Temperature microwave extraction systems

One of the extraction vessels is equipped with a temperature and pressure sensor/control unit. Figure 3.10 shows the schematic diagram of a control vessel as well as a standard vessel. A fiber-optic temperature probe is built into the cap and the cover of the control vessel. The standard EPA method requires the microwave extraction system to be capable of sensing the temperature to within +2.5°C and adjusting the microwave field output power... 

Extraction conditions +/- Decomposition of substances incomplete extraction Lower temperature of extraction system Prolong Soxhlet cycles, increase or lower temperature, prolong healing lime, break emulsion, adapt microwave oven... 

In a representative reaction, 1.0 mmol aniline derivatives, 1.1 mmol dihalides, and 1.1 mmol potassium carbonate in 2 mL of distilled water were placed in a 10 mL crimp-sealed thick-wall reaction tube equipped with a pressure sensor and a magnetic stirrer. Tlie reaction tube was placed in the MW cavity (CEM Discover Focused Microwave Synthesis System with a build-in infrared temperature sensor), operated at 120 + 5°C, power 80-100 Watt and pressure 65-70 psi, for 20 minutes. After completion of the reaction, the organic portion was extracted into ethyl acetate. Removal of the solvent... 

In the sample preparation of semi- and nonvolatile compounds, solvent extraction is typically used for extracting the analytes of interest from a sample matrix. For volatile analytes, head-space or thermal extraction are good alternatives to solvent-based techniques. Several novel extraction systems that utilize elevated temperatures of pressures in the extraction have been developed particularly for solvent-based extraction methods. These new methods typically are much faster and often more selective than older methods and consume smaller amounts of organic solvents and reagents. Commercially available systems with the ability to heat and pressurize liquids include pressurized liquid extraction (PLE), microwave-assisted extinction (MAE) and supercritical fluid extraction (SFE). Also sonication-assisted extraction (SAE) has given promising results. 

The most common extraction techniques for semivolatile and nonvolatile compounds from solid samples that can be coupled on-line with chromatography are liquid-solid extractions enhanced by microwaves, ultrasound sonication or with elevated temperature and pressures, and extraction with supercritical fluid. Elevated temperatures and the associated high mass-transfer rates are often essential when the goal is quantitative and reproducible extraction. In the case of volatile compounds, the sample pretreatment is typically easier, and solvent-free extraction methods, such as head-space extraction and thermal desorption/extraction cmi be applied. In on-line systems, the extraction can be performed in either static or dynamic mode, as long as the extraction system allows the on-line transfer of the extract to the chromatographic system. Most applications utilize dynamic extraction. However, dynamic extraction is advantageous in many respects, since the analytes are removed as soon as they are transferred from the sample to the extractant (solvent, fluid or gas) and the sample is continuously exposed to fresh solvent favouring further transfer of analytes from the sample matrix to the solvent. 

Atmospheric MAE system This second technique employs solvents with low dielectric constants. Such solvents are essentially microwave-transparent they thus absorb very little energy, and extraction can therefore be performed in open vessels. The temperature of the sample increases during extraction because it usually contains water and other components with high dielectric constants the process is thereby enhanced. Because extraction conditions are milder, this mode of operation can be used to extract thermolabile analytes. 

In a typical application, 2 to 20 g of sample is dried, weighed, and loaded into an extraction vessel. A certain amount (less than 30 mL) of select solvents is also added. Then parameters such as temperature, pressure, and extraction time are set according to the instructions from the microwave manufacturer. A preextraction heating step (typically, 1 to 2 minutes) is needed to bring the system to the preset values. Subsequently, the samples are extracted for about 10 to 20 minutes. After the extraction, the vessels are cooled, and this normally takes less than 20 minutes. Finally, the extract is filtered, concentrated, and analyzed. 

Microwave power output and sample weight seem to have minor effects on extraction efficiency. It was reported that the increase in oven power gave higher recovery of PAHs from atmospheric particles [82], The reason could be that the microwave system used in that study had no temperature control. 

The Friedlander annulation is one of the most straightforward approaches towards poly-substituted quinolines. Thus, a 22-membered library of quinolines was synthesized in a TsOH-catalyzed cyclocondensation-dehydration of 2-aminoaryl ketones and 2-aminoarylaldehydes with ketones in a household microwave oven (with power control) under solvent-free conditions [112]. It was observed that the Friedlander reaction occurred readily also in an oil-bath (at 100 °C). To compare the conventional and dielectric heating conditions precisely, a purpose-built monomode microwave system with temperature control was employed instead of the household oven. The experiments at 100 °C under otherwise identical conditions demonstrated that the dielectric heating protocol was only slightly faster. Products were isolated by a simple precipitation-neutralization sequence (in the case of solid products) or neutralization-extraction for oily or low melting point products (Scheme 43). 

A property seemingly unique to microwave chemistry is that the individual phases in multi-phase systems can be heated at different rates owing to differences in the dielectric properties. In some cases, a sizeable temperature difference can be maintained for several minutes. This technique has been applied usefully to produce aryl vinyl ketones batchwise by Hofmann elimination in a two-phase system comprising water and chloroform [77]. Although reactions took place in the aqueous phase, the thermally unstable products simultaneously were extracted and diluted into the cooler organic phase, which could be recycled. Yields were nearly quantitative and twice those obtained by traditional pyrolysis-distillation under vacuum. 

Fig. 8. ESR spectra of authentic PBN adduct formed in the Fenton system, (a) In phosphate buffer at room temperature, (b) in toluene extracts of (a) at room temperature, and (c) same as (b), but spectrum recorded at — 100°C. Spectrometer conditions scan range 100G modulation amplitude 1.0G microwave power 5 mW scan time 4 min time constant 0.25 s.

Generally, two set-ups for conducting MAE are applied in laboratories closed vessels systems allowing for temperature and pressure adjustment and control, and open vessel systems for procedures carried out under atmospheric pressure. In the open vessel systems, maximal temperature is determined by the temperature at which the extractant boils. In those systems, absorption of microwave radiation occurs in the whole sample therefore, heating is effective and homogeneous. The main disadvantage of open vessel systems is the possibility of volatile compound loss. This can be reduced by application of a reflux system fitted into the top of the extraction vessel. 

Fig. 5.5. (A) Scheme of a flow digestion system and the principle of pressure equilibration A pressure reactor, B heating zone, C cooling zone, D digestion coil, E cooling device, F connection for gas supply, G restrictor tube, H collector vial, I temperature sensor, J high-pressure pump, K injection valve, L sample loop, M sample, N and O peristaltic pumps. (Reproduced with permission of the American Chemical Society.) (B) Manifold for dynamic microwave-assisted extraction I solvent, 2 pump, 3 microwave oven, 4 extraction chamber, 5 temperature set-point controller, 6 thermocouple, 7 fluorescence detector, 8 recording device, 9 restrictor, 10 extractor. (Reproduced with permission of Elsevier.)...

Microwave-assisted extraction has also been used as a solid sample treatment prior to speciation analysis [264-266], leaving the organometallic compound moiety intact. This is a prerequisite for a successful extraction procedure to be applied prior to speciation analysis and can be met by careful optimization of the conditions of the microwave attack. Open-vessel treatment is preferred to pressurized bomb systems commonly used in the analysis for total metals because it offers milder reaction conditions — the increase in temperature is governed to a great extent by the boiling point of the solvent — and easier control of process variables [266]. 

A common commercial closed system is the Microwave Accelerated Reaction System (MARS ) 5, as supplied by the CEM Corporation, USA (Figure 5.7). This system allows up to 14 extraction vessels (XP-1500 Plus ) to be irradiated simultaneously. In addition, other features include a function for monitoring both pressure and temperature, and most notably, the system is equipped with... 

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