Extraction time, significance

A linear dependence approximately describes the results in a range of extraction times between 1 ps and 50 ps, and this extrapolates to a value of Ws not far from that observed for the 100 ps extractions. However, for the simulations with extraction times, tg > 50 ps, the work decreases more rapidly with l/tg, which indicates that the 100 ps extractions still have a significant frictional contribution. As additional evidence for this, we cite the statistical error in the set of extractions from different starting points (Fig. 2). As was shown by one of us in the context of free energy calculations[12], and more recently again by others specifically for the extraction process [1], the statistical error in the work and the frictional component of the work, Wp are related. For a simple system obeying the Fokker-Planck equation, both friction and mean square deviation are proportional to the rate, and... 

The extraction time has been observed to vary linearly with polymer density and decreases with smaller particle size [78,79]. The extraction time varies considerably for different solvents and additives. Small particle sizes are often essential to complete the extraction in reasonable times, and the solvents must be carefully selected to swell the polymer to dissolve the additives quantitatively. By powdering PP to 50 mesh size, 98 % extraction of BHT can be achieved by shaking at room temperature for 30 min with carbon disulfide. With isooctane the same recovery requires 125 min Santonox is extractable quantitatively with iso-octane only after 2000mm. The choice of solvent significantly influences the duration of the extraction. For example talc filled PP can be extracted in 72 h with chloroform, but needs only 24 h with THF [80]. pH plays a role in extracting weakly acidic and basic organic solutes, but is rarely addressed explicitly as a parameter. 

As can be seen in Figures 2 and 3, the agitation of the sample was the most important factor for almost all the analytes. In all cases, this factor has a positive effect, and it appears to be of increasing importance as the degree of chlorination of the PCBs increases. Also, the extraction time was a significant factor for all the compounds, as might be expected. 

Significant improvements in the isolation of pharmaceutical compounds from plasma, serum and urine, have been achieved using ultra low mass sorbent bed and thin disk solid-phase extraction (SPE) material. The use of low sorbent masses or disk SPE material has allowed a significant reduction in solvent usage and extraction times. Several SPE RP-HPLC methods have been developed using these materials, including... 

Selectivity. Several additional analytes were examined. Extraction efficiencies from 1 mM solutions for a 2-min extraction time were as follows glucose, 2 salicylic acid, 34 oxine, 75 phenol, 22 and p-cresol, 79. 8-Hydroxyquinoline and p-cresol are significantly more extractable than salicylic acid. 8-Hydroxyquinoline can be back extracted either by basic solution (pH >12) or acidic solution (pH <3) because of its amphiprotic behavior (pKi = 5.0 pK2 = 9.90). This experiment was performed by using either 0.1 M NaOH or 0.01 M HC1 with essentially the same results. The extraction step should be performed at pH 7-10. 

We have demonstrated in this paper that two and four samples can be extracted in parallel with supercritical carbon dioxide without significant impact on data quality. Modifications made to an off-line extractor involved addition of a multiport manifold for the distribution of supercritical fluid to four extraction vessels and of a 12-port, two-way switching valve that allowed collection of two fractions per sample in unattended operation. The only limitation that we have experienced with the four-vessel extraction system was in the duration of the extraction. When working with 2-mL extraction vessels and 50-/zm restrictors, and using the pressure/temperature conditions mentioned above, the 250-mL syringe pump allows us a maximum extraction time of 60 min. During this time, two 30-min fractions can be collected with the present arrangement. 

Extraction of Inactive Coupons with KTOX Solution. Average iron extraction rates are plotted as a function of cumulative extraction time in Figure 2. Where rates for the altered and unaltered granite differ, the confidence level at which the difference is significant is indicated in brackets. 

Few review articles have been published on microextraction procedures based on the use of a liquid-phase extractant.1314 One drawback of drop-based microextraction procedures is drop vulnerability this relates to its instability and potential dislodgement, which could be caused by sample complexity, a long extraction time, and a fast stirring speed. As a result, precision will often suffer significantly. 

One of the important aspects of any analytical method is its calibration and, therefore, much effort has been put into SPME calibration. As it is not always practicable to employ traditional calibration methods (external standards, internal standards, and standard addition) owing to the sometimes significant matrix effects in complex samples, equilibrium calibration has been suggested as an alternative. In SPME, however, it would normally take rather a long time to achieve equilibrium calibration. If sensitivity were not a concern in an analysis, reduction of extraction time would be desirable, that is, the extraction could be stopped before equilibrium but this would thus demand a new approach to calibration. In this regard, as a way of circumventing matrix effects in environmental analysis, several diffusion-based calibration methods have been recently developed for quantification in SPME.30... 

Wallace et al. (13) observed potential thermal degradation of silymarin compounds when extracting milk thistle seed meal with ethanol in a Soxhlet apparatus at 78°C, they noted that the yield of silymarin decreased at long extraction times and that the yields were higher at 60°C. The times for the yields of the silymarin compounds to reach a maximum, shown in Table 2, significantly decreased with increasing temperature. For example, the time for the yield of silychristin to reach its maximum at 140°C was about one-fourth the time at 100°C, with close yields at both temperatures. 

Lopez-Avila et al. [8] published a study in 1993 that evaluated the Soxtec extraction of 29 target compounds (seven nitroaromatic compounds, three haloethers, seven chlorinated hydrocarbons, and 12 organochlorine pesticides) from spiked sandy clay loam and clay loam. Among the five factors investigated (matrix type, spike level, anhydrous sodium sulfate addition, total extraction time, and immersion/extraction time ratio), matrix type, spike level, and total extraction time had the most pronounced effects on method performance at the 5% significance level for 16 of the 29 target compounds. The two solvent mixtures, hexane-acetone (1 1) and methylene chloride-acetone (1 1), performed equally well. Four compounds were not recovered at all, and apparently were lost from the spike matrix. Limited experimental work was performed with 64 base-neutral-acidic compounds spiked onto clay loam, and with three standard reference materials certified... 

Additional features can be found in newer systems. Some have a built-in magnetic stir bar with variable speed control for simultaneous stirring in all the vessels. Stirring enhances contact between the sample and the solvents. This reportedly results in significant reduction in extraction time and improvement in analytes recoveries [68]. The stir bar is made of Weflon, a proprietary polytetrafluoroethylene (PTFE) compound that can absorb microwave. This allows the use of nonpolar solvents for extraction since... 

In an enzyme catalysed extraction, there are many possible factors that could have an influence over the extraction efficiency, such as incubation temperature, extraction time, extraction pH, stirring rates and so on. Often 10 or more possible factors can be identified. Which are significant and should be studied or optimised further ... 

The rosemary extracts were collected as separate samples succesively in time. Each of them was analysed separatively and the results are listed in Table 4. The concentrations of a-pinene, champhene, p-cimene and limonene decreased with extraction time, while that of the oxygeneted derivatives (cineol, linalool, camphor, bomeol and verbenon) increased significantly. 

Examinations of the volatile constituents of the different oils (See Tables 2, 4 and 5) resulted in the conclusion that the concentrations of the terpene hydrocarbons (pinene, limonene) decreased with extraction time, while that of the oxygenated derivatives (champhor, verbenon, carvone) increased significantly. Thus the extraction was initially rapid for the less polar terpene hydrocarbons. 

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