The selection of a sampling method

The selection of a sampling method is dependent on the concentration and size range of the impurity/contaminant being monitored. Other evaluation criteria include sampling times, air volumes, and allowable outlet pressure restrictions.Sampling equipment should be within calibration, if applicable. Refer to equipment manufacturer’s guidelines for particle size and concentration measurement limits.Any gauges, adaptors, fittings, or tubing used to connect the sampling equipment to the sampling outlet should be clean and particle free, and should not introduce contamination to the collected sample. The preferred connection to sampling equipment should be as short as practical, constructed of stainless steel or other suitable material, and without sharp bends (approximately a 6 inch radius if bends are present), and non-restrictive. Softer metals such as brass or copper can produce particles when joined and should not be utilized.Methods include laser particle counting with a high-pressure diffuser, condensation nucleus counting coupled with scanning mobility particle sizing, differential mobility analysis, and membrane capture as covered in ISO 8573-4:2001 [32]. Other acceptable methods may be available.It is important to note that flow meters, control valves, pressure gauges, quick connect fittings and polymeric tubing can shed particles into the sampling stream. If these items must be used, specify non-shedding materials to minimize the contamination of the collected sample.When closed, valves will capture particles; and when opened the captured particles will be released. Friction on the valve seals will also release particles. If a valve is required for flow regulation, specify a high purity diaphragm or bellows valve; seal material must not be composed of rubber or other polymeric materials, with the exception of Teflon. Valves and regulators should be fully opened to minimize particle shedding as partially opened components will shed more particles.Tubing materials for minimizing particle contamination in order of preference are as follows: stainless steel, conductive polymer, polyester, vinyl (if plasticizer does not interfere), polyethylene, copper, glass, Teflon, and aluminum.