FILM AT 11 (ANGSTROMS)
In cleaning work, we hope to leave no film of residue. Practically, we hopethe residue film is no thicker than angstroms, nanometers, or submicrons.

When that practical hope is fulfilled, we need to both quantify the mass of that residue and in some cases identify it chemically.

This column is about how grazing angle FTIR spectroscopic technology can fulfill that need.

SPECTROSCOPY 101
A spectroscope measures the intensity (energy) of reflected radiation (signal) over a range of frequencies (wavenum-bers). Spectroscopy is the optical and mathematical analysis of those results (spectra). Obviously, the analysiswill be more precise when the signal is more intense.

Precision is compromised when the amount of sample is small. In this case, when the residue film is thin, the radiation beam doesn’t contact many molecules of residual soil. So, its information content, relative to a control or reference scan, is weak. Soil identification and assay are poor.

GRAZING ANGLE SPECTROSCOPY
The situation was improved after recognition that for a given input radiation signal and pristine substrate, the detected signal would be proportionalto the cosine of the angle between the radiation beam and the substrate.¹

With normal FTIR analysis of p-polarized UV light, the beam is aimed 10 to 30 degrees from perpendicular. Grazing angle FTIR uses a beam aimed 70 to 80 degrees from perpendicular. See Figure 1.

Information content of the radiation, relative to a reference scan, is substantially enhanced because the path of the light beam is extended and more molecules of soil are contacted by it. Here, the shortest distance is not preferred. Minimum film thickness can be around 1/4 of a nanometer. As with detection of nanoparticles, one has to know where to look as the sample size (area) is tiny — around 10 –⁴cm².

Figure 1: Grazing Angles

ROCKET SCIENCE
The above explanation is not rocket science. What’s been developed since the mid-1990s, and especially in this decade, is equipment to take advantage of this realization in cleaning operations. One can’t do this with a flashlight and a make-up mirror. Considerable miniaturization and weight reduction has been necessary to make practical, durable, and reliable grazing angle FTIRdevices for cleanliness verification. Yes, they are used by NASA.

CONTROLLED ENVIRONMENTS
As shown in the figure, the radiation path includes both the thin film and the environment surrounding it. Likely, tramp H₂O and CO₂ will be recognized in the FTIR analysis. Consequently, work is done in a vacuum, a controlledenvironment, or where continuous purging is done with dry N₂.

Work has also been done to detect biological residues.^2,3 Scanning over hundreds of wavenumbers allows improvement of resolution if proper calibration is done. Also, H₂O and CO₂ have been mathematically subtracted from the test sinal using valuesfrom the reference signal (scan) or by data mining techniques.

Grazing-angle FTIR is not a secondary analysis scheme — it is residue on the native surface that is examined (“in-situ”) and not residue removed with a swab. Further, like Optically Stimulated Electron Emission (OSEE),results are immediate.

AIRPLANE WINGS
How then would one verify cleanliness of an airplane wing? If the residue film was purely organic and micrometer-thick, one would probably use OSEE. The U.S. DoD has favorably evaluated⁴ one model of a portable FTIR system, but it is not believed that this technology has become “military standard.” Detectionof organic films was claimed at less than 1.0 microgram/cm².

MORE GRAZING NEXT MONTH
This technology is worth your consideration if cleanliness validation is vital. It isn’t worth consideration if you want to check external surfaces of vessels, floors, or workspace. In next month’s column, we’llreview costs and accomplishments.

References

1. Irabella:, F.M., Modern Techniques in Applied Molecular Spectroscopy, John Wiley & Sons 1998, pp. 102, 106, and 283.
2. Validation Using Mid-IR Fiber-Optics. BIOPHARM, May 2002, pp. 36–42, 71.
3. Reddy, K.B. and Teelucksingh, N. “Quantification of Active Pharmaceutical Ingredients on Metal Surfaces Using a Mid-IR Grazing-Angle Fiber Optics Probe —An In-situ Cleaning Verification Process,” Spectroscopy, October 2005, Vol. 20, No. 10.
4. Hoffard, T.A., NASA Report #20040055567, Grazing-Angle Fourier Transform Infrared Spectroscopy for Surface Cleanliness Verification, Mar. 2003, Reports: AD-A421378; TR-2217-SHR.

 

John Durkee, PhD, PE is the author of the book Management of Industrial Cleaning Technology and Processes,published by Elsevier (ISBN 0-0804-48887).He is an independent consultant specializing in critical cleaning for contamination control.Contact him at PO Box 847,Hunt,TX 78024 or 122 Ridge Road West,Hunt,TX 78024; 830-238-7610;Fax 612-677-3170;or jdurkee@precisioncleaning.com.