EXTRACTION OF SEMIVOLATILE ORGANIC COMPOUNDS FROM SOLID MATRICES
3.6. MICROWAVE-ASSISTED EXTRACTION
a quick reference to these examples, and more detailed information can be found in some recent reviews [67,68]. In principle, ASE is a universal method that can be used in any solvent extraction. However, majority applications so far have been in the environmental area, such as the extrac-tion of pesticides, herbicides, PAHs, PCBs, base/neutral/acid compounds, dioxins, furans, and total petroleum hydrocarbons. ASE has also been used to extract additives and plasticizers from polymers, additives, and active ingredients from pharmaceuticals, and contaminants/fat from food.
either 2.45 or 0.9 GHz. Domestic ovens operate at 2.45 GHz only. When mircowave radiation is applied to molecules in the gas phase, the molecules absorb energy to change their rotational states. The microwave spectrum of molecules shows many sharp bands in the range 3 to 60 GHz. This has been used in microwave spectroscopy to obtain fundamental physical–chemical data such as bond lengths and angels, and to identify gaseous molecules (e.g., molecular species in outer space).
In the liquid and solid states, molecules do not rotate freely in the micro-wave field; therefore, no micromicro-wave spectra can be observed. Molecules respond to the radiation di¤erently, and this is where microwave heating comes in. The mechanism of microwave heating is di¤erent from that of conventional heating. In conventional heating, thermal energy is transferred from the source to the object through conduction and convection. In micro-wave heating, electromagnetic energy is transformed into heat through ionic conduction and dipole rotation. Ionic conduction refers to the movement of ions in a solution under an electromagnetic field. The friction between the solution and the ions generates heat. Dipole rotation is the reorientation of dipoles under microwave radiation. A polarized molecule rotates to align itself with the electromagnetic field at a rate of 4.9 109 times per second.
The larger the dipole moment of a molecule, the more vigorous is the oscil-lation in the microwave field.
The ability of a material to transform electromagnetic energy into ther-mal energy can be defined as
tan d¼e00 e0
where tan d is the loss tangent or tangent delta; e00is the dielectric loss coef-ficient, a measure of the e‰ciency of a material to transform electromagnetic energy to thermal energy; and e0is the dielectric constant, a measure of the polarizibility of a molecule in an electric field. Table 3.11 lists the physical constants of some selected organic solvents. Polar solvents such as acetone, methanol, and methylene chloride have high tan d values and can be heated rapidly. Nonpolar solvents such as hexane, benzene, and toluene cannot be heated because they lack dipoles and do not absorb microwave.
3.6.2. Instrumentation
In general, organic extraction and acid digestion use di¤erent types of microwave apparatus, as these two processes require di¤erent reagents and di¤erent experimental conditions. A new commercial system, Mars X (CEM Corp., Matthews, NC) o¤ers a duel unit that can perform both
proce-dures. In this chapter only the instrumentation for organic extraction is discussed.
The basic components of a microwave system include a microwave gen-erator (magnetron), a waveguide for transmission, a resonant cavity, and a power supply. For safety and other reasons, domestic microwave ovens are not suitable for laboratory use. There are two types of laboratory microwave units. One uses closed extraction vessels under elevated pressure; the other uses open vessels under atmospheric pressure. Table 3.12 lists the features of some commercial MAE systems.
Closed-Vessel Microwave Extraction Systems
Closed-vessel units were the first commercially available microwave ovens for laboratory use. A schematic diagram of such a system is shown in
Table 3.11. Physical Constants of Organic Solvents Used in MAEa Vapor
Pressure Solvent
Boiling Point
(C) torr kPa e0
Dipole Moment
(debye) tan d 104
Methylene chloride 40 436 58.2 8.93 1.14 —
Acetone 56 184 24.6 20.7 2.69 —
Methanol 65 125 16.7 32.7 2.87 6400
Tetrahydrofuran 66 142 19.0 7.58 1.75 —
Hexane 69 120 16.0 1.88 <0.1 —
Ethyl acetate 77 73 9.74 6.02 1.88 —
Ethanol 78 — — 24.3 1.69 2500
Methyl ethyl ketone 80 91 12.1 18.51 2.76 —
Acetonitrile 82 89 11.9 37.5 3.44 —
2-Propanol 82 32 4.27 19.92 1.66 6700
1-Propanol 97 14 1.87 20.33 3.09 @2400b
Isooctane 99 49 6.54 1.94 0 —
Water 100 760 101.4 78.3 1.87 1570
Methyl isobutyl ketone 116 20 2.67 13.11 — —
Dimethyl formamide 153 2.7 0.36 36.71 3.86 —
Dimethyl acetamide 166 1.3 0.17 37.78 3.72 —
Dimethyl sulfoxide 189 0.6 0.08 46.68 3.1 —
Ethylene glycol 198 — — 41.0 2.3 10,000
N-Methyl pyrrolidinone 202 4.0 0.53 32.0 4.09 —
Reproduced from Ref. 85, with permission from the American Chemical Society.
a Boiling points were determined at 101.4 kPa; vapor pressures were determined at 25C, dielectric constants were determined at 20C; dipole moments were determined at 25C.
b Value was determined at 10C.
165 microwave-assisted extraction
Table3.12.FeaturesofSomeCommercialMAESystems Model/ManufacturerPower (W)Sensors
Max. Pressure (bar)
Vessel Volume (mL)Vessel Materiala
Number of Vessels Multiwave/Anton ParrGmbH, Austria
1000Pressurecontrolandinfrared temperaturemeasurement inallvessels
70 70
100 100
TFM/ ceramics TFM/ ceramics
12 6 13050TFM/ ceramics6 13050Quartz8 13020Quartz8 Mars-8/CEM, UnitedStates1500Infraredtemperaturemea- surementinallvessels35 100100 100TFM TFM14 12 Ethos900/1600, Milestone, UnitedStates
1600Pressurecontrolandtemper- aturemeasurementinall vessels
30 100 30 100
120 120 120 120
TFM/PFA TFM TFM/PFA TFM
10 6 12 10 Soxwave100/3.6, Prolabo,France250TemperaturecontrolOpenvessel Openvessel250 100or260Quartz Quartz1 1 aTFM,tetrafluoromethoxylpolymer;PFA,perfluoroalkoxy.
166
Figure 3.9. In the oven cavity is a carousel (turntable or rotor) that can hold multiple extraction vessels. The carousel rotates 360 during extraction so that multiple samples can be processed simultaneously. The vessels and the caps are constructed of chemically inert and microwave transparent materials such as TFM (tetrafluoromethoxyl polymer) or polyetherimide.
The inner liners and cover are made of Teflon PFA (perfluoroalkoxy). The vessels can hold at least 200 psi of pressure. Under elevated pressures, the temperature in the vessel is higher than the solvent’s boiling point (see Table 3.11), and this enhances extraction e‰ciency. However, the high pressure and temperature may pose safety hazards. Moreover, the vessels need to be cooled down and depressurized after extraction.
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 tem-perature to within G2.5C and adjusting the microwave field output power
Cavity Exhausted to Chemical Fume Hood
Temperature and Pressure
Sensor Connectors Wave Guide
Mode Stirrer
Magnetron Antenna
Magnetron Isolated Electronics
Room Air Inlet
Chemically Resistant Coating on Cavity Walls
Figure 3.9. Schematic diagram of a closed-vessel cavity MAE system. (Reproduced from Ref.
85, with permission from the American Chemical Society.)
167 microwave-assisted extraction
automatically within 2 seconds of sensing. The temperature sensor should be accurate to G2C.
Safety features are essential to a microwave apparatus. An exhaust fan draws the air from the oven to a solvent vapor detector. Should solvent vapors be detected, the magnetron is shut o¤ automatically while the fan keeps running. Each vessel has a rupture membrane that breaks if the pres-sure in the vessel exceeds the preset limit. In the case of a membrane rupture, solvent vapor escapes into an expansion chamber, which is connected to the vessels through vent tubing. To prevent excessive pressure buildup, some manufacturer use resealable vessels. A spring device allows the vessel to open and close quickly, releasing the excess pressure.
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
Body Cap Safety Rupture Membrane
Ferrule Nut
Linear Cover
Vent Fitting Vent Tube
Linear Body
Cap Cover Exhaust
Port
Safety Rupture Membrane Vent
Fitting Ferrule Nut
Temperature Port
Pyrex®
Thermowell
Standard Extraction Vessel Extraction Vessel for Temperature/Pressure Control Figure 3.10. Schematic diagram of a closed vessel for MAE. (Reproduced from Ref. 85, with permission from the American Chemical Society.)
heating is done through the stir bar. The same solvents used in conventional methods (both polar and nonpolar) may be adopted here, thus reducing the time for method development.
Open-Vessel Microwave Extraction Systems
Open-vessel systems are also known as atmospheric pressure microwave or focused microwave systems. An example is Soxwave 100 (Prolabo Ltd., France). A schematic diagram of such a system is shown in Figure 3.11. It uses a ‘‘focused’’ waveguide, that directs the microwave energy into a single-vessel cavity. This provides greater homogeneity of the radiation than in closed-vessel units, where microwave is dispersed into the multivessel cavity.
However, only one vessel can be heated at a time, and multiple vessels are to be processed sequentially. The vessel, typically made of glass or quartz, is connected with an air (or a water) condenser to reflux the volatile analytes and solvents. Operating somewhat like Soxhlet extraction, this type of sys-tem has been referred to as microwave-assisted Soxhlet extraction.
Magnetron
Wave Guide
Focused Microwaves
Sample Solvent Reflux System Water Circulation
Vessel
Figure 3.11. Schematic diagram of an open-vessel, waveguide-type MAE system. (Reproduced from Ref. 6, with permission from Elsevier Science.)
169 microwave-assisted extraction
3.6.3. Procedures and Advantages/Disadvantages
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 sol-vents 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.
High e‰ciency is the major advantage of microwave extraction over conventional methods such as Soxhlet. It can achieve the same recovery in a shorter time (20 to 30 minutes) and with less solvent (30 mL). The through-put is high (up to 12 samples per hour for closed-vessel system). On the other hand, MAE has several limitations. Solvents used in Soxhlet extraction cannot readily be applied to microwave extraction because some of them do not absorb microwave. Method development is generally necessary for MAE applications. Moreover, cooling and filtration after extraction pro-longs the overall process. Since MAE is quite exhaustive, normally the extract contains interfering species that require cleanup prior to analysis.
3.6.4. Process Parameters
The e‰ciency of MAE can be influenced by factors such as the choice of solvent, temperature, extraction time, matrix e¤ects, and water contents.
In general, some optimization of these conditions is necessary. Typical microwave conditions suggested in a standard EPA method are listed in Table 3.13.
Table 3.13. EPA Standard Procedure for MAE of Environmental Samples
Solvents 25 mL of acetone–hexane (1 : 1 v/v)
Temperature 100–150C
Pressure 50–150 psi
Time at temperature 10–20 min
Cooling To room temperature
Filtering/rinsing With the same solvent system
Choice of Solvent
The proper choice of solvent is the key to successful extraction. In general, three types of solvent system can be used in MAE: solvent(s) of high e00 (dielectric loss coe‰cient), a mixture of solvents of high and low e00, and a microwave transparent solvent used with a sample of high e00. Pure water was used for the extraction of triazines from soils [73], and the recovery was comparable to those using organic solvents. In the extraction of organo-chlorine pesticides (OCPs) from marine sediments, terahydrofuran (THF) yielded better recovery than either acetone or acetone–hexane (1 : 1) [74]. It was reported that dichloromethane (DCM)–methanol (9 : 1) was the most e‰cient solvent for the extraction of phenylurea herbicides (linuron and related compounds) from soils. Other solvent systems, including DCM, DCM–water (5 : 1), methanol–water (7 : 3), and methanol–water (9 : 1) gave poor performance [75]. For the extraction of felodipine and its degradation product H152/37 from medicine tablets [76], acetonitrile–methanol (95 : 5) was found to be the optimum solvent composition. Methanol was capable of dissolving the tablet’s outer covering layer, while acetonitrile broke the inner matrix into small pieces. Hexane–acetone (typically 1 : 1) has proven to be an e‰cient solvent system for the extraction of PAHs, phenols, PCBs, and OCPs from environmental samples [77,78].
Temperature and Pressure
Generally, recovery increases with the increase in temperature and then levels o¤ after a certain point. For thermally labile compounds, analyte degradation occurs at high temperatures and results in low recovery. Exces-sively high temperatures lead to matrix decomposition in polymer extrac-tions and should be avoided. In general, pressure is not a critical parameter in MAE. It changes with the solvent system and the temperature used and is acceptable below a preset limit.
It was reported that the recoveries of 17 PAHs from six certified reference marine sediments and soils [77] increased from 70 to 75% when the temper-ature was increased from 50C to 115C, and remained at 75% from 115 to 145C. In the extraction of OCPs from sediments, recovery was unchanged from 100 to 120C [74]. In the extraction of phenylurea herbicides from soils, the recovery peaked in the range 60 to 80C and decreased at lower or higher temperatures [75]. In the extraction of sulfonylurea herbicides from soils, recovery dropped from 70 to 80% to 1 to 30%, due to decomposition when temperature increased from 70C to 115C [79]. The recovery of oligomers from poly(ethyleneterephthalate) increased as temperature rose 171 microwave-assisted extraction
from 70C to 140C [80]. However, polymer fusion occurred at temperatures above 125C; therefore, 120C was chosen as the optimum.
Extraction Time
Many microwave extractions can reach maximum recovery in 10 to 20 minutes. Longer extraction time is not necessary and may lead to the decomposition of thermolabile analytes. It was reported that the recovery of sulfonylurea from soil was not a¤ected by extraction time in the range 5 to 30 minutes [79]. Similar observation was made in the extraction of PAHs from soils and sediments [6]. In the extraction of PAHs and LAHs (linear aliphatic hydrocarbons) from marine sediments, the extraction time was found to be dependent on the irradiation power and the number of samples extracted per run [81]. When the irradiation power was 500 W, the extrac-tion time varied from 6 minutes for one sample to 18 minutes for eight samples [74]. The recovery of OCPs from spiked marine sediments increased from 30% at 5 and 10 minutes to 60% at 20 minutes and to 74 to 99% at 30 minutes [82].
Matrix E¤ects and Water Content
Matrix e¤ects have been observed in MAE applications. It was reported that recoveries of OCPs from aged soils (24 hours of aging) were lower than those from freshly spiked samples [78]. Similar matrix e¤ects were also reported in the extraction of sulfonylurea herbcides from aged soils [79]. In another study, the average recoveries of 17 PAHs from six di¤erent stan-dard reference materials (marine sediments and soils) varied from 50 to 100% [77].
Because water is a polar substance that can be heated by microwave irradiation, it can often improve analyte recovery. In a study of focused MAE of PAHs from soil and sediments [6], sample moisture level showed significant influence on extraction e‰ciency, and 30% water in the sample provided the highest recovery. Similarly, the maximum recovery of phenyl-urea herbicides was obtained with 10% water in soils [75]. In the extraction of triazines from soil, water content in the range 10 to 15% yielded the highest recovery [7].
Microwave power output and sample weight seem to have minor e¤ects on extraction e‰ciency. 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.
For an extraction conducted at a controlled temperature, the oven power output may have less influence on recovery.
3.6.5. Applications of MAE
Majority MAE applications have been in the extraction of PAHs, PCBs, pesticides, phenols, and total petroleum hydrocarbons (TPHs) from envi-ronmental samples. MAE has also been used in the extraction of con-taminants and nutrients from foodstu¤s, active gradients from pharmaceu-tical products, and organic additives from polymer/plastics. Table 3.14 lists some typical applications. Readers interested in the details of MAE appli-cations can find more information in some recent reviews [85–87].
3.7. COMPARISON OF THE VARIOUS EXTRACTION TECHNIQUES