We periodically find ourselves engaged in debates over whether local (in-situ) or extractive techniques are of greater value. We see utility in three sorts of observation: overall scans (often called direct measurements), localized observation, and extractive techniques. Trade-offs include speed, cost, disruption of product flow, and whether the analysis is for general process control or to investigate a specific product performance problem.

DIRECT, OVERALL SURFACE SCANS

Analytical chemists can become enmeshed in pondering small features on a surface; considering the overall appearance of the surface may seem mundane. However, overall surface analysis is a valuable monitoring technique.

Visual observation
Start with the obvious. If there is an apparent change in the surface, further action is called for. Actually, it is wise to be on the lookout for changes in the product or component, and also to be aware of changes in process fluids. Changes in the surface can include color, films, visible particles, and texture. Other changes may be localized, such as corrosion, pitting, and discolorationat intermetallic areas.

Black light
Black light provides increased discrimination of contaminants. Many particulate contaminants will fluoresce when they absorb UV or black light, emittingvisible light that makes the particle shine like a star on a clear night.

Water break/contact angle
A water break or contact angle test can frequently determine if an organic contamination film is present. However, residual surfactants can create a false negative condition, where the contaminant contributes to a favorablecontact angle.

Optically Stimulated Electron Emission (OSEE)
In OSEE, a metallic surface is irradiated with UV light that stimulates emission of electrons through the photoelectric effect. The resulting electron current is a function of surface contamination levels. OSEE can be implemented inan on-line production monitoring station.

LOCALIZED (MICROSCOPIC) SCANNING

Optical scanning
Microscopic scanning includes a large number of techniques. In general, they identify the presence and location of contaminants but not the type of contaminant. The ability to pin-point location or identify contaminant particle size dependson the type of scanning used. Optical scanners range from large wavelength infrared (IR) through visible and ultraviolet (UV) down to the extremely small wavelengths associated with X-rays and even smaller with electron bombardment. The wavelength of the light is not the only discriminator on how precise a location can be scanned; one must also consider how the probe beam is directed and focused. IR scanning (e.g., FTIR) provides contaminant identity information sincedifferent organic molecules absorb different wavelengths of IR light.

EXAMPLES OF MORE COMPLEX SURFACE SCANS
Atomic Force Microscopy (AFM) is not an optical microscopy technique but provides the most precise location scanning. AFM employs a microscopic stylus that can detect movements down to atomic dimensions as it is scanned across asurface, analogous to a phonograph needle on a vinyl record.

Auger Electron Spectroscopy (AES) is particularly valuable for probing oxide layers in a localized, very shallow region of the surface. It is rapid, relatively low in cost, and may provide an early warning sign of trouble. It is a process where electrons stimulate the emission of other electrons; the energy of the emitted electrons can be used to identify the emitting element (contaminant).

EXTRACTION
Extraction provides a means of determining overall contamination where scanning the surface directly is impractical due to product size or configuration. Many analytical tools can be employed directly on the component surface as well as on the extracted material. In extractive techniques, the component is exposed to water or some organic solvent or blend. The extract is concentrated and then analyzed by an appropriate technique. The most common, workhorse techniques are non-volatile residue and par-ticulates. For non-volatile residue (NVR), most often, the extract is taken to dryness and weighed. Alternatively, particles are first separated, typically by filtration and occasionally by sedimentation. In this case, NVR consists of thin-film contamination. Ofcourse, thin film contamination could also include small particles that are nottrapped by the filter or that remain suspended.