Page 22 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry
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 Encyclopedia of Physical Science and Technology  En001f25  May 7, 2001  13:58







              Analytical Chemistry                                                                        561

              TABLE VIII Summary of Common Gas Chromatographic  TABLE IX Summary of Common High-Performance Liquid
              Detector Characteristics                          Chromatographic Detector Characteristics
                                               Limit of detection                                Limit of detection
                Detector type     Selectivity     (g sec −1 )    Detector type     Selectivity      (g ml −1 )

              Thermal conductivity  None           10 −9        Refractive index  None              5 × 10 −7
              Flame ionization  Combustibles       10 −12       Ultraviolet–visible  Chromophore-containing  5 × 10 −10
              Flame photometric  Sulfur, phosphorus  10 −12      absorption     species
                                species                         Fluorescence  Fluorophore-containing   10 −10
              Electron capture  Halogenated species  10 −13                     species
              Photoionization  None                10 −14       Amperometric  Electroactive functional  10 −12
                                                                                groups
                                                                Conductivity  Ionized species          10 −8

                3. High-Performance Liquid Chromatography
              This form of chromatography is very well suited for sep-  stationary phases for general laboratory glass column sep-
              arations of organic mixtures and often complements gas  arations. However, a tremendous advance in this technol-
              chromatography, since many organic species cannot be  ogy has occurred since the mid-1970s, resulting in the evo-
              volatilized readily. Separation efficiency can be similar to  lutionofhigh-performanceionchromatographscapableof
              that obtained from packed column gas chromatography  concurrent separation of monovalent and multivalent inor-
              and is usually achieved on an extremely thin stationary  ganic and organic ions in periods of minutes. A schematic
              phase coated onto small solid particles of micrometer di-  diagram of such an instrument is shown in Fig. 15. The in-
              ameter. The small size and high degree of regularity of  novations of this instrument lie in the development of new
              the packing material provide great resistance to solution  separation column resins of high efficiency and the devel-
              flow, thereby necessitating use of high pressure to force  opment of countercurrent fiber-based suppressor columns
              the mobile phase through a column. The technique has  that eliminate all but analyte ions from the mobile phase.
              greater flexibility than gas chromatography since the mo-  The elimination of all ions other than the analyte ions is ac-
              bile phase can be easily changed with dramatic effects on  complished by use of a semipermeable membrane which
              resolution. A schematic of such a chromatographic system  can only pass ions of one type of charge. A suppressor
              is shown in Fig. 14, illustrating the use of fluids as the mo-  solution is maintained on the outside of the membrane
              bile phase. Solvent programming refers to mobile-phase  and functions by exchanging ions with the mobile phase,
              solvent changes during the course of one chromatographic  maintaining charge balance and resulting in conversion of
              elution and in some respects is similar to temperature pro-  ions to nonconductive soluble compounds. For example,
              gramming. A summary of common detectors is provided  sodium ions of 2Na /CO 2−  in the mobile phase can be
                                                                                +
                                                                                     3
              in Table IX.                                      exchanged with hydronium ions of 2H 3 O /SO 2−  from
                                                                                                  +
                                                                                                       4
                                                                the suppressor solution, leaving H 2 CO 3 as a nonductive
                                                                compound in the mobile phase. The conductivity detec-
                4. Ion Chromatography
                                                                tor senses only the presence of analyte ions without a
              Ion chromatography has been known for many years, and  large background signal and can therefore provide detec-
              ion-exchange resins are perhaps the most commonly used  tion limits of 10 −10 M concentrations or better.
















                        FIGURE 14 Schematic representation of a solvent-programmed, high-performance liquid chromatograph.
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