Page 244 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

CHAPTER 8
COLUMN AND THIN-LAYER LlQUlD

CHROMATOGRAPHY

8.1 INTRODUCTION
The term 'liquid chromatography' covers a variety of separation techniques,
such as liquid-solid, liquid-liquid, ion exchange (see Chapter 7) and size-
exclusion chromatography, al1 of course involving a liquid mobile phase.
Classical liquid column chromatography is characterised by use of relatively
wide-diameter glass columns, packed with a finely divided stationary phase,
with the mobile phase percolating through the column under gravity. Although
many remarkable separations have been achie-ied, these are generally slow and
examination of the recovered fractions (e.g. by chemical or spectroscopic
techniques) can be tedious. Since about 1969, however, the development of
modern high-performance liquid chromatography (HPLC) has enabled liquid
chromatography to match the great success achieved by gas chromatography
in providing the following features:
1. high resolving power;
2. speed of separation;
3. continuous monitoring of the column effluent;
4. accurate quantitative measurement;
5. repetitive and reproducible analysis using the same column; and
6. automation of the analytical procedure and data handling.
High-performance liquid chromatography is in some respects more versatile
than gas chromatography since (a) it is not limited to volatile and thermally
stable samples, and (b) the choice of mobile and stationary phases is wider.
The present chapter is largely concerned with HPLC, together with a
summary of developments in quantitative thin-layer chromatography, but a
brief account of the various types of liquid chromatography is given first together
with a guide to the choice of appropriate separation mode.

8.2 TYPES OF LlQUlD CHROMATOGRAPHY
There are four main types of liquid chromatography which require discussion.
1. Liquid-solid chromatography (LSC). This process, often termed adsorption
chromatography, is based on interactions between the solute and fixed active
sites on a finely divided solid adsorbent used as the stationary phase. The
adsorbent, which may be packed in a column or spread on a plate, is generally
a high surface area, active solid such as alumina, charcoal or silica gel, the last