PolyAmino Acids: Quality Control Methods FAQ


  1. What methods are used to determine molecular weights reported in the Sigma-Aldrich Certificate of Analysis?
  2. What viscometer and viscometry techniques are used to measure viscosity?
  3. What brands of GPC Laser Light Scattering instruments does Sigma-Aldrich use?
  4. What is the accuracy (reproducibility) for GPC MALLS/GPC RALLS molecular weight measurement?
  5. What is the accuracy (reproducibility) for viscosity molecular weight measurement?
  6. Are there any residual protecting groups left on the final polymer?
  7. What qualitative and quantitative analytical methods are used to measure the residual protecting groups?
  8. Does Sigma-Aldrich provide polydispersity profile for polyamino acids?
  9. What is the general range of polydispersity for polyamino acids listed in the Sigma catalog/website?

What methods are used to determine molecular weights reported in the Sigma-Aldrich Certificate of Analysis?
Two methods are used - (1) Viscosity and (2) either Gel Permeation Chromatography Multi-Angle Laser Light Scattering or Gel Permeation Chromatography Right-Angle Laser Light Scattering (GPC MALLS/GPC RALLS)

Molecular weight data by viscosity is provided for all polyamino acids. GPC MALLS/GPC RALLS molecular weight is provided for only selected polyamino acids. Please see the product listing for information and contact Sigma Technical Services for further details.

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What viscometer and viscometry techniques are used to measure viscosity?
Cannon®, Cannon-Fenske or Ubbelohde viscometers are used to measure viscosity. Contact Sigma Technical Services for details of the technique used for a specific product.

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What brands of GPC Laser Light Scattering instruments does Sigma-Aldrich use?
A Wyatt DAWN EOS GPC MALLS instrument is used for most water-soluble polyamino acids.

A Viscotek™ GPC RALLS instrument is used for measuring very low molecular weight polymers (<5,000 Da) and hydrophobic polyamino acids.

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What is the accuracy (reproducibility) for GPC MALLS/GPC RALLS molecular weight measurement?
Accuracy for GPC MALLS/GPC RALLS molecular weight measurement is ± 5 to 10% (within manufacturer’s suggested range).

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What is the accuracy (reproducibility) for viscosity molecular weight measurement?
Accuracy for viscosity molecular weight measurement is ± 5 to 10% (within experimental error).

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Are there any residual protecting groups left on the final polymer?
Yes, approximately 1% or less may be present. If the customer requires a lower content of residual protecting group, Sigma-Aldrich can develop a custom process to decrease the amount of residual protecting group present in the final product.

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What qualitative and quantitative analytical methods are used to measure the residual protecting groups?
UV spectrophotometry at 250 - 300 nm is used for qualitative testing and proton NMR spectroscopy is used for quantitative testing.

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Does Sigma-Aldrich provide polydispersity profile for polyamino acids?
Polydispersity is not routinely measured on polyamino acids. Upon customer request, Sigma-Aldrich can provide polydispersity profiles for any water-soluble homopolymers with >5,000 Da molecular weight.

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What is the general range of polydispersity for polyamino acids listed in the Sigma catalog/website?
The general range of polydispersity for our polyamino acids is 1.1 to 1.5. Generally the higher molecular weights give lower polydispersity, whereas lower molecular weights give higher polydispersity. This is a general rule of thumb but it is not always true for every product.

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