Page 117 - Multidimensional Chromatography

Multidimensional Chromatography
Edited by Luigi Mondello, Alastair C. Lewis and Keith D. Bartle
Copyright © 2002 John Wiley & Sons Ltd
ISBNs: 0-471-98869-3 (Hardback); 0-470-84577-5 (Electronic)

5 Coupled-Column Liquid
Chromatography

CLAUDIO CORRADINI
Institute of Chromatography, CNR Rome, Italy

5.1 INTRODUCTION

Today, the various high performance liquid chromatography (HPLC) techniques rep-
resent one of the major parts of modern analytical chemistry and are often at the
forefront of new discoveries in chemistry, biochemistry, biology, pharmacy, clinical
chemistry, food chemistry and environmental sciences. However, HPLC, which has
had such an enormous impact on the practice of analytical chemistry, frequently
approaches its limits when applied to complex mixtures. In order to overcome this
difficulty, selectivity-enhancing steps, such as the use of more efficient columns or
the employment of appropriate gradient elutions, may be successful. In addition, the
derivatization of analytes with properly selected functional groups is usually carried
out in order to enhance detectability and selectivity. Other important methods
involve sample clean-up procedures, trace enrichment and matrix elimination.
However, sample pretreatment frequently presents difficult practical problems and
often takes up the majority of the total analysis time, as well as contributing signifi-
cantly to the final cost of the analysis, both in terms of labour and the consumption of
materials. In addition, the requisite manipulations may result in significant impreci-
sion which can greatly outweigh any variables in the actual chromatographic process
itself. Despite these complications, some type of sample treatment is frequently
mandated in order to achieve satisfactory resolution and quantitation of the analyte
of interest. Another closely related problem is column abuse, especially in trace anal-
ysis applications. In some instances, sample pretreatment can be quite effective;
however, too often the capacity of the column has been exceeded by several orders of
magnitude. Such excessive column fouling leads to poor peak shape and lower effi-
ciencies, with a corresponding loss in resolution and shorter column lifetimes. The
resolving power of HPLC may be enhanced significantly by the introduction of mul-
tidimensional liquid chromatographic techniques. These are chromatographic tech-
niques aimed at the determination of the concentration of one or more specific
analytes in a multicomponent complex matrix. When using such HPLC techniques,
the sample is separated either by more than one column or chromatographic mode,