Carrying on from Part 5 on pressure solvent extraction, we present part 6 in the E&L extraction techniques series: microwave-assisted extraction.
Microwave extraction can be performed in a very similar way to pressurised solvent extraction. In microwave extraction, the extraction typically occurs in one of two systems: Focused microwave-assisted extraction (FMAE), where the extraction occurs in an open vessel under atmospheric pressure or pressurised microwave-assisted extraction (PMAE) is a closed pressurised system and it is this approach that is very similar to pressurised solvent extraction. It is rare for a microwave extraction to be utilised as the extraction technique for a controlled extraction study but is more likely to be used as part of routine control where the material is well known and the speed advantages, and low solvent volumes can be utilised effectively.
FMAE is less common but it has been applied to certain examples as well as specific extractions such as PAHs    , PCBs, pesticides  , organometallic compounds  , and dioxins or furans  from spiked, real, and reference material samples. PMAE is far more common but does suffer from a number of disadvantages in terms of analysis time as it takes time to make sure the extraction vessels can cope with the extraction pressure and are suitably sealed. It can also take a considerable amount of time for the vessels to cool to room temperature before the pressure is reduced and they can be opened. For routine testing the low solvent volumes, short extraction times and the ability to do multiple samples at once overcome the disadvantages. Examples exist of its use in the following fields: the extractions of environmental pollutants, such as hydrocarbons (HCs) , PAHs  , organochlorine pesticides (OCPs)  , polychlorinated biphenyls (PCBs) , dioxins or furans, triazines and alkyl or aryl phosphates, in soil, sediment and sludge sample matrices.
The primary concern with microwave heating is the ability of the solvent to absorb the microwaves e.g., dielectric polarisation of the solvent must be possible. This heating efficiency depends on the dielectric constant (g) of the solvent and in most cases is proportional to the polarity of the solvent. Examples of the dielectric constant for common extractable solvents are in Table 1.
In addition to the solvents ability to absorb microwave energy, its ability to convert the energy to heat is important. The efficiency of the conversion of microwave energy to heat is given by the dielectric loss factor or loss tangent. Loss tangents of common solvents are shown in Table 2. More polar solvents are typically used in microwave extraction.
Therefore, the overall efficiency of heating using microwave energy is usually expressed by the dissipation factor which is the ratio of the dielectric loss factor and the dielectric constant of the involved matrix []. Concerning the overall efficiency, these data show a large difference between various solvents and, moreover, that methanol has been shown to be more favourable than water despite its lower dielectric constant []. Standard setups suffer from that only one solvent mix can be used at a time since the rate of heating can and does vary quite dramatically. Other possible issues are that the material to be extracted can be altered. It can be termed as ‘“accelerations of chemical transformations in a microwave field that cannot be achieved or duplicated by conventional heating’[]. A typical example would be the additional curing of rubber with microwaves. Whether this happens in standard extraction conditions has not been proved one way or another but will depend on a number of factors, including temperature, as well as possibly solvent and extraction duration. Another factor to consider with microwave heating is that for some solvents the average temperature of the solvent can be considerably higher than the atmospheric boiling point because the microwave power is dissipated over the whole volume of the solvent again potentially degrading or altering the material.
Commercial systems include the safety systems required of a microwave which are;
- No handling of pressurised vessels
- Closed rotor with protection lid
- Stable oven cavity with resealing safety door
- Pressure increase control
- Controlled pressure limits for each vessel type
- 70 bar safety disk (overpressure tolerance)
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