Thinking Outside The Box (Or At Least Outside The SEM)

By: Barbara Kanegsberg and Ed Kanegsberg
November 2008

“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

SPME
Prior to chromatography, a sample has to be extracted; and it often has to be concentrated. Solid Phase Micro Extraction has been developed as an alternative to exhaustive extraction techniques.³ The sample to be analyzed is adsorbed or absorbed onto a fused silica fiber that has been coated with a non-volatile polymer or a solid sorbent. The theory is that a representative extract can be obtained by understanding the partitioning between the sample and the coated fiber and by extracting under controlled conditions, notably, a constant temperature. The technique has been developed for liquid or vapor samples; and it has been demonstrated for in-vivo sampling. A strength of SPME is in the trace detection of volatile components. SPME is used forensically and in environmental sampling for qualitative or semi-quantitative determination of low levels of molecular contaminants. Examples include arson accelerant detection and trace odor analysis. Those anticipating using SPME, particularly where quantifiable results are needed, must use carefully-controlled conditions; and they will need to assess the variables and variability of the methodology.

PIXE
Moving back to SEM/EDX, particularly EDX, there are other possibilities for elemental analysis. One is Particle (or Proton) Induced X-ray Emission, a non-destructive technique for elemental analysis, including identification and quantification. ⁴ SEM/EDX refers to Scanning Electron Microscopy/Energy Dispersive X-Ray. By the way, EDX is also referred to as EDS (Energy Dispersive Spectroscopy). A SEM/EDX report typically includes a picture of the contaminant in situ along with basic elemental analysis. EDX uses an electron beam whereas PIXE uses high-energy ions, usually hydrogen ions (protons), produced by an ion accelerator. Compared with EDX, PIXE is more sensitive and specific. Further, it is possible to use PIXE at atmospheric pressure whereas SEM typically requires a vacuum; and it is often preferable to perform EDX in a vacuum, particularly for elements of lower atomic mass. There are also spatial limitations with SEM/EDX. This flexibility has allowed PIXE to be used in such areas as art conservation. In manufacturing, PIXE has value in assessing contamination and surface properties of larger objects.

FAVORITE LABORATORY TECHNIQUE
Most of us use favorite techniques to quantify levels of contaminants and to identify contaminants of interest. The method of choice may be specified by national or international specification. The method may be requested or required by an important customer. The technique may be used for historical reasons or it may be commonly used throughout an industry.

Given the complexity of manufacturing, the need to characterize surface properties of medical devices and other high-value products, the need for purity in complex pharmaceuticals, we all need to become more vigilant, creative analytical detectives. We hope you agree that it is instructive to understand the available options. In upcoming columns, we will provide additional detail about some of the techniques described above, and about other analytical tools. Which ones? You can help decide. If there is an analytical technique you would like to see covered in this column, let us know.

Thanks to Neil Spingarn, S&N Labs, for his helpful comments.

References

1. a – c Kanegsberg and Kanegsberg, Chromatography Systems: a) Part I – Chromatography Concepts, A2C2. January 2005; b) Part II – Chromatography Concepts, A2C2, February, 2005; c) Part III – Gas Chromatography, A2C2, March, 2005.
2. Web resources for TLC: a.inst.sfcc.edu/chemscape/catofp/chromato/tlc/tlc.htm#description b.orgchem.colorado.edu/hndbksupport/TLC/TLC.html
3. J. Pawliszyn, SPME Overview, www.science.uwaterloo.ca/chemistry/pawliszyn/Research/SPME/spme.html
4. PIXE tutorial, Harvard University Materials Research Science and Engineering Center, www.mrsec.edu/cams/PIXE.html

Barbara Kanegsberg and Ed Kanegsberg, “the Cleaning Lady and the Rocket Scientist,” are independent consultants in critical and precision cleaning, surface preparation, and contamination control. They are the editors of The Handbook for Critical Cleaning, CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.