Gas Chromatography Case Studies
- Identification of Extractables from O-Ring - Plasticizer Induced Environmental Stress Cracking
- Identification of Residue on Printed Circuit Board
- Determination of Disinfectant Ingredients for Compatibility Assessment
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O-Ring Extractables - Plasticizer Induced Environmental Stress Cracking
Engineers suspected that thermoplastic components in their assembly were cracking due to chemical exposure. The cracks were localized near an O-ring and FTIR analyses suggested compounds may be leaching from the O-ring.
A soxhlet extraction was performed to pull any extractables out of the O-rings. The soxhlet extractor (pictured at right) repeatedly washes the sample with solvent, pulling out any soluble compounds. The solvent boils in a heating mantle, condenses above the sample, falls on the sample, and periodically siphons back down. This cyclic process repeatedly washes the sample with fresh solvent, overcoming diffusion equilibrium constants and providing more complete extraction than soaking the material in a finite volume of solvent. The soxhlet extractor pulled out an oily liquid that made up roughly 20% (m/m) of the O-ring. GCMS Analysis of the O-Ring extractables showed the material had a retention time and mass-spectrum matching dioctyl terephthalate plasticizer (CAS# 6422-86-2). This is a common plasticizing oil used to make rubber softer and readily leaches/extracted when in contact with some materials. An ASTM D543 environmental stress cracking study was carried out with the plastic strained to 2% with and without the plasticizer added to the polymer surface. The plasticizer was observed to greatly accelerate the development of stress cracks (bottom right image). We performed validation of suitable replacement O-rings by FTIR & GCMS. |
GC Identification of Residue on Printed Circuit Board
A populated graphics board assembly was exhibiting intermittent contact on its mini display port connectors. Flux residues were visible on the underside of the graphics board and were expected to also be responsible for the intermittent electrical contact within the connectors. Residue from the receptacles and soldering flux residue from the underside of the PCB were collected and analyzed by Gas Chromatography (GC) and Fourier Transform Infrared Molecular Spectroscopy (FTIR). Results of both tests showed the residue inside the DisplayPort receptacles matched the composition of the flux residues on the underside of the of the PCB.
FTIR is suitable for bulk analyses, but when the samples are small or the geometries are awkward additional measures are needed. Gas chromatography offers parts per million (ppm) or better sensitivity to most small molecules. Results of the GC analysis of the flux residues by Flame Ion Detector (GC-FID) and Mass Spectrometer (GC-MS) are shown below.
GC Quantification of Disinfectant Ingredients for Chemical Compatibility Assessment
Field tests discovered an incompatibility between aerospace components and a Boeing D6-7127 approved disinfectant. FTIR and GC analysis were used to shed light on the composition of the disinfectant beyond the materials listed in the safety data sheet (SDS).
The disinfectant was mostly water which interferes with both FTIR and GC analysis. The disinfectant was evaporated down to concentrate the ingredients and facilitate further analysis. FTIR allowed rough qualitative identification of the types of compounds present in the disinfectant. The peak structures observed matched aromatic quaternary ammonium compounds and ethoxylated alcohols. GC analysis allowed for quantification of individual molecular species present in the sample, including a range of aliphatic and aromatic quaternary ammonium compounds, a range of ethoxylated alcohols, and EDTA. |