47


Figure 3. Sections of a field of view frame without (left) and with (right) z-stacking


The first requirement is to hold the filter papers flat, something that can be achieved with a glassless sample carrier plate that stretches prepared filters with a magnetic clamping mechanism (Figure 2).


High resolution is also essential for the successful measurement of particles down to 2µm in size. A combination of high quality microscope objectives and scientific grade cameras is required to ensure that even at such small sizes images contain enough pixels to confidently identify genuine particles.


Where particles cross two or more frames, as is typical for fibrous materials, the image analyser must be able to identify when this occurs. Frames must then be ‘stitched’ together before the whole edge-stitched particle is extracted from the background. Another valuable function is z-stacking, which allows images taken at different focal points to be combined before the particles are separated from the background. Large particles remain in focus, while small ones located in the well of the filter paper are also included in the measurement. Figure 3 shows a section of a frame containing a fibrous particle, with and without z-stacking.


A further necessary capacity is detection of both bright and dark particles. Without the ability to discriminate between the two, single particles with both bright and dark areas (often the case for metals) may be identified as separate particles rather than single entities.


At the end of each analysis the generation of a composite image of the whole scanned area verifies both that the filter was in focus and there were no faults in the paper. In the Morphologi G3 for example, the combination of composite image and X-Y scatter plot allows visualisation of the position


of particles on a filter, and assessment of the filtration quality. The composite image also (Figure 4) provides a permanent electronic record that demonstrates to the regulatory authorities exactly what was measured.


EXAMPLE FPM ENUMERATION


A standard operating procedure (SOP) was set up in the instrument software to analyse filter papers, and to detect and count particles in specific size classes according to the regulations governing the development and quality control of inhaled products. The area for analysis was 10mm in diameter and a circular scan of the whole area took approximately 20 minutes including z-stacking. Figure 5 shows a report generated after the measurement.


Figure 4. Composite image and X position vs Y position scattergram which show the position of particle of interest on the filter


CONCLUSION


The enumeration of FPM on filters is now a necessity in many industries to ensure compliance with a host of universally acknowledged standards and regulations. While manual microscopy techniques have traditionally been used for this purpose, automation provides a user-independent solution that delivers much-improved statistical sampling. Furthermore, SOP-driven analysis facilitates complete transferability of measurement programs.


Results obtained using the Morphologi G3 demonstrate that automated imaging systems for particle characterisation offer a powerful solution for FPM measurement on filters. Within minutes, highly detailed, reproducible and repeatable particle data was generated and automatically classified into user- definable size ranges.


Figure 5. A typical report summarising the data from the analyses.


New Work Describes Characterisation of Mineral-Based Make-Up


A new application note from Malvern Instruments outlines research and methodology for measuring the size, shape and intensity of particles in mineral-based powder make-up, using the Morphologi G3 image analysis based particle characterisation system. The freely downloadable publication ‘Morphologi G3: Understanding Mineral-based Make-up using Size, Shape and Intensity Measurements’, demonstrates how to capture and analyse data for use in optimising the brilliance and coverage of powder makeup.


The Morphologi G3 combines high quality, automated imaging with statistically significant particle shape and particle size measurements delivering more reliable characterisation than is typically obtained with manual microscopy. Images of every particle analysed are retained by the system and can be used to visually inspect the products, thereby providing a qualitative analysis of the sample.


An increasingly popular consumer care product, mineral-based powder make-up is made by blending particles of specific size and shape to obtain a product designed to perfectly hide the fact that it is actually made of discrete particles. The size and shape of constituent particles affect their light dispersing properties, and, in doing so, highly influence final product performance. Overly large particles appear powdery on the skin while those that are too small deliver an insufficient masking effect. Particle shape is equally important. For example, plate-like crystals create a pearlescent effect whereas the size of the plates determines the level of sparkle: Small plates give a more opaque smooth finish, while larger plates add a brilliant spark.


Particle imaging is a discipline that was once labour-intensive and highly subjective because it had to be performed manually. The development of automated particle imaging instruments equipped with integrated computer-controlled dispersion, advanced image processing and statistical analysis tools, such as the Morphologi G3, have taken this informative technique to a new level.


Circle no. 143


Spotlight


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