Environ  Sci  Pollui  Res  (2015)  22:19887-19895

19891

4)  Springer

Fig.  1  Scheme  of  silicone  rubber
sample  extraction,  clean-up  and
analysis

Extraction

•ASE
(100°C,  10  Min,  Hexane/DCM  (1:1))
A

i

Extract  purification

•Si0 2
Extraction  (Hexane/DCM  (70:30))
j

i

Removing
oligomers

'
•HPLC-SEC  Fractionation
(60°C,  1ml/mîn,  Hexane/DCM  (80:20))
4

\

Monitoring  extract  for
oligomers

'
•TXRF  (5|jl  on  sapphire  plate)
à

\

Quantifying
^  concentrations  ^

'
•GC/MSfor  CHC
•GC/MSfor  PAHs
à

(0-10  min)  contained  the  non-crosslinked  silicone  oligomers
and  the  second  fraction  the  CHCs  and  PAHs  fraction  (10-28
  min).  Column  temperature  was  set  to  60  °C  to  avoid  adhesion ­
  of  silicone  in  the  system.  The  second  fraction  was  concentrated ­
  to  0.2  mL  with  nitrogen  and  monitored  for  its  oligomer ­
  load  with  TXRF  before  instrumental  analysis.  For  this,
an  aliquot  of  5  )iL  was  slowly  dropped  onto  pre-cleaned  sapphire ­
  disc,  allowing  the  organic  solvent  time  to  evaporate.  The
disc  was  loaded  into  the  TXRF  and  measured  for  1000  s  for  its
silicium  load,  which  is  the  representative  element  of  the  silicone ­
  oligomer.  A  rubidium  reference  standard  was  used  for
instrument  calibration.  If  the  silicium  load  of  the  sample  was
in  the  range  of  SEC  blanks,  an  analysis  of  CFfCs  (GC-MS/
MS)  and  PAFls  (GC-MS)  was  performed.  The  complete  extraction ­
  and  analysis  process  is  illustrated  in  Fig.  1.

Results  and  discussion
Optimization  of  the  silicon  rubber  pre-cleaning
Different  solvents  showed  different  patterns  of  swelling  and  of
total  oligomer  release  (Fig.  2).  No  swelling  and  release  of
oligomers  was  induced  by  water,  while  up  to  2.3  %  of  silicone
oligomers  were  released  when  non-polar  solvents  were  used
(«-hexane/acetone  (1:1  v/v))  (Fig.  2).  The  release  of  oligomers
was  exponentially  dependent  on  the  swelling  of  the  silicone
strips  (R 2 =0.99)  (Fig.  2),  from  0  to  60  %.  In  contrast  to  a  study
of  Shahpoury  and  Flageman  (2013),  none  of  the  used  solvents
expanded  the  silicone  rubber  as  drastically  as  to  block  the
solvent  flow.  Flowever,  the  blockage  might  depend  on  the
volume  of  silicone  rubber  relative  to  the  volume  of  the  extraction ­
  cell.  Rusina  et  al.  (2007)  showed  that  PDMS  strips  became ­
  more  breakable  after  increased  swelling.  Thus,  the  1:1
combination  of  hexane/acetone  was  used  as  the  solvent  of
choice  in  further  pre-cleaning  experiments,  providing  a  high

oligomer  release  rate  with  medium  swelling  (20  %).  In  all
experiments,  PDMS  strips  regained  their  original  size  and
strength  after  the  drying  process  (evaporation  of  the  solvent).
It  was  found  that  the  amount  of  released  oligomers  from
silicone  rubber  strips  increased  with  increasing  extraction  time
up  to  70  min  with  no  further  release  with  increasing  time
(Fig.  3),  indicating  the  oligomer  release  from  the  silicone  rubber ­
  strips  to  be  fairly  exhausted.  The  final  amount  released  is
dependent  on  the  batch  from  which  the  passive  sampler  strips
were  prepared.
The  variation  of  temperature  showed  no  differences  in  the
amount  of  released  oligomers  and  hence  was  excluded  as  a
factor  for  optimization  of  oligomer  release  rates.  Thus,  the
ASE  default  temperature  of  100  °C  was  applied.
Additionally  to  the  determination  of  the  weight  of  the  PDMS
strips,  corresponding  extracts  were  analysed  by  GC-MS  to
proof  the  differences  in  weight  to  be  a  result  of  oligomer  release. ­
  All  extracts  of  silicone  rubber  pre-cleaning  showed  the

a>
E
o
cn

a>
tn
«
a>
a>
a:

n-Hexane/Acetone  T
_  -(1:1)  .  n-Hexaighyiacetatf
, n  Acetone  n-Hexane/  ■  -
z,u-  Acetone  (3:1)
r ‘  Methanol/n-Pentane  Acetone  (1:1)
(1:1)
1,5-1,0-

  -  "  Methanol/Acetonitrile  (1:2)  y  =  yO  +  A*exp(R0*x)
R 2  =  0.99
■  Oligomer  release
0,0  -  u  water  Exponential  fit
I  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1  1
0  5  10  15  20  25  30  35  40  45  50  55  60

n-Hexane/Acetone  7.
_  -(1:1)  .  n-Hexaighyiacetatt
Acetone  n-Hexane/  ■  i
Acetone  (3:1)
ri  Methanol/n-Pentane  Acetone  (1:1)
(1:1)

-  N  Methanol/Acetonitrile  (1:2)

■  “  Water

y  =  yO  +  A*exp(R0*x)
R 2  =  0.99

Oligomer  release
-  Exponential  fit

Swelling  [%]
Fig.  2  Release  of  oligomers  as  a  function  of  swelling  for  different
organic  solvents  (water,  methanol/acetonitrile,  methanol/pentane,
acetone,  hexane,  hexane/acetone,  dichloromethane/acetone,
ethylacetate)  using  ASE  (100  °C,  2x10  min)