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Study Notes: Detectors

The detector is the part of the apparatus that converts the signal from the bands eluting off the column into a readable chromatogram via a chart recorder. The characteristics of an ideal detector include the following.

  • Adequate sensitivity
  • Good stability and reproducibility
  • Linear response to analytes
  • Short response time independent of flow rate
  • High reliability and ease of use
  • Similarity in response towards all analytes
  • Non-destructive of sample
  • Minimal internal volume to reduce band broadening.

Types of detectors include the following.

  • Absorbance
  • Fluorescence
  • Electrochemical
  • Refractive index
  • Conductivity
  • Mass spectrometry
  • FT-IR
  • Light scattering.

Detectors are chosen on their ability to detect the analyte of interest in a linear fashion, with good sensitivity (detect small amounts) and good selectivity (detects only the analytes of interest). The chemistry of the analyte is important in detector selection.

The most commonly used detector is UV absorption. Basically this acts by emitting a band on UV light at a specific wavelength (such as 254 nm but others can be selected) through the mobile phase. This UV light is absorbed by chemical species in the mobile phase as they come off the column. The greater the concentration of a chemical species present, the more light is absorbed and the bigger the peak produced on the chromatogram. Obviously if the chemical species being detected did not absorb at 254 nm a different wavelength would be chosen.

 

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