What is a sample preparation process?

Sample preparation refers to the series of actions taken to transform a sample from its original state to a form that is suitable for analysis in a laboratory setting. The exact process can vary widely depending on the type of sample and the analysis to be performed. However, the main goal of sample preparation is to isolate the desired analytes (the components of interest) from the sample matrix (the bulk of the material), remove any contaminants or interfering substances, and convert the analytes into a form that is compatible with the analytical technique to be used.

1. Sampling and sample division

To ensure reproducibility in sample preparation, it is essential to extract a sub-sample that truly represents the bulk material. This means the sub-sample must share the same properties as the bulk. Given that most samples are inhomogeneous mixtures, care must be taken to avoid segregation due to varying particle sizes, especially during transport. If the entire sample isn't processed, a representative portion must be taken. The quantity of the sample is crucial; it must be sufficient for analysis and proportionate to the total sample size and grain size. These factors dictate the minimum quantity needed for the sub-sample to accurately reflect the bulk.

Some important questions should be clarified in advance:

a. Which quantity is required to ensure representativeness of the original sample?
b. From which part of the original material should the sample be taken?

A. 1. Which quantity is required?

Some industry-specific standards provide guidelines and directions on the correct sampling process, for example DIN 51701-2:1985 in the coal industry. It contains, for example, the formula

G [kg] = 0.07 [kg/mm] x z [mm]

which indicates how much sample “G” must be extracted from a bulk sample with maximum particle size “z” to obtain a representative quantity. Taking a coal sample with a maximum particle size of 5 cm as an example, the following calculation applies:

G [kg] = 0.07 kg/mm x 50 mm
G = 3.5 kg

Consequently, the extracted sample amount for a sample with max. 50 mm particles should be at least 3.5 kg to make sure it is representative. For other materials than coal a different density should be taken into account.

B. 2. From which part is the sample taken?

The sub-sample taken from the original sample must accurately represent the entire bulk, which requires careful consideration of the sampling location (middle, top, bottom, inner, or outer part of the heap). Factors such as moisture content can vary within the heap, potentially affecting analytical results. For example, the inner parts may be moister than the outer parts. Additionally, particle composition may differ between the top and bottom due to segregation. Sampling large volumes, like ship or train loads, poses challenges. To achieve a representative sub-sample, it's necessary to collect samples from multiple locations and combine them. Alternatively, samples can be drawn directly from the material flow in a production unit.

1.1. Sample Dividers

Once the sample is in the laboratory, the sample amount may be too large for the subsequent steps in the sample preparation process. Therefore, further sample division may be required. The selection of the division method and the instrument depends on the sample material and quantity. Dry, free flowing samples can be fed via vibratory feeders to rotary tube dividers and sample dividers with a rotating dividing head.

1.2. Sample Splitter

Sample splitters are used for materials with a low flowability. Manual random sampling is only acceptable if the sample is homogeneous with regards to material and grain size. However, without preliminary examination, this is difficult to ascertain.

The importance of sampling is demonstrated in the figure: Even if the analysis is carried out correctly, random sampling (e.g. with a scoop) leads to varying results which are not reproducible although the samples come from the same initial material. As shown, three different samples taken from the same initial material show variations of up to 20 % for the fraction below 2 mm. Therefore, it is essential that sampling is carried out with utmost care.

Professional sample dividers with a marginal standard deviation should be used for the extraction of representative sub-samples. The figure shows the qualitative sampling errors of the different methods. It can clearly be seen that rotary tube sample dividers produce the smallest qualitative variation (A). They achieve the highest degree of reproducibility and are clearly superior to all other methods.

3. Metal Separation and Sieving

Samples such as industrial waste, recyclable waste and secondary fuels often contain metallic components which cannot be pulverized with laboratory mills. On the contrary, metallic objects such as steel nails or iron screws can damage the grinding tools which can lead to a considerable deterioration of the mill's performance. Therefore, it is necessary to separate the metal components before switching to the next step in sample preparation: grinding. If required, the fractions have to be evaluated separately. Undesired particles like metal parts can also be removed by sieving, if their sizes differ from the particles which need to be analyzed. Here, Retsch sieving machines are the devices of choice. RETSCH sieve shakers and sieving machines not only cover a wide measuring range. Thanks to different sieving motions and sieve sizes, the RETSCH sieve shaker range is suitable for almost any bulk material. Our sieve shakers and sieving machines produce exact and reproducible results and comply with the requirements for the test materials monitoring according to DIN EN ISO 9000 ff.

5. Retsch benefit for the extraction step

The chemical extraction of analytes from ground samples is a pivotal sample preparation step in analytical chemistry, environmental science, and biochemistry, aimed at isolating specific compounds for detailed analysis. This process is essential for monitoring environmental pollution e.g. with pesticides, assessing nutrient levels in soils, detecting hazardous substances, and conducting research in various scientific fields.

The choice of extraction solvent and technique is critical and is determined based on the nature of the analytes of interest and the matrix of the ground sample. Common techniques include solid-phase extraction (SPE), liquid-liquid extraction (LLE), solid-phase microextraction (SPME), and supercritical fluid extraction (SFE), among others.

The extraction process typically involves the solubilization of the target analytes into a suitable solvent, separating them from the solid matrix and other non-target components. Following extraction, the analytes may undergo further purification and concentration steps before analysis using techniques such as gas chromatography (GC), liquid chromatography (LC), or mass spectrometry (MS) to quantify and identify the compounds of interest.

The QuEChERS method, an acronym for "quick, easy, cheap, effective, rugged and safe," streamlines sample preparation for pesticide residue analysis. Testing has shown that results from the QuEChERS method are comparable to those from other methods. During homogenization, it's important to keep the sample from overheating, as some pesticides are volatile. Cooling the sample not only prevents this but also enhances the material's breaking behavior, leading to better fineness and homogeneity. The extraction process involves taking 10 g of the food or soil sample and extracting it with 10 ml of acetonitrile. To eliminate ghost peaks in chromatograms, the extraction is performed with a salt mixture, typically sodium chloride and magnesium sulphate in a 1:2 ratio. To transfer pesticides into the organic phase, the sample is shaken with the acetonitrile and salt for 1 to 3 minutes. A laboratory mill, like RETSCH’s Mixer Mill MM 400, can be used to agitate the mixture in a 50 mL Falcon tube at up to 30 Hz, ensuring thorough and reproducible mixing, which is beneficial for the extraction process.