Sudarsanan Varaprath

Quantitative Determination of Octamethylcyclotetrasiloxane (D4) in Extracts of Biological Matrices by Gas Chromatography-Mass Spectrometry

Abstract A method was developed and validated to measure octamethylcyclotetrasiloxane (D4)† quantitatively by gas chromatography-mass spectrometry (GC-MS) at low level in extracts of several biological matrices that include plasma, liver, lung, feces and fat from rats. The key to the successful determination lay in the use of extracts dried with anhydrous magnesium sulfate. This was necessary in view of the propensity of the methyl siloxane based GC-stationary phase to generate D4 by its reaction with water present in the extracts. To enable quantiiation of D4 at parts per billion (μg/L) levels, the base ion m/z 281 resulting from the loss of a methyl group from the parent molecule was selected for monitoring by SIM mode in GC-MS. The recovery of D4 from any of the biological matrices was determined to be greater than 90% in three extractions. The D4 response for the standards in GC-MS was linear (R2 > 0.9900) and reproducible at concentrations ranging from 1—16,000 ng D4/g solvent. Precision was less than 5%.

Speciation and quantitation of degradation products of silicones (Silane/Siloxane Diols) by gas chromatography—mass spectrometry and stability of dimethylsilanediol

Gas chromatography-mass spectrometry (GC-MS) methodology was developed to speciate and quantitate several degradation products of polydimethylsiloxane (PDMS) in soil. We have demonstrated that the major degradation product,viz., dimethylsilanediol, can be readily analyzed by GC-MS without derivatization as commonly practiced in analyzing such materials. A mixture of linear siloxane diols (n = 1–5, wheren is the number of Me2SiO units), and cyclic dimethylsiloxanes (n = 4–6) was resolved by GC-MS. We also found that peak identity of various diols required that GC-MS is done in the chemical ionization (CI) mode, since the electron impact (EI) ionization mode produced similar mass fragmentation patterns for diols and cyclics containing the same number of silicon atoms. For siloxane diols, detection limits ranged from 100 pg (forn = 1) to 1 ng (for n = 5). For cyclics, the detection limit was about 1 pg. Dimethylsilanediol, known to be unstable even in the solid state, was shown by NMR techniques to be stable in aqueous solution at <0.1% concentration. A 100-ppm solution was stable for over a year. Purity check for dimethylsilanediol is best carried out by Si-29 solid-state NMR technique.

Closed-Chamber Inhalation Pharmacokinetic Studies with Hexamethyldisiloxane in the Rat

Gas uptake methods together with physiologically based pharmacokinetic (PBPK) modeling have been used to assess metabolic parameters and oral absorption rates for a wide variety of volatile organic compounds. We applied these techniques to study the in vivo metabolism of hexamethyldisiloxane (HMDS), a volatile siloxane with low blood/air (partition coefficient PB ≈ 1.00) and high fat/blood partitioning (partition coefficient PF ≈ 300). In contrast to other classes of metabolized volatiles, metabolic parameters could only be estimated from closed-chamber results with confidence by evaluating both closed-chamber disappearance curves and constant concentration inhalation studies. The constant-concentration inhalation results refine the estimate of the blood/air partition coefficient and constrain model structure for storage of the lipophilic compound in blood and tissues. The gas uptake results, from Fischer 344 rats (male, 8-9 wk old) exposed to initial HMDS air concentrations from 500 to 5000 ppm, were modeled with a 5-tissue PBPK model. Excellent fits were obtained with diffusion-limited uptake of HMDS in fat and a lipid storage pool in the blood. Metabolism, restricted to the liver, was described as a single saturable process (V max = 113.6 µmol/h/kg; K m = 42.6 µmol/L) and was affected by inhibitors (diethyldithiocarbamate) or inducers (phenobarbital) of cytochrome P-450s. Exhalation kinetics of HMDS after oral/intraperitoneal administration showed low bioavailability and significant lag times, also quite different from results of other classes of volatile hydrocarbons. In general, estimates of metabolic clearance by gas uptake studies were improved by simultaneous examination of time-course results from constant concentration inhalation studies. This conclusion is likely to hold for any volatile lipophilic compound with low blood/air partitioning.

Synthesis of 14C-labeled cyclic and linear siloxanes

Simple procedures to synthesize a variety of 14C-labeled monomeric and polymeric siloxanes are described. Specifically, the synthesis of the following siloxanes, some of which are of significant commercial importance are provided: 14C-octamethylcyclotetrasiloxane (D4), 14C-decamethylcyclopentasiloxane (D5), 14C-hexamethyldisiloxane (MM), 14C-dimethyldimethoxysilane and 14C-dimethylsilanediol (DMSD) are examples of discrete monomeric species. 14C-350 and 1000 cSt polydimethylsiloxanes (PDMS) are examples of polymeric species. Synthesis of the monomeric species with the exception of dimethylsilanediol involve reactions of Grignard reagents with the appropriate chlorosilanes, while the polymeric materials were synthesized via acid catalyzed equilibration reaction of 14C-D4 with dodecamethylpentasiloxane (MD3M). The compound 14C-DMSD was obtained by the hydrolysis of 14C-dimethyldimethoxysilane. The labeled materials listed here were synthesized for their utility as tracers in several of the ongoing environmental fate and effects studies as well as toxicological investigations.

EFFICIENT SOLID PHASE EXTRACTION PROCEDURES FOR ORGANO SILOXANES AND SILANOLS FROM AQUEOUS AND BIOLOGICAL MATRICES

Efficient solid phase extraction (SPE) procedures were established to extract a variety of organosiloxanes and organosilanols at low concentrations from aqueous and biological matrices. The organosilicon materials used in SPE experiments were all 14C-labeled. Use of 14C-labeled materials made it very convenient to determine the SPE process efficiency. For non-polar siloxanes of lower molecular weight, viz., 14C-octa-methyl-cyclotetrasiloxane (D4), 14C-decamethylcyclopenta-siloxane (D5), and 14C-hexamethyldisiloxane (MM), the recoveries from aqueous solutions, using C18 SPE cartridges, were 91 (± 2), 98.6 (± 0.5), and 96.2 (± 2), % respectively. For polymeric siloxane fluids of 350 and 1000 cSt viscosities with number average molecular weights, Mn of ∼9800 and 17,000 respectively, C4 sorbent was preferred. The recoveries for these polymeric siloxanes were 89.3 (± 1.7), and 96 (± 0), % respectively. For polar silanols, styryl-divinylbenzene based polymeric-sorbent ENV+® worked exceptionally well. Using ENV+ sorbent, samples of dimethylsilanediol Me2Si(OH)2, methylsilanetriol MeSi(OH)3, were recovered from aqueous medium with excellent efficiencies (99 and 94 %, respectively). The ENV+ cartridge was also applied to recover very efficiently the metabolites (i.e. hydroxy and hydroxymethyl functional siloxanes) of D4, D5, and MM present in rat urine. The recoveries were >90% in all cases. Recoveries of metabolites of MM and D5from plasma were ∼80 and 66% respectively. The concentrations of the analytes ranged from parts per billion to parts per million. For successful retention of the analytes to the sorbent, it was found to be necessary to place solid NaCl to the top of the SPE cartridge bed, or treat the aqueous matrices with NaCl prior to sample load. HPLC analyses of samples performed following the SPE process showed that the organosilicon materials, including the very reactive silanols, remained unchanged.

Degradation of Monophenylheptamethylcyclotetrasiloxane and 2,6-cis-Diphenylhexamethylcycloterasiloxane in Londo Soil

The natural degradation of monophenylheptamethylcyclotetrasiloxane and 2,6-cis-diphenylhexamethylcyclotetrasiloxane in soil was evaluated under laboratory conditions. Both monophenyl and 2,6-cis underwent rapid degradation in dry soil generating the same products in varying proportions. During the first 24 hr, approximately 99% of the two materials underwent significant chemical transformations forming silanols of various structures, dimethyl cyclic siloxanes of the structure (Me2SiO)x, and rearrangement products (geometrical isomers) of diphenylhexamethylcyclotetrasiloxane. Among the silanols, the following were identified as trimethylsilyl derivatives: HOSiMe2OH, HOSiMePhOH, HOSiMe2OSiMe2OH, HOSiMePhOSiMe2OH, HOSiMePhOSiMePhOH, HOSiMe2OSiMe2OSiMe2OH, HOSiMePhOSiMe2OSiMe2OH, HOSiMe2OSiMePhOSiMe2OH, HOSiMePhSiMe2OSiMePhOH, HOSiMePhOSiMePhOSiMe2OH, HOSiMePhOSiMe2OSiMe2OSiMe2OH, HOSiMe2OSiMePhOSiMe2OSiMe2OH, HOSiMePhOSiMe2OSiMePhOSiMe2OH, HOSiMePhOSiMePhOSiMe2OSiMe2OH, HOSiMePhOSiMe2OSiMe2OSiMePhOH, HOSiMe2OSiMePh-OSiMePhOSiMe2OH. Derivatization was carried out using bis(trimethylsilyl)trifluoroacetamide. Gas chromatography-mass spectrometry and liquid chromatography-mass spectrometry (atmospheric pressure chemical ionization) analyses were used to derive structures. Structures were confirmed by gas chromatography-mass spectrometry comparisons of synthetic standards. Degradation was slower in wet soil. Nevertheless, in 14 days, the chemical transformation was essentially found to be complete as soil was allowed to dry. Detection of phenol as one of the degradation products revealed the occurrence of carbon–silicon bond cleavage promoted by soil.