Figure 3
Response of the quartz crystal microbalance (QCM) during a typical ALICE exposure. a) Within 2 s of the nebulizer being turned on (t = 0) -Δf increases due to droplet deposition on the quartz crystal. At 110 s, the nebulizer is completely discharged (here: 1 mL of 6% (NH4)2SO4 solution) and the air flow is stopped. Subsequently, single particle sedimentation depletes the stagnant cloud and at 300 s, 95% of the final -Δf value (630 Hz) is reached (end of droplet deposition), which is still well below 715 Hz, the saturation value of the QCM for an infinitely thick water layer. During typical ALICE experiments, the cells were removed at 600 s (here no cells were in the ALICE). b) At 900 s dry filtered air is introduced in the exposure chamber, which dries the liquid film on the QCM. At 1500 s, the QCM deposit has completely dried, as indicated by the resistance (R) approaching 0 Ω. The dry salt mass of 72.4 μg/cm2 can then be obtained from -Δf = 4100 Hz using equation 3. For optimized timing (~10 min per exposure run) the cells could be removed from the ALICE after 300 s (5 min) and the QCM deposit can be dried more efficiently (within a few minutes) by removing the QCM from the exposure chamber and drying it with dry air.