Page 37 - Tandem Techniques

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The exit flow from the column passes to the detector. Column detector connections are designed to
minimize any extra column dispersion that might impair the chromatographic resolution. The most
commonly used detectors, in order of their popularity and versatility, are the UV detectors (single or
variable wavelength and the diode array detector), the refractive index detector, the fluorescence
detector and the electrical conductivity detector. With the exception of the refractive index detector, all
the detectors are fairly sensitive and reasonably linear in response. The most sensitive is the
fluorescence detector which unfortunately has the most restricted linear dynamic range, i.e. little more
than two orders of magnitude. The UV and electrical conductivity detectors are the next most sensitive
and have linear dynamic ranges in excess of three orders of magnitude. The refractive index detector,
although the least sensitive, has the advantage of being the closest to a universal detector and is,
therefore, frequently used for detecting the many substances that are not conducting, do not adsorb light
in the UV range of wavelengths, and do not fluoresce. All the detectors are non-destructive and are
flow-through devices and so the exit flow can be interfaced directly with another instrument. In a
similar manner to the gas chromatograph, the output of the LC detector usually passes to a computer for
data processing. In addition, the same computer will control the operation of the chromatograph as well
communicating with any tandem instrument to synchronize their operation.

Liquid Chromatography Columns

There are also two types of liquid chromatography columns, the packed column and the capillary
column. The latter, however, at the time of writing this book, is an academic novelty rather than a
practical analytical system. LC capillary columns are still in a state of development. They are used to a
small extent in supercritical liquid chromatography where they are sometimes associated with the mass
spectrometer in the tandem LC/MS technique. Unfortunately the loading capacity of the LC capillary
column is far too small for the sensitivities available from present-day LC detectors and consequently
they must be overloaded for practical use. Overloaded columns give relatively poor efficiencies and
thus, due to the