Wipe Tests for Molecular Contamination, Particles, and Fibers

Cleanroom particulate tests and fiber enumeration measure airborne particles, fibers, and other releases from low lint wipes, textiles, or other cleanroom materials.

Particulate test methods describe the extraction and enumeration of particles and fibers released from wipers (or other materials) in a wetted state, under conditions of moderate mechanical stress.

Cleanroom Wipe Evaluation Quick Links

— Particulate | Fiber Testing Methods

— Ionic Contamination, Trace Metals & Heavy Metal Contamination

— Nonvolatile Residue – NVR – in Cleanrooms

— Microbiological & Pyrogen Testing

— Profiles for Medical Device Wipedown & Electronics

— Wipe Testing Standards for ASTM, IEST & ISO Organizations

Liquid Particle Counting (LPC) Wipe Particulate Testing

Test Description: Particle Release from Wipers and Other Materials Under Conditions of Moderate Mechanical Stress

Preparing a sample for particle counting instruments or microscopic enumerations starts in an ISO 5 cleanroom. A controlled environment prevents extraneous contamination from producing false positives. The environment is similar to one required for sterile procedures. A laminar flow hood directs HEPA filtered air outward over the work surface and sweeps away trace contaminants from the working area.

When the LPC is set as outlined above, the unit will withdraw a 10 mL aliquot of fluid from the beaker (using the liquid sampling system that is part of the LPC) and determine the concentration of particles. The unit will repeat this process three more times. The software will discard the first reading and average the second, third and fourth results and report accumulative and differential particles per mL for all the established threshold (channel) ranges.

Texwipe TM22 6.2

Liquid particle counters (LPC) detect light scattered by small particles. Light is then turned into an electrical signal. Particles absorb light, which allows the machine to detect particles by inferencing the returned laser wavelength and light geometry.

Liquid Particle Counter Limitations

Historically, the limitations of particle testing equipment restricted liquid particle counts to 10,000 particles per mL in total. Samples containing larger particle counts required appropriate dilutions.

How Does Microscope Enumeration Count Particulates

A scanning electron microscope allows sample enumeration after agitating and stirring a wipe sample with the biaxial shake methods described IEST-RP-CC004. The sample is filtered and dried before evaluating the fiber counts under an optical microscope.

The use of SEM metrology for particle counting provides a method that is sufficiently sensitive to discern with great accuracy the number of particles released during sample preparation.3–5 However, the issue is how to prepare test samples in a manner that closely approximates the actual conditions of use invited further research.

Evaluating Sample Preparation Techniques for Cleanroom Wiper Testing 1997

Test Method 1.2) Fiber quantification is carried out in two ranges: 20 – 100 µm and > 100 µm. After extraction, fibers are captured on a gridded filter paper, let dry and then counted under an Optical microscope. Texwipe Test Method (TN22)

The challenge of this test is maintaining an ultra-pure extraction system.

Test Method 6.1): If the particles are not uniformly distributed over the filtered area of the filter, a new sample must be prepared. Uniformity of particle distribution over the filter is a must for accurate quantification. Texwipe Test Method (TN22)

Microscopy-based Measurement Technique for Liquid Particle and Fiber Counts

Test Description: Comprehensive Particle and Fiber Testing for Cleanroom Wipers November/December 1998

Electronic microscopes allow the enumeration of particles otherwise undetectable with an LPC for evaluating liquid particle counts, or APC used for airborne particle counts.

Novel wiper contamination tests from PAC vendors such as Texwipe showcase improved methods for enumerating particle and fiber counts within a single sample preparation.

The procedure uses a single sample preparation for the enumeration of all sizes, thereby eliminating the need for separate sample preparations for particles and fibers. The preparation involves immersing and agitating a wiper in a low-surface-tension cleaning liquid and filtering the particle-laden liquid through a microporous membrane filter.

The filter is then mounted on a scanning electron microscope (SEM) stub and first examined for the uniformity of distribution using the optical microscope. Once uniformity is determined, large fibers are also counted during this step. The sample stub is then transferred to the SEM, and particles of different size categories are counted at different magnifications. The accuracy and precision of the resultant counts are measured statistically.