Page 231 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN008B-382 June 30, 2001 18:58

698 Liquid Chromatography

FIGURE 28 Different experimental configurations for column switching. [From Snyder, L. R., and Kirkland, J. J.
(1979). “Introduction to Modern Liquid Chromatography, 2nd ed.” Wiley, New York, p. 697. Reprinted with permission.]

chromatography. Alternatively, this setup could be used means. Precolumn or postcolumn derivatization chem-
for column backflushing, which involves running the mo- istry is often an inexpensive and effective procedure to
bile phase backwards through the column. Sample com- remedy this situation. Precolumn derivatization chem-
ponents retained strongly at the front of the column can be istry is carried out prior to and usually separate from the
eluted easily to the detector. This same procedure can be chromatographic operation. The following conditions are
used to clean the front end of the column of irreversibly desirable for precolumn derivatization.
retained compounds.
1. Reaction stoichiometry and product structure are
V. SAMPLE DERIVATIZATION CHEMISTRY known.
2. The reaction should be reasonably fast and the
The previously described separation strategies are all derivatives stable in solution.
directed to improving analysis time and peak resolution, 3. The derivatives and excess reagent must be readily
assuming the separated components can be detected. separable and stable during the chromatographic step.
However, many compounds of interest, such as aliphatic
carboxylic acids or amino acids, are difficult to detect The primary advantages of precolumn derivatization is
by optical absorbance, fluorescence, or electrochemical that a wide variety of reagents are available for use since