Substance identification: FT-IR and TGA – key tools in forensic investigation
Substance identification is frequently key to establishing failure modes and suggesting root causes. ERA’s forensic toolkit includes FT-IR (Fourier Transform Infra-Red) spectroscopy and TGA (Thermogravimetric analysis) which are often used to unlock this information.
FT-IR is used widely to determine the identity of unknown substances particularly organic compounds and polymers. Moreover, the technique can also provide useful information about many inorganic materials. An infrared spectrum can provide both generic information, about the types of functional group within a compound and, by matching against reference materials or spectra, provide a proof of identity.
TGA can identify the onset temperature of events and provide quantitative weight change information while the FT-IR identifies the nature of the decomposition products associated with these events. Together, TGA with FT-IR provides a powerful and versatile means to investigate thermal decomposition and outgassing processes.
Case study – linking evidence to root cause in a pcb failure
Figure 1 shows a printed circuit board from an industrial control system. Evidence of corrosion, which eventually led to failure, can be seen clearly. This corrosion occurred below the conformal coating.
Figure 1 Failed industrial control board caused by corrosion - but what led to this corrosion?
The nearby electrolytic capacitor (far right in the figure) was found to be leaking and FT-IR was used to compare the residue found close to the corrosion site with that obtained directly from inside an unused capacitor.
Figure 2 shows the infra-red spectra resulting from the analysis. The blue spectrum is that of the fluid from within the capacitor, the other spectra arise from residue samples taken from the corroded area below the conformal coating.
Figure 2 FT-IR spectrum of samples taken from an unused capacitor (blue spectrum) and from the corrosion site on the board (other spectra)
While this data was insufficient to absolutely identify the residues it did however demonstrate that the residues were of a similar nature to the capacitor fluid and that there was a highly probable link.
Applications of FT-IR in forensics and analysis
FT-IR finds application in many areas. Here are some examples of how we have used it to great benefit at ERA:
- Production Control: Poor performance or failure often results from inappropriate material selection at the design stage or inadvertent use of the wrong material during production. FT-IR can be used to determine that the correct material has been used in a product or applications. With appropriate sampling methods only small amount of adhesives, seals, varnishing etc are required to compare with reference materials.
- Contamination identification: Unidentified peaks in an FT-IR spectrum sometimes denote contamination. This can often occur in solvents, wash solutions or lubricants. The source can often be identified by comparison but if the unidentified peaks of the spectrum can be isolated it may also be possible to identify the nature of the contaminant.
- Identification of surface treatments: Reflectance or ATR (attenuated total reflectance) spectra are extremely surface sensitive and provide information about the nature of surface treatment of coatings such as treated papers or laminated films. , market surveillance and enforcement
- Identification of samples collected from the field: Diffuse reflectance with abrasive sampling is used to examine painted surfaces, polymers and coatings. This is particularly important with large samples that cannot be directly sampled in other ways – for example field sampling of vessels and pipes.
- Analysis of complex mixtures: Where complex mixtures or multiple contaminants are involved it difficult to determine what are the various components present. Selective extraction techniques are used in this case – for example to identify the components within flux residues.
- RoHS analysis: FTIR is used to determine if polybrominated diphenyl ether (PBDE) flame retardants are present in plastics. Although the detection limit, at about 3%, is higher than the maximum concentration values this is often sufficient as a screening test.
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