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Microwave Digestion for ICP Analysis

ICP-OES and ICP-MS are common techniques for elemental analysis. For accurate results, high reproducibility, and good recovery rates, you have to prepare your samples appropriately. The most commonly used sample preparation method for ICP-OES and ICP-MS analysis is microwave digestion. Although we present more detailed information in “A Chemist’s Guide to Sample Preparation” (which you can get for free here), some of the most important parameters for this method are presented below.
 

  1. Acid mixture
  2. Digestion temperature
  3. Sample weight and heating time
  4. Microwave digestion system

Appropriate acids and acid mixtures in microwave digestion for ICP analysis

You only need a few different acids or acid mixtures to successfully perform microwave digestion for ICP analysis. What type of acid you choose depends on the sample type.

Digestion of organic samples

Organic samples need mostly nitric acid (HNO3) for digestion. Nitric acid oxidizes all carbon-hydrogen bonds to form CO2 and H2O as well as nitrous gases according to the following equation:

(CH2)n + 2n HNO3 → nCO2↑+ 2n NO↑ + 2n H2O

In some special cases, you might also need other acids as well:
 

  • Hydrochloric acid (HCl): Sometimes, this needs to be added in order to stabilize the analytes in the solution, especially if you want to analyze elements like Hg, Pb, Cd, and Fe. Otherwise, they tend to adsorb to the vessel wall, which results in recovery rates that are too low. 
     
  • Hydrofluoric acid (HF): This is used if the samples contain silicates. HF is the only acid that can dissolve silicates, so adding small amounts of HF or derivatives might be necessary to completely digest these samples.
     
  • Perchloric acid (HClO4): This introduces even more oxidative power in case HNO3 is too weak  and the sample cannot be destroyed. It is used for samples like graphite, coal, and some types of fuel.

Digestion of inorganic samples

Inorganic samples often require mixtures of different acids. The most common mixtures are the following ones:

  • Aqua Regia: You can use a mixture of 3:1 HCl:HNO3 to digest alloys or some noble metals like gold or platinum, but it cannot digest all noble metals. Aqua regia is also frequently used for leaching (i.e., the extraction of acid-soluble elements from environmental samples).
     
  • Inverse Aqua Regia: This corresponds to a mixture of 1:3 HCl:HNO3 and is less corrosive to the equipment in use. For many applications, you can replace aqua regia with reverse aqua regia.
     
  • Four-Acid-Mixture: Especially for geological and mining samples, you can use a mixture of HCl, HNO3, HClO4, and HF to totally digest minerals.

Furthermore, adding sulfuric or phosphoric acid can also help digest certain inorganic samples like ceramics and oxides (H2SO4) or metals and alloys (H3PO4). 

Sometimes, it can be difficult to find the appropriate acid mixture. So modern microwave digestion systems for ICP analysis feature comprehensive on-screen method libraries, which already suggest acid mixtures and other necessary parameters for the digestion of individual samples. 

Appropriate temperature in microwave digestion for ICP analysis

Temperature is a crucial factor in microwave digestion prior to ICP analysis, largely for two reasons.

High temperature accelerates the digestion rate

The Arrhenius Law states, as a rule of thumb, that a 10 °C temperature increase results in a two-fold reaction rate acceleration. Consequently, higher digestion temperatures result in faster and more efficient digestions (Figure 1).[1]

Figure 1: Approximate time-saving aspect according to Arrhenius’ Law. When the reaction temperature increases by 10 °C, the reaction time halves.

Figure 1: Approximate time-saving aspect according to Arrhenius’ Law. When the reaction temperature increases by 10 °C, the reaction time halves.

You have to keep in mind that when you conduct digestion for ICP analysis in open vessels (i.e., in a hot block or on a hot plate), the boiling point is soon reached. This point limits the maximum temperature in open vessel digestions. 

The boiling point of nitric acid (the most commonly used reagent for microwave digestion for ICP analysis), for example, is 110 °C. If you need to reach temperatures above 110 °C, this cannot be achieved in an open vessel system. For this, you need pressurized systems, which are instruments that safely and efficiently heat digestion matrices under closed-vessel conditions and therefore let you reach higher temperatures and pressures.

This is why modern microwave systems have been developed to support digestion for ICP analysis in an optimal way. These systems are effective heating sources, and their design lets you reach temperatures up to 300 °C and pressures up to 200 bar. They have also been designed to be corrosion-resistant as well as safe and convenient to handle. 

Figure 2 shows a comparison of closed-vessel and open-vessel heating of nitric acid. The figure focuses on the maximum temperature that can be reached.

Figure 2: Nitric acid heated under open-vessel (reflux heating) and under closed-vessel conditions (dedicated microwave system ).

Figure 2: Nitric acid heated under open-vessel (reflux heating) and under closed-vessel conditions (dedicated microwave system ).

While the boiling point limits the temperature in an open vessel setup, the temperature that can be reached in a closed vessel setup – ideally a microwave digestion system – can be far higher and is only limited by the instrumentation that has been used.

High temperature increases the oxidation potential of acids

The oxidation power of acids increases significantly with increasing temperature. This makes digestions, which would not have worked at room temperature, possible.

Check out this episode of Lab Time to see what hot acid does to textiles compared to cold acids.

Figure 3 shows the results of microwave digestion of lubrication oil at different temperatures. With increasing temperatures and therefore increasing oxidation potential of the used acid, the residual carbon decreases. When the sample is digested at 260 °C, this leads to a clear, colorless solution, which indicates that the oil has been perfectly decomposed.
 

Figure 3: Digestion of lubrication oil at different temperatures.

Figure 3: Digestion of lubrication oil at different temperatures.

Different samples require different temperatures for complete microwave digestion. Here are a few examples:
 

  • Organic samples
    Aliphatic hydrocarbons (including hetero atoms) require temperatures up to 200 °C.
    Aromatic hydrocarbons (including hetero atoms) require temperatures up to 250 °C.
     
  • Inorganic samples 
    These often need even harsher conditions: up to 280 °C for one or two hours.

Appropriate sample weight and heating time in microwave digestion for ICP analysis

When performing microwave digestion for ICP analysis with high amounts of a reactive sample, exothermic behavior during the digestion can lead to explosions if the instrument cannot control the immediate pressure increase. That´s why modern vessel designs provide overpressure release mechanisms (SmartVent Technology) to prevent parts from breaking, while letting you use higher sample amounts. 

But there are still maximum sample weights and heating times to be observed. Check out this Lab Time episode to see what happens in a kitchen microwave oven (with no safety features and pressure control) with overpressure in a vessel. 

With this video in mind … you need to optimize the sample weight and the heating profile. In general, there is one simple rule: If you are not sure about the reaction behavior of the sample, start with:

  • A very low sample weight (100 mg to 300 mg)
  • A flat heating ramp (e.g., heating to the target temperature within 20 minutes to 30 minutes)
  • A lower target temperature
  • And with small amounts of distilled water (0.5 mL to 2 mL) added to the acid mixture

Since there are so many different parameters you have to consider for microwave digestion for ICP analysis to be done successfully, modern microwave digestion systems feature pre-installed or online method libraries, which provide the parameter settings that you need, depending on your sample.

Can standard methods be applied in microwave digestion for ICP analysis?

For many analytical tasks, there are standard methods, such as ASTM, ISO / EN ISO, EPA, and USP.[2] Laboratories, charged with the responsibility of producing data used for regulatory purposes (e.g., in environmental protection or for medicinal applications), have to adopt these standard methods.

Such methods go through a long process of discussion and validation, and contain numerous rules for their application. The right standard must be used correctly and be in accord with the analytical task at hand. 

Common standard methods can be successfully performed in microwave digestion systems. Because of this, pre-installed libraries and online libraries feature standard methods that are fully complaint with the respective norms.

Where to find further information about microwave digestion for ICP analysis?

Click here to browse through a large number of application reports and sample analyses for microwave digestion for ICP analysis.

You can find detailed information about how to get started with microwave digestion in the book “A Chemist’s Guide to Sample Preparation” as well as on this website.

Instrumentation for microwave digestion for ICP analysis

There are a few easy-to-digest samples and easy analytical tasks for which it is enough to just boil the samples in acid in an open vessel setup. However, the vast majority of samples require higher temperatures between 160 °C and 280 °C. They are typically digested in dedicated microwave digestion instruments.

Here is a summary of the most important advantages of microwave acid digestion:

  • With microwave heating, you can reach your required temperature quickly. Because you can switch the microwave on and off instantly, you can precisely control the digestion temperature. This is particularly relevant in order to handle instant exothermic reactions safely.
  • Microwave digestion systems are designed to withstand high pressures and temperatures (up to 200 bar and 300 °C). This ensures maximum efficiency and top digestion quality. After the digestions are completed, microwave digestion systems actively cool down the vessels so you can handle them safely.
  • Since high temperatures increase the oxidation potential of acids, the quality of the digestion and of the analysis is significantly improved. Less residual carbon from organic samples reduces spectroscopic interferences.
  • In almost every case, there is a requirement for digesting several samples (and blanks) in parallel. Due to the common rack- or rotor-type systems, you can digest multiple samples and blanks at the same time.
  • Dedicated microwave digestion systems feature an built-in method library that suggests the sample weight, acid mixture, and a temperature/time program to successfully digest various samples.

All of this reduces the effort, time, and cost of the sample preparation step and leads to faster, more accurate analytical results. Click here for details about microwave digestion instruments.

Note

The content listed above is also valid for sample preparation for AAS analysis as well as for various sample types. 

References

[1] Stadler, A., Michaelis, M. (2021). A Chemist’s Guide to Sample Preparation Anton, 3rd Edition. Anton Paar Publishing, Graz: page 21, https://www.anton-paar.com/corp-en/a-chemists-guide-to-sample-preparation/

[2] The most-common standard methods for microwave digestion and leaching are listed in the appendix of: Stadler, A., Michaelis, M. (2021). A Chemist’s Guide to Sample Preparation Anton, 3rd Edition, Anton Paar Publishing, Graz,  https://www.anton-paar.com/corp-en/a-chemists-guide-to-sample-preparation/

 

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