PCF-­‐MS system design and development: metallic coupling devices

For  the  purposes  of  this  chapter,  the  term  ‘direct  infusion’  will  be  used  for   samples  that  have  been  directly  injected  into  a  mass  spectrometer  via  PEEK  tubing   using   a   syringe   and   syringe   pump.   This   is   a   conventional   method   for   the   introduction  of  samples  into  the  mass  spectrometer,  and  was  used  as  a  means  of   producing  a  ‘standard’  mass  spectrum  for  comparison  with  those  generated  by  the   PCF-­‐MS   system.   The   vast   majority   of   work   in   this   chapter   concerning   fibre   fabrication,   optical   coupling,   and   the   introduction   of   solutions   to   the   PCFs   was   carried  out  by  Dr  Sarah  Unterkofler.  My  work  was  concerned  with  optimisation  of   the  microfluidic  circuitry,  achieving  successful  coupling  of  the  system  to  the  mass   spectrometer,   carrying   out   control   reactions   by   ‘conventional   methods’   for   comparison  purposes  and  the  selection  of  appropriate  photoreactions  for  analysis.   It   was   first   necessary   to   establish   if   there   was   any   interaction   between   the   bare   silica   of   the   PCF   and   the   metal   complex   that   may   affect   the   mass   spectrum   observed.   Preliminary   experiments   carried   out   on   an   ion-­‐trap   mass   spectrometer   indicated   that   there   were   no   major   differences   between   the   spectra   of   a   10   μM   solution  ofcomplex  8  in  a  50:50  water/methanol  solvent  mix  when  flowed  through   a   PCF   or   PEEK   tubing   (both   when   in   the   dark   and   irradiated).   These   initial   experiments   were   then   repeated   on   a   high-­‐resolution   instrument   (Bruker   MaXis)   using  a  solution  of  100  μM  solution  of  8  in  50:50  DDW  and  acetonitrile,  with  the   addition  of  the  steel  coupling  devices;  see  Figure  4.2,  to  the  PCF.  The  solvent  was   changed  to  acetonitrile,  since  this  produced  a  lower  signal-­‐to-­‐noise  ratio  than  that   of  the  water  methanol  mix.  This  inclusion  of  metallic  coupling  devices  to  allow  the  

introduction  of  sample  and  light  to  the  PCF  (set-­‐up  shown  in  Figure  4.3)  introduced   several  new,  unknown  platinum  containing  species  at  579.2,  750.2  and  857.2  m/z,   see  Figure  4.4.  MS/MS  of  the  peak  at  750.2  m/z  shows  fragments  at  383.1,  411.1,   579.2   and     732.2  m/z.   The   ions   383.1   and   411.1  m/z   are   also   observed   in   the   spectrum   of   the   direct   infusion   of  8,   Figure   4.5.     This   would   suggest   that   the   coupling  devices  have  introduced  a  contaminant.  

Figure  4.2  An  example  of  an  initial  metallic  (steel)  coupling  device  built  in  the  Max  

Plank  Institute  of  Light,  Erlangen  used  in  the  first  PCF-­‐MS  system  (Figure  4.3).    

Figure   4.3  Schematic   of   a   PCF   sample   injection   system   when   coupled   to   405   nm  

For  completeness  of  the  comparison,  the  sample  was  then  irradiated  within   the   system   using   405   nm   laser   light.   The   photoactivation   of  8   resulted   in   the   creation   of   several   species   that   were   also   previously   detected   by   direct   infusion.   The  molecular  formulae  of  some  of  the  ions  was  determined:  382.0  m/z  PtC10H11N4,  

430.0  m/z  PtC10H12O2N5,  388.0  m/z  PtC10H13O2N2.  A  structure  is  proposed  for  one  of  

the  photoproducts  (388.0  m/z):  this  is  a  Pt(III)  ion.  Indeed,  the  photoactivation  of   Pt(IV)   diazido   complexes   have   previously   been   reported   to   generate   this   type   of   species  (see  Figure  4.6).22  The  intensity  of  the  unknown  species  579.2,  750.2  and   857.2  m/z  decreases  over  the  time  of  irradiation,  Figure  4.7.  The  system  was  then   cleaned   by   continual   flushing   with   acetonitrile.   Further   new   unknown   platinum-­‐ containing  peaks  then  appeared  at  382.0  m/z  and  619.1  m/z,  Figure  4.8.  Even  under   dark  conditions,  and  a  continual  flow  of  clean  solvent,  these  species  remained.  

Figure  4.4  Top  spectrum  is  of  complex  8,  10  μM  in  50:50  methanol  and  DDW,  in  the  

dark   flowed   through   PEEK   tubing   only;   the   bottom   spectrum   is   the   complex,   100   μM   in   50:50   acetonitrile   and   DDW,   flowed   through   the   coupling   device   and   PCF   system   in   the   dark.   Additional   unknown   peaks   appear   in   this   spectrum   at   579.2,   750.2  and  857.2  m/z.  The  peak  at  388.1  m/z  is  assigned  as  [Pt(III)(py)2(OH)2]+,  494.1  

Figure  4.5  MS/MS  of  750.2  m/z  peak  ascribable  to  contamination  introduced  by  the  

coupling  device,  possibly  from  glue  used  in  the  construction.  Peak  isolation  width  of   6  m/z  collision  energy  15  eV.  Its  isotopic  pattern  suggests  the  presence  of  Pt  in  the   ion,   as   would   the   smaller   fragments   383.1   and   411.1  m/z   also   seen   by   direct   infusion.  The  loss  of  18  m/z  is  attributed  to  the  loss  of  H2O.  

The  photostability  of  these  contaminant  peaks  was  then  tested  by  switching   the   laser   back   on.   The   post-­‐irradiation   contaminants   appear   to   be   highly   photostable,   and   did   not   break   down   after   28   min   of   irradiation   and   constant   flushing  with  acetonitrile.  The  sample  cells  provided  effective  optical  coupling  from   the  laser  to  the  fibre,  but  also  introduced  contamination  and  have  large  pre-­‐  and   post-­‐irradiation  mixing  volumes  of  60  μL  each.  These  large  mixing  volumes  increase   the  time  of  analysis,  and  the  possibility  of  photoactivated  species  interacting  with   each   other   and   solvent   molecules.   An   alternative   method   of   sample   and   light   introduction   was   therefore   deemed   to   be   required.   Preferably,   the   construction   would  be  of  a  less  reactive  material  and  the  mixing  volumes  would  be  smaller  whilst   retaining  the  optical  properties  of  the  sample  cell.  

Figure  4.6  Proposed  structure  of  the  photoproduct  of  8,  388.0  m/z.  

Pt

HO

N

N

OH

+

Figure  4.7  Irradiation  of  complex  8  (100  μM)  in  acetonitrile  over  time  (t  =  0,  5,  8.5  

min),   infused   through   the   PCF-­‐MS   system.   Ions   that   reduced   in   intensity   with   irradiation   are   highlighted   in   purple,   whilst   those   that   increase   are   highlighted   in   orange.  

Figure   4.8  The   initial   mass   spectrum   of   the   instrument   being   flushed   with   HPLC-­‐

grade  acetonitrile,  showing  the  contamination  from  the  metallic  coupling  devices,   after   irradiation.   Ions   619.1   and   382.0  m/z   are   photostable.   A   small   amount   of   943.1  m/z,  [2M  +  H]+  was  also  detected.    

4.3.1.2   PCF-­‐MS   system   design   and   development:   plastic   microfluidic   coupling