2.4. Siloxane adsorption set-up and procedure
The D4 adsorption experiments were performed under dry conditions.
During the experiments, nitrogen of 5 mL/min was used as
the carrier gas. The carrier gas first passed through a bubbler with
10 mL D4 which is maintained at 25 C in an oil bath, and then
passed through adsorbents (100 mg, around 1 cm long in a quartz
tube). The average siloxane concentration of 2000 mg/m3, higher
than the real industrial situation (50–500 mg/m3) and the highest
previously reported in the literature [14], was used in this study.
The D4 that broke through the adsorbents was trapped by hexane
in an ice bath. Samples were taken from the hexane trap regularly
and analyzed by Agilent 7820A gas chromatography with a 5975
MS detector (GC-MS) to monitor the siloxane concentration. The
GC–MS was calibrated with different concentrations of D4 for
quantitative analyses. The D5 of 10 lL was used as an internal
standard for verification of the results. GC is equipped with a 12-
meter-long HP-1 column. The temperature of the oven in GC is first
kept at 40 C for 2 min, then increased to 250 C with a 15 C/min ramp rate, and finally kept at 250 C for 5 min. The detection limit
of GC is 1 ppm. The amount of D4 adsorbed was calculated by subtracting
the D4 amount in the trap during the experiment from the
D4 amount in the trap during the blank experiment under the
same conditions. Each experiment was repeated at least three
times and the error bars are shown in the plots. The capacities of
the adsorbents were determined by the saturation points from
the D4 adsorption curves. The adsorbents with adsorbed D4 were
soaked and washed with hexane more than three times until no
more siloxanes were removable from the adsorbents. Then the
extracted siloxanes in hexane were analyzed to study the surface
polymerization reactions during the adsorption. The polymerization
ratio was calculated as the total amount of extracted D5, D6,
and D7 over the amount of extracted D4 [13].