Method development for the analysis of polydimethylsiloxane in bio-oils

Abstract

The chemistry of wood is quite complex. Wood consists of several components, such as cellulose, hemicelluloses, lignin, and extractives, which are all important during pulping. The components are either used as the final product or collected as by-products in the black liquor. Some of the by-products can still be separated from the black liquor and be utilized. One common example of a by-product which is utilized to a much smaller extent than its actual potential is the tall oil. Typically, the tall oil contains small amounts of polydimethylsiloxane (PDMS) in different forms, e.g., high, medium, or low molecular weight (MW), which originate from the antifoaming agents that are added during pulping.

The objective of this work was to develop an improved analytical method for the analysis of PDMS in bio-oil matrices. The experimental procedures utilized during this work were solvent extraction and solid phase extraction (SPE). The analytical techniques for detecting PDMS were high-performance size exclusion chromatography (HP-SEC) and gas chromatography (GC). The focus of this work was to study the properties of the PDMS as it eluted through SPE cartridges, in order to find an improved method for the fractionation. When using the pure high MW PDMS as model compound, it was concluded that the best choice of eluent was dichloromethane (DCM), and the best choice of cartridge material was florisil. Florisil was chosen due to the high yield of eluted PDMS and the relatively good separation between the PDMS and the bio-oil (BO) 1. For the previously developed SPE method, the eluent was also DCM, however, the cartridge material was silicone hydroxide (SiOH).

Another goal of the experimental work was to separate and concentrate the eluted PDMS from a matrix of bio-oil. In total, there were three different bio-oil matrices utilized. The experiments also comprised utilizing three different PDMS model compounds (i.e., high, medium, and low MW). All three model compounds were spiked separately in all three matrices and fractionated using the florisil cartridge. According to the used solvent combinations during SPE fractionation, the first fractions eluted the low MW PDMS, the middle fractions eluted the medium MW PDMS, and the last fractions eluted the high MW PDMS. The developed florisil SPE method was also successfully applied to separate the highly volatile D3 and D4 PDMS degradation products from BO 1. The detection of the volatile D3 and D4 PDMS products and the low MW PDMS was possible by GC, while the medium and high MW PDMS were detected by HP-SEC.

The properties and potential of the florisil cartridge were compared with the SiOH cartridge throughout this work. When considering the separation of the PDMS from the bio-oil matrices, the florisil cartridge performed a slightly better separation than the SiOH cartridge. The limit of detection (LOD) and limit of quantitation (LOQ) were determined for the SPE method, using the florisil cartridge, as 37 ppm and 113 ppm, respectively. These values were in the same range as for the previously developed SPE method, which utilized the SiOH cartridge (30 ppm and 92 ppm). The main conclusion from the comparison of the cartridges is that the florisil cartridge resulted in a better separation and isolation of the PDMS from the bio-oil matrices, although based on the LOD and LOQ for the cartridges, the SiOH cartridge was overall slightly better than the florisil cartridge.