Injection/extraction time

While the decrease in extraction time is favourable for laboratories in general, it can be critical when laboratory analyses are used in feedback control of production cycles and quality control of manufacturing processes. The volume of solvents used in PFE can be some 10 times less than traditional extraction methods (cf. Table 3.36). PFE cuts solvent consumption by up to 95 %. Because so little solvent is used, final clean-up and concentration are fast direct injection in analytical devices is often possible. Automated PFE systems can extract up to 24 sample cells. 

Parameters that should be tested in HPLC method development are flow rate, column temperature, batch and supplier of the column, injection volume, mobile phase composition and buffer pH, and detection wavelength [2], For GC/GLC methods, one should investigate the effects of column temperature, mobile phase flow rate, and column lots or suppliers [38], For capillary electrophoresis, changes in temperature, buffer pH, ionic strength, buffer concentrations, detector wavelength, rinse times, and capillaries lots and supplier should be studied [35, 36], Typical variation such as extraction time, and stability of the analytical solution should be also evaluated [37],... 

Wall [23] has discussed recent developments including timed-split injection, extraction and detection systems in SFC. 

For determining the robustness of a method a number of parameters, such as extraction time, mobile-phase pH, mobile-phase composition, injection volume, source of column lots and/or suppliers, temperature, detection wavelength, and the flow rate, are varied within a realistic range and tlie quantitative influence of the variables is determined. If the influence of a parameter is within a previously specified tolerance, this parameter is said to be witliin the robustness range of the method. These method parameters may be evaluated one factor at a time or simultaneously as part of a factorial experiment. 

If sample does not contain fat or oil, the duration of the extraction should not exceed 1 hr. If it contains fats one must verify the extraction time that is necessary for an efficient aroma recovery. Depending on the matrix, one must also verify the appropriate extraction time, prior to beginning. Generally, this time does not exceed 1 hr. Solvent injection and quantification by GC/MS after the SDE is usually used to verify the minimum extraction time needed. 

FIGURE 13.9 Principle of SPME. (a) Extraction in a closed vessel by DI or the use of an SPME device, (b) Desorption of analytes from the fiber in the GC injection port. The graph in the middle corresponds to the amount of substance introduced in the GC. The signal due to the analytes increases with increasing hydropho-bicity and extraction time. 

To reduce matrix effect, it is important to obtain cleaner extract and use adequate separation. In the presence of late-eluting components that cause matrix effect, adjustment of retention time, run injection cycle time, and/or postinjection column washing should be used to eliminate or reduce their impact. 

Calibration is carried out using standard calibration curves. The simplicity, repeatability, and low cost of the method have allowed its use for routine determination of trihalomethanes in tap water. SOME has also been compared with solid phase microextraction (SPME), purge and trap (P T), and direct aqueous injection (DAI) [10]. This technique offers accuracy comparable with that obtained using P T and DAI. With respect to conventional LEE, the SDME method is more accurate. In contrast to DAI and P T, it requires no special equipment. SDME has been used for extraction of chlorophenols [II], pesticides [12, 13], warfare agents [14], and butanone derivatives [15], and for control of food products [16]. The low costs of the SDME method (typical GC syringe and 2-3 pL of solvent), simplicity, and short extraction time (approximately 15 min) make it particularly suitable for preliminary analyses of organic pollutants in water samples. It can also be an effective alternative to SPME, as it does not require the use of expensive instrumentation. 

The method is different from conventional SPE in that SPE isolates the majority of the analyte from a sample (> 90%) but only injects about 1 to 2% of the sample onto the GC. Solid-phase microextraction isolates a much smaller quantity of analyte (2-20%), but all of that sample is injected into the GC. The extraction efficiency of the fiber is a combination of extraction time, the thickness of the stationary phase, and the magnitude of the partition coefficient for the stationary phase. 

SPME can either be performed by head-space extraction (HS-SPME) by placing the fiber in the vapour above a gaseous, liquid or solid sample, or by direct immersion extraction (DI-SPME), by immersing the fiber in a liquid sample. After a certain extraction time, the SPME needle is removed from the septum and inserted into the injection port of the GC or into the desorption chamber of the SPME-HPLC interface. The desorption is performed by heating the fiber in the GC inlet, or by pumping a solvent through the desorption chamber of the SPME-HPLC interface. The main advantages of SPME compared to LLE and solid phase extraction (SPE) are that no or little solvent is... 

Organophosphate esters have been analyzed mainly in the indoor air samples. SAE has been relatively popular in the extraction of organophosphates from air samples, which have been collected either on filters or adsorbents. Both static and dynamic extraction can be used. An example of dynamic SAE (DSAE) is the extraction of OPEs from quartz filters by hexane MTBE (7 3). The flow rate was 0.2 ml/min, and the total extraction time was only 3 min, at a temperature of 70°C. The recoveries were compared with static SAE (2 X 20 min) and PEE and the recoveries obtained with DSAE (>95%) were at the same level or better than those obtained with other methods. No further purification or concentration was needed before GC-NPD analysis of organophosphates. The GC column was a 30 m X 0.32 mm i.d. DB% column with a phase thickness of 0.1 /rm. Splitless injection was applied in the sample introduction. The LODs were better than 0.4 ng/m. The system was developed further, and the DSAE was connected online with GC using PTV and large volume injection during the transfer. The most abundant compound found in the air was tri(w-butyl) phosphate. ... 

There are both nonpolar and polar coatings available. Dimethylpolysi-loxane is the most popular nonpolar one, and the 7 pm thin film is best for high molecular weight analytes the 30 jum film is preferable for mid-range (pesticides), and the 100 /tm film for volatiles. Extraction efficiency depends on many factors the chemical nature of the analyte, the sample matrix and the polymer coating the extraction time and temperature the degree of stirring and the analyte concentration. The desorption step depends primarily on the injection port temperatures, the volatility of the analyte, and the film thickness. 

Oellig, C. and Schwack, W. 2014, Planar solid phase extraction clean-up and microliter-flow injection analysis-time-of-flight mass spectrometry for mnlti-residue screening of pesticides in food, J. Chromatogr. A, 1351 1-11. 

High-Throughput Planar Solid Phase Extraction Coupled TO Microliter-Flow Injection Analysis-Time-of-Flight Mass Spectrometry for a Fast Pesticide Screening... 

Required extraction times are usually short in DLLME, since the extraction equilibrium is quickly reached due to the large transfer area for the extraction procedure. The efficient recovery of the sedimented or floating extractant and its further injection into analytical instrumentation for the quantification of the target compounds is the most troublesome part of the procedure, mainly... 

In Figure 7.53 a flow-injection interface for fluorometric monitoring of focused microwave-assisted Soxhlet extraction is represented [195]. This assembly allows real-time online monitoring of the PAHs extracted from solid samples in each Soxhlet cycle and provides qualitative and semi-quantitative information from natural and spiked samples. The method has been applied to a certified reference material (CRM 524, BCR, industrial soil/organics) for quality assurance/validation. The proposed technique is as efficient as conventional Soxhlet to extract PAHs from soils but with a drastic reduction of both extraction time and organic solvent disposal. 

Other alternative options for PAHs extraction coupled to flow injection are static, dynamic, and static-dynamic-pressurized liquid extraction. They have been applied to contaminated soils analysis, allowing the partial automation of the proposed approaches. Efficiencies close to 100% have been obtained with the three operational modes. However, the static-dynamic mode has proved as the most suitable alternative providing the shortest extraction time (25 min) versus the static (30 min) and the... 

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