Comprehensive, Non-specific Methods
Extractive gravimetric methods, for example, are comprehensive but very non-specific techniques to determine contamination by weighing. Examples include the newly proposed ASTM method for detecting contamination levels in permanent implantable metal devices and most non-volatile residue (NVR) methods. Weighing the residue detects almost everything that can be extracted from a component, including, potentially, some of the component substrate as well.
These methods do not detect volatile contaminants (important where adsorption to the component is an issue). Like cleaning, extractive gravimetric techniques are dependent on the specifics of extraction including time, temperature, cleaning force, and the solvent used. Further, the technique of evaporating the mixture extracted into the solvent prior to analysis can influence the weight of the residue. Typically, heat is used for evaporation. With high-boiling solvents, high heat can result in a caramelized mixture of solvent residue and contaminant, resulting in a falsely elevated contamination level.
Gravimetric methods are comprehensive in that an array of organic and inorganic contaminants, thin film, and particulate, are detected. With appropriate and consistent techniques they can be accurate and show high inter-test precision. While gravimetric techniques do not elucidate the specific contaminant, they are useful as a relatively rapid early warning technique. Similarly, OSEE, a non-extractive technique for overall surface residue can detect essentially everything on a relatively flat surface, but there is no discrimination or identification. Contact angle determination can also be a good early warning signal of contamination. For all such methods, the technique and limitationsmust be well understood1.
Separation and Detection
The past three columns were devoted to an overview of chromatographic methods. Current chromatography systems are “black boxes” consisting of an injection port, a column, and a detection system. It is important to keep in mind that the power of chromatography is as a separation technique. Early forms of chromatography were also detection methods in that one could visually see many of the separated bands on the column. While some identification could be made based on elution time or position on the column, in order to definitivelyidentify chemical species, appropriate detection techniques are required.
Detectors are quite varied; there is no “universal” detector. A typical particle counting system discriminates by size, but does not characterize the particle. For characterization, other detection methods such as FTIR are needed. Then there is the issue of how specific a detector is and how quantifiable the results are. For example, FTIR identifies contaminants by circumstantial evidence. However, limited identification and quantification can be achieved with FTIR with careful data analysis. FTIR can help track the usual suspects,without the cost and time associated with more definitive techniques
Limited, Discriminating, or Standard Methods
We should also remember that some detectors are pre-set to detect a subset of chemical species. This is particularly true for field testing techniques. There is the temptation to use a detector used by the facilities department to detect refrigerant leaks to test for other volatile contaminants; this may not provide accurate information.
Standard methods provide a common basis of understanding and potentially lower undesirable inter-laboratory variability; a test performed in Cleveland can be readily compared with one obtained in San Francisco. The problem is that there is a tendency to look at such standardized methods as universally applicable, as absolute, as infallible. However, standard methods may not be applicable to your particular set of contaminants. Whether you are considering an EPA method, an ASTM method, or any other recognized test method, you have to understandthe purpose and limitations of the specific standard method.
Standard tests are often optimized to detect a particular category of contaminants present on a particular matrix. For example, some methods geared to environmental contaminants are based on extraction with a single solvent (sometimes the late, lamented CFC-113). If the standard extraction solvent is non-polar, like hexane, while the suspect contaminants are relatively polar and do not readily dissolve in hexane, the method may be performed correctly, but the contaminants will be missed. Standard gas chromatography methods may employ a pre-set injection temperature, perhaps because there is no concern with detection of materials above a certain boiling point. If you have a mixture containing high boilers and want to know what is actually present, the standard method will not providecomplete results.
1 B. Kanegsberg & M. Chawla, “Measuring Thin Film Surface Contamination”, A2C2™ Magazine, September, 2001
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press.Contact them at BFK Solutions LLC., 310-459-3614; email@example.com; www.bfksolutions.com.