“Gee, we ran SEM/EDX, and we couldn’t identify the organic contamination; so we added another cleaning step.” We hear this comment on a regular basis. The reason is that most manufacturers have access to SEM/EDX; and it provides useful information. However, sometimes we expect more from SEM/EDX than the technique is capable of providing, such as molecular identification of a specific organic residue. Attempting to identify an organic compound by SEM/EDX is an exercise in futility. To achieve a quality product, it is important not simply to do critical cleaning but to clean critically—to choose the correct cleaning process. Critical cleaning has to be customized to the residue, to the surface, to the required purity of the liquid or solid product. To customize, we have to understand the contaminants. We have discussed a number of techniques in residue determination and surface characterization in previous columns. Here, we introduce a few additional techniques to get you thinking beyond the SEM.

SFC
It is easier to accurately detect and identify single materials than it is to sort out the contents of complex mixtures. Supercritical Fluid Chromatography is a separation technique. We have discussed chromatography.^1a While chromatography was originally based on visual observation of the colors of the separated materials, chromatography systems include instrumentation for both separation and detection. ^1b Just as in Gas Chromatography (GC) or High Pressure Liquid Chromatography (HPLC), SFC works by differential interaction of components of the mixture between the stationary phase and the mobile phase. In the case of SFC, the mobile phase is carbon dioxide, either liquid or at a temperature and pressure above the critical point, where the distinction between vapor phase and liquid phase disappears. The CO2 may be used alone, or, more commonly, it may be combined with other organic compounds to change the elution profile (the way the mixture separates).

SFC is considered to be a green chemistry because of the reduced reliance on organic solvents. However, for those who consider the concept of a practical green methodology to be an oxymoron, please consider that SFC is commercialized and utilized in applications ranging from purification of pharmaceuticals to analytical testing of petrochemicals. In contrast with GC, SFC inherently operates at lower temperatures; the critical temperature of CO₂ is 31 degrees C, with a corresponding pressure of 72 atm. Therefore, if some contaminants of interest are thermolabile and if they are part of a complex mixture, SFC may be a more effective approach than GC to separate, identify, and quantify the actual contaminant rather than a product of decomposition. ^1c SFC has also been used in separating chiral compounds, those that have both left- and right-handed optical isomers. Because left- and right-handed molecules may differ in biological activity, identifying and separating chiral compounds are of importance in medical and pharmaceutical applications.

TLC
Thin Layer Chromatography is a less sophisticated but potentially powerful separation technique that should be considered for process development and monitoring.² As with other chromatographic techniques, TLC consists of a stationary phase and a mobile phase. The stationary phase is a thin layer of sorbent on a flat plate. The sample is spotted on the sorbent; and the TLC plate is placed in a container of solvent so that the bottom edge of the plate is in contact with the solvent. As the solvent rises up through the sorbent layer, the compounds separate. Sometimes, the technique is run in a two dimensional mode. The mixture is spotted near one corner; and exposed to solvent A. After the initial separation, the plate is turned 90 degrees and exposed to solvent B, one with different solvency properties than solvent A. This allows a second chromatography and additional separation of the mixture.

TLC is a rather humble technique compared with HPLC, Ion Chromatography, GC, or SFC; and it should be considered to be a screening technique rather than a technique that gives complete answers in a full-scale contamination investigation. The separated materials are typically detected visually, so what you see are a series of spots; you do not definitively obtain molecular identification. However, it is sometimes possible to identify a suspect contaminant by its position on the TLC plate. In that sense, you have to know what you are looking for; the contaminant is identified circumstantially. TLC is also accessible in that the investment in equipment is relatively low and, if you do not have immediate access to an analytical laboratory, TLC can be a viable “do-it-yourself” approach. The “do-it-yourself” aspect involves some method development. Even those having access to very sophisticated instrumentation find TLC to be a valuable technique in process development and materials purification