Study of Cigarette Smoke Filtration by Means of the Scanning Electron Microscope

The method developed by Peck (8) for observing smoke deposits on cigarette filters with the scanning electron microscope was extended to two techniques to determine how the particulate phase of smoke is deposited on celluIose acetate filters and on individual cellulose acetate fibers.

Technique A: Immediately after the smoke particles were deposited on the fibers, the filter was exposed to methyl 2-cyanoacrylate vapour; the methyl 2 cyanoacrylate monomer polymerized rapidly and formed a very thin film (0.05 µ thick) over the partially volatile particles so they could be examined in the vacuum chamber of the scanning electron microscope. This technique was used to observe smoke deposits on single fibers oriented either parallel or perpendicularly to the smoke stream.

Technique B: Methyl 2-cyanoacrylate vapour was drawn into a mixing chamber in front of the filter as each puff of smoke was taken. The monomer coated the particles and polymerized. The coated particles were subsequently trapped by the fibers and observed with the scanning electron microscope.

From techniques A and B, it was observed that single fibers oriented parallel to the smoke stream showed a heavy deposition of small particles (<< 0.1 µ in diameter). This observation qualitatively confirms the theory that diffusion is one of the predominant mechanisms of filtration. Relatively smalI numbers of large smoke particles (> 0.5 µ in diameter) were trapped by single fibers oriented perpendicularly to the smoke stream. These large particles were trapped by interception on fibers which were perpendicular to the smoke path. The edge of each Y-cross-section fiber, where interception is most likely to occur, was more heavily coated than other parts of the fiber. All of the large particles in a 28- × 45-µ area on a single fiber oriented perpendicularly to the smoke stream were counted. The total number of particles on the fiber were calculated and compared to the amount expected from the totaI number of particles per puff, the fraction of particles larger than 0.5 µ, and the single fiber efficiency. Good agreement between the experimental and calculated values was obtained.