GC-MS/SIM and HPLC method development for monitoring polydimethylsiloxane and its degradation products

Abstract

Polydimethylsiloxane (PDMS) is an abundant and highly persistent polymer used in many applications. One of these applications is as an antifoaming agent in the kraft pulping process. This chemical pulping process generates tall oil as a by-product that can be used for producing biodiesel. PDMS has been detected as a contaminant in the biorefineries that is causing challenges in the processes. This work aimed to develop a GC-MS/SIM and HPLC method to detect and monitor the contaminants in different bio-oils. Furthermore, pyrolysis GC-MS was to be utilized for PDMS degradation studies, and an automated normal-phase flash chromatography was to be tested as a potential sample-cleanup procedure. Two GCMS instruments equipped with different dimensioned columns were used for the detection of the PDMS degradation products hexamethylcyclotrisiloxane (D3), octamehtylcyclotetrasiloxane (D4) and decamethylcyclopentasiloxane (D5) present in biooils. One GC-MS instrument was equipped with an HP-1 column and the other with an HP5MS column. The condition of the first-mentioned instrument was better maintained compared to the second one and, therefore, lower concentrations were detectable. Additionally, the more sensitive instrument was able to detect contaminants of D3–D5, which were found to originate from the silicone-based inlet septum. The contaminants made the validation of the method more difficult and were taken into consideration in the interpretation of the results. The method’s linearity, accuracy and precision were determined by utilizing the HP-5MS instrument. The linearity was found to be good for all three cyclic compounds. The accuracy determination showed that the matrix of the bio-oils somehow affects the response in the detection of D3–D5. Precision was difficult to determine, as too few data points were collected. The HP-1 instrument was utilized for determining the lowest detectable concentration, however, as the contaminants affected the detection, it could only be determined that at least a concentration of 2 ppm D3–D5 in relation to the bio-oil was detectable. GC-MS/SIM analyses of different bio-oils showed that it could be possible to quantify the cyclic compounds directly from the bio-oils. With an RP-HPLC-ELSD, low (5cSt), medium (50 cSt) and high (1000 cSt) molecular weight PDMS were analyzed. For 5 and 50 cSt PDMS, the separation of components within the molecular weight groups was possible, and each molecular weight group was separable from the others. When spiked in different bio-oils, the matrices interfered completely with the detection of 5 cSt PDMS and slightly with 50 cSt PDMS. The lowest detectable concentration of 1000 cSt PDMS in three different bio-oils, was 1% PDMS in relation to the bio-oil. For lower detectable concentrations, sample cleanup or fractionation should be performed. The normal-phase flash chromatography, equipped with an ELSD, was not suitable for the detection of PDMS in bio-oils. The different molecular weight groups were not separable and detectable when spiked in bio-oils. Automated reverse-phase flash chromatography or preparative HPLC should be tested as potential sample cleanup procedures.