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Microwave-assisted sample preparation

Acid digestion is one of the most important sample preparation techniques. Analytical equipment which is able to determine small concentrations of toxic metals requires a clear, particle-free, slightly acidic solution.

Analysis of e.g. food, soil, or pharmaceuticals cannot be done directly; first the sample needs to be transformed into an analyzable liquid. This process is called sample preparation. By means of acid digestion a sample is destroyed or dissolved by the use of acids and only the (trace) metals remain in solution.

In other words: sample preparation is the important link between sample and analysis. It has to be highly reliable in order not to compromise analytical data[1]. Microwaves are used as effective heating sources that help save time and allow the efficient control of the whole process. 

Tabla de contenido

  1. Acid digestion
  2. Acid leaching
  3. Why use microwaves for sample preparation?
    1. Fast heating rates
    2. Instant turn-on and turn-off
    3. No contact to heating core required
  4. What to consider in microwave-assisted acid digestion
    1. But how can microwaves speed up the digestion time?
    2. What happens at lower temperatures?
    3. Pressure and the impact of sample weight
  5. Microwave systems
    1. Monomode microwave systems
    2. Multimode microwave systems
    3. Directed Multimode Cavity (DMC)
    4. Pressurized Digestion Cavity (PDC)
  6. References

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Acid digestion

Acid digestion is the most common wet-chemical sample preparation technique. By using concentrated acids or mixtures thereof, the matrices of organic and inorganic samples can be totally destroyed or dissolved, and the whole sample can be brought into solution. Subsequently, the concentration of elements or species can be determined with an adequate analytical technique, e.g. AAS (atomic absorption spectroscopy), MIP-OES (microwave-induced plasma optical emission spectrometry), ICP-OES (inductively coupled plasma optical emission spectrometry), ICP-MS (inductively coupled plasma mass spectrometry)[2].

Find the most commonly used acids for acid digestion and their typical concentrations below[3]:

  • Nitric acid, HNO3 (65 %)
  • Hydrochloric acid, HCl (30 % to 37 %)
  • Hydrofluoric acid, HF (40 % to 48 %)
  • Sulfuric acid, H2SO4 (95 % to 98 %)
  • Perchloric acid, HClO4 (70 % to 72 %)
  • Phosphoric acid, H3PO4 (85 %)
  • Hydrogen peroxide, H2O2 (30 %)
  • Aqua regia, HCl + HNO3 (volume ratio 3:1)
  • Reverse aqua regia, HCl + HNO3 (volume ratio 1:3)
  • Boric acid, H3BO3 (approx. 5 %)

Acid leaching

Acid leaching is similar to acid digestion, but without completely destroying or dissolving the sample matrix. As determination of the (bio-) availability of elements is an important question, acid leaching is also a typical environmental application. For leaching of species from the sample matrix acids with different strengths – preferably a (diluted) mixture of HCl and HNO3 (3:1, aqua regia) – will be used.

Why use microwaves for 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]. 

Figure 1: The spectrum of electromagnetic waves

Microwaves are particularly well-suited for sample preparation tasks due to:

Fast heating rates

With microwaves the conversion of electromagnetic energy into heat energy works highly efficiently and results in extremely fast heating rates – these heating rates are not reproducible with conventional heating. While in conventional heating the heat comes from the outside and goes into the reaction mixture by convection currents (resulting in a very hot vessel wall), microwaves pass through the (almost) microwave-transparent vessel wall and heat the reaction mixture on a molecular basis – by direct interaction with the molecules.

Instant turn-on and turn-off

Microwave radiation can be instantly turned on and off, whereas conventional heating cannot which means the vessels continue to be heated even after the heating source has been turned off. This capability for instant turn-on and turn-off allows the instruments to put in power when needed, but also to shut off in cases of temperature overshoots.

No contact to heating core required

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.

Figure 2: Simplified schemes of the heat distribution in a conventionally heated reaction mixture (a) and in a microwave heated reaction mixture (b)

What to consider in microwave-assisted acid digestion

The most important parameter for acid digestion and acid leaching is the temperature. Pressure is not important; it is something the microwave systems are designed to cope with. High temperature is important for mainly two important aspects: to speed up the digestion reaction and to improve the digestion quality. 

But how can microwaves speed up the digestion time?

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.

Figure 3: The influence of temperature on the digestion performance

Figure 4: Digestion at different temperatures but the same hold time

What happens at lower temperatures?

It is not just possible to substitute high digestion temperature by increasing the run time at lower temperatures as there is a lack of a higher oxidation potential. This is demonstrated well in Figure 4, where the same amount of sample has been digested at different temperatures but the same hold times. 

Pressure and the impact of sample weight

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. 

total pressure = vapor pressure of the acid + reaction pressure from the sample

As a consequence, increasing the sample weight will result in a higher amount of gaseous products, thus limiting the achievable temperature. Sample weight is therefore another very important parameter in an acid digestion.

This short video explains the correlation between temperature and sample weight in a completely closed system.

SmartVent technology, a technology available in some microwave digestion devices , was invented to be able to safely release reaction gases and thus maintain higher temperature levels for better digestion quality. Due to the release of gases, less headspace is required, which allows a compact vessel and rotor design. This approach allows easier handling while increasing the throughput. 

Microwave systems

The cavity is the part of a microwave reactor where the reaction vessel is placed and irradiated. There are different irradiation modes depending on the design of the instrument:

Monomode microwave systems

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.

Figure 5: Schematic illustration of a monomode cavity with a small sample

Multimode microwave systems

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.

Directed Multimode Cavity (DMC)

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). 

Figure 7 Schematic illustration of a DMC (Directed Multimode Cavity)

Pressurized Digestion Cavity (PDC)

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.

Figure 8: Schematic illustration of a Pressurized Digestion Chamber (PDC)

Conclusion

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). 

Figure 9: Time consumption in the analytical process

Thanks to microwave-assisted sample preparation and its fast heating effect and instant on-and-off the run time compared to open-vessel techniques is significantly lower as a higher temperature correlates with a shorter digestion time. There are various systems available for microwave-assisted sample preparation. These differ in the technology and in the setup of the cavity used and provide varying levels of sample throughput.  

References

  1. Cammann, K. (2001). Instrumentelle Analytische Chemie. Spektrum Lehrbuch, p. 3-1.
  2. Bulska, E., Matusiewicz, H. (2018). Inorganic Trace Analytics: Trace Element Analysis and Speciation.
  3. Harris, D. C. (2014). Lehrbuch der Quantitativen Analyse. 8. Auflage, p 796.
  4. https://en.wikipedia.org/wiki/Microwave [Online]
  5. Meyer, H., Riedel, E. (2018). Allgemeine und Anorganische Chemie. 12. Auflage, p. 180.
  6. https://www.techopedia.com/definition/722/waveguide [Online]

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