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LC Detection Systems (1)

LC Detection Systems (1). Generally the most expensive part of the instrument Depends on analyte properties, and required sensitivity (and money) Choice may be assisted by obtaining spectra on traditional instruments A) Ultraviolet/Visible absorption (may extend to IR)

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LC Detection Systems (1)

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  1. LC Detection Systems (1) • Generally the most expensive part of the instrument • Depends on analyte properties, and required sensitivity (and money) • Choice may be assisted by obtaining spectra on traditional instruments A) Ultraviolet/Visible absorption (may extend to IR) • Most commonly used, Mass LOD = 100pg-1ng (1pg), 0.1-1mg for IR • Three major types: • The simplest (and cheapest) uses a mercury source & can only detect 254nm and 280nm • Tungsten and Deuterium sources used, together with interference filters • The best (and most expensive) uses a diode array to monitor the spectrum of the eluent over a wide range of wavelengths B) Fluorescence • Mass LOD = 1 -10pg (10fg) • Monitors emission at specific wavelengths, in response to excitation with a specific wavelength

  2. LC Detection Systems (2) C) Refractive Index • Mass LOD = 100ng-1mg (10ng) • Monitors changes in the refractive index of the solvent, caused by the presence of analytes • Universal, but non-specific, not very sensitive D) Mass Spectrometric • Mass LOD = 100pg-1ng (1pg) • Often found as a tandem or “hyphenated” technique E) Nuclear Magnetic Resonance Spectroscopic • A recent innovation in tandem techniques F) Electrochemical • Mass LOD = 10pg-1ng (100fg) • May be potentiometric, conductometric or amperometric • Most common is conductivity (esp for ion exchange and capillary electrophoresis) – see over

  3. LC Detection Systems (3) Conductivity: Simple cheap robust but… …need to solve the problem of detecting analyte ion in presence of large quantities of other ions (eluent) Suppressor Column • Ion exchange column of the opposite type to the analytical column • Converts eluent ions (but not analyte) to a non-charged (non-conductive) form Anion exchange The eluent is often sodium carbonate (Na2CO3), and the suppressor column is a strong acid type cation resin. 2Na+(aq) + CO32-(aq) + 2Resin-H+(s) 2Resin-Na+(s) + H2CO3(aq) Charged Neutral Cation Exchange The eluent is often hydrochloric acid (HCl), and the suppressor column is a strongbase type anion resin. H+(aq) + Cl-(aq) + Resin+OH-(s) Resin+Cl-(s) + H2O Charged Neutral

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