The packaging and transportation of evidence or substances that require laboratory testing are extremely important steps. Unfortunately, packaging and transportation can be given less priority than necessary. When testing for the material composition of an unknown substance, incorrect packaging and handling can affect the outcome of testing resulting in contamination. One common area where the outcome of testing is frequently affected due to incorrect packaging, is the sampling technique of Fourier-Transform Infrared Spectroscopy (FTIR) known as Attenuated Total Reflection (ATR). What type of incorrect packaging could affect the technique of ATR though?
A common practice in the field of forensics is to place samples in reusable bags often under brand names such as Ziploc®.
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Polyolefin Films & Slip Agents
Slip agents vary, but one commonly observed slip agent is a fatty acid amide known as stearamide
< Figures 1 and 2 >
Stearamide is also frequently used because it exhibits properties of an anti-blocking aid, a substance which helps reduce the adhesion between films . Slip agents and anti-blocking aids may aid in the production of polyolefins, but can contaminate submitted samples. The migration of a slip agent such as stearamide to the surface of a polyolefin can cause transferable contamination. Migrated slip agents can be rubbed onto any substance placed into a reusable bag, even if contact is made for a brief period of time.
To demonstrate slip agent contamination, a mock sample submission was created at Micron inc.
- Figures 1 & 2: A customer submits an unknown polymer for testing to determine the material composition
- Figure 3: After analysis using FTIR-ATR the sample was determined to be polytetrafluoroethylene (PTFE), but additional absorptions are present that do not correlate with PTFE
- Figure 4: An additional analysis was performed on the reusable bag which the unknown polymer was received in. The reusable bag was identified as the polyolefin known as polyethylene using FTIR-ATR
- Figure 5: Again additional absorptions were present that do not correlate with polyethylene. A slip agent was suspected, isolated, analyzed using FTIR-ATR, and was identified as stearamide
- Figure 6: A final comparison spectra of the unknown polymer, a reference of PTFE, and a reference of stearamide displays the contamination of the slip agent from the reusable bag onto the sample submitted for testing (Figure 6).
Figure 3 & 4
Figures 5 & 6
1. Chen, J., Li, Hu, & Walther. (2007). Fundamental study of erucamide used as a slip agent. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 25(4), 886-89
2. Zahedi, A., Ranji, A., & Asiaban, S. (2006). Optimizing COF, Blocking Force, and Printability of Low Density Polyethylene. Journal of Plastic Film & Sheeting, 22(3), 163-176.