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.