Continuous extraction mode

Principles and Characteristics Water is an interesting alternative for an extraction fluid because of its unique properties and nontoxic characteristics. Two states of water have so far been used in the continuous extraction mode, namely subcritical (at 100 °C < T < 374 °C and sufficient pressure to maintain water in the liquid state) and supercritical (T>374°C, p>218 bar). Unfortunately, supercritical water is highly corrosive, and the high temperatures required may lead to thermal degradation of less stable organic compounds. However, water is also an excellent medium for extraction below its critical temperature [412], Subcritical water exhibits lower corrosive effects. 

Example Regardless of the manufacturer of the hardware, the effect of a time lag on resolution is quite dramatic. The resolving power of linear instruments is improved by a factor of 3-4 and reflector instruments become better by a factor of about 2-3. [36] The advantages are obvious by comparison of the molecular ion signal of Ceo as obtained from a ReTOF instrument with continuous extraction (Fig. 4.7) and from the same instrument after upgrading with PIE (Fig. 4.12), or by examination of MALDI-TOF spectra of substance P, a low mass peptide, as obtained in continuous extraction mode and after PIE upgrade of the same instrument (Eig. 4.11). 

Fig. 4.11. MALDI-TOF spectra of substance P, a low mass peptide, as obtained from a Bruker Biflex ReTOF in continuous extraction mode (left) and after PIE upgrade (right).

Schematic description of a continuous extraction mode and a delayed pulsed extraction mode in an linear time-of-flight mass analyser, o = ions of a given mass with correct kinetic energy = ions of the same mass but with a kinetic energy that is too high. Delayed pulsed extraction corrects the energy dispersion of the ions leaving the source with the same mjz ratio.

Supercritical fluid extraction can be performed in a static system with the attainment of a steady-state equilibrium or in a continuous leaching mode (dynamic mode) for which equilibrium is unlikely to be obtained (257,260). In most instances the dynamic approach has been preferred, although the selection of the method probably depends just as much on the properties of the matrix as those of the analyte. The potential for saturation of a component with limited solubility in a static solvent pool may hinder complete recovery of the analyte. In a dynamic system, the analyte is continuously exposed to a fresh stream of solvent, increasing the rate of extraction from the matrix. In a static systea... 

All these problems could be convincingly overcome by application of the continuous operation mode [63 d)]. Most interestingly, this unusual continuous biphasic reaction mode enabled the quantitative separation of relatively high boiling products from the ionic catalyst solution under mild temperature conditions and without use of an additional organic extraction solvent. More details of this process are included in Chapter 8, Section 8.2.2.3. 

The ability of the ReTOP to compensate for the initial energy spread of ions largely increases the resolving power of TOP instruments. While a typical continuous extraction TOP instmment in linear mode cannot resolve isotopic patterns of analytes above about m/z 500, it will do when operated in reflector mode (Pig. 4.7). At substantially higher m/z, the ReTOP still fails to resolve isotopic patterns, even though its esolution is still better than that of a linear TOP analyzer. 

The essential independence of mean ion velocities on the molecular weight of the analyte leads to an approximate linear increase of the mean initial kinetic energies of the analyte ions with mass. High-mass ions therefore carry tens of elec-tronvolts of translational energy before ion acceleration. [33,41,50] The initial velocity of the ions is superimposed onto that obtained from ion acceleration, thereby causing considerable losses in resolution with continuous extraction TOP analyzers, in particular when operated in the linear mode. 

Figure 1.3 shows a typical semi-batch (semi-continuous) distillation column. The operation of such columns is very similar to CBD columns except that a feed is introduced to the column in a continuous or semi-continuous mode. This type of column is suitable for extractive distillation, reactive distillation, etc. (Lang and coworkers, 1994, 1995 Mujtaba, 1999). Further details of semi-batch distillation in extractive mode of operation are provided in Chapter 10. 

In some cases continuous extraction of products allows extended operation and high volumetric efficiency. Reactors run in this mode are referred to as continuous-stirred batch reactors... 

To reduce the kinetic energy spread among ions with the same m/z ratio leaving the source, a time lag or delay between ion formation and extraction can be introduced. The ions are first allowed to expand into a field-free region in the source and after a certain delay (hundreds of nanoseconds to several microseconds) a voltage pulse is applied to extract the ions outside the source. This mode of operation is referred to as delayed pulsed extraction to differentiate it from continuous extraction used in conventional instruments. Delayed pulsed extraction, also known as pulsed ion extraction, pulsed extraction or dynamic extraction, is a revival of time-lag focusing, which was initially developed by Wiley and McLaren in the 1950s, shortly after the appearance of the first commercial TOF instrument. 

The instrumentation should allow for two extraction modes one dynamic and the other static. In the dynamic mode, the extraction vessel is constantly supplied with fresh supercritical fluid, while the recovery container is continually supplied with the solute. In the static mode, the outlet of the extraction vessel is closed and extraction takes place without the regeneration of supercritical fluid. On completion of the extraction, the vessel is rapidly rinsed by the supercritical fluid in order to permit the recuperation of the solute. At this point we should return to our discussion of the four components of the instrumentation. 

Which extraction mode is the better remains a controversial issue. While the static mode provides longer contact between the sample and solvent, swells the matrix and facilitates penetration of the extractant in its interstices — thereby increasing its efficiency — the dynamic mode allows the analyte to be continuously exposed to the pure (clean) solvent, thus favouring displacement of the analyte s partitioning equilibrium to the mobile phase. Most SFE methods use both modes a static step is employed to ensure close contact between the sample and supercritical fluid without consuming much extractant that is followed by a dynamic step where the extracted analytes are driven to the restrictor and equilibrium is allowed to complete. 

Extraction columns usually are operated in a steady-state continuous-flow mode of operation with one liquid dispersed in the other. Mass transfer is then promoted by using various fixed or moving elements (various types of packings, trays, or agitators). These elements are... 

In the differential extraction mode, the concentration profile changes continuously as the two phases have no exact stepwise phase separation and a continuous movement towards each other is present. Here, an ideal contacting pattern for the two phases corresponds to a perfect countercurrent plug flow (e.g. extraction towers, Podbiehiiak... 

Extraction can be performed in either a static or dynamic mode. In the latter mode, the extractant is continuously flowing through the vessel, whereas the static mode allows the sample to soak in it. Problems associated with continuous extraction are restrictor plugging (6) and that dissolved water modifies the SCF, making extraction conditions difficult to predict, whereas static extraction can be inefficient and slow. In practice, a combination of both has been found to be the most effective to achieve maximum efficiency (7). 

An Sl E system can be operated in one of two ways. In the dynamic extraction mode, the valve between the extraction cell and the restrictor remains open so that the sample is continually supplied with fresh supercritical fluid and the extracted material flows into the collection vessel where depressuriz-ation occurs. In the static extraction mode, the valve between the extraction cell and the restrictor is closed and the extraction cell is pressurized under static conditions. After a suitable period, (he exit valve is opened and the cell contents arc iransferred through the rcsiricior by a dynamic flow of fluid from the pump. The dynamic mode is more widciv used than the static mode. 

In the normal FI extraction mode the enrichment factor is limited by the phase ratio which, in turn, is restricted by practical factors. A relatively complex extraction system was described by Atallah et al.[26] in which the continuously pumped sample is extracted by a small volume of oiganic phase trapped in a closed loop which incorporates the segmentor, extraction coil, phase separator and detector flow-cell. A simplified schematic diagram of the circulated extraction part of the manifold in shown in Fig. 

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