Choice of Reagents
A substance that absorbs microwave energy strongly is called a sensitizer. The latter preferentially absorbs the radiation and passes the energy on to other molecules. Polar molecules and ionic solutions (usually acids) will absorb microwave energy strongly in relation to non-polar molecules. This is because they have a permanent dipole moment that will be affected by the microwaves. If extraction between non-polar molecules is required, then the choice of solvent is the main factor to consider. If the solvent molecule is not sensitive enough to the radiation, then there will be extraction. This is because the substance will not heat up.
Solvent Effects
A correct choice of solvent for microwave-assisted extraction is essential. The solvent must be able to absorb microwave radiation and pass it on in the form of heat to other molecules in the system. The following equation [8] measures how well a certain solvent will pass on energy to others:
e"/e' = tan 5
where 5 is the dissipation factor, e" is the dielectric loss (a measure of the efficiency of conversion of microwave energy into heat energy), and e' is
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the dielectric constant (a measure of the polarizability of a molecule in an electric field).
Polar solvents, such as water, acetone and methanol, all readily absorb microwaves and are heated up when subjected to microwave radiation. Nonpolar solvents, such as hexane and toluene, do not heat up when they are subjected to microwave irradiation.
microwave system is shown in Figure 5.6. The sample and acid (sulfuric acid can be used) are introduced into a glass container, which has the appearance of a large boiling/test tube, and is then fitted with a condensor to prevent loss of volatiles. The sample container is placed within the microwave cavity and heated.
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
, Glass sample container / connected to a / water-cooled reflux
Figure 5.6 Schematic of an atmospheric, open-focused microwave digestion system.
Figure 5.7 Schematic of a pressurized microwave digestion system.
an alarm to call attention to any unexpected release of flammable and toxic material. The microwave energy output of this system is 1500 W at a frequency of 2450 MHz at 100% power. Pressure (up to 800 psi) is continuously measured (measurements taken at the rate of 200 s-1), while the temperature (up to 300�C) is monitored for all vessels every 7 s. All of the sample vessels are held in a carousel which is located within the microwave cavity. Each vessel has a vessel body and an inner liner. The liner is made of �TFM� fluoropolymer and has a volume of 100 ml. A patented safety system (AutoVent Plus�) allows the venting of excess pressure within each extraction vessel. The system works by lifting of the vessel cap to release excess pressure and then immediately resealing to prevent loss of sample. If solvent leaking from the extraction vessel(s) does occur, a solvent monitoring system will automatically shut off the magnetron, while still allowing the exhaust fan to continue working.
What advantage in terms of digestion time could the use of a microwave system offer over a conventional hot-plate?
It would be faster using a microwave oven (consider the domestic microwave oven and a conventional electric or gas cooker), particularly if the digestion is carried out in sealed vessels (pressure and temperature effects).
The use of open vessels for digestion can lead to additional problems associated with loss by volatilization of element species. This can be rectified by the correct choice of reagents and the type of digestion apparatus to be used.
5.4.2	Microwave Digestion Procedure
Unless the microwave system used is capable of temperature feedback control, thus allowing it to replicate effectively the desired temperature profile, then the system will need to be calibrated. The procedure for calibration of microwave equipment is shown in Box 5.2. The typical operating procedure for digestion of a sample of sediment, sludge or soil is shown in Figure 5.8, while for siliceous and organically based matrices, see Figure 5.9. However, it should be borne in mind that all digestion vessels and volumetric flasks to be used should be acid cleaned prior to use in order to reduce the risk of contamination. In addition to
Figure 5.8 A typical procedure used for the microwave digestion of sediments, sludges and soils [9].
Figure 5.9 A typical procedure used for the microwave digestion of siliceous and organically based matrices [10].