Ultimately, you will choose a technique based on any of several factors:
Size of contaminating particles
Heterogeneity of particle population
Number of samples/throughput needs
Skill/experience level of the analyst
Need to determine source of contamination
Follow these links for technical information on these methods:
Microscope-Based Techniques Microscope-based techniques, including optical microscopy and scanning electron microscopy, are best for determining the size distribution and nature of contaminating particles and the source of contamination.
Particle Identification This section provides a high-level overview of particle characteristics commonly used to identify contaminants by microscopy and other methods.
Particle Gravimetric Analysis This section describes how to select filters, prepare samples, and calculate results of gravimetric analysis, one the most widely used methods for air and liquid monitoring.