety. To a lesser extent, solid-phase microextraction (SPME), supercritical fluid extraction (SEE), semi-permeable membrane device (SPMD), and molecularly imprinted polymer (MIP) techniques have been reported. 

In the last several years, on-line extraction systems have become a popular way to deal with the analysis of large numbers of water samples. Vacuum manifolds and computerized SPE stations were all considered to be off-line systems, i.e., the tubes had to be placed in the system rack and the sample eluate collected in a test-tube or other appropriate vessel. Then, the eluted sample had to be collected and the extract concentrated and eventually transferred to an autosampler vial for instrumental analyses. Robotics systems were designed to aid in these steps of sample preparation, but some manual sample manipulation was still required. Operation and programming of the robotic system could be cumbersome and time consuming when changing methods. 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

Brack [81] has illustrated the analysis of antioxidants in a CB-free vulcanisate of unknown composition according to Scheme 2.7. Some components detected by off-line TD-GC-MS (cyclohexylamine, aniline and benzothiazole) were clearly indicative of the CBS accelerator other TD components were identified as the antioxidants BHT, 6PPD, Vulcanox BKF and the antiozonant Vulkazon AFS. In the methanol extract also the stabiliser ODPA was identified. The presence of an aromatic oil was clearly derived from the GC-MS spectra of the thermal and methanol extracts. The procedure is very similar to that of Scheme 2.3. 

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