Extraction time

As mentioned earlier, extraction time should be chosen to ascertain equilibrium. In this case, the best detection limits are reached. In addition, increase in extraction yield is highest in the first part of the extraction and becomes less when time evolves. When sampling using short extraction times, repeatability is often poor, but is improved when extraction times are dose to or at equilibrium time. However, when high throughput (i.e., analysis of many samples in a relative short time span) is demanded or when equflibration times are very long, long extraction times are not always chosen. In these cases, extraction of the analytes is carried out under nonequUibrium conditions, which has an effect on the recovery. 

A more detailed description of SPME concepts as well as experimental considerations can be found in Ref [3]. 

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We present a novel method, called VIGRAL, to size and position the reflecting surface of a flaw. The method operates on standard B-scan recorded with traditional transducers, to extract Time-of-Flight (ToF) information which is then back-projected to reconstruct the reflecting surface of the flaw and characterize its radiation pattern. The VIGRAL method locates and sizes flaws to within k/2, and differentiates between flat and volumetric defects. 

The dependence of the mean work performed in the extraction for diflferent extraction times is shown in Fig. 3. One sees that in very rapid extractions a very large amount of work is required to overcome the friction, which decreases as the extraction is done more slowly ultimately, only a small amount of work remains to be done to compensate the free energy change for transferring the hydrophobic ligand into the solvent. In a simple system, the friction... 

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... 

Pectin. Pectin [9000-69-5] is a generic term for a group of polysaccharides, mainly partially methoxylated polygalacturonic acids, which are located in the cell walls of all plant tissues. The main commercial sources of pectin are citms peel and apple pomace, where it represents 20—40% and 10—20% of the dry weight respectively. The pectin is extracted, the extract purified, and the pectin precipitated (50) increased extraction times lead to the production of low methoxyl pectins. 

The sfe of chlorpyrifos methyl from wheat followed by on-line Ic/gc/ecd has been investigated (93). Extraction profiles were generated to determine the maximum analyte recovery and the minimum extraction time. Using pure CO2, a 65% recovery of chlorpyrifos methyl spiked onto wheat at 50 ppb was reported. When 2% methanol was added to the CO2, the recovery from a one gram sample averaged 97.8% (n = 10, 4.0% RSD). 

Recently, SPE cartridges and disks have been widely and successfully used in preconcentration processes [1-3]. They reduce solvent usage, disposal costs, and extraction time for sample preparation and obtain large enrichment factors. 

The influence of temperature, solution s pH and other parameters in formation of ionic associate is investigated. As a result, optimal conditions of determination are established pH 4,0 volume of acetate buffer - 0,5 ml volume of 0,1% aqueous solution of CV - 0,3 ml extraction time - 3 minutes. The ratio of aqueous and organic phases is 1 1. Photometric measurement of toluene layer is carried out at = 606,0 nm. The accuracy of procedures checked by the method of additives. 

The effect of different pai ameters such as temperature, pressure, modifier volume, dynamic and static extraction time on the SFE of the plant were investigated. The orthogonal array experimental design method was chosen to determine experimental plan, (5 ). In this design the effect of five parameters and each at five levels were investigated on the extraction efficiency and selectivity [4]. 

Figure 11.15 also shows that cytochrome C is displaced by lysozyme during extraction, i.e. at longer extraction times (cf. Figure 11.15(b) and (c)) the amount of lysozyme is increased as the amount of cytochrome C is decreased. 

One example of such a comparison for several dehydrated foods against a vacuum-oven method is shown in Table IV. The data for the Fischer method are given for three different extraction times and are also shown graphically in Figure 4. The drying conditions were arbitrarily chosen to be 38 hours at 70° C. 

This is a slow process, and the extraction time depends on the type of extractor used. With stirring as described in Note 10, practically quantitative extraction of jb-hydroxy phenyl pyruvic acid can be achieved within 6 hours. Extremely long extraction times may cause decomposition of the product. 

Center in Wyndmoor Pennsylvania is developing advanced technologies for the analysis of endosulfan in meat, poultry and eggs (FEDRIP 1999). This technique will include the use of a supercritical fluid extractor in order to reduce the amount of organic solvent use and to speed up extraction times. 

Classical extraction is achieved by mixing the samples with an organic solvent (solid-liquid extraction) such as acetonitrile, methanol, or ethanol, used either in the pure form or as a mixture or aqueous solution." Extraction time can be reduced by sonicating the samples. ... 

The aim of the present investigation is to study the influence of microwave pretreatment of fresh orange peels on the extraction time of orange pectin and the influence of the intensity and duration of microwave heating on pectin yield. 

Influence of extraction time on the yield and characteristics of pectin from fresh Navel I orange peels pretreated by microwave heating... 

Sam- ple Nr Kind of initial material for pectin extraction Extraction time min Yield of pectin, g per 200 g fresh material AUAC, % DE, % Molecul mass Mv Huggins constant Kh Intrinsic viscosity [ft] dl.g- Gel. strengtl TB... 

The method procedures are very time-efficient by always using a well defined portion of the extracts, thus avoiding multiple extractions, time-consuming rinses and overloading of the chromatographic cleanup systems with co-extracted sample matrix. 

The concept of SPME was first introduced by Belardi and Pawliszyn in 1989. A fiber (usually fused silica) which has been coated on the outside with a suitable polymer sorbent (e.g., polydimethylsiloxane) is dipped into the headspace above the sample or directly into the liquid sample. The pesticides are partitioned from the sample into the sorbent and an equilibrium between the gas or liquid and the sorbent is established. The analytes are thermally desorbed in a GC injector or liquid desorbed in a liquid chromatography (LC) injector. The autosampler has to be specially modified for SPME but otherwise the technique is simple to use, rapid, inexpensive and solvent free. Optimization of the procedure will involve the correct choice of phase, extraction time, ionic strength of the extraction step, temperature and the time and temperature of the desorption step. According to the chemical characteristics of the pesticides determined, the extraction efficiency is often influenced by the sample matrix and pH. 

The solvent, microwave energy applied, and extraction time selected are the main parameters controlled in MAE. The user should use proper extraction vessels and equipment in MAE because very high pressures can be achieved and explosions may result if appropriate precautions are not taken. 

Finally, it is very difficult to keep standard solutions for monitoring or recalibration purposes. Control batches should be purged and kept with a dry, inert gas. As long as results are reproduced, the system is considered to be in calibration. The relatively long extraction times usually prohibit the use of these methods for quality control analysis applications in a plastics manufacturing plant. 

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. 

More recently, some novel pressurised procedures for extraction of additives from polymers have been developed (Table 3.3). The principal objectives of all these techniques, such as SEE [89], MAE [90] and pressurised fluid extraction (PEE), is to replace the conventional extraction methods by shortening the extraction time, reduction in solvent use and automation. 

Figure 3.1 Loss of stabilisers from 0.5 mm HDPE plaques through Soxhlet extraction with n -heptane as a function of extraction time. , Anox 20 , Anox PP18 V, Silanox (n = 10) , Silanox (n = 8.5) x, Silanox (n = 7). After Gray and Lee [144]. Reproduced by permission of Blackwell Science...

Applications Extraction is typically accomplished by refluxing the polymer in an appropriate solvent for l-48h [84,199]. In many cases, ultrasonic exposure reduces the extraction time [90,200], According to Table 3.5 there are several reports of US extraction from polymers. Ultrasonic extraction has been used for HDPE/(BHT, Irganox antioxidants, Isonox, Cyasorb, Am 340, MD 1024, Irgafos 168) [56], LDPE/Chimassorb 81 [201], SBR/tri(nonylphenyl) phosphite [200], HDPE/(Tinuvin 770, Chimassorb 944) [114], etc. Nielson [90] compared the recoveries obtained for a variety of analytes from PP, LDPE and HDPE with Soxhlet, ultrasonic bath and microwave oven. For all samples, the ultrasonic extraction could be achieved within 1 h. For LDPE and PP most compounds (except Irganox 1010) were extracted within 10 min. Further experiments by Nielson [56] on extraction from HDPE confirmed these results. Where phosphite antioxidants (such as Irgafos 168) are present the use of the solvent mixture DCM-cyclohexane was preferred as it prevented hydrolysis of the phosphite by extraction solvents such as alcohols [56]. Similarly, phosphite esters also undergo hydrolysis... 

As diffusion to the surface of a polymer is one of the limiting steps in extraction, the particle size or film thickness of a sample is also important [278,333,337-340]. With the typical diffusion coefficients of additives in polymers a particle diameter of about 0.3 mm is required for an extraction time of about 1000 s at 40 °C. An exception to this is the extraction of thin films and foams, for which the shortest dimension is small. It is not surprising that no more than 50 % of antioxidants could be extracted from PP pellets as opposed to 90 % recoveries from the same polymer extruded into film [341]. Grinding of the polymer is usually an essential step before extraction. Care should be taken to avoid loss of volatile additives owing to the heat generated in such processes. Therefore, cryogrind-ing is preferred. 

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