Separation of analytes

Separation of analytes - context
Analyte mixtures and separation - 1
Analyte mixtures and separation - 2
Analyte mixtures and separation - 3
Introduction to chromatography
Chromatographic columns
Principle setup of a chromatographic unit
Example of separation
Chromatographic system - 1
Chromatographic system - 2
Standard gas chromatograph
(Old) liquid chromatography unit
(New) liquid chromatography unit
HPLC columns
Comparison GC and HPLC
Selftest
     1) Purpose of chromatographic process
           Answer
     2) Unsuitable compounds
           Answer
     3) HPLC vs. GC
           Answer
     4) Detectors
           Answer
     5) Solvent properties
           Answer
     6) Retention time
           Answer
     7) Analyte identification
           Answer
     8) Irreversible or reversible sorption process
           Answer
     9) Short analysis time
           Answer
     10) Retention factor
           Answer
     11) Breakthrough-curve
           Answer
Problems
End of lesson

4) Detectors

 

Question:

Which detectors are used for which compounds and in connection with which type of chromatography?

 

Answer:

HPLC: UV/VIS, Fluorescence, Diode array detectors (DAD). The compounds need to contain chromophores (chemically speaking: moieties with conjugated II-electron systems): these are groups that absorb UV- or visible light. NOTE that a compound that fluoresces always also absorbs UV-visible light. Yet, not every compound that absorbs light also fluoresces.

GC: Electron capture detector (ECD) for halogenated compounds or compounds with NO2 groups. Must have electronegative groups that capture electrons (hence the name 'electron capture detector').

Flame ionization detector (FID): has a pretty broad spektrum. However, there may be difficulties for compounds, which contain C-atoms that are already highly oxidized (e.g., a C-Cl bond). Why? In the FID, the chemical that eludes from the column is combusted in a flame. Combustion is an oxidation process. If the C-atoms in the chemical are already highly oxidized (as in C-Cl), there is less left to oxidize. Hence, one measures a smaller signal.

Mass spectrometers (MS) are the most universal detectors. They can be used for all compounds and in connection with both, GC and HPLC. MS are not discussed as part of this class. You can find information on MS in the script (advanced reading).