Digestions
Sample digestion
Sample digestion is one method of sample preparation. Instrumental analysis usually requires a pre-treated sample in which the analytes are present in dissolved and measurable form. This applies to both solid and many liquid samples. The aim of sample digestion is to produce the required analytic sample in the lab. As with all steps in sample preparation, errors can occur during sample digestion that affect the sample and therefore also the analysis results. Reliable digestion methods and processes are therefore necessary to ensure high quality results in the laboratory. Optimum sample digestion contributes to another target dimension in the analytical laboratory: economic efficiency. While conventional methods of sample digestion can be very time-consuming, microwave digestion in particular offers very short digestion times, which increases sample throughput.
Acid digestion: Multiple types
When sample digestion is complete, the analytes are present in the sample in fully digested form. Ideally, the matrix is completely decomposed. This is a prerequisite for the subsequent analysis, which can be performed with ICP-OES, ICP-MS or AAS. In addition to converting the analytes into a water- or acid-soluble form, the digestion must also meet the general requirements for successful sample preparation. These include, in particular, avoiding loss of analyte and preventing contamination. From an economic point of view, there is potential for optimization by reducing the time required for digestion. Together with less effort for sample handling due to more automation, the right digestion process makes work in a chemical laboratory more economical.
Acid digestion of a sample can be done in a number of ways. While acid digestion in open vessels can be carried out with comparatively simple equipment, microwave digestion (acid digestion a closed vessel) requires more extensive laboratory equipment. Regardless of the type of digestion, temperature and pressure are the governing parameters for the process.
Acid digestion is carried out with the aid of mineral acids such as nitric acid or hydrochloric acid. To achieve this, the mixture comprising sample and acid is generally boiled over a period of multiple hours. Alternatively to an open container, laboratory staff can also use a closed container. This increases the pressure during digestion, as the steam cannot escape. The reaction velocity increases thanks to the ensuing higher temperature. From an economic perspective, pressurized digestion is preferable to digestion in an open container. In addition to the faster digestion time, a lower risk of contamination is another factor in favor of pressurized digestion.
Microwave digestion
A laboratory microwave unit is another instrument available for optimizing the time required for sample digestion. Vessels for pressure-aided digestion receive thermal energy from electric heating jackets. It can take up to an hour for the sample-acid mixture to reach the desired temperature in an open container. With microwave heating, this time is much shorter. Here, the energy does not take the long way though the container, but instead is transferred directly to the sample. This makes it possible to attain the target temperature for the digestion reaction much more quickly. And because microwave digestion greatly reduces the heating time, the total digestion time ends up being much shorter than with traditional acid digestion.
Diverse sample materials
Acid digestion used various reagents. Which one you choose depends on the nature of the sample at hand. Organic substances can be digested using oxidizing substances such as nitric acid and hydrogen peroxide or sulfuric acid. Organic samples occur mainly in the following areas of application:
In geology and materials science, inorganic samples are the main object of investigation. Various acid mixtures are available for decomposing these samples for analysis. Again, which one you choose depends on the sample. Hydrochloric acid, nitric acid or aqua regia, for example, are suitable for the digestion of pure metal samples.
Digestion machines
Even a sparsely equipped chemical laboratory can carry out digestion. This simply requires an open vessel where the sample and the acid can be brought to a simmer. Acid digestion becomes a bit more complex at the reflux stage, in which the evaporated acid is collected again as condensate. However, for larger-scale applications, sample preparation with microwave machines has become well established. These devices can be divided into three main modules.
Microwave unit
The inside of the microwave unit contains the magnetron, which generates high-frequency waves. To ensure the microwaves spread out evenly, the oven chamber in modern microwave machines has a round shape. Gas released from the containers is siphoned off by a central gas collection system. These gases do not reach the oven chamber. The oven chamber itself can be equipped with a vacuum system that helps the samples cool faster following digestion.
Rotor insert with digestion vessels
Inside the digestion machine is the rotor insert. Laboratory staff can conveniently insert the digestion vessels from above. During the digestion process, the rotor turns the vessels within the microwave field. The vessels themselves play an important role in microwave digestion within the system as a whole. High-quality vessels should be durable. They are made of a material that is invisible to microwave radiation, yet resistant to the acids involved. Containers that can be handled safely and without the need for tools further complement efficiency. Optimized surfaces make cleaning easier and thus help to avoid contamination. These sample digestion requirements are overall best addressed by PTFE and quartz as container materials.
The controller
With microwave digestion, monitoring pressure and temperature in each vessel is critical. During the microwave digestion process, the various samples usually do not behave identical to one another, meaning that each vessel must be considered individually. The speedwave XPERT pressure-assisted microwave digestion system implements this feature with a contactless approach: Optical Pressure Control (OPC). To do this, the specialized vessels are equipped with a glass ring. The glass ring changes its optical characteristics depending on the pressure inside the vessel. An optical sensor reads the pressure inside each individual vessel. This removes the need for a reference vessel to monitor pressure. A rupture disc on each of the speedwave XPERT's vessels provides an additional layer of safety. If the pressure becomes excessive, the disc shatters, allowing the process to be aborted immediately. Monitoring of individual vessel means higher safety and more reliable results.
Both these factors are further supported by the temperature monitoring function. During sample digestion, the speedwave XPERT system continuously records the surface temperature of the vessels, as well as the sample temperature. Contactless methods are used here as well, making it easy to detect exothermic reactions early. If necessary, the machine can regulate (or abort) the digestion process using the parameters of pressure and temperature. This function is possible thanks to the controller, which continuously adjusts the microwave output power. A microwave digestion system is only as quick and safe to use as its controller. That is why the speedwave XPERT offers an external touch controller with a graphical user interface. In addition, the machine can be controlled with a PC or any mobile device. And of course, the controller is not only responsible for safety. It serves to select the right digestion program for every scenario; and it documents process parameters while the digestion is in progress. In this way, it is possible to fully reconstruct how each sample was digested.
Which digestion method for which application?
Microwave digestion is suitable for practically every field of application in analytic chemistry. Despite its advantages with respect to throughput time and process stability, other types of sample digestion are still in use. This is most often the case where maximum sample throughput is less of a priority.
Food and agriculture
One application of chemical analysis is predicting the shelf life of fruits. Their mineral content is decisive for shelf life.1 One criterion is the fruit's ratio of K and Ca. When analyzing fruit samples, digestion is required after homogenization, and microwave digestion is often the sample preparation method used. The measurement itself is then performed using ICP-OES. Microwave digestion can finish the sample preparation step in under an hour. This makes it possible to analyze many samples per day.
Conservation
Environmental analysis often involves measurements of substances in extremely low concentrations. Digestion of samples with aqua regia is very common in these scenarios. It is used, for example, in the analysis of wastewater. This type of digestion can take place in a system with reflux. Here, the acid is heated and channeled past the sample. The digestion system collects the condensate so that less acid is consumed than with a system without reflux. This method of digestion takes several hours.2 However, aqua regia digestion can also be carried out using a microwave digestion machine. Taking a program time of 25 minutes as an example, the throughput time of such samples can be reduced to a fraction of their original time using microwave digestion.
Pharmaceuticals
The European and American pharmacopoeia set out requirements for the level of heavy metals in pharmaceutical products. Their aim is to limit the concentration of these metals to harmless levels. Manufacturers are obliged to test their products for these characteristics. Heavy metals can get into the product through abrasion during the manufacturing process or through the use of certain catalysts for synthesis reactions, for example. Here too, the sample must be converted into a dissolved form during sample preparation. This can be accomplished with microwave digestion, or by incineration. High sample throughput is once again a reason to choose microwave digestion in this scenario. ICP-MS, meanwhile, is the most suitable choice for the analysis itself.
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