Page 21 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry
P. 21

P1: GJB Revised Pages
 Encyclopedia of Physical Science and Technology  En001f25  May 7, 2001  13:58






               560                                                                                  Analytical Chemistry


                 Stationary phases can be solid surfaces or surfaces  rate at which equilibrium is attained include the volume
               coated with liquids, while mobile phases can be liquids or  and average thickness of the liquid stationary phase and
               gases. Standard forms that stationary phases take are often  diffusion coefficients and relative equilibrium concentra-
               very fine granular powders spread evenly on planar glass  tion ratios of the mobile and stationary phases.
               or polymer supports (e.g., thin-layer chromatography) or  A feature of fundamental interest for any chromato-
               packed in columns. Also common are porous polymeric  graphic work is the resolution or separation capability of
               solids usually containing a microcoating of water (liquid  the technique. One equation useful for quantitative mea-
               stationary phase) or capillary tubes whose inside walls are  surement of resolution R can be written
               coated with a liquid stationary phase.
                 Thetheoryofchromatographyhasbeenreasonablywell                R = 2	Z/(W 1 + W 2 ),
               established with a “kinetic” or “rate” theory that describes
                                                                 where W 1 and W 2 represent the base width of two different
               the broadening of the bands or zones of separated com-
                                                                 peaks in a separation and 	Z represents the difference in
               ponents on a stationary-phase bed, describes their time
                                                                 elution of the maxima of the two peaks.
               of appearance at any particular point, and provides de-
               tails of the separation power or resolution of the particular
               system employed. Numerous equations describing zone  2. Gas Chromatography
               broadening have been proposed, the simplest being the
               general form of the van Deemter equation as derived for  This technique is one of the cornerstones of chromato-
               gas–liquid chromatography,                        graphic analysis, being suitable for the separation of very
                                                                 complex organic and inorganic gas samples. The mobile
                             H = A + B/V + Cv,                   phase is a carrier gas chosen to maximize separation ef-
               where A, B, C represent three kinetically controlled pro-  ficiency as per the van Deemter equation, while concur-
               cesses known as eddy diffusion, longitudinal diffusion,  rently maximizing the sensitivity of the detector. The sta-
               and nonequilibrium mass transfer, respectively. The term  tionary phase is either a solid or a viscous organic liquid
               v represents the mobile-phase flow rate, and H is defined  coated on a solid support (hundreds available) or an open
               as the height equivalent of a theoretical plate. This ter-  tubular capillary column (for very high efficiency separa-
               minology derives from the plate theory of distillations,  tions).
               where a distillation tower can be divided into a number of  A schematic representation of a gas chromatograph is
               equilibria steps or plates. In the chromatographic sense,  shown in Fig. 13, and it indicates that the sample is first
               the height equivalent of such a step represents simply the  vaporized by injection into a heated port, then passed
               length L of the separation bed divided by the number of  through a heated separation column, and finally detected.
               steps in the separation bed N (determined from efficiency  The small sample size that can be passed by capillary
               of separation):                                   columns usually necessitates supplementation of special-
                                                                 ized splitters at the injection port. A splitter accurately and
                                  H = L/N,
                                                                 reproducibly reduces the volume of sample that enters the
               The three kinetically controlled factors responsible for  column from standard sample injections done by syringe.
               zone broadening can be readily analyzed to improve sep-  Numerous detection devices exist, and the most common
               aration efficiency, which is equivalent to minimizing the  are summarized in Table VIII. Separation times of vari-
               value of H. Eddy diffusion refers to the torturous path  ous species eluting from the column are often controlled
               that sample species must take through a packed particle  by varying column temperature by a technique known as
               stationary-phase bed. The distance traveled by different  temperature programming.
               molecules as they migrate across the bed must vary since
               different paths are highly probable. This results in zone
               broadening and is critically dependent on average particle
               diameter and size distribution. Longitudinal broadening
               refers to the natural diffusion of molecules from a concen-
               trated band toward areas of lower concentration on either
               side. The most important parameters are those that con-
               trol molecular diffusion rates in the stationary phase and
               in the mobile phase. Nonequilibrium mass transfer recog-
               nizes the fact the mobile phase may move too quickly to
               allow concentration equilibrium to be established between  FIGURE 13 Schematic representation of a single-column gas
               the mobile and stationary phases. Factors that control the  chromatograph.
   16   17   18   19   20   21   22   23   24   25   26