Page 228 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS
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7   ION EXCHANCE

       thus allowing one column to be regenerated while the other is in use. A more
       convenient arrangement, however, is the use of a hollow-fibre suppressor, such
       as that developed by Dionex Ltd, which allows continuous operation of the ion
       chromatograph.  In the  case  of  anion  analysis,  the  suppressor incorporates  a
       tubular  cation exchange membrane which is constantly regenerated by  dilute
       sulphuric acid flowing through  the outer casing.
         It is  appropriate  to  refer  here  to  the  development  of  non-suppressed  ion
       chromatography.  A simple chromatographic  system  for  anions  which  uses  a
       conductivity detector but requires no suppressor column has been described by
       Fritz and CO-~orkers.'~ The anions are separated on a column of macroporous
       anion exchange resin which has a very low capacity, so that only a very dilute
       solution (CU 10-4M) of  an aromatic organic acid  salt (e.g. sodium phthalate)
       is required as the eluant. The low conductance of the eluant eliminates the need
       for a suppressor column and the separated anions can be detected by electrical
       conductance.  In  general,  however,  non-suppressed  ion chromatography  is  an
       order of magnitude less sensitive than the suppressed mode.
       Detectors.  Although electrical conductance has been widely used for detecting
       ions in ion chromatography, the scope of the technique has been considerably
       extended by the use of other types of detector. It is convenient broadly to classify
       detectors into two series.
       1. Detectors employing electrochemical principles:
         (a) Conductimetric detectors. Conductance is a fundamental property of ions
             in  solution making it  an ideal technique  for monitoring  ion exchange
             separations because of its universal and linear response. It is the optimum
             mode  of  detection  for  strong  acid  anions  (pK, < 7),  providing  high
             sensitivity in the absence of background electrolyte.
         (b)  Amperometric detectors. This type of detector may be used to detect ions
             which are electrochemically active but not readily detected by conductance
             measurement, e.g. weak  acid anions such as CN-, HS-  (pK, > 7). The
             detector commonly features interchangeable silver or platinum  working
             electrodes  and may be  used  alone (no suppressor then being required)
             or simultaneously with a conductivity detector.
       2.  Detectors based  on established optical absorption and emission techniques,
         are typified by:
         (a) Spectrophotometric  detectors.  The  operation  of  spectrophotometric
             detectors  is  based  on  the  absorbance  of  monochromatic  light  by  the
             column effluent in accordance with the Beer-Lambert  law (Section 17.2).
             As most organic species have significant absorption in the UV region of
             the spectrum, these detectors  have  wide application.  Sensitivity clearly
             depends  on  how  strongly  the  sample  absorbs  at  the  wavelength  of
             maximum  absorption,  but  detection  limits  in  the  low- (or even  sub-)
             nanogram  range may be achieved in favourable conditions.
               An alternative approach using a spectrophotometric detector has been
             described  by  Small and  Millerzg The essential  feature here  is that  the
             eluting ion, which is commonly phthalate, must absorb ultraviolet light
             thus allowing its concentration to be monitored at a suitable wavelength
             as it emerges from the column. When a non-absorbing sample ion elutes
             from  the  column,  the  concentration  of  eluting  ion  decreases  and  the
             detector  registers  a  negative  response  which  is  proportional  to  the
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