Instant Notes: Analytical Chemistry

● Fluorescence detectorsare based on filter-fluorimeters or spectrofluori-

meters. They are more selective and can be up to three orders of magnitude

more sensitive than UV absorbancedetectors. The detector responds selec-

tively to naturally fluorescing solutes such as polynuclear aromatics, quino-

lines, steroids and alkaloids, and to fluorescing derivatives of amines, amino

acids and phenols with fluorogenic reagents such as dansyl chloride (5-

(dimethylamino)-1-naphthalene sulfonic acid).

● Refractive index(RI) monitors are the closest to being universal HPLC

detectors, as nearly all dissolved solutes alter the refractive index of the

mobile phase. They are differential detectors, generating a signal that

depends on the difference between the RIof the pure mobile phase and

the modified value caused by the dissolved solute, which can, therefore, be

positive or negative.

They are several orders of magnitude less sensitive than UV absorbance

detectors, but are invaluable in the separation of saturated solutes such as

carbohydrates, sugars and alkanes. They are highly temperature sensitive

and are very difficult to use with gradient elution because the sample and

reference cells cannot be continuously matched.

● Electrochemical detectorsare based on measuring either the conductance of

an aqueous mobile phase containing ionic solutes, or the currentgenerated

by the electrochemical reduction or oxidation of solutes at a fixed applied

potential (amperometry) (Topic C9 ).

164 Section D – Separation techniques

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Fig. 7. 3-D display mode for a diode-array detector (DAD).