Sample Preparation

From microwave digestion to separation and extraction - our sample preparation systems for analytical chemistry applications optimally prepare your precious samples for analysis. All sample preparation systems are ideal companion devices to our analyzers and spectrometers.

Sample preparation

Established procedures in analytic chemistry have reached an extreme level of sensitivity and precision. Even so, laboratory equipment vendors continue to work on improving efficiency and precision. Analysis itself is not the only lever for improving the quality of results: Sample preparation likewise plays an essential role. Even a highly precise analyzer cannot compensate for an improperly prepared sample. Sample preparation is also a critical factor in the time required for analysis. It typically takes several times longer to prepare the sample than it does to actually run the analysis.¹

Due to its outsize effect on the quality of results, sample preparation requires particular care. Sample preparation includes all the steps that take place in the laboratory before the actual measurement (for example with an ICP-OES machine). The goal: Take a chemically and physically representative portion of the sample and prepare it in such a way that it is suitable for the selected test method. Depending on the composition of the sample and the analytic method, sample preparation can comprise many steps, including

• Creating a solution
• Separating out interfering substances
• Cleaning and enriching the analytes
• Adjusting the concentration required for the measurement

Sample shelf life

After they are extracted, samples are prepared and then sent to the laboratory. These process steps can become staggered out in time if the sample is stored. Systematic errors can occur in sample preparation, which is usually a multi-step process. These errors can be remedied by using an internal reference substance. The substance must be as close as possible to the analyte itself so that it is affected similarly by the sample preparation steps. During the analysis, laboratory staff can compare the amount measured in the reference substance with the amount that was present at the start. The change in concentration can then be applied to the analyte, thus allowing systematic errors in sample preparation to be corrected.

Measurement distortion is also an issue when it comes to the shelf life of samples. Sample shelf life refers to how long a sample remains usable. The determining factor here are the permissible deviations in the measured variable. For example, there are types of samples in which the measured variable changes steadily at a known rate. In such case, it is possible to specify with certainty how long the sample remains usable. This figure must be specific to the sample, as samples and measured variables exhibit great variation. Therefore, a critical factor in result quality is taking the necessary precautions for sample stability. These precautions include selecting the right sample quantity, adhering to transportation and storage times, and documenting processes.

Homogenization

Sampling often involves obtaining significantly larger sample quantities than are necessary for a single analysis. During sample preparation, it is therefore not uncommon to have to take quantities in the milligram range from a sample quantity of several hundred grams. Thus, it is especially important to make sure the sample is homogenous. This means that the analytes are distributed uniformly throughout the sample. Homogenization procedures in the lab during sample preparation include mixing, grinding and sieving.

Filtration / Centrifugation

Samples in solution often include solid impurities. During analysis, these can cause issues such as clogs and blockages at bottlenecks in the analyzers. It is equally important to remove irrelevant substances from the sample which could otherwise complicate the analysis. Filtration and centrifugation serve this purpose within the scope of sample preparation. With filtration, a membrane catches particulate from the solution while the rest of the sample passes through the filter. An alternative method for separating particles from the dissolved sample is centrifugation.

Digestion / Extraction

If the sample is solid, it must be turned into a solution before any further steps. Chemical decomposition is often required for this. For example, aqua regia is often used to prepare mineral and soil samples for analysis using wet chemical digestion. Labs are now adopting microwave-assisted digestion as a significantly simpler and time-efficient method.

Meanwhile, extraction serves to remove undesired dissolved substances from the sample. For this, sample preparation uses liquid-liquid extraction (LLE), solid phase extraction (SPE) and solid phase microextraction (SPME). These sample preparation methods purify the sample, but they can also cause the analytes to become more concentrated.

Purification

Extracts, for example those from solid phase extraction, are not always suitable for analysis directly after. Even after the extraction, there could still be substances in the sample that interfere with the analysis. In this case, column chromatography is one way to further treat the sample. Here it is enough to simply separate the interfering substances from the analytes without having to fully decompose the sample into its components.

Sample preparation equipment

• Extraction system
  Solid phase extraction (SPE) is suitable for preparing samples in the range of a few milliliters for chromatographic analysis, as well as other analytic methods. This process requires only a small amount of solvent, a choice that is both environmentally and economically sound. The basic principle comes from column chromatography. SPE uses a cartridge with a solid phase. The sample is placed on the solid phase and the analytes become concentrated on the phase. The interfering matrix passes unimpeded through the solid phase. Then, the analytes can be eluted from the solid phase using a suitable solvent. When preparing samples in the laboratory, negative pressure serves to draw the sample through the cartridge. Depending on the level of automation, it may also be possible to extract multiple samples at once. A solvent-free alternative is SPME, which can also be automated.
• Homogenizers
  Laboratory machines for creating homogenous samples are designed to transmit energy to samples in the most effective way possible. This energy serves to mix liquids, or to disperse solids within liquids. Homogenizers are also responsible for crushing solid sample components into uniform particle sizes.
• Sample splitters
  Sample splitters (also known as sample dividers) ensure that the sample is representative of its origin material. One type of splitter functions using rotation, in which a bulk material is fed into a hopper, underneath which are multiple steadily rotating basins. The stream of material from the hopper divides equally between the basins, resulting in partial samples with a uniform composition. Suspensions, too, can be divided into representative samples using this equipment.
• Filters / Centrifuges
  Pleated filters or industrial strainers are suitable for large sample quantities. Syringe filters, meanwhile, are often used for small sample quantities. These can be placed on the tip of a syringe. The pressure applied to the syringe plunger pushes the sample through the filter. Labs often feature pressurized filter systems for semi-automatic operation. Lab centrifuges are another alternative that accommodate smaller samples. Centrifugal forces within ensure that solid components of the solution pass to the bottom of the container. A pipette can then be used to remove the liquid from the upper part of the sample container.
• Digestion
  The principle behind digestion is the destruction of the matrix, and with it, extraction of the analytes. Microwave-assisted digestion machines are commonly found in analytic chemistry laboratories. The sample is fully broken down in a microwave with the aid of acid; the microwave rapidly heats samples to high temperatures. Microwave digestion thus a time advantage compared to other analytic methods. The short digestion times ensure high efficiency in everyday laboratory work. Microwave digestion is also suitable for almost all organic and inorganic sample materials.

Sample preparation in various industries

Sample preparation holds great potential when it comes to increasing the efficiency of laboratory analyses. Using the right sample preparation equipment can greatly reduce throughput times. Meanwhile, the right level of automation will combine high quality results with economic advantage. This requires application-specific solutions, however.

Environmental analysis

In environmental analysis, uniformly defined parameters are used to monitor the quality of wastewater and sludge. One example of this is the parameter AOX (adsorbable organic halides). These are hazardous compounds that have toxic, mutagenic and carcinogenic properties. Even when wastewater contains these toxins in small concentrations, they can concentrate in the sludge from wastewater treatment plants. When recycling the sludge, care must be taken not to exceed the limits for AOX.

AOX in sludge is measured according to EN 16166. This method assumes that the substances being sought are first enriched in activated carbon. Then the carbon/sample mixture is combusted, producing hydrogen halides which can be measured with microcoulometry. The right laboratory equipment can make sample preparation in AOX measurements highly efficient.

The analytes are enriched on the activated carbon using the agitation method, in which the carbon, sludge and a nitrate solution must be shaken intensively for one hour. Filtration prepares the sample to be fed into the respective analyzer. The analysis process can be greatly shortened by automating the agitation. An entire batch of samples can be prepared with an orbital shaker. Meanwhile, filtration of multiple samples in parallel saves valuable time during sample preparation in the lab.

AOX in wastewater is measured according to DIN EN ISO 9562. Here, the column method is the preferred method of sample preparation.  The machines from the AOX and AOF sample preparation series can enrich between six and 28 samples simultaneously, thus enabling high sample throughput.

Foodstuffs analysis

Food samples often present stringent requirements for laboratory preparation. From fruit juices to baked goods to dairy products, this industry features diverse samples and matrices. Sample preparation procedures are equally varied. Oftentimes, it is necessary to break apart multiple matrices to get the analytes out of each matrix and put them in a form that can be measured.    Microwave digestion is suitable in many of these cases. Microwave digestion saves a lot of time and effort compared to wet chemical digestion. At the same time, microwave digestion is suitable for almost all samples, meaning that a suitable laboratory device can be used for a wide range of applications. This is especially true of the speedwave XPERT pressurized microwave digestion system. Its extremely long service life and intuitive operation make the machine a mainstay of the efficient laboratory.

Materials analysis

Rising requirements for the sensitivity and precision of analyses drive increasingly stringent standards for sample preparation, too. After all, sample preparation directly affects the quality of results.

Materials analysis tests samples that either serve as raw materials or are considered the intermediate or final product. When analyzing end products, the presence of impurities is the primary parameter of interest for analysis. Impurities can arise during the production process and adversely affect the quality of the product. Monitored elements include hexavalent chromium in toys, for example. This material can be separated from trivalent chromium using a liquid chromatography (LC) system. A mass spectrometer then precisely measures both chromium species in the trace range. This application is well served by the PQ LC Series combined with a PQ MS.

Quality factors in sample preparation

Sample preparation is of central importance for the quality of analytic results. After all, sample preparation steps in the laboratory influence the quality of the sample. Therefore, sample preparation procedures must always be selected with regard to the accuracy of the subsequent analytical method. Problems with sample preparation cannot be remedied later in the process.

That is why purely cost-driven process optimizations can never be allowed to compromise the quality of sample preparation. For example, in foodstuffs analysis, EC Regulation no. 333/2007 mandates that the main requirement for sample preparation is to obtain a representative and homogenous sample without any secondary contamination or loss of analytes.² The following sample preparation quality factors can be deduced from this:

• Mix components in a reproducible fashion
• Avoid contamination
• Digest solid samples
• Separate out interfering substances
• Safely handle small sample quantities

To ensure a high quality level, laboratories must always employ the most appropriate machines in each of the various steps in sample preparation. They should periodically review operating procedures and laboratory equipment for potential optimization opportunities. Cutting-edge sample preparation systems make a significant contribution to quality by avoiding cross-contamination, creating defined and monitored process conditions, and by offering intuitive handling to users.

Sources

¹ https://cem.de/theorie-zum-mikrowellen-aufschluss
² https://eur-lex.europa.eu/legal-content/DE/ALL/?uri=CELEX%3A32007R0333, C.2.1