Spotlight Particle Characterisation


The number of international standards relating to Foreign Particulate Matter (FPM) continues to grow steadily, reflecting an increasing awareness of the importance of FPM in numerous industries. Products as diverse as hydraulic fluids and pharmaceutical inhalers are now covered by regulations that adopt or refer to such standards.


This article presents data from an automated image analysis method (Morphologi G3, Malvern Instruments, Figure 1) for the fast, accurate, and highly repeatable measurement of FPM contamination on filters. By automating analysis, removing operator subjectivity and enabling the rapid classification of particles into specified size brackets, image analysis delivers significant methodological improvements over conventional manual microscopy.


Automated Detection of Foreign Particulate Matter on Filters


AUTOMOTIVE COMPONENTS


Modern automotive fluid components and systems are highly sensitive to the presence of residual particles from manufacturing and assembly processes. Such particles increase wear rates and enhance the likelihood of system failure, and their strict measurement and control falls within the ISO 16232 series of standards. These enable a company to verify that automotive components meet specific levels of cleanliness.


Figure 1. The Morphologi G3 automated particle characterisation system combines light microscopy and image analysis


DEFINING FPM


“United States Pharmacopeia (USP) include USP 788 that specifies acceptable levels of FPM in injectables, and USP 789, which defines limits for ophthalmics.”


The term FPM describes extraneous contaminant particles such as glass, transparent synthetic fibres, stainless steel, rubber, aluminium and plastic that can be introduced at any stage in a manufacturing process or during operation. Such contamination potentially has serious implications. The direct health consequences of particulate contaminants in injectable drugs, for example, include blockage of blood vessels and inflammatory or allergenic reactions. How much impact particulate contamination has in any situation depends on a variety of factors, including the chemical composition of the particles, their size, number and rate of introduction.


PARTICULATE STANDARDS


Standards and regulations that have been established to address the issue of FPM contamination are there to ensure quality and safety, and to provide an internationally recognised universal reference system for manufacturers and users. This allows comparison of particulate contamination data and the exact specification of required levels of cleanliness.


HYDRAULIC FLUIDS


In hydraulic systems, power is transmitted and controlled through a liquid under pressure that acts both as a lubricant and power transmitting medium. Particulate contamination in the liquid can interfere with its ability to lubricate and is often the primary cause of wear-related damage, ultimately leading to machine failure and downtime. The most widely used international standards for industrial fluid power systems are: ISO 4407, which specifies how optical microscopy should be used to analyse the fluids; and ISO 4406 which is the standard for coding the level of particles larger than 5µm and those greater than 15µm allowed in a 1ml fluid sample. Another standard, ISO 11218, also relates to the cleanliness classification of hydraulic fluids but applies to aerospace applications.


Compliance with ISO 16232 requires manufacturers to enumerate and record particles 5µm or larger. Again, optical microscopy is identified as being an efficient and accurate method for particle analysis. Analysis of the component cleanliness in fluid circuits is described also in the VDA 19 directive, which establishes a process for quantifying contamination arising during manufacture. It applies to automotive components used in a variety of fluid systems and the directive specifies methods for both particle extraction and subsequent analysis.


PHARMACEUTICALS


The presence of FPM is especially a concern in the pharmaceutical industry where extremely rigorous checks are required. Standards published by the United States Pharmacopeia (USP) include USP 788 that specifies acceptable levels of FPM in injectables, and USP 789, which defines limits for ophthalmics. The US Food and Drug Administration requests that FPM in a wide range of pharmaceutical inhalation devices is subject to quality control limits in defined size ranges. The International Pharmaceutical Aerosol Consortium on Regulations and Science has also discussed the testing for FPM in orally inhaled and nasal drug products.


MEASURING FPM


Conventionally, FPM analysis for compliance with ISO and other standards has relied on the use of manual light microscopy to count contaminant particles collected on filters. However, the many limitations associated with this method mean that it is less than ideal. In particular, it is prone to human error and operator subjectivity, especially given the considerable length of time required to manually count sufficient particles.


Now, however, advances in image analysis techniques are making it possible to automatically classify particulate contaminants in fluids such as hydraulic transmission and fuel injection systems. Automated imaging systems for particle characterisation offer a user-independent solution that combines light microscopy and image analysis with new levels of speed and objectivity. Importantly, there are now systems that allow the analysis of FPM directly on the filter used to collect it.


Author Details:


Ulf Willén & Debbie Huck, Malvern Instruments Ltd Enigma Business Park Grovewood Road Malvern, Worcestershire, WR14 1XZ, UK Tel: +44 (0)1684 892456 Fax: + 44 (0)1684 892789 www.malvern.com


Automation unquestionably overcomes the drawbacks of conventional manual microscopy: automated systems measure hundreds of thousands of particles within minutes, providing statistically relevant data; operator bias and human subjectivity are eliminated; and the use of Standard Operating Procedures (SOP) gives complete transferability of analysis methods - from the research laboratory to the QC department for example. In addition, the high-resolution images generated allow advanced particle characterisation in terms of the size and morphology of individual particles. Such information allows detailed investigation and effective troubleshooting.


AUTOMATION SOLUTION


Figure 2. A glassless sample carrier allows two filters to be mounted at once, for successive analyses without manual intervention (Morphologi G3)


Given the advantages, it is unsurprising that there is a trend away from manual measurement of contamination levels towards automated inspection of filters. Fully automated imaging systems do however need suitable hardware and software in order to perform the specialised task of measuring FPM on filters.


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