Method for analyzing a particle accumulation on a membrane, device for automated analysis and sample preparation unit therefor

Abstract

Method for analyzing a collection of particles (3; 5; 6; 7) on a membrane (1) using an optical microscope (100; 200; 303a), wherein, in order to detect particles (3; 5; 6; 7) of different contrast (light and dark), the permeability of the membrane (1) for light radiation is increased before analyzing the particle collection in such a way that the permeability of the membrane (1) is at least 50% lower before the increase than afterward, and that the analysis of the particle collection for detecting particles (3, 5, 6, 7) of different contrast is carried out under a first and a second illumination condition, wherein the first illumination condition is generated by introducing a first background (106; 106a; 106b) or optical filter into the beam path of the optical microscope (100; 200; 303a).

Description

Technical area

[0001] The present invention relates to a method for analyzing an accumulation of particles on a membrane using an optical microscope.

[0002] Furthermore, the present invention relates to a device for the automated analysis of an accumulation of particles on a membrane, comprising: (a) a sample preparation unit for automated preparation of particle collection, and (b) an optics unit with an optical microscope for automated analysis of the particle accumulation.

[0003] Furthermore, the present invention relates to a sample preparation unit for a device for the automated analysis of an accumulation of particles on a membrane, wherein the sample preparation unit is designed for the automated preparation of the particle accumulation on the membrane.

[0004] The method, device, and sample preparation unit according to the invention can be used for automated particle counting, measurement, and classification, particularly for classification based on metallic luster and / or shape. They are suitable for determining technical cleanliness according to "Testing of Technical Cleanliness: Particle Contamination of Functionally Relevant Automotive Parts. 2nd, revised edition. Berlin: Association of the Automotive Industry, 2015 (Quality Management in the Automotive Industry; 19.1)" and "Standard ISO 16232 2018-12-00. Road vehicles - Cleanliness of components and systems," as well as for determining particulate contamination according to "Standard VDI 2083 Sheet 1 2013-01-00. Cleanroom Technology - Particle Cleanliness Classes of Air."

[0005] The term "membrane" is to be understood in the context of the invention as a generic term for a carrier on whose surface a particle accumulation can be deposited; it includes in particular filter membranes but also adhesive carrier layers, such as those used in sedimentation traps. State of the art

[0006] Determining the technical cleanliness of products or components, as well as rooms or production processes, through particle monitoring is one of the standard tools in quality assurance in many industrial sectors. Optical microscopes are frequently used for this purpose, usually in the form of fully automated particle counting microscopes. The advantage of microscopically determining technical cleanliness is that, in addition to counting particles, it also allows for simultaneous measurement and classification of the particles. This allows for differentiation between metallic and non-metallic particles and / or the shape of the particles to be used as a distinguishing criterion.

[0007] There are standards for determining technical cleanliness. These are defined, particularly for the automotive industry, in VDA Volume 19.1 (Testing Technical Cleanliness: Particle Contamination of Functionally Relevant Automotive Parts, 2nd revised edition, Berlin: German Association of the Automotive Industry, 2015; Quality Management in the Automotive Industry; 19.1) and the ISO 16232 2018-12-00. Road vehicles - Cleanliness of components and systems. In the medical field, for example, by the VDI 2083 Sheet 1 2013-01-00. Cleanroom Technology - Particle Cleanliness Classes of Air. All procedures described in these standards are based on the detection of dark particles on a light background using threshold determination.

[0008] In very few cases is it possible to directly determine technical cleanliness, for example, on the surface of the product or component being examined. Therefore, technical cleanliness is usually determined indirectly.

[0009] For products or components, particle analysis for indirect determination usually involves cleaning the particulate contaminants from the surface to be examined. The cleaned particles are then deposited on a filter membrane, which is then examined microscopically. Cellulose membrane filters or mesh filters made of PET or polyamide are typically used for this purpose. These create a bright, usually white, background under microscopy, which is particularly suitable for detecting dark particles. Since a rinsing fluid is usually used for cleaning, this process is often referred to as extraction.

[0010] To indirectly determine the technical cleanliness of rooms or production processes, sedimentation traps are typically used. These traps bind sedimented particles to an adhesive layer. The sedimentation traps are opened during the application and closed again after a specified period of time. The particles bound to the adhesive layer during the application period are analyzed using light microscopy, usually with an automated particle counting microscope. The adhesive layer is also usually located on a light, usually white, background.

[0011] From DE 10 2005 062 439 B3, a particle analysis system and a particle analysis method are known in which the particles adhering to a surface of the component to be analyzed are removed with a cleaning solution and then analyzed in the filter residue of the cleaning solution with regard to their size, distribution and chemical nature (metallic / non-metallic).

[0012] However, the particle clusters to be analyzed often contain more than just dark particles. In many manufacturing processes, light or transparent particles can also be generated alongside dark particles. Light particles include, for example, plastic or ceramic particles; transparent particles include, in particular, glass particles. Complete detection of these particles against a bright background is difficult using optical microscopy.

[0013] While there are also filters of other colors, such as gray, black, or yellow, that could enable the detection of light-colored particles, all of these filters lead to the same problems as light (white) filters: transparent particles and particles of the same color as the filter are barely detectable. They always have the disadvantage that not all particle types can be detected on a single filter.

[0014] DE 10 2018 207 535 A1 discloses a method for the indirect analysis of the technical cleanliness of an automotive part using a particle counting microscope. In this method, the particles deposited on a filter medium are fixed with an adhesive before being microscopically analyzed. To increase the reliability of the analysis, the analysis is performed using two light sources, one generating visible light and the other UV light. While the use of two light sources may increase the reliability of the analysis, it does not guarantee that all particles can be detected. This applies particularly to transparent particles, but also to particles in filter color.

[0015] US 2015 / 0 211 976 A1 discloses a method and device for determining the dirt particle load of workpieces. In this method, the dirt particles are absorbed in a fluid volume and subsequently deposited on a litmus-coated filter membrane. The dirt particles deposited on the filter membrane are analyzed using an optically magnifying camera. Through the litmus-coated filter membrane, the camera simultaneously determines the pH value of the cleaning fluid, namely based on the discoloration of the litmus-coated filter membrane.

[0016] DE 10 2019 103 551 B3 discloses a method for analyzing a particle accumulation on a filter membrane using a light microscope and an SEM-EDX analysis system. The filter membrane, with the particles fixed thereon, is coated with an electrically conductive mass prior to analysis and then tightened. DE 20 2005 020 273 U1 describes a particle analysis system in which a first and a second image of the same measurement area of ​​the particle accumulation are taken to detect metallic particles. The images differ in that they are generated with differently polarized light.

[0017] DE 10 2018 121 948 A1 teaches a method for fixing particles to a substrate with an adhesive solution, in which the contact of the particles with the adhesive solution takes place via the underside of the substrate through the pores. Technical task

[0018] The present invention is therefore based on the object of providing a method which enables the most complete parallel determination possible of all particles on one and the same membrane and which, moreover, can be carried out simply and inexpensively.

[0019] Furthermore, the present invention is based on the object of providing a device for the automated analysis of an accumulation of particles on a membrane, which enables the most complete parallel determination possible of all particles on one and the same membrane.

[0020] Finally, the present invention is based on the object of specifying a sample preparation unit for the device. Summary of the invention

[0021] With regard to the method, the above-mentioned object is achieved according to the invention on the basis of a method of the type mentioned at the outset in that, in order to detect particles of different contrast, the permeability of the membrane for light radiation is increased before the analysis of the particle accumulation in such a way that the permeability of the membrane is at least 50% lower before the increase than after, and that the analysis of the particle accumulation in order to detect particles of different contrast is carried out under a first and a second illumination condition, wherein the first illumination condition is generated by introducing a first background or optical filter into the beam path of the optical microscope.

[0022] Conventional membranes used for particle analysis have the disadvantage of having an intrinsic color, which prevents the parallel determination of all particles on a single membrane. However, the intrinsic color of the membrane cannot be easily changed without simultaneously affecting other important membrane properties, such as mechanical or thermal stability, porosity, or surface smoothness.

[0023] The mechanical stability of the membrane, both in the dry and moistened state, is necessary to withstand the pressures encountered during a filtration process and to be transportable in both dry and moistened states, for example, to a drying oven. Furthermore, the membrane surface must not be too smooth to prevent particles from "floating back and forth" on the surface during the extraction and analysis process. Furthermore, the membrane porosity should not be too low to ensure a rapid filtration process.

[0024] Any change in these properties will impact the performance of the analysis procedure and the analysis results. For example, common transparent polycarbonate membranes have low porosity and a very smooth surface. Furthermore, their thinness makes them difficult to handle. This is particularly evident when attempting to adhere a polycarbonate membrane to a glass surface. In practice, this often results in adhesion inhomogeneities that significantly interfere with subsequent optical analysis.

[0025] The present invention is therefore based on the idea of ​​increasing the optical transmittance and thus changing, in particular reducing, the intrinsic color of the membrane only before the process step of analyzing the particle accumulation. At this point in time, the accumulation of particles is already present on the membrane. Preparatory process steps such as filtration of a particle-containing rinsing liquid, transferring a membrane to a glass support, or fixing the particles on the membrane have already taken place before the time of reducing the intrinsic color. According to the invention, the intrinsic color of the membrane is reduced by increasing the permeability of the membrane to light radiation. An increase in light transmittance can be achieved, for example, by chemical and / or physical treatment of the membrane.Physically, for example, by filling the pores with a liquid, whereby the refraction of light on membrane components is reduced, or chemically by dissolving or breaking down the membrane.

[0026] Increasing the optical transmittance by reducing the intrinsic color of the membrane only before the process step of analyzing the - optionally pre-fixed - particle accumulation has several advantages: For example, due to the absence or reduction of the membrane's inherent color, any filter or background can be introduced into the beam path. Using different filters or backgrounds makes it possible to examine the same membrane under different conditions, allowing for the parallel determination of particles with different contrasts (i.e., light and dark) on the same membrane.

[0027] A membrane with increased permeability is partially or completely transparent to light radiation. Unlike conventional, non-transparent membranes, this transparency also allows for the analysis of particles located on it using transmitted-light microscopy. Transmitted-light microscopy offers significant advantages in the detection of transparent particles.

[0028] A further advantage is that proven membranes can be used in the preparatory steps for optical analysis. This is simple and cost-effective, as existing knowledge can be utilized and these membranes are inexpensively available.

[0029] In a preferred embodiment of the method according to the invention, increasing the permeability of the membrane for light radiation comprises applying an ionic liquid to the membrane.

[0030] With regard to increasing the permeability of a membrane for light radiation, it has proven particularly useful if the ionic liquid has a melting temperature below the standard temperature, preferably below 15°C.

[0031] Ionic liquids are suitable for both filling the pores of the membrane and partially dissolving or breaking it down. Both of these actions reduce light refraction at the membrane, especially at fibers such as cellulose fibers. The ionic liquid can be applied to the membrane as a pure substance or as a mixture.

[0032] A particularly time- and cost-efficient process is obtained when the application of the ionic liquid is accompanied by a fixation of the particle accumulation.

[0033] The ionic liquid may be suitable for dissolving the membrane, forming a gel-like mass that contributes to the partial embedding and fixation of the particle collection. Thus, the fixation of the particles and the increase in the light permeability of the filter membrane occur in a single process step.

[0034] Particularly good results can be achieved with an ionic liquid if the membrane is a cellulose membrane, especially a cellulose nitrate membrane.

[0035] The use of an ionic liquid also has the advantage that the membrane is optimized for examination of the same membrane with both an optical microscope and a SEM-EDX system by applying the ionic liquid.

[0036] Due to increased demands, more precise material analysis is increasingly required, which is often performed using SEM-EDX (scanning electron microscope, EDX, or EDS (energy dispersive X-ray spectroscopy). SEM-EDX analysis provides more detailed structural information as well as the ratios of the chemical elements. The atoms of the particle sample are excited by an electron beam of a specific energy, causing them to emit X-rays characteristic of the respective chemical element.

[0037] This poses problems when the particles to be examined rest on an electrically non-conductive substrate, such as a filter membrane. Electrical charging of the substrate and particles poses the risk of particles moving or flying off uncontrollably, and electromagnetic fields can deflect the electron beam. Furthermore, spontaneous discharges from the substrate and particles can occur, resulting in a brief signal overload of the imaging detectors. This occurs particularly in SEM-EDX analyses, as the particles must be bombarded with electrons for a relatively long time and in a focused manner to obtain sufficiently high count rates for the SEM-EDX spectrum.

[0038] By making the membrane permeable to light radiation using an ionic liquid, the intrinsic conductivity of the ionic liquid eliminates the need to coat the particle cluster with a conductive coating prior to SEM-EDX analysis. However, this requires that glass covers are not applied during sample preparation, as glass covers are not permeable to the electron beam in SEM-EDX.

[0039] It has proven beneficial if the ionic liquid contains ethylammonium nitrate (EAN) and / or 1-ethyl-3-methylimidazolium acetate (EMIM OAc).

[0040] The ionic liquids mentioned are suitable, for example, for increasing the permeability of cellulose fibers and membranes, and they have a comparatively low melting point. In the simplest case, the ionic liquid is applied to the substrate as a pure substance. However, it has proven particularly effective to apply the ionic liquid in diluted form, preferably as an aqueous solution. The advantage of aqueous solutions is that they can penetrate particularly well into the pores of a hydrophilic membrane and fill them. This facilitates a rapid, homogeneous distribution of the ionic liquid in the membrane and, consequently, a uniform and consistent increase in optical transmittance.

[0041] Advantageously, the ionic liquid is applied to the filter membrane diluted with water, whereby the dilution ratio (ionic liquid:water) in volume fractions is in the range of 1:1 to 7:1.

[0042] If the ionic liquid is diluted more than 1:1, this impairs the permeability-enhancing effect of the ionic liquid. At a dilution of less than 7:1, the effect of adding water is lost. Alternatively or in addition, a filter membrane that is not yet completely dry is wetted with the ionic liquid. The residual moisture in the filter membrane can potentially compensate for a small amount of water added to the dilution.

[0043] Increasing the membrane's permeability to light radiation requires a certain exposure time of the ionic liquid to the membrane. Under standard conditions (SATP conditions), the exposure time is approximately 6 to 8 hours.

[0044] In a preferred modification of the method according to the invention, it is provided that the membrane with the particle collection is heated to a temperature in the range of 50°C to 85°C for a period of 1 hour to 4 hours after the application of the ionic liquid and before analysis and is then analyzed.

[0045] By heating, the exposure time is reduced to 1 hour to 4 hours and the process is accelerated overall.

[0046] According to the invention, the particle accumulation is analyzed under a first and a second illumination condition, and the first illumination condition is created by introducing a first background or optical filter into the beam path of the optical microscope.

[0047] The background is a metallic or non-metallic body, preferably opaque to light radiation, which has its own color.

[0048] The method described above primarily concerns reflected-light microscopy; however, it can also be applied to transmitted-light microscopy.

[0049] In reflected-light microscopy, the background is assigned to the underside of the object. The illumination beam first strikes the top side of the object.

[0050] Due to the increased light transmittance of the membrane according to the invention, the illumination beam at least partially penetrates the object, striking the background. The light of the illumination beam reflected by the object and the background is, at least partially, reflected back into the imaging beam path.

[0051] Two lighting conditions can be created by introducing two different backgrounds, but also with a single background. In the latter case, the analysis is performed once with and once without a background.

[0052] Analysis under two illumination conditions enables the most complete detection of all particle types in a sample, regardless of their contrast. For the analysis of light and / or transparent particles, a background should be inserted that provides good contrast to the light and / or transparent particles. A gray or black background is preferably used for the analysis of light and / or transparent particles. A light, preferably white, background is particularly suitable for the analysis of dark particles. It has proven advantageous to use a non-metallic background for the analysis of dark particles. This makes it possible to continue to differentiate between metallic and non-metallic particles.

[0053] Advantageously, the second illumination condition is generated by introducing a second background different from the first background into the beam path.

[0054] The second background can be specifically adapted to the expected particle spectrum. This allows the illumination conditions and thus the entire analysis to be optimized when the contamination pattern is known.

[0055] When measuring with transmitted light microscopy, there is no background or the background must be transparent; for example, made of glass that is inserted into the beam path between the light source and the object to be analyzed.

[0056] With regard to the device for the automated analysis of a collection of particles on a membrane, the above-mentioned object is achieved according to the invention on the basis of a device of the type mentioned at the outset in that the permeability of the membrane for light radiation can be increased by means of the sample preparation unit in such a way that the permeability of the membrane is at least 50% lower before the increase than afterward, wherein the device is designed in such a way that, in order to detect particles of different contrast, the particle collection can be analyzed under a first and a second illumination condition, wherein, in order to generate the first illumination condition, a first background or optical filter can be introduced into a beam path of the optical microscope.

[0057] Known devices for the automated analysis of a collection of particles on a membrane are also called automated optical microscopes or particle counting microscopes. They can be used for automated particle counting, measurement, and classification based on metallic luster or textile fiber shape. Common membranes used for particle analysis have the disadvantage of having an intrinsic color, which makes it difficult to determine all particles on a single membrane in parallel. However, the intrinsic color of the membrane cannot be easily changed, as any change in the intrinsic color of the membrane would simultaneously affect other important membrane parameters.

[0058] The present invention is therefore based on the idea of ​​modifying the sample preparation unit of the device in such a way that the intrinsic color of the membrane can be automatically reduced before microscopy of the - optionally pre-fixed - particle accumulation.

[0059] According to the invention, the inherent color of the membrane is reduced by increasing its permeability to light radiation. Increasing light permeability can be achieved through chemical and / or physical treatment of the membrane. Purely physical: for example, by filling the pores with a liquid that reduces light refraction at membrane components, or chemical: by dissolving or partially dissolving the membrane.

[0060] Preferably, the sample preparation unit comprises a pipetting unit with which an ionic liquid can be applied to the membrane.

[0061] It has proven useful if the optical microscope has a stage, with the stage being assigned a holder for the automated introduction of a background into the beam path.

[0062] The recording allows for the automated insertion of one or more backgrounds into the beam path one after the other. Two lighting conditions can be created by inserting two different backgrounds.

[0063] Analysis under two illumination conditions enables the most complete detection possible of all particle types in a sample, regardless of their contrast.

[0064] With regard to the sample preparation unit, the above-mentioned object is achieved according to the invention starting from a sample preparation unit of the type mentioned at the outset in that the permeability of the filter membrane for light radiation can be increased by means of the sample preparation unit in such a way that the permeability of the membrane is at least 50% lower before the increase than afterward.

[0065] The sample preparation unit is preferably designed for retrofitting existing optical microscopes. Reference is made to the above comments on the device and method.

[0066] It has proven useful if the sample preparation unit is designed for the automated application of an ionic liquid to the filter membrane.

[0067] Ionic liquids are suitable for both filling the pores of the membrane and partially dissolving or breaking it down. Both of these actions reduce light refraction at the membrane, especially at fibers such as cellulose fibers. The ionic liquid can be applied to the membrane as a pure substance or as a mixture.

[0068] The use of an ionic liquid also facilitates the examination of the same membrane with both an optical microscope and a SEM-EDX system. Definitions and measurement methods

[0069] Individual terms from the above description are defined below for additional information. These definitions are part of the description of the invention. In the event of a conflict between one of the following definitions and the rest of the description, the provisions of the description shall prevail. Light transmittance

[0070] The light transmittance of the membrane at a measuring wavelength is determined by measuring the transmission of the membrane or of a test sample containing the membrane at the measuring wavelength. Since the light transmittance depends on the viewing angle, it is determined in the direction of the surface normal to the membrane surface. For this purpose, a monochromatic light beam with the measuring wavelength and intensity I0 is directed onto the membrane perpendicular to the area spanned by the membrane, and the intensity I1 of the light beam is determined after it emerges. The light transmittance of a membrane is increased if the transmission of the membrane or of a test sample containing the membrane is at least 50%, preferably at least 80%, lower before the process step that increases the light transmittance than after it. Ionic liquid

[0071] An ionic liquid is a salt in liquid form. Ionic liquids are usually organic salts. Ionic liquids are essentially composed of positively and negatively charged ions. background

[0072] For transmitted-light microscopy, transparent objects, such as glass, can be used as backgrounds. For reflected-light microscopy, opaque, non-metallic, or metallic objects are suitable as backgrounds. Standard conditions

[0073] The standard conditions (SATP conditions) are 298.15 K (25 °C, 77 °F) for temperature and 100 kPa (14,504 psi, 0.986 atm) for absolute pressure. Example

[0074] The invention is explained in more detail below using an exemplary embodiment and drawings. The schematic representation shows: Fig. 1 an accumulation of different particles on a filter membrane, Fig. 2 to 5 process steps of a first method according to the invention for analyzing an accumulation of particles on a filter membrane, Fig. 6 to 8 process steps of a second method according to the invention for analyzing an accumulation of particles on a filter membrane, and Fig. 9 an embodiment of a device according to the invention for the automated analysis of an accumulation of particles on a filter membrane, which comprises a sample preparation unit according to the invention.

[0075] To determine the technical cleanliness of a component of a machine element, the component is cleaned with a rinsing fluid. The collected rinsing fluid contains the particles from the cleaning process; it can contain both dark particles and light and / or transparent particles. The particle-containing rinsing fluid is then filtered through a cellulose filter membrane, which retains particles with a particle size above 2 µm. The particles contained in the rinsing fluid are deposited on the top side of the filter membrane.

[0076] It is understood that the methods according to the invention are not limited to the filter membrane type described above, but other commercially available filter membranes can also be used as an alternative filter membrane.

[0077] Fig. 1 shows a schematic view of the filter membrane 1 with a top side 2 and a bottom side 4. This and the following schematic representations are not to scale for illustrative purposes.

[0078] After the particle-containing rinsing liquid has been filtered through the filter membrane 1, several particles accumulate on the upper surface 2, of which only particles 3, 5, 6, and 7 are shown in the figure. Particles 3, 5, 6, and 7 differ from one another in their color and translucency: Particle 3 is a black, dark particle, particle 5 is a white, light particle, particle 6 is transparent to light radiation, and particle 7 is gray. Particles 3, 5, 6, and 7 adhere loosely to surface 2.

[0079] The procedures described below are based on an analysis of the filter membrane 1 from Fig. 1 described.

[0080] The Fig. Figures 2 to 5 schematically show a first procedure in which the particle accumulation is analyzed using a reflected light microscope.

[0081] The filter membrane 1 with the particles 3, 5, 6, and 7 located thereon is first prepared for analysis using light microscopy. In a first step, the particles 3, 5, 6, and 7 are fixed to the filter membrane 1, while simultaneously increasing the light transmittance of the filter membrane 1.

[0082] Fig. Figure 2 shows the provision of a glass support in the form of a slide frame 8, onto which 0.3 ml of a water-diluted solution 15 of ethylammonium nitrate (EAN) is applied, forming a drop 9. The water-diluted EAN solution 15 was obtained by mixing EAN and water in a ratio of 3:1. Alternatively, EAN can also be applied undiluted to the glass support. However, the water-diluted EAN solution 15 has the advantage over undiluted EAN in that it has a lower viscosity. This results in the water-diluted EAN solution 15 being better distributed, especially on hydrophilic filter membranes.

[0083] The filter membrane 1 with the particles 3, 5, 6, 7 located thereon is then placed with its underside 4 onto the drop 9. The water-diluted EAN solution 15 of the drop 9 moves from the underside 4 of the filter membrane 1 through the pores to the top side 2 as a result of capillary force and comes into contact with the surfaces of the particles 3, 5, 6, 7 that are in contact with the filter membrane 1. Due to capillary force and surface tension, the water-diluted EAN solution 15 is drawn slightly upwards along the particle surfaces. The water-diluted EAN solution 15 fulfills two functions. On the one hand, it increases the transparency of the filter membrane 1 by filling the pores of the filter membrane 1 and simultaneously partially dissolving the structure of the filter membrane 1. This reduces the refraction of light at the cellulose fibers of the filter membrane 1 and increases the permeability of the filter membrane to light radiation.On the other hand, the water-diluted EAN solution 15 together with the dissolved cellulose fibers forms a mass 12 fixing the particles 3, 5, 6, 7 on the filter membrane 1, as shown in . Fig. 3. The slide frame 8 is closed with a removable and framed glass cover 10. The glass cover 10 is clipped on. This protects the filter membrane 1 from further contamination. The protective glass of the glass cover 10 is selected so that it does not change the polarization state of the observation light and does not affect the optical analysis under dark-field illumination.

[0084] However, increasing the light transmittance of filter membrane 1 requires several hours under standard conditions. The process of increasing light transmittance can be accelerated by applying heat. At a temperature of 70 °C, approximately 2 hours are required to increase the light transmittance of filter membrane 1. Since the water-diluted EAN solution 7 remains as a liquid in the filter membrane, the achieved transparency is permanent. The light transmittance of the filter membrane achievable with this process step is more than 5 times higher than in its original state and is reflected in a higher transparency that is not just "frosted glass" but sufficient for undisturbed imaging of the particles under an optical microscope, even at high magnification and in transmitted light.

[0085] A filter membrane prepared as described above is referred to as sample 105; it can be analyzed with both a reflected-light microscope and a transmitted-light microscope.

[0086] The Fig. 4 and Fig. 5 show the analysis of the sample 105 using a reflected light microscope 100. The reflected light microscope 100 comprises an optical imaging device 101 for imaging the particle accumulation on the filter membrane 1, an illumination device 102 arranged in a ring around the optical imaging device 101, an optical polarizer 103, an optical analyzer 104, and a holder for a sample to be examined under the microscope, which can be moved in all spatial directions. Fig. 4 and Fig. 5 shows only the sample 105 placed in the holder, the movable holder itself is not shown.

[0087] Below the recording there is an insertion possibility (not shown) for a background 106. Instead, the Fig. 4 and Fig. 5 only the inserted background 106.

[0088] In Fig. 4, the background 106 is a bright (white) background 106a. Brightfield or darkfield illumination can be selected as the illumination type. To distinguish between metallic and non-metallic particles, polarized light and darkfield illumination are used. Against this background, the gray particle 7 and the black particle 3 are easily detectable. In contrast, the white particle 5 and the transparent particle 6 are barely detectable.

[0089] In Fig. 5, the background 106 has been changed. Instead of the bright background 106a, the sample 105 is now assigned a dark, black background 106b. Alternatively, a metallic background can be inserted instead of the black background, which also appears black in linearly polarized light and with the polarizer-analyzer position crossed. Against the black background 106b, the gray particle 7 and the white particle 5 are easily detectable. The black particle 3 and the transparent particle 6, however, are barely detectable.

[0090] In this analysis, the size distribution is determined by counting and measuring the particles. Typically, a qualitative distinction is also made between metallic and non-metallic particles, or a differentiation based on shape to detect fibrous particles.

[0091] By examining the sample under two illumination conditions—i.e., with a white and a black background—particles 3, 5, and 7 can be easily detected. Regarding the transparent particle 6, it is difficult to predict whether this particle will be more easily detectable against a light or dark background. The two illumination conditions increase the probability of detecting the transparent particle 6, thus improving the overall detectability of transparent particles.

[0092] The Fig. 6 to 8 schematically show another procedure in which the filter membrane 1 is placed on a slide 210 instead of a slide mount 8, and ethylammonium nitrate (EAN) 205 diluted with water is applied to the top side 2 of the filter membrane 1. The filter membrane 1 is then covered with a coverslip 11 and analyzed with a transmitted-light microscope 200.

[0093] Fig. Figure 6 shows the process step of applying diluted ethylammonium nitrate (EAN) to the filter membrane 1. To avoid covering particles 3, 5, 6, and 7 with ionic liquid, the application is preferably carried out at the edge or at another location on the filter membrane surface 2 that is not covered with particles 3, 5, 6, and 7, or that is not required for the subsequent analysis. The application is continued until the ethylammonium nitrate (EAN) 205 has spread sufficiently to wet the entire filter membrane.

[0094] As in Fig. As shown in Figure 7, the filter membrane 1 is then covered with a coverslip 11 and stored at 70 °C for 2 hours to increase the light transmittance of the filter membrane 1. The filter membrane 1 thus prepared is hereinafter referred to as Sample 215.

[0095] The sample 215 is then analyzed in an optical transmitted light microscope 200. Fig. Figure 8 shows the transmitted-light microscope 200 used to analyze the sample 215. The transmitted-light microscope 200 has an optical imaging device 201 for imaging the particle accumulation, an LED lamp 202 with a diffuser 203, and a mount for the sample 215 to be examined, which can be moved in all spatial directions. Fig. Figure 8 shows, in simplified form, only the sample 215 placed in the holder; the movable holder itself is not shown.

[0096] In transmitted light, all particles (except flat, transparent ones) cause shadows, as the light is refracted from the beam path. While this method does not allow for the differentiation between metallic and non-metallic particles, it is advantageous for the analysis of particle clusters with transparent particles (e.g., glass beads from blasting material), since glass beads, for example, produce clear shadow patterns in transmitted light due to their refraction behavior, and would be difficult to detect in reflected light.

[0097] Fig.Figure 9 shows a device 300 for the automated analysis of a particle collection on a filter membrane. The device 300 comprises a sample preparation unit 301, a microscope sampler 302, and an optical unit 303 with a reflected-light microscope. The device 300 is designed to enable fully automated sample preparation and analysis of a filter membrane 1 with a particle collection resting thereon.

[0098] The device 300 can be divided into six functional sections. Section I contains a storage container 308 for glass supports 310. To simplify the illustration, the device is described below using the preparation and analysis of a single sample. First, a glass support 310 is automatically fed from the storage container by a transport device 305 to a sample preparation unit 306 in Section II. There, the glass support 310 is sprinkled with 0.3 ml of an ethylammonium nitrate-water mixture 308 (mixing ratio 3:1). The transport device transports the sprinkled glass support to Section III. There, a sample, i.e., a filter membrane, on the upper side of which is a particle accumulation to be analyzed, is applied to the sprinkled glass support 310. This also occurs automatically by means of a sample feeder 304, to which samples can be fed automatically or manually.The samples are stored in the sample feeder 304 under standard conditions until their analysis can begin. When the analysis begins, the sample is applied to the dripped glass support 310, so that the ethylammonium nitrate-water mixture on the glass support 310 passes through the pores to the top side 2 by capillary force and there comes into contact with the surfaces of the particles that are in contact with the filter membrane 1. Furthermore, the sample is covered with a coverslip. The sample is then fed to section IV, where it is heated to 70°C for 120 minutes using a continuous furnace 311. Finally, the sample is fed by the transport device 305 to the microscope sample feeder 302, where it is stored until its microscopic analysis. The microscope sample feeder 302 automatically prepares the sample for microscopic analysis using the reflected-light microscope 303a.

[0099] The reflected light microscope 303a has a field of view between 0.1 mm 2 and 100 mm 2 It is equipped with a digital camera connected to a computer (not shown). The computer is used for the automated evaluation and analysis of images transmitted from the digital camera to the computer. The reflected-light microscope 303a is equipped to enable automated switching between a bright background 304a and a dark background 304b.

Claims

[1] Method for analyzing a collection of particles (3; 5; 6; 7) on a membrane (1) with an optical microscope (100; 200; 303a), wherein in order to detect particles (3; 5; 6; 7) of different contrast (light and dark) the permeability of the membrane (1) for light radiation is increased before analyzing the particle collection in such a way that the permeability of the membrane (1) is at least 50% lower before the increase than after, and that the analysis of the particle collection for detecting particles (3, 5, 6, 7) of different contrast is carried out under a first and a second illumination condition, wherein the first illumination condition is generated by introducing a first background (106; 106a; 106b) or optical filter into the beam path of the optical microscope (100; 200; 303a). [2] Method according to claim 1, characterized bythat increasing the permeability of the membrane (1) for light radiation comprises applying an ionic liquid (15; 205; 308) to the membrane (1). [3] Method according to claim 2, characterized by that the application of the ionic liquid (15; 205; 308) is accompanied by a fixation of the particle accumulation on the membrane (1). [4] Method according to claim 2 or 3, characterized by that the ionic liquid (15; 205; 308) contains ethylammonium nitrate (EAN) and / or 1-ethyl-3-methylimidazolium acetate (EMIM OAc). [5] Method according to one of the preceding claims 2 to 4, characterized by that the ionic liquid (15; 205; 308) is applied to the membrane (1) diluted with water, wherein the dilution ratio (ionic liquid:water) in volume fractions is in the range from 1:1 to 7:

1. [6] Method according to one of the preceding claims 2 to 5, characterized bythat the membrane (1) with the particle collection is heated to a temperature in the range of 50°C to 85°C for 1 hour to 4 hours after application of the ionic liquid (15; 205; 308) and before analysis and then analyzed. [7] Method according to one of the preceding claims, characterized by that the second illumination condition is generated by introducing a second background (106; 106a; 106b) different from the first background (106; 106a; 106b) into the beam path. [8] Device (300) for the automated analysis of an accumulation of particles (3, 5, 6, 7) on a membrane (1), comprising: (a) a sample preparation unit (301) for the automated preparation of the particle accumulation on the membrane (1), and (b) an optics unit (303) with an optical microscope (100; 200; 303a) for the automated analysis of the particle accumulation, wherein the permeability of the membrane (1) for light radiation can be increased by means of the sample preparation unit (301) in such a way that the permeability of the membrane (1) is at least 50% lower before the increase than afterward, wherein the device (300) is designed in such a way that, in order to detect particles (3, 5, 6, 7) of different contrast, the particle accumulation can be analyzed under a first and a second illumination condition, wherein, in order to generate the first illumination condition, a first background (106; 106a; 106b) or optical filter can be introduced into a beam path of the optical microscope (100; 200; 303a). [9] Device (300) according to claim 8, characterized bythat the optical microscope (100; 200; 303a) has a stage, wherein the stage is assigned a holder for the automated introduction of a background (106; 106a; 106b) into the beam path. [10] Sample preparation unit (301) for a device (300) for the automated analysis of an accumulation of particles (3, 5, 6, 7) on a membrane (1) according to one of claims 8 or 9, wherein the sample preparation unit (301) is designed for the automated preparation of the particle accumulation on the membrane (1), characterized by that by means of the sample preparation unit (301) the permeability of the membrane (1) for light radiation can be increased in such a way that the permeability of the membrane (1) before the increase is at least 50% lower than after. [11] Sample preparation unit (301) according to claim 10, characterized bythat the sample preparation unit (301) is designed for the automated application of an ionic liquid (15; 205; 308) to the membrane (1).

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