Sometimes, it is possible to select the appropriate extraction agent (or cleaning agent, for that matter), by simply verifying the solubility of the soil in the extraction chemistry. However, soils may change chemically during the manufacturing process; or there may be mixed soils. Particles, particularly particles below one micron, are difficult to remove. For all particles, adherence may be both physical and chemical. Therefore, there is no substitute for pragmatic evaluation of extraction agents and protocols.
Swabbing can be effective for readily-accessible surfaces. Because, swabbing is operator-dependent, qualification of swabs, techniques, and performance is essential.
Extraction of complex surfaces requires a variety of techniques. For example, transfer lines and tubing may have to be extensively flushed. Complex, ornate components may require soxhlet extraction, where the component is placed in a vessel and held in the vapor zone of a solvent. Such extraction is analogous to vapor phase cleaning/degreasing, but the extraction typically takes place for extended periods of time. Ultrasonic extraction may also be used. However, as with ultrasonic cleaning, more extraction is not always better. The part may itself erode; large particles can break down. Excessive force may produce artifactual suspensions.
From: The Right Extraction