Page 299 - Inorganic Mass Spectrometry - Fundamentals and Applications
P. 299

Analysis of ~oncond~ct~~e  Sa~ple   Types                     285

          the R value  from  dc  powering  was 98% vs. 75% for the rf. Operation of the rf dis-
          charge  at  higher  powers  was  projected  to  produce  more  comparable  values.  Analy-
          sis of a mixture of rare earth elements (REEs) indicated  that  the  degree of atomic
                                         to
          ion  production  was  again  very  sensitive sampling  conditions  (i.e.,  position,  pres-
          surelpower)  in  the case of rf, and  not so much so for the dc case. In fact, R values
          between the REEs varied  with rf conditions,  but  not for dc, seemingly  depending
          on the M-0 bond  strengths.  Relative  sensitivity  factors for the rf and dc plasmas
          were  found  not  to  be  appreciably  different  under  controlled  conditions,  as  the  rf
          source  showed  very  good  stability (-5%  RSD)  and  reproducibility (~15% RSD).
               As  described  in  Sec.  7.3, which  dealt  with the use of the  secondary  cathode
          methodology,  one of the unexpected  areas  in  which  GD-MS  may be applied (so
          long  as  the  sample can  be analyzed intact) is the direct  analysis of polymeric  ma-
                                                                        in-
          terials. A simple  method  providing  elemental,  end-group,  or  molecular  weight
          formation  could  have  impact  in  a number of  industrial situations.  Coburn  and
          coworkers  actually  reported the production of molecular  fragment  mass  spectra
                                                                         of
          fluoro-  and  hydrocarbon  polymers  by rf GD-MS [69]. Twenty  years  later,  Marcus
          and  coworkers  noted a number of unexpected spectral features in the analysis of
          small  glass  samples  mounted to the DIP  via  double-sided  adhesive  tape.  On fur-
          ther  study, it was  shown  that  these  species  were in fact  representative of the  com-
          position of the tape. This observation  has  opened  up  an  entirely new line of study
          in the  use of rf GD-MS  as a tool that  provides  high-sensitivity  determinations of
          polymer  composition.
                                                          of
               The first  studies  by  this  group  involved  an  evaluation discharge  parame-
          ters  and  their  respective  roles the  structure  and  intensity mass  spectra  derived
                                 in
                                                        of
          from  PTFE-based  polymers  [70]. The spectra  were  found to be nearly  identical to
          those  obtained by SUMS, as  shown  in  Fig. 7.9, but  with  ion  beam  currents that were
          six  orders  of magnitude  more  intense  vs.  A).   The polymer  fragment
                                                               by rf GD-MS.
          ion  currents are in fact similar  to  those  obtained  in  metals  analysis
          Plasma  stabilization  times  of  less than 5 min  were  easily  achieved,  mdcing  the
          analysis  very  competitive  with  SIMS. In contrast to those  in the work of Schelles
          and Van Grieken [24], the spectra  were  rich  with  molecular  fragments  as  opposed
                                                the
          to  mainly  atomic ions, with  the  CF3+  fragment  in rf spectrum -1000  times  more
          intense than the C?  in the dc case.  Discharge  power  was  not  seen  dramatically  to
          change the spectral character,  only  the  ion signal levels.  On the other hand,  changes
          in  discharge  gas  pressure,  and  more  correctly the subsequent  changes in dc  bias,
          cause  substantial  variations  in  the  spectral  character.  Specifically,  high  Ar pres-
          sures (i.e., lower  dc-bias  values) are less effective  in  liberating  polyatomic  species,
          and so the spectra tend  to be more  atomic  in  nature. A depth  profile of a metallic
          layer on a polymer  substrate  was  presented
                                            to demonstrate  applications  such  as  the
          analysis of printed circuit boards.
               Two additional  studies  have  focused  on the area of polymer  analysis by rf
          GD-MS. The problem of sample  melting  was  addressed  through  the  design of a
          cryogenically  cooled  sample  holder  [7 l]. Just  as  in  the case of Schelles  and Van
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