Analysis of Carbohydrates - Detectros

10 important questions on Analysis of Carbohydrates - Detectros

Why are UV detectors not used for carbohydrate detection?

Because they are non-chromophoric compounds.

How is the refractive index (RI) used?

  • Two cells present: reference and sample cell

  • Reference cell filled with eluent without any compound
  • Quite sensitive to environmental conditions (T, buffer, impurities)
  • Universal detection / quantification

  • No gradient elution allowed

How does multi angle light laser scattering work?

  • Reflection of light by molecules/ particles measured at different angles
  • Accurate measurement of Mw of molecule or aggregate
  • Information on molecules shape and flexibility
  • Discriminative towards small molecules
  • Over-estimating aggregates and high Mw populations
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How does viscometric detection work?

  • Pressure over different coils is measured and correlated to the viscosity
  • Discriminative towards small molecules
  • Keeping a constant temperature is very important.

How does a evaporative light scattering detector (ELSD) work?

The eluent (which needs to be volatile) and without salt is evaporated. Then the light scattering of the carbohydrate without solvent can be measured.

  • ELSD is prefered over RI since you can use a elution gradient because the eluent is evaporated.
  • Non linear response for OS above DP4 

How does the Pulsed Amperometric Detector (PAD) work?

Charged carbohydrates are oxidized under alkaline conditions at the gold surface of the detector. This gives a change in electrical current at the surface that can be converted to a signal.
  • PAD gives different responses for different sugars.
  • Lower for monosaccharides 

How does laser-induced fluorescence detection (LIF) work?

  • Molecules are first excited to higher energy levels by laser light and then emit light of lower wave lengths.
  • Carbohydrates have to be labeled with APTS since they are not fluorescent of them selves.
  • Good in combination with MS in order to identify carbohydrates.  

Why can you analyse arabinose oligomers easyere with PGC-HPLC then with HPAEC-PAD?

With HPAEC-PAD you need at lot of salt and therefore it can not be analyzed with the MS. This is not the case for PGC-HPLC.

Why is there a difference in response and/or shape of peaks and the different elution time(s)?

RI: all molecules are taken into account is used to determine the concentration of the molecules.
LS: Firs half gives more signal because it positively discriminates the big molecules, underestimates the intermediate ones and overlooks the small ones
IV: You don't see the small molecules, discrimination is based on MW.





Why are at least two detectors needed for a good evaluation? Which ones?

RI to determine the concentration and LS to identifie the big one. UV could be a usefull addtion.

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