Getting Started in HPLC

Section 2E. HPLC Detectors

Other types of detectors are less commonly used in HPLC:
  • Refractive Index (RI) detectors monitor the index of refraction of the column effluent (the mobile phase leaving the column). Every liquid has a characteristic index of refraction, and this index changes when a solute (the analyte) is dissolved in the liquid. The change in refractive index thus indicates elution of analytes from the column. Because RI detection is based on a property of the mobile phase, it is universal. In this respect, it differs from UV detection which is sample-specific. RI is used primarily for analytes which do not absorb in the UV. It is sensitive to small changes in mobile phase flow or composition and can be very sensitive to small changes in ambient temperature (good temperature control is required for successful operation of RI detectors).
  • Fluorescence detectors, like UV detectors, are sample-specific; they depend on a particular characteristic of the analyte. Because not all compounds fluoresce, fluorescence detection is much more selective than UV detection. This selectivity is enhanced because two wavelengths are used: the excitation wavelength and the emission wavelength. In many cases, these wavelengths can be tuned to allow discrimination between even structurally similar analytes. Because fluorescence detection measures emitted light (rather than transmitted light), it can be much more sensitive than UV detection (it is easier to measure an absolute quantity than a small difference between two larger values).
  • Electrochemical detectors, like fluorescence and UV detectors, are sample-specific; they rely on oxidation or reduction of the analyte at an electrode surface. They are used primarily for easily oxidized/reduced analytes such as catecholamines or sugars. Electrochemical detectors can be even more sensitive and selective than fluorescence detectors, but require scrupulous attention to reagent and solvent purity for successful operation.

Some HPLC "detectors" are actually complete analytical instruments in their own right. Examples of these "hyphenated" detectors include LC-MS (mass spectrometry) and LC-FTIR (Fourier-transform infrared spectroscopy). In many such systems, the "detector" is more complex and powerful than the LC system. The LC can often be considered as a sample cleanup/inlet system for the MS or FTIR.



2000, LC Resources Inc. All rights reserved.
Last revised: April 02, 2001.