Molecular Characterization of Secondary Aerosol from Oxidation of Cyclic Methylsiloxanes

Abstract

Cyclic volatile methylsiloxanes (cVMS) have been identified as important gas-phase atmospheric contaminants, but knowledge of the molecular composition of secondary aerosol derived from cVMS oxidation is incomplete. Here, the chemical composition of secondary aerosol produced from the OH-initiated oxidation of decamethylcyclopentasiloxane (D5, C10H30O5Si5) is characterized by high performance mass spectrometry. ESI-MS reveals a large number of monomeric (300 < m/z < 470) and dimeric (700 < m/z < 870) oxidation products. With the aid of high resolution and MS/MS, it is shown that oxidation leads mainly to the substitution of a CH3 group by OH or CH2OH, and that a single molecule can undergo many CH3 group substitutions. Dimers also exhibit OH and CH2OH substitutions and can be linked by O, CH2, and CH2CH2 groups. GC-MS confirms the ESI-MS results. Oxidation of D4 (C8H24O4Si4) exhibits similar substitutions and oligomerizations to D5, though the degree of oxidation is greater under the same conditions and there is direct evidence for the formation of peroxy groups (CH2OOH) in addition to OH and CH2OH.

Keywords: Organosilicon aerosol; Oxygen-containing functional groups; Particle formation; Particle phase; Secondary organic aerosol.

Figure. 1

Representative ESI mass spectra of secondary aerosol from D₅ oxidation.

Figure. 2

Plots of C/Si versus O/Si for saturated products of D₅ oxidation in the monomer (a) and dimer (b) regions. Each dot represents an assigned molecular formula of a saturated oxidation product. The arrows in the monomer plot extend from unoxidized D₅ and represent the two types of substitution that can occur. The arrows in the dimer plot take into account different possible linkages between monomers.

Figure 3

Product ion spectra for isolation of a) 373 m/z(+), b) 371 m/z(−) and c) 387 m/z(+). These precursors correspond to the oxidation product of D₅ having one OH substitution for a CH₃ group (Fig.3a and Fig. 3b), and D₅ having one CH₂OH substitution for a CH₃ group (Fig.3c).

Figure 4

Product ion spectrum of 727 m/z(+). a) Complete product ion spectrum. b) An expansion of 350 to 380 m/z(+). Peaks corresponding to an O linkage are marked with red. Peaks corresponding with a CH₂ linkage are marked with blue. Peaks consistent with both are marked in black. c) Expansion of the product ion spectrum of 795 m/z(+) between 380 and 470 m/z(+).Peaks corresponding with a CH₂CH₂ linkage are marked with blue.

Figure 5

Plots of C/Si versus O/Si for saturated products of D₄ oxidation in the a) monomer and b) dimer regions. Each dot represents an assigned molecular formula of a saturated oxidation product. The arrows in the monomer plot extend from unoxidized D₄ and represent the two main types of substitution that occur. The arrows in the dimer plot take into account different possible linkages between monomers. Circles represent monomer products where all eight CH₃ groups have been substituted. Formulas to the right of the circles (same C/Si but higher O/Si) must contain peroxy groups.

Scheme 1

Dimer structures with a) −O−, b) −CH₂−, and c) −CH₂CH₂- linkages. The OH group in b) could be located anywhere around the siloxane ring. The locations of the two CH₂OH groups in c)can be anywhere on the left hand ring, while the location of the OH group can be anywhere on the right hand ring. The C₁₁ product confirms the existence of a CH₂CH₂- linkage. Other labeled ions are consistent with, but not unique to, this linkage.

Scheme 2

Possible pathways for product formation from OH oxidation of D₅.