Microwave-assisted sample preparation

Microwaves are electromagnetic waves in the range of 300 MHz (0.3 GHz) to 300 GHz, with a typical frequency of 2.450 MHz for both domestic microwave ovens and laboratory equipment (see Figure 1)^[4]. 

Due to the way microwave heating works (see Figure 2), no direct contact to a heating core is necessary, enabling the heating of different vessel sizes, shapes, and amounts with the same microwave.

According to the Arrhenius equation^[5] an increase in temperature will lead to a decrease in reaction time. This is the key to why microwaves shorten the digestion time: The very fast heating effect of microwaves, the instant on-and-off, and the optimized workflow lead to an enormous time-saving effect.

Higher temperature also increases the oxidation potential of the acids, which has a positive effect on the digestion quality, as an increase of the temperature leads to a decrease in the amount of residual carbon. The residual carbon content is a good parameter to characterize the digestion performance as it will result in less interference during the analysis.

The ultimate target – reaching high temperatures – is limited by the boiling point of the reagents. From a physical point of view a closed system will be required to reach higher temperatures. This will lead to an increasing vapor pressure of the reagents.

In addition to the vapor pressure, the digestion reaction also contributes to the pressure inside the vessel. Depending on the sample material which has to be digested, gaseous products are formed during the digestion. 

The microwave energy is created by a single magnetron and directed through a waveguide^[6] to the reaction mixture (see Figure 5). This results in a “standing wave” and the reaction vessel is located in a hot spot where efficient heating of small volumes is possible. The instruments can have a space-saving design but only one sample can be digested at once.

One or two magnetrons create microwave irradiation, which is directed into the cavity through a waveguide and distributed by a mode stirrer (see Figure 6). Microwaves are reflected from the walls thus interacting with the reaction mixture in a random manner. Additional rotation of the reaction vessel in the cavity prevents temperature inhomogeneity and formation of hot spots.

This mode allows for heating larger volumes (>20 mL) of reaction mixtures and for heating several samples simultaneously (up to 64 samples). To enable homogenous heating of several samples the multimode reactors need to have a bigger footprint compared to monomode reactors.

The DMC combines the benefits of monomode and multimode reactors. Like in a monomode system, the microwaves are directed to the samples providing highly efficient heating in a small footprint system, but like in a multimode system more than one sample (up to 12 samples) can be digested in a single run (see Figure 7). 

The microwave energy is created by a single magnetron and directed to the PDC (Pressurized Digestion Cavity). A PTFE liner, filled with a load solution, is placed in the cavity and gets heated with the reaction media. Heating a load solution instead of a specific vessel ensures an optimized temperature distribution and compensates exothermic reactions. The reaction temperature is permanently controlled by a temperature sensor on the bottom of the PDC. This enables the use of one up to 28 thin-walled vials (made of quartz, glass, or fluoropolymer) closed with simple plug-on caps without a required minimum filling volume.

Prior to heating, the cavity is pressurized with an inert gas (e.g. nitrogen or argon), which prevents boiling and evaporation of the reagent mixture. In this way the pressure in the cavity seals the vials.

The importance of sample preparation for subsequent analytical results

There has been a rapid increase in the demand of analytics over the last few decades. The exponential growth of regulations and norms as well as an increase in human population along with the global distribution of goods pose enormous challenges regarding safety, quality, and the authenticity of various products, all of which can be investigated using analytical techniques. However, although analytical instruments have continued to offer high resolution, fast analysis, robustness, miniaturization, and portability, analysis using these state-of-the-art instruments still requires extensive sample preparation, e.g. microwave-assisted digestion and leaching. Obtaining precise results requires good sample preparation and errors made in sampling, sample preparation, or sample introduction (injection) cannot be corrected by using even the most advanced analytical systems.

Despite many advances in recent years, sample preparation is still often considered to be the bottleneck in the analytical workflow, as it takes up more than 60 % of the time needed from sampling to the end result (see figure 9).