Particle analysis – preparation and examination of various sample types

preparation and examination of various sample types

A wide range of different samples can be examined in the GWP. The preparation is crucial here. The following types of samples are usually examined using scanning electron microscopy with energy-dispersive X-ray micro-area analysis (SEM-EDX) for inorganic components and partly using Fourier transform infrared spectroscopy (FT-IR) for organic components as part of ParticleCheck:

• dusts and powders (REM-EDX and FT-IR possible)
• coatings and deposits (REM-EDX and FT-IR possible)
• Particles and residues on filters and membranes (usually only REM-EDX possible)
• Oils and lubricants (particles usually only possible with REM-EDX, oil itself with FT-IR)
• Water and various liquids (such as oils)
• chips and individual particles (REM-EDX and FT-IR possible)

The sample preparation for each sample type is explained below.

dust and powder

Dust is generated in almost all technical areas. This can be abrasion from machines, "normal" ambient dust or other contaminants. As part of ParticleCheck, the material of unknown samples can be analyzed and, with the appropriate background knowledge, assigned. Samples of this type require little preparation. For SEM-EDX analysis, the samples must be conductive. Carbon pads are available for this purpose. They are adhesive on both sides and can be attached to a sample plate. The powder or dust can now be sprinkled or stamped onto the other side. The sample is now ready for analysis; no further preparation is necessary.

Customers have the option of ordering a sampling kit. Stamp samples of dust or powder can be taken on site and the prepared samples sent to GWP. Please feel free to contact us about this.

Dust or powder can also be analyzed for organic components using FT-IR, provided that it can be assumed that the material inside is homogeneous or of equivalent quality. The sample can be applied directly to the device's diamond measuring cell (ATR). This has a radius of 1mm and should be completely covered by dust or powder. Therefore, a sufficient amount is required for FT-IR, whereas individual particles are sufficient for SEM-EDX analysis.

deposits and deposits

The chemical composition or origin of deposits and deposits such as corrosion products is often of particular interest in cases of damage or damage prevention. The preparation is comparable to that of dust and powder. If they are already loose, the procedure is the same as for the "powder and dust" sample type. If they are still stuck to surfaces, you can use a scalpel to try to scrape off some particles and transfer them to a C-pad. Another option is to put the sample in an ultrasonic bath with a solvent. The deposits dissolve in the liquid, which in turn can be filtered through a membrane or a core pore filter (see "Filtration" and "Water and various liquids").

Even deposits and residues can be partially transferred to the ATR measuring cell of the FT-IR, provided that enough material is available and can be scraped off (see “Dusts and powders”).

particles and residues on filters and membranes

In many technical applications, particles are retained in filters. The question often arises as to what kind of materials they are and where they come from. Depending on their size and symmetry, filters and membranes can be examined directly. A small piece can be taken for this purpose. To make it conductive, it is vapor-coated with gold or carbon and fixed to an aluminum sample plate. The sample is then ready for analysis using SEM-EDX.

However, the particles are often stuck between several membrane layers or the mesh or pores. Here, it is possible to prepare the samples in an ultrasonic bath with a solvent. The liquid obtained in this way contains the particles fixed in the membranes and can then be filtered (see "Filtration").

oils and lubricants

A particularly common question is the chemical composition of residues in oils and lubricants. The materials found can provide an indication of the wear and tear of certain components. However, oils and greases are very thick and viscous, so they cannot be easily filtered. Therefore, dilution is necessary.

Oils are pre-diluted using light petrol until the concentration and flow properties are suitable. Lubricants and greases are often more difficult to dissolve. In this case, dilution tests are carried out using various solvents, sometimes acids and bases. A centrifuge is also used to separate the poorly soluble components from the easily soluble ones. A number of very complex work steps are repeated several times until the grease is diluted enough to be able to be filtered. This process is very sample-specific.

water and various liquids

In some cases, water-like liquids are provided, such as washing liquids. If they have a suitable viscosity, they can be filtered without further preparation (see "Filtration"). If they are too viscous, a dissolution attempt must also be carried out until a suitable dilution occurs and the particles of the sample can be recovered.

chips and individual particles

Depending on their size and material, chips and individual particles can first be cleaned (rinsing and dispersing with acetone or ethanol) and then (see “Dusts and powders”) prepared on a conductive C-pad.

If the particles are large enough to be handled macroscopically, they can also be transferred to the diamond measuring cell of the FT-IR and analyzed for possible organic components.

The filtration

Particles can be extracted from liquids using core pore filters or filter membranes. For this, the samples must be as thin as possible. This can be achieved using various solvents, which depend on the matrix material. The liquids are then sucked off via a filter unit using a pumping effect and by achieving a negative pressure. The filter is located on a ceramic or metallic support filter, which prevents the wafer-thin material from being sucked in or destroyed. As standard, we use core pore filters made of polycarbonate that have already been vapor-coated with gold. This eliminates the need for subsequent sputtering to make the samples conductive. The main advantage of this is that the gold adhering to the particles does not influence the element analysis and evaluation too much using the Au peaks measured in the spectrum. The pore size is 0,8 µm, so that even the smallest particles can still be measured. For specific questions, other pore sizes (up to 0,1 µm) or non-vaporized filters (e.g. for gravimetry) as well as various membranes are available.

During filtration, attention must be paid to the particle concentration in the sample. In the case of heavily contaminated liquids, a high number of particles can clog the pores and suction is no longer possible. Therefore, only a small proportion of the sample can be filtered at any one time. This depends on the particle concentration and experience.

After particle extraction, the samples are dried and fixed to an SEM sample plate for analysis using a self-adhesive C-pad.