High Pressure/Performance Liquid Chromatography (HPLC) is very common and can be used to estimate purity of parylene dimers.
The main issue with this method is that to get accurate values or percent composition for each component, you should calibrate the detector response [typically absorbance peaks with an ultraviolet light (UV) detector] with pure compounds for each of the component’s individual peaks that you’re expecting.
This means you should have 100% pure symmetrical and unsymmetrical dichloro, monochloro, trichloro, and tetrachloro dimers to get the most valid numbers for your HPLC results. The challenge is that 100% pure samples don’t exist.
Another test method that provides more accurate and repeatable results is Gas Chromatography-Mass Spectrometry (GC-MS). The advantage with GC-MS is that the MS detector is measuring the mass (or more accurately the mass-to-charge ratio) or the compounds that are separated into individual peaks.
GC-MS gives actual chemical structural information, while a UV detector is much more relative since one compound may absorb much more strongly at a lower concentration as compared to another compound. VSi only uses GC-MS testing performed by a third party for all of our dimers.