Fundamentals of microwave enhanced sample preparation

How microwaves work

Microwave ovens are an alternative method to conventional conductive heating and for introducing energy into chemical reactions. The first device for generating fixed frequency microwaves, originally called the Magnetron, was a spin-off from the development of RADAR during World War II, originally designed by Randall and Booth at the University of Birmingham. It first began to be used commercially and domestically in the 1950s and gained wide spread domestic use in the late 1970s and 1980s. 

Microwave radiation lies in the electromagnetic spectrum between infrared and radio frequencies corresponding to wavelengths between 1 centimeter and 1 meter, and frequencies between 30 GHz and 300MHz. Domestic microwave ovens are required to operate at either 2.45 GHz or 900MHz in order to avoid interference with telecommunications.

The way microwave ovens generate heat lies in the ability of an electric field to polarize charges in a material and the inability of the polarized molecules to follow the rapid reversing oscillations of an electric field. The field provides the energy necessary for the molecules to rotate into alignment. Some of the energy however is transferred into random motion. The  response of a material to electromagnetic radiation depends on the time scales of the orientation and disorientation of polar molecules relative to the frequency of the radiation. For example, the dipole polarization of water is due to its dipole moment, which results from the differing electronegativities of the oxygen and hydrogen atoms. When the electric field oscillations are slow the time taken by the electric field to change directions is longer than the response time of the water molecules so the dielectric polarization keeps in phase with the oscillations of the electric field. If the electric field oscillates too rapidly it changes direction faster than the response times of the dipoles. In this situation the dipoles do not rotate, no energy is absorbed, and the water does not heat up. In the microwave frequency range the time in which the field oscillates is about the same as the response times of the dipoles but the diploes lag behind. 

Dipole rotation and ionic conductance are the two primary mechanisms for the adsorption of microwave energy by a solution. Molecular dipoles align with the applied electric field. The electric field oscillates forcing the dipole molecules to move and the resulting friction heats the solution. Laboratory microwave ovens (oscillating at a frequency of 2.45 GHz) forces the dipole molecules to align and then randomize 5 billion times per second. In ionic conduction the ionic species migrate in one direction and then the other according to the polarity of the electromagnetic field. Heating is the natural consequence when accelerated ions meet resistance to their flow. These two mechanisms heat solutions much faster than conduction and convection.

When water is heated in a high-pressure vessel in a convection oven an equilibrium vapor pressure is established. This equilibrium is dependant on the rates of evaporation and condensation. When the temperature increases the evaporation rate increases and the condensation rate decreases because the vessel walls are heating both the solution and the gas phase. As a result, more water is in the vapor phase, so pressure within the vessel is high. When water is heated to the same temperature in a closed-vessel in the microwave the pressure is significantly lower because the vessel is microwave-transparent and has little insulating capacity. The vessel remains relatively cool during the heating process, which allows for an increase in condensation rate and removal of water from the vapor phase, which results in a lower internal pressure. 

Microwave ovens and vessels

Traditionally chemists have heated solutions during sample preparation using flames, hot plates, heating mantels, ovens and heating blocks. The heat is transferred via conduction to parts of the solution that are in contact with the heat source. The maximal achievable solution temperature is limited by the vessel's thermal transfer properties, the solution's boiling point, colligative properties of the solution, and the pressure in the vessel.

Crude metal extraction methods using mineral acids are known to have occurred as far back as the  14th century, many of the methods that we currently use were developed in the 19th century. For example, a method for determining nitrogen content in protein, developed in by Kjeldahl 1883, is still being used today. Although the method is now incorporated in more streamlined and automated instrumentation, the method has changed little for 100 years. Analytical chemists first began using domestic microwaves in 1975 to aid in digestion of biological samples in standard laboratory glassware. By the mid 1980s closed reaction vessels were being used and reaction temperatures were exceeding atmospheric boiling point. Since then there have been many advances in vessel design and in the monitoring and control of temperature and pressure. 

The first closed-vessel digestion vessels were made entirely of Teflon and had a pressure limit of about 7 to 10 atmospheres, but these deteriorated with age and use. The second generation vessels had an inner perfluoralkoxy fluoropolymer liner and cap encased in a shell of polyetherimide. The vessels were capable of pressure limits of about 30 atmospheres through the use of a calibrated pressure relief mechanism. Some of the more current vessels are composed of a tetrafluormethoxil fluoropolymer liner and cap and casings made of polyetherimide or other microwave transparent materials. Their working temperatures approach 260 degrees centigrade and pressure limit is between 60 and 100 atmospheres.

The biggest challenge to the advancement of microwave enhanced chemistry was in developing probes that could enter the microwave environment safely and with little disruption to the reaction. Pressure feedback controls were first introduced in 1989, and temperature feedback control by 1992. These controls enabled controlled digestion and the development of studies in digestion mechanisms as well as standardization of methods. Because the condensation rate within the vessel varies greatly with the material digested there is no method for predicting internal pressure within a closed-vessel, hence, pressure control is never used in standardized microwave preparation methods.

The laboratory microwave ovens are different from their domestic counterparts in offering additional safety features like explosion-proof and safety interlocking doors, corrosion-resistant cavity, and computer controlled temperature and pressure feedback control mechanisms. Some ovens have NO_x and organic solvent detectors that shut down the oven in the event of a leak. Some oven systems have specialized adapters to add reagent at programmed step-wise intervals.

Sample preparation

The ability to control temperature and heating rate have allowed scientists to both study the decomposition process and develop standardized methods for preparing samples for subsequent molecular analysis. Three general standard sample preparation methods have been developed by the EPA using microwave energy; Method 3052, Method 3051A, and Method 3051.

Samples prepared for digestion should be dried. For inorganic samples microwave drying  methods can be more efficient than conventional drying in an oven. Drying samples with organic compounds that are to be analyzed can be a problem especially if your analytes are volatile. Hot plate drying techniques can lead to overheating and losses but microwave assisted heating allows for lower the temperatures because the sample dries as the temperature decreases helping to retain volatile analytes.

Organic analytes often require extraction and separation from a matrix in a suitable solvent. The disadvantage is that large volumes of solvent are generated and must be concentrated and disposed. The procedure can be lengthy, taking up to days to complete. Using closed-vessel microwave methods organic solvents can be heated several fold higher than their atmospheric boiling points without the associated pressure increases. The reflux action brings the sample into contact with fresh solvent, and twelve to fourteen samples can be run simultaneously. Microwave organic sample preparations are equal to or better than the Soxhlet method.

For acid digestion of organic samples, a slow and even temperature increase or " ramp" is required for oxidizing the sample, shorter ramp and longer hold times, or multiple cycles of heating are required for more refractory samples.

Limitations

The ability to control temperature and pressure has also lead to improvements in reproducibility and thereby increased ability to analyze sources of error. Most of the error in determining analyte concentrations can be corrected by digesting blanks and subtracting the amount of analyte contaminant found in the reagents. A more difficult source of error to analyze is from incomplete digests, especially in recalcitrant samples such as soil or slag. Silicate minerals are very difficult to decompose without the aid of more powerful oxidizers and reducing agents.

Contamination of the samples by ions and elements found in the reagents is an important concern when the analyte of interest in found in ultra-trace amounts. In these circumstances ultra-pure certified reagents should be used and clean-room techniques should be employed. Vessels should be acid washed, and blanks with water source and blanks with reagents alone should be run.

References

Mingos, D.M.P., and D. R. Baghurst. Applications of Microwave Dielectric Heating Effects to Synthetic Problems in Chemistry. In Microwave-Enhanced Chemistry, H.M. Kingston, and S. J. Haswell (Eds) 1997, American Chemical Society, Washington, DC.

Richter R.C., D. Link, and H. M. Kingston, 2001. Microwave-enhanced chemistry, Analytical Chemistry Jan: 31A-37A

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