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•  Ions  from  different  formula  groups  including  their  isotope  signals  have  the  same
nominal  mass  and  overlapping  retention  time  ranges  and  can  therefore  cause
interferences,  when  low  resolution  mass  spectrometry  is  applied.
First  methods  for  PC  As  analysis  were  developed  during  the  1980ies  (Gjps  and
Gustavsen,  1982;  Schmid  el  al.,  1985).  Electron  capture  negative  ion  (ECNI)  detection
was  selected  due  to  strongly  decreased  detection  limits  caused  by  restricted  molecule
fragmentation.  This  technique  is  based  on  the  formation  of  anions  due  to  the  high
electron  affinity  of  polychlorinated  compound  by  capture  of  electrons  (in  thermal
equilibrium).  Matrix  compounds  and  other  interferences  have  a  low  electron  affinity  and
do  not  form  anions  (Oehme,  1998).  Moreover,  high  resolution  mass  spectrometry
(HRMS)  was  chosen  to  suppress  disturbances  from  PCAs  themselves  as  well  as  from
other  not  completely  removed  polychlorinated  pollutants  such  as  PCB  (Tomy  et  al.,
1997).  ECNI  was  still  the  method  of  choice  when  this  project  was  started  in  2002
(Froescheis  and  Ballschmiter,  1998).
•  However,  ECNI  generates  additional  problems  influencing  the  reliability  of
results  (Reth  and  Oehme,  2004;  Reth  et  al.,  2005):
•  Response  factors  are  strongly  dependent  on  the  number  of  Cl  atoms  and  could
vary  by  one  order  of  magnitude.  Moreover,  co-elution  of  compounds  with
different  degree  of  chlorination  can  influence  the  overall  response.
•  Lower  chlorinated  isomers  with  <3-4  Cl  atoms  have  low  response  factors  and
could  normally  not  be  detected.
•  Molecular  ions  M"'  as  well  as  [M-Cl]"  and  the  chlorine  adduct  [M+C1] +  are  formed
simultaneously  and  increased  the  risk  of  interfering  mass  overlap.
•  The  overall  degree  of  chlorination  influences  the  overall  response  factor  of  the
PCA  mixture  present  in  an  environmental  sample  and  required  references  with
similar  composition  for  quantification,  which  normally  are  not  available.
Nevertheless,  the  quantification  procedures  developed  of  Tomy  et  al.  (1997;  Tomy  and
Stern,  1999)  allowed  to  minimise  the  risk  of  systematic  errors  and  gave  rather  reliable
results.  However,  this  required  HRMS  as  well  as  many  runs  up  of  the  same  sample  (up
to  twenty)  to  detect  all  possible  formula  groups  (chain  lengths  from  Cio  n  and  number